Introduction
With the advance of exploration and development technology and the deepening of geological understanding, important progress has been made in the exploration of ultra-deep oil and gas all over the world. By the end of 2018, About 187 ultra-deep oil and gas reservoirs have been found below 6 000 m, of which 68 oil and gas reservoirs are deeper than 8 000 m. Ultra-deep reservoirs are becoming an important field for the growth of global oil and gas production [1⇓-3]. At present, large-scale oil and gas reservoirs found in ultra-deep clastic rocks around the world are almost gas reservoirs, but oil reservoirs are relatively few. The typical reservoirs are mainly distributed in three series of strata and have the following characteristics: (1) Ordovician-Jurassic marine strata deeper than 6 000 m, with primary intergranular pores and secondary dissolution pores, such as those in the Anadarko Basin in U.S. and the Central North Sea Basin in England [4]; (2) Cretaceous strata at 6 000-8 000 m, with enlarged intergranular dissolution pores, and (ultra-) low porosity and permeability, such as those in the Kuqa Depression in the Tarim Basin, NW China [5-6]; and (3) Neogene strata at 6 000-7 132 m, with primary intergranular pore, and medium to high porosity and permeability, such as those in the Gulf of Mexico Basin in U.S. [7]. However, there are few reports on the discovery of large- scale reservoirs with primary pores and good reservoir space in the field of Paleogene ultra-deep clastic rocks. Late (5.3 Ma) and fast burial, weak compaction and relatively lagged diagenesis made the ultra-deep Paleogene strata in the Linhe Depression of the Hetao Basin favorable reservoirs with a large number of primary pores, good physical properties, and low thermal evolution.
The ultra-deep layers of Linhe Depression in Hetao Basin are mainly distributed in the Naoxi trough belt, with an exploration area of nearly 2 000 km2. High- quality source rocks and various types of large lithologic- structural traps are developed in the ultra-deep Paleogene Linhe Formation, with great exploration potential. Since 2021, a risk exploration well Hetan 1 [8] and a pre-exploration well Hetan 101 have been drilled in the Guangming structure in the northern part of the Linhe Depression, and breakthroughs have been made in exploration. In particular, Well Hetan 101 produced oil of 1084.8 t/d and gas of 1.07×104 m3/d from the ultra-deep Paleogene Linhe Formation at depth deeper than 6 500 m, making a major breakthrough to oil and gas exploration in the ultra-deep clastic rocks in the Hetao Basin. The daily oil production hit the highest record of ultra-deep clastic rocks in China, indicating a huge exploration potential in the ultra-deep clastic rocks in the Linhe Depression. The controlling factors on ultra-deep oil and gas accumulation are diverse, and rarely studied on clastic oil accumulation in faulted basins, especially on ultra-deep clastic oil and gas accumulation with ultra-high yield. Based on drilling, logging, geological experiment and burial evolution history data, this paper studies the geological characteristics and controlling factors on the enrichment and high yield of oil and gas in ultra-deep clastic rocks in the Linhe Depression of the Hetao Basin, with the intent to improve the theoretical understanding of oil and gas accumulation in ultra-deep clastic rocks and provide technical support for future oil and gas exploration in ultra-deep clastic rocks.
1. Geological setting
The Linhe Depression is a northeast trending asymmetric double-fault depression on the basement of Archean-Proterozoic metamorphic rock series located in the west of the Hetao Basin (Fig. 1a , 1b ). The depression, about 230 km long and 70 km wide, is geographically in the south of the Seerteng Mountain, the northwest of Yimeng Uplift, Zhuozi Mountain and Helan Mountain, and the east of Langshan Mountain and Bayanwula Mountain. The area of the depression is about 2.24×104 km2 [9-10]. According to the latest two-dimensional and three-dimensional seismic data, the basin structure and fault distribution was re-described. In general, it is characterized by north-south zones and east-west belts. Along the Jilantai structural belt-Dengkou low uplift, the depression is divided into southern Jilantai sag and northern Bayannaoer sag. The western part of the Bayannaoer sag is Naoxi trough belt, the central part covers Wulanbuhe strike-slip structural belt, Nalinhu faulted buried-hill belt, Xinglong fault belt and other major structural belts, and the eastern part includes Yellow River fault trough belt and Wuyuan slope belt. The western part of Jilantai Sag is Jilantai structural belt, Jixi trough and Shabu structural belt, and the eastern part is Jidong slope (Fig. 1b ). The Paleogene target layer in Naoxi trough has a buried depth of more than 6 000 m, which is the key area of this study.
Fig. 1. Tectonic units and comprehensive histogram of the Linhe Depression in the Hetao Basin (modified after Reference [11]). |
The basement of the Hetao Basin is the metamorphic rock series of the Archean Wulashan Group. From the bottom to the top, there are the Lower Cretaceous Guyang Formation, the Paleogene Eocene Wulate Formation, the Oligocene Linhe Formation, the Neogene Miocene Wuyuan Formation, the Pliocene Wulantuke Formation and the Quaternary Hetao Group (Fig. 1c ). The Eocene Wulate Formation and the Oligocene Linhe Formation contain high-quality source rocks of saline lacustrine facies. Oil and gas are mainly distributed in the Paleogene Linhe Formation, the Wulate Formation and the Archaeozoic strata. The Wuyuan Formation is dominated by red mudstone, which is a regional cap rock, forming reservoir assemblages characterized by self-source and self-reservoir, side-source and side-reservoir, and new-source and ancient-reservoir are developed in the Linhe Depression.
