Introduction
Bohai Bay Basin is an important oil and gas basin in eastern China. By the end of 2017, 139 gas fields had been discovered in the Bohai Bay Basin, and the proven geological reserves of natural gas were 3 600×108 m3, including the discovery of natural gas geological reserves 305×108 m3, and condensate oil 896×104 t[1] in the largest Qianmiqiao condensate gas field. After more than half a century of exploration, no large natural gas fields have been found, so it has long been considered large-scale natural gas fields are unlikely to occur in the Bohai Sea area with strong late tectonic activity.
Previous studies have attributed the causes of limited natural gas discovery in Bohai Bay Basin to two aspects, hydrocarbon source rocks and caprocks: the main source rocks of paleogene Shahejie Formation in Bohai Bay Basin are humus type and mixed type, moderate in maturity, and mainly oil-producing, which is not conducive to the formation of large oily gas reservoirs[2,3,4]; the strong tectonic activities in the Bohai Bay Basin damaged the caprock, unfavorable for the preservation of natural gas[3,5]. In recent years, a lot of research on gas-forming material foundation, trap formation and evolution, high-quality reservoir genesis and natural gas preservation conditions[6,7,8,9,10,11,12,13,14,15,16] has been carried out with regard to Bohai Oilfield in Bohai Bay Basin. These research results have guided the successful discovery of Bozhong 19-6 large gas field of hundred billion cubic meters order, which marks the opening of a new chapter of deep natural gas exploration in Bohai Sea area.
In this study, based on a myriad of data such as drilling, seismic, core and geochemistry etc., the basic conditions of deep natural gas accumulation in Bohai Sea are analyzed, the reservoir formation characteristics of 19-6 gas field in Bozhong is summarized, and the reservoir formation pattern for deep buried low buried hill-sandy conglomerate complex condensate gas field is established, which provides reference for natural gas exploration in Bohai Bay Basin and other similar oil-type continental faulted basins.
1. Regional geological overview
Bozhong Sag is in the middle and east Bohai Sea area of Bohai Bay Basin, and the Bozhong 19-6 gas field is located in the southwest of Bozhong Sag (Fig. 1a). The 19-6 structure is located in the Bohai 19 tectonic ridge, which, together with the Bohai 13 tectonic ridge on the west side and the Bohai 21-22 tectonic ridge on the east side, constitutes a low buried hill group with an area of nearly 400 km2 (Fig. 1b). These 3 tectonic ridges are complex tectonic ridges developed on the Paleozoic basement and formed by the cutting of the Tanlu strike slip fault. The 19-6 gas field in Bozhong is cut into two parts, i.e. east and west, by the NS strike Tanlu fault, and further cut into complex fault blocks by the near EW secondary faults (Fig. 1b).
Fig. 1.
Fig. 1.
Location map (a) and structural outline map (b) of the study area.
In Bozhong Sag, the distribution of buried hill formation varies widely in vertical and horizontal directions, from the Mesozoic, Lower Paleozoic and Paleozoic 3 strata in the north to the Paleozoic 1 strata in the south, and the thicknesses of lower Paleozoic and Mesozoic change widely between 50 m and 1 200 m and between 100 m and 1 600 m respectively. The overlying Cenozoic is up to 4 500 m thick, including Paleogene Kongdian Formation, Shahejie Formation and Dongying Formation, Neogene Guantao Formation and Minghuazhen Formation, as well as Quaternary. The Kongdian Formation is dominated by conglomerate of 400-700 m thick, and the lower part of Shahejie Formation and Dongying Formation are mostly thick dark gray and gray lake mudstone, with thin layers of siltstone and fine sandstone. The upper member of Dongying Formation to Neocene is composed of mainly interlayers of sandstone, sandy conglomerate and mudstone of different thicknesses, representing river facies and delta facies.
The main gas layers in Bozhong 19-6 gas field are Paleozoic and the Kongdian Formation above low buried hill. The west, east and northeast of the gas field are surrounded by the southwest subsag, south subsag and main subsag. In the subsags, develop 3 sets of high quality source rocks, i.e. Sha 1 Member, Sha 3 Member and Dong 3 Member. The source rocks, in the mature-post mature stage, overlie the low buried hill and conglomerate or contact directly with them through large fault sections.
2. Basic conditions for the formation of 19-6 gas field in Bozhong
2.1. Multiple sets of thick high-quality humic-sapropelic source rocks in Bozhong Sag
Since the Oligocene, the Bozhong sag has been the settlement and sedimentary center of the whole Bohai Bay basin, with hugely thick Cenozoic depositing, and it is the position thinnest in crust in the whole Bohai Bay basin, with obviously higher terrestrial heat flow value, which creates good regional geological conditions for the formation of the gas source rocks and their high-evolution degree necessary for high quality natural gas field.
The subduction of the Pacific Plate to the Eurasian Plate resulted in mantle uplift and thinning of the crust, and superimposed strike slip effect gave rise to the Cenozoic Rift Basin[17,18]. The Bozhong Sag is located at the intersection of the Tanlu strike slip fault, Zhangjiakou-Penglai fault and Qinhuangdao-Lüshun fault, which is the end of the development and evolution of the whole Bohai Bay Basin. It gradually became the settlement and sedimentary center of the Bohai Bay Basin in the sedimentary period of the Kongdian Formation (Fig. 2), and accepted the hugely thick deposits of Shahejie-Dongying formations of Eocene-Oligocene and the Neocene Guantao-Minghuazhen formations [19]. With the migration of sedimentary center, 3 sets of lacustrine source rocks, Sha 1, Sha 3 and Dongying members developed in Bozhong Sag[20], among which the source rock of Dongying Formation is the major set of hydrocarbon source rock in Bozhong Sag. Hydrocarbon source rocks in Shahejie Formation and Dongying Formation combined are more than 1 000 m thick in general, and can be over 3 000 m at maximum, which constitutes large-scale gas source rock in Bozhong Sag.
Fig. 2.
Fig. 2.
Regional geologic profile of Bohai Bay Basin (for section location,
The mantle uplift and crustal thinning in Bozhong Sag are intense, the minimum burial depth of the Moho surface between the crust and mantle is only about 25 km[21], which is the thinnest area of the crust in the Bohai Bay Basin (Fig. 3). The special deep structure leads to high terrestrial heat flow background. The terrestrial heat flow value in the sag area is 60-65 mW/m2, and the terrestrial heat flow value in the surrounding salient area often exceeds 70 mW/m2[19]. Some previous studies concluded that the main reason why no large natural gas fields had been found in the Bohai Bay Basin was that natural gas resources were not abundant there[3]. To be specific, the hydrocarbon source rock organic matter in the Paleogene Shahejie Formation is mainly sapropelic type and mixed type, and mostly in the mature or high mature stage, and thus mainly generate oil and small amount of gas, making it hard to form large oil-type gas reservoir[2,4].
Fig. 3.
Fig. 3.
Crustal thickness of Bohai Bay Basin.
The high quality source rocks in the Shahejie Formation and Dongying Formation of Bozhong Sag have good types and high abundance of organic matter, TOC value up to 6%, and HI value up to 800 mg/g. But different from the hydrocarbon source rocks of the Sha 4 member in the continental Jiyang subsag formed in strong reduction and brine deposition environment with humus type kerogen[22,23], the high quality source rocks in Bozhong Sag have a ratio of pristane to phytane of 0.1-2.7, ratio of gammacerane to C30 hopane of 0.05-0.30, and paleosalinity (from trace element recovery) of 5‰-9‰, which indicate the source rock is mainly formed in the reduction-weak oxidation and brackish water-semi-brackish water environment. Moreover, the microscopic components of kerogen have higher contents of exinite and sapropelinite both, reflecting aquatic organisms and terrestrial higher plants both make great contribution to the organic matter in the source rock. Taking Well CFD23-3-1 as an example, the kerogen type index of the source rock is 29-87 (average 62), suggesting the kerogen is humic-sapropelic type. Under the whole rock optical analysis, the algae body is dispersed or unclear lamina stage, has high activation energy of hydrocarbon generation, (average oil-producing activation energy of 219-222 kJ/mol)[24], which is conducive to gas generation of the source rock in high thermal evolution stage. Based on the results of hydrocarbon generation simulation experiment, in combination with the evolution profile of the hydrocarbon source rock in drilled wells, it is concluded that the oil generation threshold depth of the Bohai Sag is 2500 m, at the source rock Ro of 0.5%; the lower limit depth of oil generation peak is 4 100 m, at the source rock Ro value of 0.9%; and the liquid hydrocarbon window range is 2100-4 750 m, at the source rock Ro of 0.5%-1.5%. The burial depth of source rocks in Bozhong Sag is over 5 000 m in general and up to 12 000 m. The large burial depth and high heat flow make the source rocks in the sag reach mature-over-mature and gas generation stage, with vitrinite reflectance of 1.2%-3.6%.
The hydrocarbon expulsion intensity of the Dong3 Member and Sha1 Member in the south of Paleogene Bozhong Sag is about 1000×104 t/km2[13], in which the gas expulsion intensity exceeds 20×108 m3/km2. The gas expulsion intensity of the subsag area is even higher, which satisfies the formation conditions of large gas field[25]. It is concluded through study that the amount of natural gas resources in Bozhong Sag may exceed trillions of cubic meters, with sufficient natural gas resources to form large gas field.
2.2. Large-scale low amplitude buried hill Kongdian Formation conglomerate massive composite trap generated by multi-stage tectonic evolution
The Bozhong 19-6 structure is a large, multi-layer system, multi-structure, deep buried low buried hill composite trap developed on the Paleozoic basement (Fig. 4), complex in type and genesis.
Fig. 4.
Fig. 4.
Geological profile in the southwest of Bozhong Sag (for section location,
From the stratum point of view, the Bozhong 19-6 trap group is above Paleozoic buried hill, and in some areas, it covers the Mesozoic and Tertiary Kongdian Formation sandy conglomerate. It can be divided into two types from the perspective of structure, namely, the single-layer structure type that only develops Paleozoic buried hill, and the type of double-layer structure including the sandy conglomerate mass of Paleozoic buried hill-Mesozoic buried hill and Paleozoic buried hill-Kongdian Formation. The traps of single-layer structure type are mainly distributed in the middle and north of the Bozhong 19-6 structure, the Paleozoic buried hill-Mesozoic buried hill double-layer structure is mainly distributed on the west side of the Bozhong 19-6 structure, and the double-layer structure traps of sandy conglomerate mass in Paleozoic buried hill-Kongdian Formation are mainly distributed in the southern part of the Bozhong 19-6 structure. The traps are mainly faulted block, anticline and nose structural traps, with a single trap area of 2.0-73.7 km2, total area of 250.8 km2, amplitude of 75-1 225 m, and high point buried depth of 3825-5 475 m.
