Introduction
Carbonate breccia is a kind of coarse clastic rock formed by a series of brecciated geological processes on carbonate parent rock. Researchers have done a lot of research on its petrological characteristics and genetic mechanism, and have obtained a lot of research results. Based on the difference of genetic mechanism, carbonate breccia can be summarized into four categories[1]: sedimentary breccia, such as carbonate gravity flow, broken debris flow and storm deposition[2,3,4,5,6,7,8,9,10]; non-depositional genesis breccia, mainly related to karst-collapse[11,12,13,14]; tectonic breccia, formed by strong tectonic fracturing[15,16]; diagenetic breccia, formed by pressure dissolution and recrystallization during burial diagenesis[1]. In petroleum exploration, carbonate breccia is a kind of high-quality hydrocarbon reservoir rock, which could form large-scale and high-yield plays with huge exploration potential[17,18].
The Yingxi region of the Qaidam Basin has recently been a hotspot for oil and gas exploration in lacustrine carbonate rock of China. Oil and gas exploration in this area began in the early 1980s, an industrial well named S20 drilled in the early stage produced more than 1 000 tons oil per day at the beginning. After more than 30 years of exploitation, it has produced a total of 3×105 m3 of oil and a total liquid of more than 7×105 m3. However, due to limited data, there are still controversies on the types and origins of high- and stable-yield reservoirs in this region. Some researchers suggested that the reservoir is fracture-type[18,19], but fracture-type reservoir cannot explain 30 years of stable oil production. Some other researchers considered that the reservoir space is mainly intercrystalline pores, which are connected by the cracks[20,21], but long-term high and stable yield cannot be achieved by nanometer intercrystalline pore contribution. Based on comprehensive analysis of a large number of cores, thin slices, experimental tests, logging, seismic and productivity data in the area from 2014 to 2017, many types of carbonate breccia were identified in this area. Based on detailed description of their characteristics and classification, the genesis and spatiotemporal distribution of various types of breccia were explored. We concluded that the tectonic breccia interval is the high and stable yield reservoir section in deep zone of Yingxi area, which is of great significance for the high-efficiency exploration in this region.
1. Geological settings
The Yingxi area is located at the northwestern end of the Yingxiong range, one of petroleum-rich structural belts, Qaidam Basin. Affected by the continuous contraction and strike-slip superimposed effect of the Kunlun Mountains and the Altyn tagh Mountains during the Himalaya movement, there are many groups of NW-trending thrust faults were developed in the area (Fig. 1a). The Cenozoic sedimentary sequences in the Qaidam Basin have been divided into seven stratigraphic units from bottom to top (Fig. 1b)[22]: the Paleogene Lulehe Formation (E1+2) and Lower Ganchaigou Formation (E32), the Neogene Upper Ganchaigou Formation (N1) and the Lower Youshashan Formation (N21), the Upper Youshashan Formation (N22), Shizigou Formation (N23) and the Quaternary Qigequan Formation (Qp1+2). The upper section of the Lower Ganchaigou Formation (E32) is the main lacustrine carbonate exploration target layer in the study area. The geochemical analysis of cores and elements show (Table 1) that the depositional environment of this formation featured arid and hot climate, material source under-compensation, and hypoxia reduction[23]. As the lake basin water oscillation, then salinized and eventually turned into a brine lake basin[24], four types of sedimentary rocks, including lacustrine carbonate rock, gray mudstone, rock salt and thin sandstone, with obvious mixing characteristics were deposited. The reservoir rock is mainly carbonate rock, accounting for more than 95% of the total thickness of the reservoir.
Fig. 1.
Fig. 1.
Location and stratigraphic columnar section of the Yingxi area, Qaidam Basin.
Table 1 Element testing data of upper member of Lower Ganchaigou Formation in Yingxi and adjacent areas.
| Well | Depth/ m | Sr/ (mg•kg-1) | Ba/ (mg•kg-1) | Sr/ Ba | V/ (mg•kg-1) | Ni/ (mg•kg-1) | V/ Ni | Co/ (mg•kg-1) | Cu/ (mg•kg-1) | Sr/ Cu | Th/ (mg•kg-1) | U/ (mg•kg-1) | Th/ U |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| C4 | 3 237.0 | 1 167 | 15 870 | 0.07 | 77.5 | 30.4 | 2.6 | 15.40 | 34.5 | 33.9 | 8.87 | 4.78 | 1.86 |
| C6 | 3 409.0 | 929 | 1 100 | 0.84 | 69.9 | 34.8 | 2.0 | 11.40 | 32.5 | 28.6 | 7.41 | 5.04 | 1.47 |
| H39 | 2 668.0 | 701 | 668 | 1.05 | 95.8 | 36.5 | 2.6 | 14.50 | 32.2 | 21.7 | 10.90 | 6.28 | 1.73 |
| Q30 | 2 415.0 | 955 | 1 539 | 0.62 | 67.7 | 31.2 | 2.2 | 10.80 | 26.6 | 35.9 | 8.44 | 3.95 | 2.14 |
| QN1 | 3 610.0 | 366 | 545 | 0.67 | 77.9 | 31.9 | 2.4 | 14.00 | 21.5 | 17.0 | 11.80 | 4.28 | 2.76 |
| ST1 | 3 368.5 | 134 | 460 | 0.29 | 97.5 | 37.7 | 2.6 | 16.10 | 36.1 | 3.7 | 14.90 | 3.18 | 4.69 |
| S15 | 4 075.0 | 195 | 516 | 0.38 | 136.0 | 37.3 | 3.6 | 15.50 | 12.9 | 15.2 | 13.00 | 3.40 | 3.84 |
| S23 | 4 138.0 | 1 505 | 9 863 | 0.15 | 59.7 | 25.7 | 2.3 | 9.36 | 22.9 | 65.8 | 7.66 | 4.19 | 1.83 |
| S25 | 4 006.0 | 268 | 220 | 1.22 | 80.1 | 25.3 | 3.2 | 10.70 | 22.1 | 12.1 | 7.74 | 4.72 | 1.64 |
| S29 | 4 535.5 | 1 045 | 16 730 | 0.06 | 68.4 | 28.9 | 2.4 | 11.30 | 30.5 | 34.2 | 8.91 | 4.41 | 2.02 |
| S35 | 2 806.0 | 565 | 3 190 | 0.18 | 84.3 | 33.8 | 2.5 | 13.60 | 28.8 | 19.6 | 12.50 | 4.81 | 2.61 |
| SB1 | 3 871.0 | 889 | 4 345 | 0.20 | 60.1 | 25.0 | 2.4 | 9.93 | 23.8 | 37.3 | 7.70 | 4.15 | 1.86 |
| Y6 | 3 580.0 | 789 | 3 759 | 0.21 | 56.6 | 24.9 | 2.3 | 9.81 | 23.4 | 33.6 | 7.40 | 3.14 | 2.36 |
2. The carbonate breccia
According to the core and thin section data from new wells, three types of carbonate breccias, sedimentary breccia, diagenetic breccia and tectonic breccia, were identified in the Yingxi area. According to the different occurrence characteristics, tectonic breccia can be further divided into fault breccia and interlayer slip breccia.