2. Hydrocarbon exploration breakthrough
In 2020, a risk exploration well Linhua 1X, and a key pre-exploration well Xinghua 1, were successively drilled in the middle-shallow and middle-deep layers of the Xinglong faulted structural belt in the northern part of the Linhe Depression. High-yield industrial oil flows of 305 m3/d and 274 m3/d were obtained during oil test at 3 374-3 379 m and 4 370-4 378 m, respectively (Table 1 ). It reveals that the Paleogene Linhe Formation in the northern part of the Linhe Depression has self-source and self-reservoir plays with characteristics of late accumulation and near-source enrichment.
Table 1. Well test yield and physical properties of crude oil of the Paleogene Linhe Formation in Linhe Depression |
| Structural belt | Location | Well | Depth/m | Well test production | Physical properties of crude oil | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Oil/ (m3∙d−1) | Gas/ (104 m3∙d−1) | Density/ (g∙cm−3) | Viscosity/ (mPa∙s) | Freezing point/°C | Wax content/% | Sulphur content/% | Resin + asphaltene content/% | ||||
| Xinglong faulted belt | Outer | Linhua 1X | 3 374.0- 3 379.2 | 305.00 | 0.877 0 | 24.310 | 50 | 19.10 | 0.88 | 28.50 | |
| Middle | Xinghua 1 | 4 370.4- 4 378.6 | 274.00 | 0.869 0 | 23.720 | 39 | 18.30 | 0.43 | 20.93 | ||
| Inner | Hetan 1 | 6 112.4- 6 120.2 | 302.40 | 0.871 6 | 19.470 | 46 | 21.85 | 0.36 | 15.05 | ||
| Hetan 101 | 6 557.0- 6 566.2 | 1 285.77 | 1.07 | 0.841 2 | 6.894 | 42 | 22.10 | 0.17 | 9.55 | ||
In ultra-deep layers of the risk exploration trough area in 2021, a risk exploration well Hetan 1 was drilled at the anticline of the reverse traction structure in the high part of the Guangming structure. Well test was carried out in the 6 112.4-6 120.2 m well section corresponding to sand group II in the first member of the Paleogene Linhe Formation (Lin 1 Member for short) by opening 7.0 m/2 layers. The lithology of the reservoir is fine sandstone. The porosity is 12.2%-20.2%, and permeability is (1.11- 47.32)×10−3 μm2 from logging interpretation. Through an 8 mm nozzle at tubing pressure of 3.730-3.809 MPa, a high-yield oil flow of 302.4 m3/d was obtained. The formation pressure coefficient is 1.57, the crude oil density is 0.871 6 g/cm3 (at 20 °C), the viscosity is 19.47 mPa·s (at 50 °C), the freezing point is 46 °C, the wax content is 21.85%, the sulfur content is 0.36%, and the asphaltene and resin content is 15.05% (Table 1 ). The crude oil is medium oil with low sulfur and high wax contents.
In order to continuously explore the potential for ultra-deep strata in trough area and clarify the size of oil and gas resources, Well Hetan 101 was drilled in the low sag of the Guangming structure in the Xinglong faulted structural belt in 2023, and well test was carried out in the 6 557.0-6 566.2 m well section at sand group III of the Paleogene Lin 1 Member by opening 9.2 m/layer. It's found that the reservoir is fine sandstone with porosity of 15.7%, and permeability of 182.5×10−3 μm2 from logging interpretation. Through a 10 mm nozzle at tubing pressure gradually increasing from 10.52 MPa to 12.66 MPa, the oil production was 1 285.77 m3/d, equivalent to 1 084.8 t, and the gas production was 1.07×104 m3/d, which has achieved a major breakthrough in ultra-deep 1 000-ton oil wells in clastic rocks. The original formation pressure in the producing section is 104.2 MPa, and the pressure coefficient is 1.62. The density of the crude oil is 0.841 2 g/cm3 (at 20 °C), the viscosity is 6.894 mPa•s (at 50 °C), the freezing point is 42 °C, the wax content is 22.10%, the sulfur content is 0.17%, and the resin and asphaltene content is 9.55% (Table 1 ). The crude oil is light oil with low sulfur and high wax contents.