The formation and evolution of Bozhong 19-6 low buried hill traps mainly experienced 4 tectonic stages: Indosinian - early Yanshanian period extrusion thrust orogenic stage, mid-Yanshanian tensile block uplift stage, late Yanshanian inverted folding uplift stage, and Himalayan period transformation and burial stage (Fig. 5).
Fig. 5.
Fig. 5.
Schematics of the tectonic evolution history of Bozhong 19-6 low buried hill (for section location,
Before the Indosinian period, the Caledonian and Hercynian movement experienced by the North China platform[26,27] mainly caused vertical uplifting, forming only wide and gentle folds, the Upper Ordovician-Lower Carboniferous deposits were missing, and low amplitude anticline traps were developed (Fig. 5a).
During Indosinian-Early Yanshanian Period, under the effect of continuous strong extrusion[28,29], a large number of near east-west thrust faults came up. At the same time, accompanied by the strong sinistral torsion activity of the NS strike slip fault, the Bozhong 19-6 tectonic region fold uplifted and suffered denudation, with almost the whole lower Paleozoic denuding away, and the Paleozoic metamorphic rock exposing to surface. The early anticline traps were complicated by faults, forming anticline, fault block and broken nose trap groups with larger amplitude and area (Fig. 5b, 5c).
In the middle of Yanshanian stage, there was a fundamental change in the characteristics of tectonic system and stress field in North China[30,31,32,33,34,35], and the study area changed from the previous compresso-torsional shear stress field to the tensile shear stress field. The previously near EW faults experienced stretching & inversion, and the strike slip faults experienced large-scale sinistral strike slip. In the 19-6 tectonic region in Bozhong, which was the faulted block differential uplift area, the anticline, faulted block and broken nose trap groups were further fragmented and complicated (Fig. 5d).
In the late Yanshan stage, the stress field of the study area turned into near SN weak extrusion one, and the Bozhong 19-6 structure area uplifted further, forming the tectonic pattern high in the north and low in the south. In the north, are mainly faulted anticline and broken nose traps of anticline morphology, and in the south, faulted block traps held between faults (Fig. 5e).
In the early stage of Himalayan stage, on one hand, under the effect of strong rifting, the existent faults reactivated and expanded. In sedimentary period of Kongdian Formation, the northern part of the area suffered denudation, and the southern part received sandy conglomerate deposits; on the other hand, there was episodic compressive torsion, the southern part experienced reverse uplifting, and complex faulted block trap groups in the south and north with anticline setting basically fixed in shape. In the late Himalayan period, the study area turned into a relatively calm depression period, with little transformation, and the buried hill was quickly covered and buried by overlying sediments, forming low buried hill structure (Fig. 5f).
2.3. High quality reservoirs of diverse lithologies
The analysis of regional tectonic evolution shows that the Bozhong 19-6 structure in the Indosinian-Yanshan period and Himalaya period witnessed strong Tanlu strike slip fault activity, which controlled the formation of fractures in Paleozoic metamorphic rocks and sandy conglomerate in the Kongdian Formation. Affected by the continuous strike slip effect of Tanlu fault since the Indosinian period, multi-stage fractures and dynamic metamorphic rocks such as fractured rocks developed in the reservoir of Paleozoic metamorphic rock, forming large fractured high-quality reservoirs in the buried hill. Affected by the activity of Tanlu fault in Himalayan period, a large number of fractures formed in the sandy conglomerate of the Kongdian Formation, which provided condition for the formation of deep buried sandy conglomerate fracture-pore type high-quality reservoir.
2.3.1. Characteristics and main controlling factors of Paleozoic metamorphic rock reservoir
There are a variety of rocks in Paleozoic buried hill of Bozhong 19-6 structure, mainly metamorphic rock and late intrusive veins. The metamorphic rock is dominated by gneiss (Fig. 6a, 6b), metamorphic granite (Fig. 6c), mixed gneiss (Fig. 6d), fractured rock (Fig. 6e) and porphyry (Fig. 6f). The invading veins are mostly diabase (Fig. 6g), granitic porphyry (Fig. 6h) and ivernite (Fig. 6i), which mostly intersperse in rock branches in the metamorphic rock. The reservoir space in Paleozoic metamorphic rock can be divided into 3 types, weathering and leaching pore (fracture) (Fig. 6j), fracture in mineral particle crystal (Fig. 6k) and tectonic fracture. The observation under the microscope shows that the fractures were formed in multiple periods, (Fig. 6l), and provide effective reservoir space for gas accumulation. The porosity and permeability test results of 228 metamorphic rock cores show that the porosity is 0.2%-21.9% (4.4% on average) and the permeability (0.003-614.784)×10-3 μm2 (on average 5.050× 10-3 μm2), which indicates that metamorphic rock reservoirs in this area are strongly heterogeneous.
Fig. 6.
Fig. 6.
Rock types and fracture characteristics of buried hill in Bozhong 19-6 gas field. (a) Well BZ19-6-2sa, 4 277 m, biotite granodiorite gneiss, core photograph; (b) Well BZ19-6-1, 4 475 m, biotite plagioclase gneiss, plane polarized light; (c) Well BZ19-6-4, 4 574 m, biotite monzonitic granite, orthogonal optical light; (d) BZ19-6-2sa, 4 218.5 m, mixed gneiss, core photograph; (e) Well BZ19-6-2Sa, 3988.5 m, oblique long slices of hemp fractured rock, orthogonal optical light; (f) Well BZ19-6-1, 4 157 m, shredded porphyry, shredded plaque structure, orthogonal optical light; (g) Well BZ19-6-2sa, 4 076 m, diabase, plane polarized light; (h) Well BZ19-6-2sa, 3 939.5 m, granitic porphyry, plane polarized light; (i) Well BZ19-6-2sa, 4 025 m, ivernite, orthogonal optical light; (j) Well BZ19-6-1, 4 030 m, early weathering and leaching seams; (k) Well BZ19-6-1, 4 052 m, crystal internal micro-fracture, orthogonal optical light; (l) Well BZ19-6-1, 4167 m, multi-stage fractures cutting each other, plane polarized light.
Different from the conventional zonation mode, the buried hill reservoir of metamorphic rock in Bozhong 19-6 gas field is divided into 2 segments, and a tight zone of about 200 m thick is held between the 2 reservoir segments (Fig. 7), which will be one of the research focuses in the next stage.
Fig. 7.
Fig. 7.
Zonation of metamorphic rock reservoir in Well BZ19- 6-7. GR—natural gamma; Rd—deep lateral resistivity; ϕCNL—compensating neutron; ρ—density.
The high-quality reservoir of Paleozoic metamorphic rock is mainly controlled by 3 factors, lithology, weathering leaching and tectonics[36,37,38,39]. The lithology of 19-6 tectonic Paleozoic metamorphic rock in Bozhong 19-6 structure is mainly composed of monzonite gneiss, metamorphic granite and mixed granite. With high content of felsic brittle minerals, the rock would generate fractures easily under the effect of later tectonic movement, furnishing a good lithologic foundation. The metamorphic rock buried hill experienced long weathering and denudation, as a result, a large number of weathering pores and fractures emerge, greatly improving the physical property of the reservoir. Paleozoic buried hill has experienced multi-stages of tectonic movements since the Indosinian period, so fractures of different strikes were created, forming a fracture network, which provided reservoir space for the later oil and gas accumulation. In the study area, the Paleozoic fractures were mainly formed in Indosinian, Yanshanian, early Himalayan and late Himalayan (Fig. 8). In the Indosinian period, affected by the collision between the Yangtze Plate and the North China Plate, a large number of near NWW thrust faults, and a large number of near NWW extrusion fractures came about. During the Yanshanian period, the pacific plate subducted toward the East-Asia continent along NNW direction[40,41], the Tanlu fault experienced sinistral spin extrusion, deriving a large number of EN extrusion fractures. In the early Himalayan period, mantle column activity caused the basin rifting, giving rise to a large number of near SN tensile faults, and further deriving near EW tensile fractures. during late Himalayan period, the Pacific changed in subduction direction, dextral strike slip happened to the Tanlu fault, deriving a large number of NE tensile fractures. On the whole, the 4-stage tectonic fractures form 3 sets of fracture systems, which constitute the main reservoir space of the metamorphic rock reservoir.
Fig. 8.
Fig. 8.
Diagram of tectonic stress and fracture development in Paleozoic of Bozhong 19-6 gas field in different periods.
The first reservoir segment has a gas layer of 40-300 m thick, net gross ratio of 0.18-0.68, porosity of 0.6%-17.0% (on average 7.1%), and permeability of (0.05-90.30)×10-3 μm2 (on average 7.40×10-3 μm2). Different well areas differ widely in reservoir development degree. Under the influence of weathering and leaching, the high-quality reservoir is concentrated in the top 120m of buried hill. The reservoirs below are affected by lithology and fracture development, and show the characteristics of high quality reservoir alternate with poor reservoir. The first reservoir segment is divided from the top down into weathered sandy conglomerate zone, weathered fracture zone and internal fracture zone (Fig.7). The weathered sandy conglomerate zone is distributed at the top of the buried hill, mainly formed by weathering and leaching. The components of weathered sandy conglomerate are mainly metamorphic rock particles. The reservoir space is pore type (Fig. 9a, 9b) with a small number of fractures, and the reservoir physical property is the best. The weathered fracture zone is in the upper part of buried hill, and influenced by tectonic action and weathering leaching, and the reservoir space type is pore-fracture type and fracture type (Fig. 9c), with a large number of dissolution expansion pores along the fractures observed under the microscope (Fig. 9d). The inner fracture zone is mainly caused by tectonic action, in which the feldspar minerals deformation under the action of tectonic stress (Fig. 9e) and shredded matrix filling in the fault zone are also found (Fig. 9f) .
Fig. 9.