3. Petroleum geological characteristics
3.1. Source rocks
Drilling in the northern part of the Linhe Depression in the Hetao Basin reveals two sets of important source rocks in Paleocene Linhe Formation and Lower Cretaceous Guyang Formation [11⇓-13]. The Linhe Formation develops a set of high-quality source rock of salty lacustrine facies, with high abundance of organic matter (about 1% on average). The organic matter is mainly of types I-II1, distributed in the central and northern parts of the Linhe Depression, with an area of 5 000 km2. It is vertically concentrated in the middle and upper parts of the second member of the Linhe Formation and the first member of the Linhe Formation. The source rock in the northern trough area is 100-450 m thick, and buried at 6 000-10 000 m. The thermal evolution degree is high and the resource potential is large. The analysis of tectonic sedimentary evolution shows that the sedimentary center of the Linhe Formation has a characteristic of northward migration. The gypsum-salt rock is developed in the north of Hangwu fault, especially near the Xinglong fault, which is of salty lake facies. And the south of the Hangwu fault is of saline lake facies. Paleoenvironmental studies suggest that the Paleogene Linhe Formation was mainly deposited in a semi-arid and evaporative environment [14]. In the sedimentary center, gypsum-salt rock is developed as saline/salty lake deposits which provide an important guarantee for the enrichment and preservation of organic matter and a super hydrocarbon generation capacity (the hydrocarbon generation intensity is 400×104 t/km2).
Early studies suggest that the source rocks of the Linhe Formation in the Hetao Basin have the characteristics of high sulfur content and rich phytoplankton [11,15]. However, the latest study reveals that the content of gymnosperm pollen in the source rocks of the Linhe Formation is the highest, and the content of pinaceae is as high as 88% [16]. Further maceral analysis of kerogen found many resinites and sporophytes from the exinite of higher plants. Pyrolysis analysis shows that among the discovered hydrogen-rich hydrocarbon generating components (resinite, sporophyte, algae and matrix vitrinite), resinite has the highest hydrocarbon generation potential (94.35 mg/g) and hydrogen index (508.33 mg/g), and the lowest activation energy (223-235 kJ/mol). The cumulative generated hydrocarbon is large, up to 40%, just following algae, but the vitrinite reflectance (Ro) at the peak of hydrocarbon generation is only 0.61%, resulting in a comprehensive Ro value of 0.68% at the peak of hydrocarbon generation. Therefore, the rich resin in the source rock is the most important factor controlling the early-mature and high-quality source rock characterized by rapid expulsion and efficient hydrocarbon generation in the Linhe Formation of the Hetao Basin [17]. It is also an important factor for early and continuous hydrocarbon generation, high resource conversion rate and great resource potential of the source rocks in the trough area. At the same time, the above research shows that the organic matter components of the Linhe Formation source rocks not only contain aquatic algae, but also mix with terrigenous resin and sporophyte. This not only explains why C29>C27 sterane in the chromatography and mass spectra with contribution from higher plants (Fig. 2 ), but also explains the origin of high wax content (18%-22%) and high freezing point (39-50 °C) in the crude oil of the Linhe Formation. Compared with the saline lacustrine source rocks at home and abroad, the organic matter from in the study area has the characteristics of early maturity, early hydrocarbon generation peak, early expulsion and efficient hydrocarbon generation. The research results have important reference significance for resource potential evaluation and oil and gas exploration in the basins with middle and low thermal evolution, late rapid subsidence, late hydrocarbon generation and late accumulation.
Fig. 2. Chromatography and mass spectra of core and crude oil samples from key exploration wells in the Linhe Formation, Linhe Depression. (a) Crude oil, Lin 1 Member, 3 374.00-3 379.2 m, Well Linhua 1X; (b) Gray mudstone, Linhe Formation, 4832.00-4 843.00 m, Well Xinghua 4; (c) Crude oil, Lin 1 Member, 6 557.00-6 566.20 m, Well Hetan 101; (d) Crude oil, Lin 2 Member, 5 480.00-5 487.80 m; Well Zage 1. |
Gas chromatography analysis shows that with the increase of crude oil/source rock maturity, heavy components gradually decrease and light components gradually increase. Therefore, the gas chromatogram of mature crude oil/source rock shows a single and front peak, such as in Well Zage 1. However, due to a high content of heavy components and a low content of light components in the gas chromatogram of immature to low mature crude oil/source rock, it shows bimodal or back peaks, such as the crude oil from Well Linhua 1X and the source rock from Well Xinghua 4. It is worth noting that the maturity of the crude oil from Well Hetan 101 drilled in the ultra-deep layer in the trough area is higher than that from Well Zage 1, and the burial depth is also much deeper than the latter, but the content of heavy components in the gas chromatogram is relatively higher than the latter. The reason is that Well Hetan 101 is located in the Guangming Structure in the northern part of the Linhe Depression, where there are ultra-deep and ultra-high pressure oil and gas reservoirs, the oil and gas system is relatively closed, and the oil and gas come from the mixture of early low-mature oil in the near-source trough area and late mature-high mature oil and gas with the increase of thermal evolution degree. Well Zage 1 is located in the Zage Structure in the Nalinhu faulted buried-hill zone with deep brial depth. The reservoir at normal pressure is relatively far from oil sources, and the oil and gas mainly come from the mature-high mature crude oil generated in the late stage of the trough area.
3.2. Sedimentation
During the sedimentary period of the Linhe Formation in the Linhe Depression, the basin was in depression-fault transformation as a whole [18]. The strata in the basin are gentle in occurrence and less developed in faults. The sedimentary center was located in the north-central part of the Naoxi trough belt, and its sedimentary evolution was mainly controlled by structure and paleoenvironment. Influenced by the Mesozoic uplift of the Helan Mountain and the Zhuozi Mountain in the south, the subsidence center of the basin gradually migrated from south to north during the Paleogene, and the structural transformation of the Helan Mountain and the Zhuozi Mountain in the southern basin margin controlled the formation of a large provenance system. Because the Paleogene was a weak extensional fault depression period, the stratigraphic occurrence was relatively gentle, and large braided river delta deposits were developed from south to north. Affected by distant sources and long-distance transportation, sand bodies entered the trough area and then the lake. Affected by the buoyant sedimentary environment with brackish and saline water, the sand bodies kept advancing to the center of the lake, making the composition and structural maturity of the sand bodies high. Drilling data revealed that the quartz content of sand bodies in trough area of the Lin 1 Member is as high as 75%, the ratio of net-to-gross (NTG) is 20% to 30%, the thickness of a single sand body is 1-10 m, the source and reservoir are interbedded, and the general characteristic of the deposits is coarse in the lower part and fine in the upper one. The upper submember of the second member of Linhe Formation (Lin 2 Member for short) is mainly salty lake facies, while the sand bodies in the lower submember (non-drilled) may be more developed according to drilling data and regional sedimentary cycles. The paleoenvironment of the sedimentary period of the Lin 2 Member was characterized by semi-arid to arid, brackish to saline water, weak oxidation and weak reduction to strong reduction environment [14]. The lithology is mainly medium-fine sandstone, siltstone and mudstone, and the color is brownish red, brownish brown-gray and dark gray. During the sedimentary period of the Lin 1 Member, the climate of the study area was semi-humid to arid, and the lithology was from coarse to fine. Due to the overall migration of the sedimentary center toward north and east, the study area gradually moved away from the sedimentary center. The color of the sedimentary rock changed from gray to brown, the water became shallow, and the salinity and the reducibility gradually became low and weak. Therefore, on the whole, the development and evolution of the sedimentary system in the study area was controlled by the paleostructure and paleoenvironment. In the semi-humid climate, the supply from external sources was more abundant, so the sedimentary sand bodies were developed extensively, making favorable sedimentary facies in the trough area. In the arid climate environment with the decrease of precipitation and rivers, sand bodies were underdeveloped, and plus enhanced evaporation, gypsum-salt rocks were more developed in the trough area, and the water salinity increased, so that saline and salty lacustrine deposits appeared (Fig. 3 ).
Fig. 3. Sedimentary facies distribution and prognostic map in the north of Linhe Depression. (a) Sedimentary facies map of sand group II in Lin 2 Member; (b) Sedimentary facies map of sand group IV in Lin 1 Member. |
3.3. Reservoirs
The reservoir space of the Linhe Formation in the Linhe Depression is dominated by primary intergranular pores, and a small number of secondary dissolved pores. The reservoir physical properties are good [11]. The porosity of ultra-deep high-quality reservoir cores from measurement and logging interpretation is 14.4%-18.2%, and the permeability is (23.0-182.5)×10−3 μm2. From logging interpretation, the porosity is 15.7% at 6 557.0-6 566.2 m in Well Hetan 101, and the flow permeability during well test is 226×10−3 μm2. Good reservoir physical properties provide an important basis for extra-high production from this well. In view of the genesis of high-quality reservoirs in ultra-deep clastic rocks, many scholars have done a lot of research and analysis. The genesis of high-quality reservoirs dominated by primary pores mainly includes three types: (1) primary pores with medium- shallow fluid overpressure and medium-shallow oil and gas filling [19]; (2) primary porosity with shallow chlorite cladding and medium-shallow/deep oil and gas filling [20]; and (3) primary pores at overpressure after early long- term shallow burial and late rapid deep burial, and with medium-deep oil and gas filling [21]. The unique burial, thermal evolution history and oil and gas filling history of the Hetao Basin [22] make the ultra-deep high-quality reservoirs both common and unique.
3.3.1. High rigid particle content and thermodynamic field under weak compaction
Large-scale shallow braided river delta deposits are developed in the northern part of the Linhe Depression. The surrounding source areas are generally dominated by high-quality parent rocks such as granitic gneiss and quartz schist. The total amount of three rigid particles of quartz, feldspar and siliceous debris in sandstone exceeds 85%, of which the quartz content is 54%-76%. The composition and structural maturity is high, and the compaction resistance is strong. In addition, the geothermal gradient of the Hetao Basin is low, which is (2.3-2.6) °C/100 m. The compaction rate of sandstone decreases rapidly with the decrease of geothermal gradient, which is conducive to the preservation of primary pores. The Linhe Formation has been shallowly buried for a long time, and has been rapidly buried since 5.3 Ma, with short compaction period [22]. Therefore, the compaction strength is weak, which provides an important material basis and conditions for the preservation of primary pores.