Fig. 9.
Reservoir space characteristics of different zones of Paleozoic metamorphic rock reservoirs in Bozhong 19-6 gas field. (a) Well BZ19-6-4, 4 417 m, weathered sandy conglomerate, rich in pore, core photograph; (b) Well BZ19-6-8, 4 572 m, weathering leaching dissolution pores, core photograph; (c) Well BZ19-6-2sa, 4 442 m, fractures, plane polarized light; (d) Well BZ19-6-, 4 050 m, dissolution pores along fractures, plane polarized light; (e) Well BZ19-6-4, 4 671 m, feldspar particles deformed by tectonic stress, orthogonal optical light; (f) Well BZ19-6-4, 4 697 m, shredded base filling in fracture zone, orthogonal optical light; (g) Well BZ19-6-7, 5 403.5 m, fractures, orthogonal optical light; (h) Well BZ19-6-7, 5 351-5 352 m, fractures, imaging logging.
The second reservoir segment is mainly a fractured reservoir formed by tectonic action, with a thickness of about 230 m, a net gross ratio of 0.52, a porosity of 0.2%-10.9% (on average 2.8%) and a permeability of (0.04-0.05)×10-3 μm2 (on average 0.05×10-3 μm2). A large number of fractures are found in both microscopic observation and imaging logging (Fig. 9g, 9h), which have no obvious zonation.
The bedrock zone is located in the lowest part of buried hill, mainly fresh rock minimally affected by weathering and tectonic action, with few fractures. It is metamorphic rock buried hill reservoir with the worst physical property, which is non-reservoir layer.
There are two obvious reservoir segments in the Paleozoic metamorphic rock buried hill in the Bozhong 19-6 gas field, therefore the gas field still has a good reservoir more than 1000 meters below the top of the buried hill. The total thickness of the reservoir is huge, which greatly broadens the exploration field of buried hill.
2.3.2. Characteristics and main controlling factors of sandy conglomerate reservoir in Kongdian Formation
The sandy conglomerate reservoirs in the Kongdian Formation are mainly distributed in the south of Bozhong 19-6 gas field. During the sedimentary period of the Kongdian Formation, the lake basin was in the period of initial fault depression, the area of lake basin was limited, and the range of sources was large. Sag controlled fault activity proceeded for long, with the development of fan delta deposition, providing a large number of coarse debris material close-range displacement. With the rise of the relative lake level, the fan delta had the characteristics of regressive development, forming a multi-stage superimposed, large thickness of coarse clastic sediments in the vertical upward manner (Fig. 10). Drilling revealed that the maximum thickness can reach 736 m. The sandy conglomerate in the Kongdian Formation mainly originates from the Paleozoic metamorphic rock with small particle size and better sorting, and a small number of particles are the Mesozoic volcanic rock with large size and subangular-angular shape (Fig. 11a, 11b). That means the sources of materials are mainly Paleozoic, followed by Mesozoic. The sandy conglomerate has a porosity of 3%-14.5%, on average 7.8%, permeability of (0.02-14.50)×10-3 μm2, on average 4.39×10-3 μm2 (Table 1), belonging to the ultra-low porosity-ultra low permeability reservoir. The reservoir space is mainly composed of pores and fractures, in which the pores mainly include primary intergranular pores (Fig. 11c), intergranular dissolution pores (Fig. 11d), and dissolution intragranular pores (Fig. 11e). The fractures mainly include internal fractures in gravels (Fig. 11f) and fractures penetrating gravels (Fig. 11g).
Fig. 10.
Fig. 10.
Sedimentary facies of sandy conglomerate in the Kongdian Formation of Bozhong 19-6 gas field.
Fig. 11.
Fig. 11.
Lithology and reservoir space types of the Kongdian Formation sandy conglomerate in Bozhong 19-6 gas field. (a) Well BZ19-6-3, 4 211.00 m, sandy conglomerate, core photograph; (b) Well BZ19-6-3, 4 048.13-4 048.43 m, sandy conglomerate, core photograph; (c) Well BZ 19-6-3, 4 051.80 m, intergranular primary pore, single polarized ; (d) Well BZ 19-6-3, 3 818.59 m, intergranular dissolution pore, single polarized; (e) Well BZ 19-6-3, 3 850.61 m, granular dissolution pore, single polarized; (f) Well BZ 19-6-1, 3 576.00 m, conglomerate internal fractures, single polarized; (g) Well BZ 19-6-1, 3 585.00 m, conglomerate penetrating fractures, single polarized.
Table 1 Statistics on physical property of sandy conglomerate samples from Kongdian Formation in Well BZ19-6-1.
| Depth/m | Porosity/% | Permeability/10-3 μm2 |
|---|---|---|
| 3 571 | 9.0 | 9.800 |
| 3 585 | 8.9 | 6.410 |
| 3 595 | 9.1 | 14.300 |
| 3 617 | 6.6 | 1.800 |
| 3 637 | 12.2 | 0.503 |
| 3 660 | 11.4 | 7.830 |
| 3 675 | 14.5 | 8.380 |
| 3 685 | 9.3 | 3.110 |
| 3 695 | 3.2 | 0.458 |
| 3 710 | 11.8 | 14.500 |
| 3 735 | 9.8 | 12.500 |
| 3 765 | 7.3 | 1.010 |
| 3 780 | 7.2 | 2.040 |
| 3 793 | 7.5 | 0.840 |
| 3 829 | 3.3 | 0.385 |
| 3 848 | 4.7 | 0.021 |
| 3 860 | 6.9 | 0.172 |
| 3 890 | 4.2 | 0.971 |
| 3 920 | 3.6 | 0.328 |
| 3 940 | 6.4 | 3.460 |
| 4 011 | 7.1 | 3.270 |
The formation and evolution of sandy conglomerate reservoirs are influenced by sedimentation, tectonics and diagenesis, among which sedimentation is the foundation, and tectonic action and diagenesis are critical control factors. The main material source of sandy conglomerate in the study area is Paleozoic metamorphic rock, and the parent rock in the provenance area was subjected to weathering and leaching, with a large number of fractures and dissolved pores forming. In the process of transportation, fractures and pores in the gravels further expanded. After deposited, these pores could be preserved, forming inherited reservoir space to store oil and gas. In addition, in the Himalayan period, Tanlu fault activity controlled the formation of high-quality reservoir in the deep sandy conglomerate. Through slice observation, it is found that in the sandy conglomerate are a lot of fractures penetrating particles under the effect of tectonic stress formed in late diagenetic stage, which play a critical role in improving the permeability of sandy conglomerate reservoir and provide good pathways for the dissolution of reservoir by CO2 and other acidic fluids in the later stage as well.
2.4. Thick lacustrine overpressure mudstone caprock provides good preservation conditions for natural gas accumulation
Lacustrine mudstone of 490-1 200 m thick in the Dong2 submember-Shahejie Formation in Bozhong 19-6 structure (Fig. 12). Drilling data and pressure simulation analysis revealed that this set of mudstone generally developed overpressure with a distinct double overpressure layer structure, which can be divided into upper overpressure layer (Dong 2 submember + Dong 3 mudstone) and lower overpressure layer (Shahejie Formation mudstone). With a formation pressure coefficient of 1.2-1.8, upper overpressure layer is widely distributed in Bozhong 19-6 structure and a regional high quality caprock. In contrast, the lower overpressure layer has a higher formation pressure coefficient of up to 2.0, but limited distribution range mostly in the slope area of the subsags and the slope of buried hill structure, thus it is a local caprock.
Fig. 12.
Fig. 12.
Development characteristics of Paleogene thick mudstone in Bozhong 19-6 structure (see
The regional tectonic analysis shows that there is a great difference in the intensity of the neotectonic movement between the Bozhong 19-6 structure region and the east branch of the Tanlu fault in the east of the Bohai Sea. The neotectonic movement of the eastern branch of the Tanlu fault was active, resulting in rich fractures, which is conducive to the migration and accumulation of oil and gas along the active fractures to the shallow layers[42,43,44]; in contrast, the new tectonic movement was weak in Bozhong 19-6 structure region, with few faults developed in the late period, most faults failed to penetrate the thick mudstone segment (Fig. 12), which is not beneficial for oil and gas to migrate upward along the faults, but is conductive for oil and gas to accumulate as reservoir in deep zones. In the tectonic region, the stable distribution of the giant thick lake over-pressure mudstone caprock in the Bozhong structure area controls the migration and accumulation of natural gas under it on one hand, and prevents natural gas from dissipating upward at a faster speed on the other hand, providing a good preservation condition for natural gas accumulation.
3. Characteristics and model of reservoir formation in Bozhong 19-6 gas field
3.1. Gas reservoir features
3.1.1. Natural gas characteristics and gas sources
The Bozhong 19-6 gas field is a special condensate gas reservoir with ultra-high condensate oil content, with a gas oil ratio of 951-1 658 m3/m3, and ground condensate oil content of greater than 700 g/m3. The condensate oil has a density of 0.792 6-0.808 9 g/cm3 (average 0.799 9 g/cm3) at 20 °C, viscosity of 1.244-2.136 mPa•s at 50 °C, (on average 1.677 MPa•s), sulfur content of less than 0.03%, wax content of 11.80%-18.26% (on average 14.49%), colloid+bitumen content of 0.17%-1.42% (on average 0.88%), and freezing point of 12-22 °C (on average 18 °C). Compared with the condensate oil in other oil and gas fields of Bohai Sea area, it has the characteristics of "higher wax content and freezing point".
The natural gas has a alkane gas content of 83.58%-90.85% (on average 89.25%), CO2 content of 9.15%-16.27% (on average 10.64%), H2S content of (10.44-36.63)×10-6, representing "medium-high carbon dioxide content, trace hydrogen sulfide" natural gas (Table 2).