3.3.2. Low interstitial material (cements) content and early hydrocarbon filling
The analysis of rock slices shows that the average content of cements in sand bodies less than 1 m thick is higher than 12%, that in sand bodies 1-3 m thick is 5%-12%, and that in sand bodies more than 3 m is generally lower than 5% [22]. On the one hand, it is affected by the favorable factors such as distant sources, high composition and structural maturity, high quartz content and thicker sand bodies, so that the content of cements in the early diagenetic stage of Linhe Formation is very low, and the cementation is weak. At the same time, affected by early cementation and pore preserving like particle cladding and ring edge on the micro scale, and indirect action of pore preserving such as carbonate cementation crust on the edge of sand bodies on the macro scale [19], with weak cementation. On the other hand, in the process of reservoir burial diagenesis, oil and gas occupy a large amount of pore throat space after early hydrocarbon filling, which can effectively hinder the circulation and exchange of pore water in the reservoir, resulting in slow pore water flow and difficult ion supply, thereby inhibiting or even stopping cementation and effectively preserving reservoir space [23-24]. The early-mature and early-expulsion high-quality source rocks are developed in the Linhe Depression, and a large number of hydrocarbons were generated at the low evolution stage, followed by deep burial, thermal evolution and efficient hydrocarbon generation. The analysis of homogenization temperature and fluorescent oil inclusions of hydrocarbon-bearing brine inclusions at 7 039.61 m in the Lin 2 Member in Well Hetan 101 found that the fluid inclusions have both low-temperature and high-temperature peaks characterized by early and continuous charging of oil and gas. Under the same fluorescence, both yellow and blue-green show the characteristics of low-mature and early charging to high-mature and continuous charging. Early charging and accumulation may play an important and positive role in the preservation of ultra-deep primary pores and permeability. Therefore, although temperature reached the middle diagenetic stage A or even stage B due to rapid and deep burial in the late stage, cementation is not strong, and obviously lags behind the diagenetic evolution in the Hetao Basin. In particular, the content of cements in the high-quality reservoirs in the Xinglong faulted structural belt in the northern part of the Linhe Depression is generally low, and the cements do not occupy too much primary pores, so that the primary pores could be retained by a large quantity. In conclusion, the physical properties of the ultra-deep clastic reservoirs are controlled by weak compaction, early hydrocarbon filling and weak cementation, which make them better than those in other basins in the same period.
3.3.3. Ultra-high pressure on pore preservation, fracture expansion and permeability increase
3.3.3.1. Cause of ultra-high pressure
Drilling data reveals that ultra-high pressure strata are developed in the Guangming Structure in the northern part of the Linhe Depression. The formation pressure coefficient of the high-yield interval in Well Hetan 1 is 1.54, and that in Well Hetan 101 is 1.62. Both belong to the abnormally high pressure system. It is believed that the ultra-deep abnormally high pressure is mainly controlled by three factors: hydrocarbon generation, tectonic compression and overpressure caprock sealing.
Pressure increase by hydrocarbon generation. The source rock of the Linhe Formation in the Guangming structure entered the hydrocarbon generation threshold at the end of the deposition of the Neogene Wuyuan Formation, and began to generate low-mature conventional medium oil. Then after starting the deposition of the Wulantuke Formation, the basin entered the rapid subsidence stage. With the increase of burial depth and thermal evolution degree, the source rock of the Linhe Formation quickly entered the peak period of hydrocarbon generation and expulsion, and a large amount of oil and gas was generated in a short time. The pressure difference between source and reservoir increased abruptly, and high-intensity hydrocarbon generation and expulsion, migration and accumulation induced pressure transfer from the source rock to the reservoir, making the pressure in the reservoir gradually change from normal to ultra-high. The development of high-quality source rocks with early maturity, early expulsion and high hydrocarbon yield in the Linhe Formation provides an important material basis for the formation of abnormally high pressure reservoirs in the ultra-deep Guangming structure.
Structural compression. In addition to hydrocarbon generation and pressurization, under the influence of the strong extension and rapid subsidence and gravity of the Neogene Wulantuke Formation and the development of plastic gypsum-salt rock in the Lin 2 Member, the Xinglong fault in the northern part of the Linhe Depression experienced large-scale detachment. The detachment fault is steep in the upper part and gentle in the lower part. The detachment section is located at the junction of the plastic gypsum-salt rock and the underlying sandstone in the Lin 2 Member. The vertical detachment distance is 1.5 km, and the lateral is 3 km. The detachment process was affected by the gravity of the overlying strata, with a part of the gravity acted on the parallel section, resulting in a rolling anticline above the section. Deformation and compression made the formation at the root of the detachment fault thick obviously, the space compressed, and the pressure increased, which is beneficial to the formation of abnormally high pressure.
Overpressure caprock sealing. Drilling data show that the Guangming structure located in the sedimentary center of the lake basin and away from the source has a low NTG (less than 21%), relatively developed mudstone (i.e., source rock), and ultra-high pressure caprock affected by strong hydrocarbon generation and primary migration, which provides favorable conditions for the sealing of high-quality reservoirs. Aunder the influence of late rapid subsidence, the overlying thick Neogene Wuyuan Formation mudstone (the mud-to-formation ratio is greater than 90%) developed into an ultra-high pressure caprock affected by under-compaction. Therefore, the Linhe Formation reservoir is preserved well under the sealing by itself and the Neogene ultra-high pressure formation.
3.3.3.2. Pore-preservation, fracture expansion and permeability increase
Early studies suggest that ultra-high pressure may greatly reduce the compaction strength on reservoir, inhibit the development of authigenic quartz, and promote the dissolution of feldspar [25-26], so that the reservoir properties can be well preserved. However, with the breakthrough in the exploration well Gaotan 1, which produced oil of more than 1 000 m3/d from the deep clastic reservoir in the southern margin of the Junggar Basin [27], the geologists began to conduct deep research on the causes for extra-high production. It is believed that the ultra-high pressure pore preservation effect (18% porosity) is not enough to support the daily production of crude oil exceeding 1 000 m3. Therefore, physical simulation experiments were carried out on the low-mature sandstone samples from the outcrop area, focusing on the burial history and temperature and pressure changes in the basin, to simulate the influence of burial depth and ultra-high pressure on the physical properties of the low-mature sandstone. It is found that a large number of micro-fractures are developed in the sandstone reservoir under the action of high pore pressure (greater than 90 MPa) [28], and the permeability of effective micro-fractures is 10-1 000 times the matrix permeability [29], thus controlling the distribution of high-quality reservoirs and the enrichment and high production of oil and gas. The experimental study further found that the development of micro-fractures is closely related to the rapid burial process in the late stage and the formation of ultra-high pressure strata [30].
The burial history of the ultra-deep clastic rocks in the Hetao Basin [22] shows that until the sedimentary period of Neogene Wulantuke Formation (since 5.3 Ma), the basin began to rapidly subside and be buried at a rate of up to 1 000 m/Ma. The rapid subsidence and deep burial caused under-compaction and overpressure in the Neogene middle and shallow layers. The rapid thermal evolution in the Paleogene and explosive hydrocarbon generation and expulsion formed the ultra-high pressure system, and the source and reservoir are interbedded, with efficient hydrocarbon expulsion and migration in a short time, pressure transmitted to the reservoir in time, made the current reservoir pressure up to 96-120 MPa, with a pressure coefficient of 1.6-2.2. Such ultra-high pressure is very close to the fracturing pressure threshold which is about 70%-80% of the static pressure according to experimental results, thus inducing a large number of micro cracks. Ultra-high pressure not only plays an important role in preserving primary intergranular pores (i.e., the porosity as high as 15.7% at 6 660 m in Well Hetan 101), but also induces micro-fractures in the late rapid deep burial stage, which make the reservoir permeability a qualitative leap. For example, the flow permeability tested in Well Hetan 101 reached 226×10−3 μm2, while the permeability calculated is only one tenth of the air permeability of the core. Therefore, the measured reservoir permeability may reach the ultra-high permeability level due to the development of micro-fractures. This is the most important factor for the thousand tons of oil production of the well drilled in ultra-deep clastic rocks. The Hetao Basin, as an extensional faulted basin, can produce a large number of micro-fractures at ultra-high pressure because it has a high content of rigid particles and brittle minerals (quartz content in the sandstone is greater than 70%). In addition, cementation was weak before rapid and deep burial, and the matrix pores were particularly developed, which are more than 25% above 3 000 m. Combined with some scholars' experiments, this type of reservoir is affected by micro-fractures formed under late ultra-high pressure, and the permeability can be increased by 55%. The ultra-high pressure caused by rapid subsidence and deep burial in the late stage has a good effect on pore preservation and permeability increase of the reservoir. In particular, improving the permeability of ultra-deep reservoirs provides an important guarantee for high and stable oil and gas production.
3.4. Traps and accumulation
The Guangming Structure is a large reverse traction fault-nose facing the trough area, controlled by a large forward slip fault (Xinglong fault). It is separated by a secondary fault inside, forming a high-position Hetan 1 trap and Hetan 101 trap facing the trough area (Fig. 4 ). During the Paleogene fault-depression period, the Xinglong fault controlled the distribution of the early sedimentary (subsidence) center, and gypsum-salt rock of salty lake facies was developed in the Lin 2 Member, while the sedimentary range of gypsum-salt rock in the Lin 1 Member migrated toward northeast along the Xinglong fault. In the Neogene fault depression period, the subsidence center gradually migrated to the downthrown side of the Langshan fault, while the downthrown side of the Xinglong fault gradually changed from an early negative structure to a positive structure. At the same time, affected by the strong extensional fault depression and strike-slip action in the late Neogene, the Xinglong fault evolved from an early normal fault to a large-scale decollement fault that slipped along the bottom of the gypsum-salt rock in the Lin 2 Member, and forming a reverse traction structure. Large-scale negative flower-like structures were developed in the core of the reverse traction, and separating the large-scale fault nose structures into two structural traps. In addition to the fault nose structure formed by the Xinglong fault, the Hetan 101 trap is also controlled by the western section of the nearly east-west trending Hangwu strike-slip fault, forming a fault-block trap controlled by the two faults.