Table 2 Statistics on natural gas components and carbon isotopes in Bozhong 19-6 gas field.
| Well | Depth/m | Horizon | Natural gas component/% | H2S content/ 10-6 | Natural gas carbon isotope/‰ | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| CH4 | C2H6 | C3H8 | CO2 | CH4 | C2H6 | C3H8 | CO2 | |||||
| BZ19-6-1 | 3 566.8-3 634.0 | Kongdian Formation | 76.75 | 8.68 | 2.98 | 9.37 | 11.23 | -38.50 | -27 | -25.50 | -7.0 | |
| BZ19-6-1 | 4 043.4-4 142.0 | Archaeozoic | 70.85 | 8.04 | 2.82 | 16.27 | 14.19 | -38.80 | -27 | -25.60 | -3.6 | |
| BZ19-6-2Sa | 3 873.7-3 923.5 | Archaeozoic | 78.27 | 8.19 | 2.59 | 9.35 | 11.76 | |||||
| BZ19-6-2Sa | 3 879.0-3 998.7 | Archaeozoic | 77.78 | 8.22 | 2.78 | 9.19 | 19.81 | |||||
| BZ19-6-4 | 4 411.0-4 499.8 | Archaeozoic | 75.41 | 8.83 | 3.01 | 10.49 | 36.63 | |||||
| BZ19-6-5 | 3 500.0-3 566.0 | Kongdian Formation | 77.40 | 8.21 | 2.95 | 9.15 | 10.44 | |||||
| Average | 76.08 | 8.36 | 2.85 | 10.64 | 17.34 | -38.65 | -27 | -25.55 | -5.3 | |||
The carbon isotope composition of methane, ethane and propane in natural gas of Bozhong 19-6 gas field are -38.8‰ - -38.5‰, -27.0%, and -25.6‰ - -25.5‰ respectively. The contents of methyl cyclopentane and dimethylcyclopentane in C7 light hydrocarbon are 37% and 13% respectively. According to relevant standards[45,46,47,48,49,50], it is identified as oil type gas from the humic-sapropelic source rock (Fig. 13a).
Fig. 13.
Fig. 13.
Identification of genesis type of natural gas in Bozhong 19-6 gas field (modified as per the references [45-46, 51-52], I—coal-derived gas zone; II—oil type gas zone; III—inverted mixed gas zone; IV—coal-derived gas and oil gas zone; V—coal-derived gas, oil gas, mixed gas zone; VI—biogas and sub-biogenic gas zone).
The mean values of ln (C1/C2) and ln (C2/C3) of natural gas in Bozhong 19-6 gas field are 2.2 and 1.1 respectively, which are similar to those of the Upper Triassic gas in the central Sichuan region[51]. According to plates proposed by Guoliguo et al.[52], the gas is identified as kerogen degradation gas (Fig.13b, 13c).
By using the regression formula of carbon isotope composition of oil type gas methane and the vitrinite reflectance established by Dai Jinxing[45], the vitrinite reflectance corresponding to natural gas maturity of Bozhong 19-6 gas field calculated is 1.64%-1.71%, indicating high maturity, similar to the maturity of hydrocarbon source rock in the Sha 3 Member. Therefore, it is inferred that the main source rock is that in Sha 3 Member, meanwhile, it is speculated that the CO2 may come from the mantle. As the overpressure mudstone expulses hydrocarbon downward and laterally along fault planes and unconformity surfaces, part of the natural gas originates from the hydrocarbon source rocks in the Sha 1 Member and Dongying Formation.
CO2 carbon isotope composition δ13c is -7.0‰ - -3.6‰, in combination with CO2 content, it is inferred that the CO2 is of inorganic genesis (Table 2).
3.1.2. Distribution of natural gas
The main gas-bearing layers in Bozhong 19-6 gas field are Paleozoic buried hill and Paleogene Kongdian Formation. Drilling revealed that the buried hill gas layer is 40-400 m thick, featuring: (1) on the plane, the gas layer at structure high (106-271 m) is thicker than that at the structure low (40-45 m); (2) vertically, the gas layer is mainly distributed at the top of the buried hill, secondly, inside the buried hill. For example, in the south block, in the top 120 m of the buried hill, the net gross ratio is 0.80, the gas layer thickness is 96.5 m; while in the 470 m of the inner buried hill, the net gross ratio is 0.37, and the gas layer thickness is 174.5 m. In the north block, the top 120 m of buried hill has a net gross ratio of 0.98, and gas layer thickness of 117.9 m; the 210 m inner buried hill has a net gross ratio of 0.49 and gas layer thickness of 102.2 m.
The Paleogene Kongdian Formation gas layer has a thickness of 200-300 m and total gas-bearing area of nearly 10 km2. According to the reservoir quality and tested productivity, the reservoir is divided into two types: type I reservoir is distributed at the top of the Kongdian Formation, with a thickness of 160-200 m, better reservoir quality and higher productivity. Type II reservoir is distributed in the lower member of Kongdian Formation, with a thickness of 30-130 m, poorer reservoir quality and lower productivity than the type I reservoir.
In Bozhong 19-6 gas field, the Paleozoic buried hill is a massive gas reservoir, with the gas column height of up to 1194 m; the Paleogene Kongdian Formation is a layered gas reservoir, with a gas column height of 465 m. The 3P natural gas geological reserves of Paleozoic buried hill and Kongdian Formation sandy conglomerate reservoirs are over hundred billion cubic meters, and the geological reserves of 3P condensate oil are tens of millions of cubic meters, making it a rare large condensate gas reservoir in Bohai Bay Basin.
3.1.3. Gas reservoirs
The stratigraphic test data of Bozhong 19-6 gas field shows that the Kongdian Formation has a formation pressure of 45.57-46.96 MPa, formation pressure coefficient of 1.21- 1.36, formation temperature of 134.1-134.9 °C, representing weak overpressure and normal temperature gas reservoir. There are some differences in formation pressure of Paleozoic buried hill reservoir in the south and north block. The top of the buried hill in the south block has a formation pressure of 46.93 MPa, formation pressure coefficient of 1.26, and formation temperature of 152 °C; while that in the north block has a formation pressure of 48.72 MPa, formation pressure coefficient of 1.15, and formation temperature of 171.4 °C. Overall, the reservoir is of normal-slight overpressure, and normal temperature.
3.2. Reservoir formation mode
The Bozhong 19-6 gas field features near source and multi- kitchen overpressure hydrocarbon supply. In Bozhong Sag, the hydrocarbon source rock of the Paleogene Sha 3 member directly overlies the sandy conglomerate and the low buried hill, or the hydrocarbon source rocks in the Shahejie Formation and Dongying Formation contact with the low buried hill through faults. The oil and gas generated by the source rocks can get into and accumulate in the traps nearby through weathered crust and faults. The Bozhong 19-6 gas field is surrounded by the southwest subsag, south subsag and main subsag in the Bozhong Sag, each of which is a hydrocarbon generation center, so the gas field has hydrocarbon supply from multi-kitchens. The hydrocarbon source rocks in the stage of high evolution in the subsag zone generally have overpressure, which can provide sufficient power for oil and gas accumulation.
The Bozhong 19-6 gas field has a "golden source-reservoir- cap assemblage" of overpressure gas source rock, high quality caprock and normal-weak overpressure reservoir. The main reservoirs are sandy conglomerate and low buried hill metamorphic rock in the Kongdian Formation, and the overlying stratum is overpressure mudstone of 1 000 m thick[53].
The Bozhong 19-6 gas field features ultra-late rapid accumulation of natural gas. The current condensate gas field has experienced two main stages in geological history, e.g. early (24-5 Ma from present) reservoir formation and destruction, late-ultra late (5-0 Ma from present) reservoir adjustment and condensate gas reservoir conversion (Fig. 14). At the end of Paleogene (24 Ma from present), a small range of hydrocarbon source rocks in the south and central southwest subsags of Bozhong sag entered the mature stage and began to generate hydrocarbon, and oil and gas began to gather In Bozhong 19-6 structure into a small-scale reservoir, but it was destroyed by tectonic movement due to the shallow burial depth (about 2000 m) and poor cap conditions. The oil and gas suffered biodegradation and broke through the lower diagenetic degree mudstone cap of Dongying Formation and dissipated. The small amount of 25-norhopane in the condensate oil now proves the early reservoir shallow burial and biodegradation (Fig. 14a). During the initial period of Neotectonic movement (5 Ma from present), the source rocks in the south and southwest of Bozhong extensively entered mature-high maturation stage and generate a large amout of hydrocarbons. The Bozhong 19-6 structure began to gather oil at large scale. The neotectonic movement made some of the crude oil move upward along the faults and gathered again in the traps in shallow Guantao Formation and Minghuazhen Formation, forming the 19-4 medium-sized oilfield. The homogeneous temperature of saline inclusions symbiotic with hydrocarbon inclusions in the reservoir of Bozhong 19-6 gas field is 110-150°C, according to the burial history, the accumulation period is determined to be at around 5 Ma[54,55] (Fig. 14b) from present. In the late stage of Neotectonic movement, the source rocks in the south and southwest subsags of Bozhong area all entered high mature stage and generated large amount of gas. In the Bozhong 19-6 structure, natural gas began to gather on a large scale. The natural gas under high temperature and pressure invaded into the early oil reservoir, resulting in the dissolution of the soluble components of crude oil into natural gas and the deposition of left asphalt in the reservoir, that is conversion from the early oil reservoir to the condensate gas reservoir. In the upper part of the Bozhong 19-6 gas field trap, asphalt is commonly seen. Based on the Jacob[56] formula on asphalt reflectivity to vitrinite reflectance, the calculated equivalent asphalt vitrinite reflectance is only 0.9%, reflecting the asphalt is of gas invasion origin. The development abundance of oil inclusions in the reservoir of Bozhong 19-6 gas field is very high, with GOI value of up to 80%. The development abundance of gas inclusions is low, and the source rocks in the slope zone are still in the stage of massive gas generation, reflecting that the natural gas of the Bozhong 19-6 gas field is ultra-late accumulation. Ultra-late rapid accumulation is beneficial to the preservation of the gas field (Fig. 14c).
Fig. 14.
Fig. 14.
Buried hill gas reservoir accumulation in Bozhong 19-6 gas field (Ar—Archaeozoic; E2-3s—Shahejie Formation; E3d3— Dong 3 Member; E3d1-2—Dong 1 Member-Dong 2 Member; N1g—Guantao Formation; N1-2m—Minghuazhen Formation).
To sum up, the hydrocarbon source rocks in the southwest subsag, south subsag and main subsag in the Bozhong area have experienced the hydrocarbon generation and discharge process of early oil and late gas. The oil and gas discharged from the source rocks, under the control of high-quality mudstone cap of large thickness and stable regional distribution, move along the unconformity surfaces and faults. Especially, when the main source rock and the low buried hill juxtapose laterally, the lateral hydrocarbon supply window would be big. Meanwhile, the general overpressure of hydrocarbon source rocks provides strong dynamic condition for natural gas migration. The Bozhong 19-6 gas field went through the process of early oil and late gas accumulation. The natural gas gathered in large scale in ultra-late stage and transformed the oil reservoir to condensate gas reservoir (Fig. 15).
Fig. 15.
Fig. 15.
Reservoir formation pattern of Bozhong 19-6 gas field (for profile location,
4. Conclusions
The Bozhong Sag has unique geological conditions for the formation of large gas field. At the regional sedimentation and subsidence center, Bozhong Sag has several sets of thick humic-sapropelic high quality hydrocarbon source rocks with high maturity, which provides sufficient gas source. Multi-stage tectonic evolution gives rise to several types of composite traps. Tanlu fault activity causes the formation of reservoirs of different lithologies, dominated by buried hill metamorphic rock and Kongdian Formation sandy conglomerate. The buried hill reservoir is a massive gas reservoir and the sandy conglomerate in the Kongdian Formation is a layered gas reservoir. The thick lacustrine overpressure mudstone caprock provides good conditions for natural gas preservation.
The Bozhong 19-6 gas field is a condensate gas reservoir with super high condensate oil. The natural gas is the degradation gas of humic-sapropelic kerogen. The gas field features large gas layer thickness and gas column height, and first oil then gas accumulation. The reservoir features multi-source hydrocarbon supply, multiple normal pressure-slightly overpressure reservoirs, thick overpressure mudstone caprock, and ultra-late rapid accumulation. The discovery of Bozhong 19-6 gas field has opened up an area for natural gas exploration in Bohai Bay Basin, which has important reference significance for natural gas exploration in similar basins.
Reference
Characteristics and formation conditions of natural gas accumulation areas in Bohai Bay Basin
The geological characteristics and formation conditions for the main natural accumulation belts in Bohai Bay basin of East China are analyzed. The larger scale natural gas accumulation belts of the basin are all related to Permian\|Carboniferous coal\|genetic gas. The main oil\|type gas accumulation belts can be distributed in the shallow layers, and they are some secondary gas accumulation belts forming in the course of petroleum migration up along faults because of separating out of the resolved gas. The distribution of the natural gas accumulation belts are mainly controlled by the secondary structural belts. The main gas accumulation belts can be divided into four types of the uplift belt, prominence, fault bench belt and slope belt. The main controlling factors for generating natural gas accumulation belts in the basin include the excellent gas\|generating conditions, the secondary structural belts with long\|term developing and good sealing conditions.
Gas exploration potential in Bohai Sea
Hydrocarbon exploration in Bohai Sea lasted more than thirty years, but for gas discoveries, only middle or small fields are found. Based on a synthetical analysis of basin type, gas origins and gas prone sag evolution, Bohai Sea is considered as an oil dominant basin, where most gas is from oil type and terrigenous source rocks. Now there is still a considerable potential to discover gas because of less exploration in Bohai Sea.
Characteristics of helium isotopic composition in the Bohai Bay Basin and the Ordos Basin and their significance in gas-bearing
//CHEN Yong, WANG Shui, QIN Yunshan, et al.
Types, distribution sequence model and enrichment of natural gas pools in Bohai Gulf Basin
On the basis of the regional geological characteristics andnatural gas pools found in Bohai Gulf Basin,this paper studies the types,distribution sequence and model of natural gas pools.The analysis of three typical gas pools/fields-the Wenliu gas pool in Dongpu sag,the Suqian gas field in Jizhong depression and the Banqiao conden sate field in Huanghua depression leads to a discussion on the enrichment of gas pools,and the favourable exploration of finding the large or medium-sized gas fields in Bohai Gulf Basin.
Gas exploration potential in offshore Bohai Bay Basin
The exploration orientation of large and medium sized natural gas pools in Bohaiwan polymerized basin
DOI:10.7623/syxb200106001
URL
[Cited within: 1]
Bohaiwan Basin is a composite basin polymerized by a Mesozoic and Cenozoic rift basin and a Carboniferous remnant coal-bearing basin .The Mesozoic-Cenozoic-covering Carboniferous-Permian remnant coal-bearing b asins such as the northeast of Jizhong,the south of Jizhong,Huanghua,Dongpu,Jiya ng and Kaiping Depressions make up of the composite multi-source oil-gas-bear ing system consisting of Carboniferous-Permian coal-formed hydrocarbon source rock,Eogene-Mesozoic dead color shale source rock and Ordovician limestone sour ce rock.This system has the geosyncline-horst alternative geological configurat ion with much resource.It has 9 sets of source-reservoir-seal assemblage verti cally and particular gas reservoir sequence horizontally.It can form the fractur e-buried hill complex oil-gas accumulation combination characterized by the pa ttern of Palaeozoicera-Cenozoic generating_Palaeozoicera-accomulating_Mesozoic sealing.After the high accuracy gravitational prospecting,magnetic exploration, electrical survey,geological seismic integrated exploration and interpretation are carried out,a group of large and medium-sized natural gas pools may be fou nd through drlling.
Discussion on the source rock in Suqiao buried hill hydrocarbon reservoirs
DOI:10.7623/syxb200106004
URL
[Cited within: 1]
The Suqiao buried hill hydrocarbon is a representativ e's example of both primary reservoir in Paleozoic and breakthrough of explorati on history in coal-formed oil/gas fields at North China plateform in early 80 y ears.It was the focus reservoir made more attention by our domestic petroleum g eologists at that time.Based on the study of geological structures and potential reservoir conditions of Wenan slope Baxian depression,author concludes that h ydrocarbons of Suqiao reservoirs were mainly generated by lacustrine claystone o f Tertiary in Baxian depression,perhaps only few hydrocarbon generated by coal b eds.This paper also analyzed some public papers in which conclusions were the Su qiao reservoir generated from coal beds according to both the geochemistry and s o called evidents of coal formed hydrocarbon studies.Finally,the paper said so f ar there are no real strong evidents support to the coal formed gas and for some key items there are no satisfactorily explanation.
Hydrocarbon-generating potential and history of Carboniferous-Permian coal measure strata in Wenliu area
DOI:10.11743/ogg20040407
URL
[Cited within: 1]
The Carboniferous-Permian coal measure strata in Dongpu sag is a fair source rock unit of Wenliu gas pool due to its stable distribution and high organic content. There are two hydrocarbon-generating periods in the burial history of coal measures. The first one occurs when the T1-2 formation is deposited. Vitrinite reflectance, apatite fission track and fluid inclusion analyses indicate that the burial depth of coal measures is about 3500 m, with the palaeotem-perature of 118 and Ro of 0.80%-0.85% .Hydrocarbon generation of coal measures was interrupted in the Meso-zoic uplift and erosion period (about 1.8 million years) .During the Paleogene rifting period,the coal measures was entered into its secondary hydrocarbon-generating period as the burial depth increases, reaching its hydrocarbon-generating peak after the deposition of Dongying formation. The gas-prone source rock is mainly distributed in the region of Wendong-Qianliyuan, with the buried depth of 5290-6360m. The discovery maturity (percentage of ultimately proven reserve with respect to total resources) of gas resources in Wenliu area is more than 90% . In the effective gas supply area of Wenliu gas pool, the hydrocarbon-generating volume and hydrocarbon-expelling volume of Carboniferous-Permian coal measures are estimated to be 3253.3 million m3 and 2657.0 million m3 respectively, with a gas accumulation coefficient of 5.92%.
Main controlling factors and mode of natural gas accumulation in Bohai Bay area
Case of discovery and exploration of marine fields in Chin (Part 9): Qianmiqiao Ordovician condensate field in Bohaiwan Basin
Major controls on natural gas accumulations in deep-buried hills in Bozhong Depression, Bohai Bay Basin
An analysis of offshore gas exploration potential in terms of comparison between offshore and onshore areas in Bohai Bay Basin
Bohai Bay Basin is composed of onshore and offshore areas in geography.Oil and gas exploration results have revealed that it is mainly an oil production basin.For the onshore gas discoveries,most gas reservoirs are dominated by associated gas and less than 5 billion cubic meters in reserves,and other 4 bigger ones (over 10 billion cubic meters in proven reserves) are dominated by gas generated from coal material or gas in maturation to overmaturation.For the offshore area, gas fields have been finding continuously,in which JZ20 2 has been brought into development.Compared to the onshore area,there are much better conditions for gas accumulation in the offshore area:(1)The depocenter of the entire Bohai Bay Basin has gradually moved to the offshore area since Oligocene,resulting in much thicker Tertiary sedimentation, with the thickest section over 10,000 meters.(2)The offshore area may correspond a center of mantle bulge and is higher in geothermal flux than the onshore area.(3)In contrast to the onshore area, there is a Lower Dongying Formation source rock with higher gas generating ability in the offshore area.(4)There are two Tertiary regional seals offshore,forming a better sealing condition than the surrounding onshore area.The total offshore gas resources is estimated at 1229.3 billion cubic meters,according to a recent assessment,and it is believed that more and bigger gas fields will be discovered in the future,due to the present lower degree of exporation.
Geochemical characteristics and natural gas sources of Bozhong Depression, Bohai Bay Basin
Among the natural gases discovered in Bozhong depression, hydrocarbon gas is dominant and non hydrocarbon gas is small. Of the gases found below the Dongying cap rocks, most are wet gas and a few are dry gas. This dry gas was formed by pyrolysis. But among the gases found beneath the Minghuazhen cap rocks, most are dry gas and a few are wet gas. This dry gas was formed by biodgradiation. The gases from Shahejie and Dongying source rocks are mostly oil type gas. The gas of QHD30 1 is mixed gas and mainly from the coal derived gas from pre Tertiary source rocks. The formation of natural gas in the Bozhong depression is characterized by multiple sources, multi periods and continuous generation and expulsion. So its exploration potential is good in Bozhong depression. The regions where the Dongying cap rocks well developed are the major exploration targets of natural gas.