Fig. 4. Spatial distribution and accumulation model of oil and gas reservoirs in ultra-deep Linhe Formation in the north of Linhe depression. |
Due to the development of mudstone caprock thicker than 1 000 m (mud-to-formation ratio is greater than 90%) in the Wuyuan Formation, the fault-mud smearing effect is significant and the trap preservation conditions are good. In the Late Neogene, source rocks began to generate and expel hydrocarbon, and the generated oil and gas migrated and accumulated to the high part of the trap, forming early reservoirs. With the rapid subsidence and deep burial of the Pliocene-Quaternary and the increasing degree of thermal evolution, the source rocks rapidly generated and expelled hydrocarbon, forming ultra-high pressure that transmitted from the source to the reservoir. With the increase of formation pressure and pressure coefficient, a large number of micro-fractures were induced in the source rocks and reservoirs under strong pressure. The development of micro-fractures not only increases the reservoir space and improves the permeability, but also promotes the formation of ultra-deep supersaturated oil and gas reservoirs. This may be an important reason for the oil saturation in Well Hetan 101 as high as 88% from logging interpretation, and the daily output of more than 1 000 t. At the same time, affected by the fault-reservoir-cap assemblage controlled by the secondary fault, the sand group III of the Lin 1 Member is divided into two independent oil and gas systems. The sand group III of Well Hetan 101 in the low part is an ultra-high-yield and ultra-high-pressure pay, while the same set of sand bodies in Well Hetan 1 is a water layer, indicating that the same set of sand bodies in the upper and lower walls of the secondary fault is not connected, and the formation water in the high part was not discharged due to the failure of oil and gas supply in the trough area. From the difference of crude oil properties in different test layers in the two wells, it shows that with the increase of burial depth, the degree of thermal evolution of source rocks and the intensity of hydrocarbon generation and expulsion, the formation pressure coefficient continues increasing, the maturity of crude oil and the gas content (associated gas) increase significantly, while the density, viscosity, asphaltene, resin and sulfur contents of the crude oil decrease significantly, but the wax content and freezing point are higher (Table 1 ), reflecting that the oil and gas reservoirs have the characteristics of near-source enrichment and accumulation. The first row of large structural traps in the trough area is located in the same structural setting of the Hetan 101 trap. Combined with the fact that the southeast sedimentary-source sand body is thinning and pinching out to the northwest and gradually transiting to the salty lake facies, the trap has the characteristics of structural and lithologic dual control. The area of structural-lithologic traps reaches 150 km2, forming large-scale ultra-high pressure structural-lithologic reservoirs, and showing a great exploration potential. It is expected to become a high-efficiency and high-yield oil and gas reservoir with reserves of 100 million tons in ultra-deep clastic rocks in China.
4. Controlling factors on enrichment and high yield of oil and gas in ultra-deep clastic rocks and favorable exploration direction
4.1. Controlling factors on oil and gas enrichment and high yield in ultra-deep clastic rocks
Zhao et al. [31] proposed that the formation of oil and gas reservoirs requires three major elements, namely, geological elements, accumulation process and energy field. Geological elements are oil and gas carriers. The process of hydrocarbon reservoir formation covers the whole process from hydrocarbon generation, migration, accumulation to preservation and transformation. The energy field provides the environmental conditions for the formation of geological elements and the action of the process of hydrocarbon accumulation. The enrichment and accumulation of oil and gas requires the three elements to have a good spatial and temporal configuration relationship. The study on the oil and gas accumulation and enrichment rules in the ultra-deep clastic rocks of the Hetao Basin found several factors control the enrichment and high yield of ultra-deep clastic reservoirs including sufficient oil source and strong source supply, good structural background and near-source trap conditions, high-quality reservoirs with a high quartz content, good sealing and source-reservoir-cap assemblage, and good spatial and temporal configuration of the above factors.
4.1.1. High hydrocarbon generation rate and strong hydrocarbon supply
The development of source rocks with high-yield hydrocarbons and strong source supply is the premise of oil and gas enrichment and high yield of the ultra-deep clastic rocks. The ultra-deep formation located in the sedimentary center of the Linhe Formation developed high-quality source rocks of salty lacustrine facies with early maturity, early expulsion and high hydrocarbon yield, and had great resource potential. With the rapid subsidence and deep burial thermal evolution in the late stage, the explosive hydrocarbon generation-expulsion and ultra-high pressure strata were formed, showing strong hydrocarbon supply, which provide an important basis for oil and gas enrichment in ultra-deep clastic rocks.
4.1.2. Late tectonic activity and good trap conditions
Favorable structural settings and trap conditions provide a good place for oil and gas enrichment in ultra-deep clastic rocks. Affected by the strong tectonic activity of the Langshan fault during the late strong extensional fault depression period, the Linhe Depression became a steep structure faulted in the west and thrusted in the east. In particular, the large Xinglong fault controlling the sag occurred during this period. The strong extensional detachment resulted in a large nose-uplift structural trap facing the trough area, which provided a good structural background for the accumulation and enrichment of oil and gas. At the same time, the detachment compressed the space of the trough area, which effectively promoted the formation of ultra-high pressure strata and indirectly provided favorable conditions for the convergence of oil and gas.
4.1.3. Weak diagenesis preserving pores and overpressure inducing fractures
Pore preservation under weak diagenesis and fracture expansion under ultra-high pressure are important factors guaranteeing the enrichment and high yield of oil and gas in ultra-deep clastic rocks. When the ultra-deep Paleogene Linhe Formation deposited in trough area, distal shallow-water large-scale braided river deltas were developed, where the sand composition and structural maturity were high, the cement content was low, the cementation was weak, and the compaction resistance was strong. Early long-term (5.3-30.0 Ma) shallow burial, late short-term (since 5.3 Ma) rapid deep burial, early oil and gas filling and late ultra-high pressure jointly inhibited cementation and compaction, so that deep sand bodies (4 500-5 500 m) are still relatively loose and weak at present. At the same time, explosive hydrocarbon generation and expulsion of high-quality source rocks after late deep burial induced ultra-high pressure and consequently micro-fractures which greatly improved the reservoir permeability. This changed the conventional understanding that the physical properties of ultra-deep clastic reservoirs decrease sharply with the burial depth, and greatly expands the field of ultra-deep oil and gas exploration. It is also the key to producing 1 000 tons of crude oil per day from the ultra-deep clastic rocks in the Hetao Basin.
4.1.4. Good source-reservoir-caprock assemblage
Good source-reservoir-caprock assemblage and high- quality sealing conditions are important guarantees for oil and gas enrichment in and high yield from ultra-deep clastic rocks. During the deposition of the Linhe Formation, the source and reservoir were interbedded in ultra-deep strata in the trough area, and the NTG was relatively low (20%-25%), forming a good source-reservoir- cap assemblage. The mudstone is not only a good source rock but also a good cap rock. Affected by the rapid subsidence and deep burial in the late stage, the ultra-deep clastic rock provides ultra-high-pressure sealing condition, and at the same time, the overlying Neogene mudstone over a kilometer thick not only serves a good cap rock, but also provides a favorable condition for the lateral sealing of large-scale consequent slip faults. Therefore, from the comparison and analysis of the chromatographic mass spectra in different zones (Fig. 3 ), it can be found that part of the oil and gas in Well Hetan 101 is low mature oil early generated, indicating that the Guangming Structure is an independent oil and gas system with good sealing conditions after oil and gas generation, which provide an important guarantee for the formation of ultra-high pressure and the enrichment and high yield of oil and gas from ultra-deep clastic rocks.
4.2. Favorable fields of ultra-deep oil and gas exploration
The major breakthrough to oil and gas exploration of ultra-deep clastic rocks in the Linhe Depression in the Hetao Basin not only changed the original understanding of the lower limit of effective ultra-deep clastic reservoirs (especially matrix pores) in faulted basins, but also answered questions for the efficient exploration and development of ultra-deep clastic reservoirs in faulted basins. More importantly, the major discovery of the 1 000-ton oil well Hetan 101 shows a great exploration potential of the ultra-deep formations in the trough area, and points out the important direction for future oil and gas exploration in the Hetao Basin. The favorable exploration area of ultra-deep clastic rocks around the trough area in the Linhe Depression is more than 1 000 km2, and of which, the most favorable field is adjacent to the hydrocarbon generating center of the Linhe Formation with a good structural background and development of structural traps, including the Xinglong faulted structural belt and the Nalinhu faulted buried-hill belt. The ultra-deep Wulanbuhe strike-slip structural belt located in the central and southern trough area and far away from the hydrocarbon generating center of the Linhe Formation is adjacent to the hydrocarbon generating center of the Paleogene Wulate Formation of saline lacustrine facies. The ultra-deep layers are potential to explore with fewer wells drilled by now. Especially, the ultra-deep layers are located in the trough area with moderate and low NTG. The sediments mainly came from the southeast. The distribution direction of the sedimentary sand bodies (SE-NW) and the main structural direction (NE-SW) are nearly vertical, which provides a favorable condition for hydrocarbon accumulation in structural traps and structural-lithologic traps. Therefore, the ultra-deep field in the trough area may form various types of structural and structural-lithologic oil and gas reservoirs, which are favorable targets for future exploration in the Linhe Depression.
5. Conclusions
The Paleogene Linhe Formation in the Linhe Depression of the Hetao Basin is rich in high-quality source rocks, with the characteristics of early maturity, early expulsion and efficient generation of hydrocarbon. This is the important basis for the ultra-high pressure oil and gas reservoirs with early low-mature oil filling and mature-high-mature oil and gas filling with increasing thermal evolution degree in the trough area.
A new reservoir formation mechanism characterized by pore preservation under weak digenesis showing weak compaction and cementation and permeability increase by fracture expansion induced by ultra-high pressure is proposed. In the ultra-deep interval deeper than 6 500 m, a medium porosity and medium permeability reservoir with porosity of more than 15% and permeability of 226×10−3 μm2 was discovered. This discovery greatly expands the space for oil and gas exploration and is the key for oil and gas exploration of ultra-deep clastic rocks and obtaining ultra-high yield oil flow.
The enrichment and high yield of oil and gas in the ultra-deep clastic rocks in the Linhe Formation is attributed to the high-quality source rocks which have the potential of high hydrocarbon yield and continuous and strong hydrocarbon supply, with good structural background and dominant traps as the basis, weak diagenesis pore-preserving and ultra-high pressure fracture expansion as key factors, high-quality sealing conditions and good source- reservoir-cap combination as important guarantees.
The area adjacent to the hydrocarbon generating center of the Linhe Formation with good structural background and structural traps is the most favorable exploration target, including the Xinglong faulted structural belt and the Nalinhu faulted buried-hill belt.