Natural gas distribution, main controls over accumulation and exploration targets in Bohai Sea
For the discovered gas reservoirs in Bohai sea,their distribution characteristics are analyzed and summarized,the main controls over their accumulation are discussed and the favorable formations and areas for the future gas exploration are determined.There is still a great potential of gas exploration in Bohai bay,in spite of its oil-prone nature as a basin,where the gas distribution are mainly controlled by four factors: gas-rich sag,Tanlu fault zone,high-quality seal,and palaeohigh.Eogene and Pre-Tertiary should be the main formations for the future gas exploration,with Neogene to be also considered.The low highs around Liaozhong and Bozhong sags,the uplifts in these sags and various structural zones and sedimentary bodies on the slope areas are the most important targets of gas exploration in the future.
Cenozoic tectonic evolution in Bohai Bay Basin province
The tectonic evolution of Bohai Bay basin province can be divided into two stages of rifting and post-rifting. and basically completed a cycle of taphrogenesis.The riftingstange during Paleogene is composed of three extensional periods which correspond theformations of Kongdian and 4th number of Shahejie,3rd number of Shahejie,and 2nd-1stnumbers of Shahejie and Dongying. There are obvious disparity in tectonic styles andkenematics between the three extensional periods.The evolution of extensional tectonics caninclude 2 paths:(1) from domino systems to listric systems and ramp-flat normal faultsystems:(2)from non-rotation normal fault systems to listric systems and ramp-flat normalfault systems.
The anatomy about the tectonic cause of formation of Bohaiwan Basin
At present,the explanations about the tectonic formative mechanism of bohaiwan basin mostly emphasize the action of manifold stress fashion or many periodical tectonic stress field,including extension + slip and extension + pull-apart. Based on the cause of formation of fault and the theory of rock distortion on the condition of microdistortion,these explanations are only speculations,and it isn't succinct about the explanation of dynamics mechanism. Indicated form the sandbox experiments,boundary geometry condition and rift extensional direction are the key factors that control the formative characters and distribution of the rift basin. Some evidences indicate that the complicated tectonic visage of bohaiwan basin is related to the complicated geometry conformation of the basin boundary. Sandbox experiments may provide revelation and bases for the embed studies of the Cenozoic tectonic cause of formation and dynamics mechanism of the bohaiwan basin.
Charging of the Neogene Penglai 19-3 field, Bohai Bay Basin, China: Oil accumulation in a young trap in an active fault zone
DOI:10.1306/09080808092 URL [Cited within: 1]
The organic-rich hydrocarbon source rocks and their resource potentials in Jiyang Deresssion
DOI:10.1007/BF02873097
URL
[Cited within: 1]
The organic-rich hydrocarbon source rocks in Jiyang Depression are analyzed for their main sedimentary and geochemistry characteristics and resource potentials. The E (2-3)s~l 3 organic-rich hydrocarbon source rocks was found first during the seventh five year plan in the well Niu38, then followed by E (2-3)s~u 4 and E (2-3)s~l 1. The three sedimentary unit all belong to the lacustrine transgression system tracts of different third-order sequence formed during the basin intense rift stage. The lithology assemblages consist mainly of deep-lacustrine shale, oil shale and laminated mudstone with strong sedimentary rhythm, which indicate that the sedimentary environments bear some characteristics of closed lake and favor the formation and preservation of organic matter. The hydrocarbon source rocks all have high abundance of organic matter, high hydrocarbon potential and show great heterogeneity. The resource evaluation history analysis and oil and source correlations show that the giant oil fields in Jiyang depression are mainly associated with the organic-rich hydrocarbon source rocks.
Gold tube pyrolysis study of source rock hydrocarbon generation in Bozhong area, offshore Bohai Bay Basin
Sealed gold tube pyrolysis was carried out for 3 source rock samples from the third(E_2s~3) and first(E_3s~1) members of Shahejie Formation,and the third(E_3d~3) member of Dongying Formation in the Shanan Sag,and 2 source rock samples from the E_2s~3 in the Qinnan Sag in Bozhong area.These samples are lacustrine shale with a high organic matter content and low maturity,and contain type I kerogen.Minerals in the source rock samples are mainly clay minerals and contain a significant amount of calcite,quartz and feldspar.The gold tube pyrolysis experiments focus on liquid hydrocarbon generation.The mean hydrocarbon generation kinetics of E_2s~3,E_3s~1 and E_3d~3 source rock samples range between 219 and 222 k J/mol,and the kinetics of all samples have a concentrated distribution.Samples from same sag but different intervals,and samples from different sags but same interval have no significant difference,but the samples with a relatively low content of clay minerals and a high content of calcite have a relatively higher mean hydrocarbon activation energy.Carbon isotopes of C_(14+) liquid products during peak hydrocarbon generation can be 11.3% heavier than extracts from unpyrolyzed samples,and oil-source correlation with extracts from low maturity source rocks must consider this.TOC loss of residual solids can reach up to 48%.It is necessary to restore organic matter abundance and kerogen type when evaluating high maturity source rocks with a high organic abundance and favorable organic type.
Formation conditions and main controlling factors of large gas fields in China
Conspectus on Mesozoic Basins in Bohai Bay province
According to the remaining of Mesozoic strata and structural deformation on the basement rocks exposed beneath the Cenozoic basin in Bohai bay province, the Mesozoic basins can be divided into 5 periods of development. The first period was an intra-craton large depressional basin developed during the Early-Middle Triassic time. The second period during Late Triassic was a craton interior sag just developed on the southern Bohai Bay. The third period during Early-Middle Jurassic was a compress-flexing basin which developed within the core of the syncline deformed by Late Indosinian movement. The fourth period during Late Jurassic-Early Cretaceous was a rifting basin and the rifting center was on the western Bohai Bay province. The fifth period during Late Cretaceous was a post-rifting sag. As a response to Indosinian and Yanshanian movements, these basins were induced to inversion and erosion. The Early Indosinian movement formed some large-scale gentle foldings trending in east-west in Bohai Bay province , but the deformation during Late Indosinian movement was different from Western to Eastern Bohai Bay province. In the Western part the deformation was weak only EW-trending open folds occurred while in the Eastern part deformation was stronger but some NE folds and thrusts superposed on the EW folds. Some large-scale gentle folds trending in northeast direction developed during Early Yanshanian movement in Bohai Bay province and had resulted in deformation and inversion of Early-Middle Jurassic basin. Middle and Late Yanshanian movements had not caused any substantial foldings in the Jurassic and Cretaceous basins, however there are regional inversion uplifts in the Late Jurassic-Early Cretaceous basins and the Late Cretaceous basins. After Early-Middle Jurassic the tectonic regime had a fatal change from successive sag and compresso-flexing into the period of mainly rifting basin.
Discussion on Mesozoic Basins patterns and evolution in the eastern north China Block
DOI:10.1016/S1872-5791(07)60010-4
URL
[Cited within: 1]
The Mesozoic basin evolution in the eastern North China Block was controlled by both the compression and amalgamation between the Eurasian plates and the Paleo-Pacific oceanic subduction and collision with the Eurasian Continent, and was closely related to the formation and evolution of deep faults (e.g., the Tancheng-Lujiang fault zone) and intracontinental and peripheral orogens including the Hinggan-Mongolian orogen, Qinling-Dabie orogen, Taihang uplift and so on. During early to middle Triassic, there possibly existed a NWW-trending large-scale sedimentary basin across the North China Block. This basin was inherited from the structural style and sedimentary characteristics of the Late Hercynian. The topography was high in the northwest and low in the southeast. The compressional amalgamation between the Yangtze and North China Blocks in a scissors-shape pattern during late Triassic, leading to the heterogeneous uplifting of the North China Block, i.e., the extent of uplift was less in the west than that in the east. The scope of basin was shrunk to the west, and the eastern part became a source provenance for erosion and denudation. Since early or middle Jurassic, the eastern part of North China Block was in the transition from the Paleo-Asian tectonic domain to the Paleo-Pacific tectonic domain. Formation of the Taihang Mountain divided the North Chima Block into two large-scale basins. In the west was the Ordos basin, whereas the eastern one comprised a series of small intermontane basins in the Bobai Bay area at the early stage. The structural styles included the Indosinian thrust faults and gentle folds. At the late stage, the sediments were widely dispersed. From Late Jurassic to Early Cretaceous, the influence caused by the Paleo-Pacific plate motion was predominant over the tectonic evolution of the North China Block, leading to formation of large-scale rifts or faulted basins. For instance, along the east of Yanshan-Qikou-Lankao-Liaocheng fault system there occurred a series of NW-and NWW-trending rift basins in response to the sinistral strike-slip stress field of the Tancheng-Lujiang fault zone. Between this fault zone and the Taihang Mountain there developed several NE-and NNE-trending rift basins due to the diminishing of the sinistral strike-slip stress field. Within the Tancheng-Lujiang fault zone, NNE-trending slip-extensional basin was formed as a result of extensional stress field, e.g., the Xialiaohe Depression. In late Cretaceous, the areas to the west of the Tancheng-Lujiang fault zone were uplifted and denuded, with deposition of red fluvial-lacustrine sediments in a few basins, such as the Xinyang basin, Jizhong Depression, Linqing Depression and Huanghua Depression.
Study for evolution of CB30 buried hill structure and rule of oil-gas accumulation
Through pre-stack depth migration data and combing study of drill data with study of regional stress field, the paper pointed out that undergoing transformation of local stress field from Indo-Chinese stage(NW) into Yanshan stage (NEE) and regional depression in Ximalaya stage,CB30 buried hill formed reversal structure in Paleozoic,leading to extreme development of reversed fault and fracture in buried hill inside and a lot of losses of Paleozoic; analyzing combination relation of reservoir and cap formation according to formation development, the paper pointed out that there exist multiple set of combination of reservoir and cap formation in buried hill,so constituting a good trap for oil-gas because of extreme development of fracture; also pointed out that pore-style weathered crust reservoirs mainly formed in Mesozoic and lump-shaped reservoir,mainly being structural fracture and dissolution cave,are formed in Paleozoic based on structural amplitude,formation lithology development degree of faults and cap formation data; finally considered that CB30 buried hill reservoir belongs to typical light con-densate reservoir and Zhuangdong Depression is main oil-gas source according to geochemical analysis.
Effects of Indosinian Movements on tectonic formation and evolution, Jiyang Depression
DOI:10.1007/s11442-006-0415-5
URL
[Cited within: 1]
The Indosinian movement, beginning from the mid-Jurassic to the early Jurassic, is the product of the convergence of North China and South China plate, and imposing a far greater effect on the tectonic conditions in Jiyang depression through the immense NE—SW direction stress. Firstly, it leads to the formation and widespread of compressional structure with west—north direction, a process giving rise to NW oriented draping uplift belt and concomitantly wide and low angle syncline. Secondly, several NW oriented extrusion thrustings are caused latterly, which are mainly reverse thrust and the less abundant drape structures. Otherwise, the Indosinian movements results as well in the considerable relief in the study area, a phenomena, relatively uplifts aligned alternatively with the relative low-lying area, caused by the erosion difference. Finally, the upper Paleozoic is well preserved.
Effect of Mesozoic tectonic evolution on hydrocarbon accumulation in buried hills in Huanghua depression
DOI:10.7623/syxb200202004
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Huanghua Depression is a structurally complicated rift basin formed during Tertiary in Bohai Bay Region.Its pre_rift structural system,mainly generated during Mesozoic era,dramatically affected the rifting tectonics and the structural styles of buried_hills in Huanghua Depression.A ccording to seismic interpretations and tectonic analysis,Mesozoic tectonic deformation can be clarified into Indosinian and Yanshanian episodes.A complexes N?S trending strike_slipping fault and fold system in Huanghua Basin which developed during Jurassic_Cretaceous overprinted Indosinian anticlinorium and syclinorium.It formed the basic structural sketch of"relics" and "hybrid" buried_hill and made a great influence on the evolution of Tertiary faulted_block hills.Thus,Mesozoic tectonic events in Huanghua Depression can be treated as an most important factor for evaluating hydrocarbon bearing potential in buried_hills,due to their domination in buried_hill traps,reservoirs,oil types,and the scale of oil pools in Huanghua Depression.
Development of the Bohai Bay Basin and destruction of the North China Craton
DOI:10.1017/S0004972710001772
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The Bohai Bay Basin is a Mesozoic and Cenozoic basin located on the eastern block of the North China Craton. It is the central region of the destruction of the North China Craton. The architectures and structures of the Bohai Bay Basin recorded the structural processes of destruction of the craton in the Meso-Cenozoic. Combined the structures revealed by 3D seismic profiles across the Bohai Bay Basin in recent 10 years with geochemical results and Meso-Cenozoic structures around the Bohai Bay Basin in recent 20 years,we propose that during the destruction of the North China Craton,the shallow tectonism of the Bohai Bay Basin was the Mesozoic extrusion tectonics and the Cenozoic NW-directed crustal extension. The Cenozoic deep-seated tectonism is the regional far-field eastern propagation of the western asthenosphere due to the India-Eurasia plate collision accompanying with eastern jumping and back-rolling of subduction zones of the Pacific Plate. The Mesozoic deep-seated tectonisms are the local delamination and magma underplating.
Destruction of North China Craton: Insights from temporal and spatial evolution of the proto-basins and magmatism
DOI:10.1007/s11430-012-4534-9
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Temporal and spatial evolution of proto-basins and magmatism in the North China Craton might provide information of its destruction.Overall,the destruction of the North China Craton is a heterogeneous process of bottom upward and from margin toward interior,related to multiple interactions between the craton and its surrounding plates.The interior of craton would be initially destructed during the Early-Middle Jurassic.Subduction of the Paleo-Asian Ocean Plate may have destructed the northern margin of the North China Craton.Collision of the Yangtze Plate with the North China Craton is significant for the magmatic and tectonic activities during the Late Triassic-Middle Jurassic.Subductions of the Izanagi and Paleo-Pacific plates lead to the ultimate destruction of the North China Craton.Temporal and spatial evolution of the proto-basins and related magmatism in the North China Craton indicate that the dominant mechanism of the cratonic destruction may be thermal mechanical-chemical erosion.
Late Mesozoic evolution history of the Tan-Lu Fault Zone and its indication to destruction processes of the North China Craton
Late Mesozoic evolution history of the Tan-Lu Fault Zone is important record for destruction processes of the North China Craton. The Tan-Lu Fault Zone suffered local sinistral faulting at the end of Middle Jurassic. This event,i. e. the episode A of the Yanshan Movement,was accompanied with formation of series of NNE-striking,shortening structures in the North China Craton. This event suggests initiation of the Izanagi Plate subduction. The fault zone showed no activity during Late Jurassic while the North China Craton experienced local extension and magmatism as well as regional uplifting. Weak back-arc extension should be responsible for the Late Jurassic events. The fault zone was subjected to intense,sinistral movement at the beginning of Early Cretaceous while northern and eastern parts of the North China Craton presented a series of structures produced by nearly N-S compression. It is suggested that this shortening event,i. e. the episode B of the Yanshan Movement,is related to both the final closure of the Okhotsk Ocean and highspeed subduction of the Izanagi Plate. The eastern North China Craton turned into peak destruction under the setting of back-arc extension during the Early Cretaceous while the Tan-Lu Fault Zone experienced intense extensional activity. Regional compression at the end of the Early Cretaceous terminated the peak destruction of the North China Craton and caused another sinistral faulting event in the Tan-Lu Fault Zone. This compressive event occurred under the important plate adjustment from the Izanagi Plate to Pacific Plate.
Reservoir characteristics and development controlling factors of JZS Neo- Archean metamorphic buried hill oil pool in Bohai Sea
DOI:10.1111/j.1754-4505.2008.00068.x
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JZS is the biggest buried hill oil pool in the Liaoxi uplift of Bohai Sea area,with dominated light grey gneiss and cataclasite.The laboratory test reveals that reservoir properties of JZS buried hill are reasonably good,but still very heterogeneous.Reservoirs in this area can be classified into 4 types with FMI and porosity log information:①network-dissolved fracture reservoir;②cataclasite reservoir;③dissolution fraction reservoir and ④micro-fracture tight reservoir.The Neo-Archean metamorphic buried hill reservoir in the JZS underwent diagenetic period→pre-buried→erosion-deformation→hypergenesis→post-buried period.The key controlling factors in JZS pool are Cenozoic palaeo-geomorphology and tectonic stress of Mesozoic and Cenozoic.The weathered eluvium is distributed in flat slopes of different highlands.Tectonic stress,weathering and dissolution during catagenesis,and organic acid dissolution during oil and gas infilling are the key to affect the evolution of metamorphic reservoir of the JZS oil pool.The evolutionary pattern of Neo-Archean metamorphic reservoir of the JZS oil pool can be applied to similar buried hills in the Bohai Sea area.
Geological significance in discovering large Jinzhou 25-1 light oilfield in Liaoxi sag
Jinzhou 25-1 is a recently-discovered large field with light oil,and so far it is the largest light oilfield in Bohai Sea and the first field with oil reserves over 109 tons in Liaoxi sag.The main target interval in the oilfield is Paleogene Shahejie Formation and its trap type is complex fault-blocks,with a perfect reservoir-seal combination.The reservoir is characterized by shallow buried depth,great oil reserves,high abundance,low oil density and high tested deliverability.The discovery of Jinzhou 25-1 is important in geological significance.
Reservoir characteristics of the crystal basement in the Xinglongtai buried-hill, Liaohe Depression
Reservoir evaluation and fracture characterization of the metamorphic buried hill reservoir in Bohai Bay
DOI:10.1016/S1876-3804(12)60015-9
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With core observation, thin slice identification and imaging logging data, reservoir evaluation and fracture characterization are studied in the Archaean metamorphic buried hill reservoir in the JZ25-1S field in the Bohai Bay Basin. The reservoir is divided into three zones vertically: weathered crust, semi-weathered crust and base rock, belonging to three reservoir types of fracture-pore pattern, pore-fracture pattern and micro fracture-compact pattern. The fracture effectiveness is estimated using crude oil inclusions testing technology, the direction of regional geostress, and full wave train acoustic logging data. It is concluded that the reservoir in the semi-weathered crust is most developed and the reservoir of pore-fracture pattern is the main oil producing layer. The main trend of fractures is nearly parallel to the main fault trend in the buried hill. The inclined fractures of late opening-mode parallel to the current direction of maximum geostress are the most effective fractures. At last, in combination of imaging logging interpretation with actual production, the qualitative and semiquantitative reservoir classification criteria are proposed and the buried hill reservoir is divided into three levels of I, II and III.
Structure overlap and tectonic setting of Yanshan Orogenic Belt in Yanshan era
Extensive magma activity and intensive tectonic deformation are two typical characteristics in Yanshanian orogenic belt, East China. The distribution of igneous, especially syn-tectonic intrusive core-complex and the spread of igneous basins, has a good correlation with the direction of syn-deformation belts. This paper discussed the structure overlap, the conversion of structure stress field, and the tectonic setting through the integrated study of igneous structure associations, structure deformation and magma-tectonic event sequence etc. We think that the acy folds and the folded thrust-nappes etc. record multi-episode compressive deformation. The five different directions of igneous distribution (Late of Early Jurassic (J~3_1), late of mid-Jurassic (J~3_2), middle of Late Jurassic (J~2_3), late of Late Jurassic (J~3_3) and early of Early Cretaceous (K~1_1) are totally in accordance with the direction of compressive deformation. It is proposed in this paper that there is a regional northwest compression structure, and as a rule there is a compression situation in north China in the early stage of Early Cretaceous (K~1_1). And we think that Yanshanian orogenic belt of North China was formed and evolved in the assemblage setting defined by four tectonic boundaries, i.e., Mongolia-Okhotsk tectonic belt, Verkhoyano-Chukotsk orogenic belt, Izanagi ocean subduction, and Tethys ocean subduction.
Major Mesozoic tectonic events in the Yanshan Belt and the plate tectonic setting
Our field work and geochronologic studies in northern Hebei and western Liaoning areas indicate that the major Mesozoic contractional events in the Yanshan belt are of (1) pre-Middle Jurassic (Indosinian?) , (2) Late Jurassic and (3) Early Cretaceous age. The first N-S shortening could be a consequence of the collision of the Mongolian arcs along the Solon suture against the North China plate. Siberian-North China plate collision across the Jura-Cretaceous Mongo-lo-Okhotsk Sea more than 800-1000 km to the north could be responsible for the regional N-S intraplate contractional deformation in the Yanshan belt during Late Jurassic and Early Cretaceous time. The NNE-structural trends, the regional patterns of Mesozoic magmatism in eastern China and the close spatial and temporal ties existing in the middle crust between the S-directed Early Cretaceous basement nappe and voluminous plutonic intrusion suggest the results of the influence of thermal regimes resulting from the westward Pacific plate subduction and related magmatism.
Relationship between faulting and hydrocarbon pooling during the Neotectonic movement around the central Bohai Bay
DOI:10.1073/pnas.0909708106
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The area around central Bohai Bay is an important prolific zone,where faults are well developed and faulting during the neotectonic movement played a significant role in migration and preservation of hydrocarbons.The fault activity rate (FAR) method was used to study the relationship between faulting activity and hydrocarbon accumulation/preservation in the area based on analysis of activity changes along the strike of main fault and combinations of faults with different FAR within the zone.In addition,the controlling mechanisms of hydrocarbon accumulation were also discussed in respects of the locations of spill points,the definition of fault carrier systems and FAR,and the relationship between fault throw and thickness of regional caprock.The results indicate that hydrocarbon accumulations in the area are mainly located near faults with moderate or weak activities(FAR25 m/Ma),while they seldom occur near faults with strong activities(FAR25 m/Ma) because these faults served mainly as vertical migration pathways,making it hard to form shallow oil accumulations.For these faults,only in areas with relatively weak fault activities can hydrocarbon accumulation occur.
Neotectonic movement and its petroleum geology significance in northern Liaozhong sag
Liaozhong sag is an important hydrocarbon rich sag in Bohai sea,in which a lot of large-middle scale oil and gas fields are discoveried.However,there is no important breakthrough in Neogene exploration in Liaodong depression and it is regarded as an early faulted and early decline basin with inactive neotectonic movement for a long time.Analysis of high resolution 3D seismic and drilling data show that strong neotectonic movement happens and the faults with the characteristics of both stretch and strike-slip structures develop in late stage of northern Liaozhong sag because of the late stage rise in Liaodong uplift.Under the neotectonic movement,large scale traps in the shallow layer develop in northern Liaozhong sag and becomes the center of tectonics and sediment in Neogene Liaodong Bay depression,which improves reservoir-seal assemblage in Neogene northern Liaozhong sag.Moreover,the enhanced late stage faults activity not only offers hydrocarbon migration channels but also driving force for oil and gas migration for the late hydrocarbon accumulation,providing good accumulation conditions in Neogene northern Liaozhong sag.The new recognitions of neotectonic movement and its hydrocarbon accumulation guide some important discoveries in shallow layer in northern Liaozhong sag.
Accumulation models and regularities of large-middle scale oilfields in Bohai Sea, Bohai Bay Basin
Thirty-one large-middle scale oilfields including 9 oilfields with hundreds of millions of tons reserves are discovered in Bohai sea,Bohai Bay basin.Combined with the previous research,hydrocarbon accumulation models and laws of the 31 large-middle oilfields are summarized,which can provide an important role to guide continuous discoveries of large-middle scale oilfields and ensure the reserve growth.Studies show that there are five hydrocarbon accumulation models of the large-middle scale oilfields in Bohai sea including drape anticline type,strike-slip inversion type,uplift of the sag-draped type,steep slope-fault block type and gentle slopefault nose type,which possess the unique accumulation regulations:adjacent to(potential)hydrocarbon rich sag is the essential;the superior reservoir-seal assemblages are the important guarantee;active fault zone,especially Tan-lu fault zone is favorable hydrocarbon accumulation belt;the coupling of time-spacesource in late hydrocarbon accumulation is the key.
Identification of various alkane gases
Origin and identification of natural gases
DOI:10.1016/j.molcatb.2005.02.001
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In the past 20 years, many researchers were engaged in tackling the key problems about natural gases, and brought about the theory and methods on origins and identification marks of natural gases. The natural gases include organic gases, inorganic gases and hybrid gases according to their origins. Gases of organic origin can be subdivided into biogenic, biogenic-thermal catalyzed, pyrogenic, and cracked gases, and can also be subdivided into coal derived gases and petroliferous gases according to the types of parent material. Coal derived gases include coal derived pyrogenic gases and coal derived cracked gases, which play a dominant role in natural gas resources. Petroliferous gases mainly refer to the associated gas of crude oil, including petroliferous pyrogenic gases and petroliferous cracked gases. Inorganic gases give priority to CO_ 2 , including petrochemistry gases and mantle derived gases. Hybrid gases are the mixture of gases from two or more origins: mixture of gases generated during different thermal evolution stages from the same source rock; mixture of gases generated from different source rocks; and mixture of inorganic gas and organic gas. The identification marks in common use for genetic types of natural gases mainly refer to the composition of natural gas, carbon isotopic composition of alkane and CO_ 2 , and parameters of light hydrocarbon. The carbon isotopic compositions are the most efficient tool and widely applied in gas typing.
Identification of coal formed gas and oil type gas by light hydrocarbons
The development and application of fingerprint parameters for hydrocarbons absorbed by source rocks and light hydrocarbons in natural gas
Discussion on light hydrocarbon index for identifying natural gas origin
DOI:10.1007/s11430-008-5001-5
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C value of individual light hydrocarbon between humic-sourced gas and sapropelic-sourced gas have been founded. The humic-sourced gas has the distribution ofCCCCCδ13tolueneδ13cyclohexaneδ13methyl cyclohexane7C), methyl cyclohexane (MCH) and dimethyl cyclopentane (ΣDMCP), etc, relative contents of C and MCH are influenced mainly by the source organic matter type of natural gas. Therefore, it is suggested that the gas with relative content of C of more than 30% and relative content of MCH of less than 70% is sapropelic-sourced gas, while gas with relative content of nC7 of less than 35% and relative content of MCH of more than 50% is humic-sourced gas. Among components of C aliphatics, the gas with relative content of C normal alkane of more than 30% is sapropelic-sourced gas, while the gas with relative content of C normal alkane of less than 30% is humic-sourced gas. These paremeters have been suggested to identify humic-sourced gas and sapropelic-sourced gas.
New indexes and charts for genesis identification of multiple natural gases
DOI:10.1016/S1876-3804(17)30062-9
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Identification of natural gas genesis and source for high-matured multiple natural gases is a great challenge in the exploration of deep ltra deep and unconventional natural gases. In this paper, the genesis identification method system of multiple natural gases is enriched through new experimental techniques and comprehensive analysis of geological data. New indexes and charts of genesis identification for multiple natural gases were determined to distinguish the sapropelic kerogen degraded gas and crude oil cracking gas, accumulated and scattered liquid hydrocarbon cracking gas in different evolution stages, nitrogen, carbon dioxide of organic and inorganic origins, inert gases of crustal and mantled origins, coal-formed gas and oil-type gas by helium, nitrogen, carbon dioxide and mercury content in natural gas. These indexes and charts have been successfully applied in the Sichuan, Tarim and Songliao basins to identify the natural gas genesis and source for complicated gas reservoirs. The research results have provided effective support for the natural gas exploration in the Sinian ambrian ancient carbonate formations in the Sichuan Basin, deep formations in the Kuqa depression of the Tarim Basin, and deep volcanic formations in the Songliao Basin.
Laboratory studies of differences between oil-derived and kerogen maturation gases
DOI:10.3969/j.issn.1001-6112.2011.04.019
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An immature type-I mudstone collected from the third section of the Tertiary Shahejie Formation(Es33) in the Dongying Sag of the Bohai Bay Basin was studied with laboratory experiments.Products were extracted,fractionated into chemical groups,and re-mixed to obtain synthetic oil(s-oil) with a group composition similar to reservoir oil and a pseudo-kerogen(p-kerogen) composed of 83% kerogen and 17% residual soluble organic matter.The two samples were pyrolyzed in sealed gold tubes under constant pressure and non-isothermal heating conditions and the generated gases were analyzed.The two gases are quite different in their chemical and isotopic composition.Compared with the gas derived from p-kerogen,the s-oil-derived gas is rich in C2-C5 hydrocarbons during the early cracking stages and C1-C3 hydrocarbons are depleted in δ13C throughout the cracking stages.The carbon isotopic ratio differences between the two gases can reach 10‰(δ13 C1),14‰(δ13 C2),and 9‰(δ13 C3).The δ13C2-δ13C3 value of the s-oil gas is much more sensitive to thermal stress than that of the p-kerogen gas and the plots of(δ13C2-δ13C3) versus δ13 C1 and(δ13C2-δ13C3) versus ln(C2/C3) are effective in identifying the two gas types.These results provide a guide to differentiate gases derived from oil cracking from those derived from kerogen maturation.
Study on accumulation mechanism of Ya13-1 efficient natural gas reservoir in Qiongdongnan Basin
The accumulation environment of the efficient natural gas reservoirs are differ from large-medium sized,which require simultaneously efficient gas source rocks,accumulating process and energy match of reservoir rocks and cap rocks.Due to fast subsidence rate and great sediment thickness in Ya13-1 structure zone Qiongdongnan basin from Neocene to this day,the large area gas source rocks rapidly came into generation/migration phases.Universal dynamic overpressure afforded stronger power for liquid migration in 3D conduit system.The reservoir sandstones are normally-pressured and the cap rocks are overpressured,and such favorable energy match relation is very important for gas reservoirs conservation.Ya13-1 gas field is a late,overpressure,fast charge,3D carrier and ideally energy match,efficient natural gas reservoir.
The Neotectonism and major Neogene oil and gas field in Bohai Sea
Pliocene-Holocene Neotectonism has played a key role in formation of Tertiary oil and gas fields in Bohai Sea. A "centripetal" evolution of the subsidence and sedimentation center and gradually lowering geothermal flux since Neogene have resulted in later hydrocarbon generation and drainage from source rocks in the Sea. The Neotectonism has created a lot of Neogene structures which can provide favourable places for hydrocarbon accumulation. A large number of late faults has intersected sag-controlling faults, unconformities and sandbodies, which may form excellent pathways for hydrocarbon migration and control final hydrocarbon accumulation. These Neogene oil and gas pools can be divided into three major types according to structural position and trap origin: anticlines in uplifts, rollover anticlines and fault blocks in fault-terrace zones between uplifts and sags, and inherited and inverted structures in sags.
The material and energy effects of Neotectoics/Late-stage tectonics and petroleum accumulation
Disperse solid bitumens as an indicator for migration and maturity in prospecting for oil and gas