2.1. The sedimentary breccia
Sedimentary breccia is thin in single layer and has been encountered in very few wells and layers, i.e. Well S43 only revealed 5 cm thick sedimentary breccia, and Well S41-6-1 encountered sedimentary breccia of about 20 cm in thickness. It is in abrupt contact with both the gypsum lime-dolomite below and the mudstone or muddy lime-dolomite above. The gravel component is mainly muddy lime-dolomite, with poor sorting. The particles are 2-30 mm in size, non-directional in arrangement, and largely sub-angular, with a few gravels weakly rounded. Sand- silt-grade matrix and authigenic minerals such as anhydrite and calcite fill between the gravels (Fig. 2). The sedimentary breccia shows the overall feature of semi-consolidated lime-dolomicrite with slump, transportation and re-deposition characteristics, and has a small amount of dissolved pores and micro-cracks (Fig. 2b).
Fig. 2.
Fig. 2.
Photographs of sedimentary breccia in upper member of Lower Ganchaigou Formation of Yingxi area. (a) Well S41-6-1, 3857.7-3 857.9 m, sedimentary breccia, with mud impregnation on surface, various sizes of gravel, sandy- and muddy-matrix and anhydrite, calcite authigenic mineral filling between gravels; (b) Well S41-6-1, 3 857.78 m, microphotograph of sedimentary breccia, with sandy- and muddy-matrix and anhydrite, calcite authigenic mineral filling between gravels. The gravels are angular, with local weak rounding. There are a small number of dissolved holes and micro-cracks, polarized light.
2.2. The diagenetic breccia
Diagenetic breccia is ubiquitous in the gypsum- rich layer in this area. It is 10-50 cm thick in single layer, and in normal depositional contact with the upper and lower strata. The occurrence of them is basically the same, and there is no obvious abrupt interface and structural transformation characteristics. The main characteristic of the breccia section is the large amount of autogenous mineral aggregates such as euhedral large-crystal anhydrite and glauberite in the original sedimentary muddy lime-dolomite, which prop open and cut the surrounding rock into breccia shape (Fig. 3). Among them, the glauberite mineral is easily dissolved by the drilling fluid, which makes the structural characteristics of the surrounding rock breccia more obvious (Fig. 3a, 3b). The anhydrite has lower solubility, thus retaining the complete crystal growth structure and cutting the surrounding rock (Fig. 3c-3e). The diagenetic breccia only contains nano-size intercrystalline pores in dolomite of the surrounding rock and few large-scale effective dissolution pores.
Fig. 3.
Fig. 3.
Photographs of diagenetic breccia in upper member of Lower Ganchaigou Formation in Yingxi area. (a) Well S41-2, 4203.96-4 204.36 m, diagenetic breccia, euhedral anhydrite- glauberite mineral growth in penecontemporaneous period propping open and causing deformation of weak consolidated argillaceous limestone, and the argillaceous lime-dolomite appears breccia-like shape after being washed and dissolved by mud during drilling; (b) Well S41-2, 4 120.93-4 121.19 m, diagenetic breccia, with the same characteristics as (a) ; (c) Well S41-2, 4 204.3- 4203.8 m, diagenetic breccia, needle-strip self- shaped anhydrite mineral growth cutting argillaceous lime-dolomite into breccia-like shape; (d) Well S41-6-1, 3 864.58-3 864.68 m, diagenetic breccia, needle- and plaque-like euhedral anhydrite mineral growth cutting argillaceous lime-dolomite into breccia-like shape; (e) Well S41-6-1, 3 861.48-3 861.61 m, diagenetic breccia, with the same characteristics as (d) .
2.3. The tectonic breccia
2.3.1. Fault breccia
Fault breccia is common in the core of the thrust fault zone, and is mainly developed in the carbonate interval adjacent to the fault. The core of the breccia is rigidly fractured and has no plastic deformation characteristics. The particles are various in size and consistent with the surrounding rock in composition (Fig. 4a, 4c). The fault breccia is in normal depositional contact with the upper and lower strata, but the bottom stratum has a large dip angle and high-angle tectonic fractures (Fig. 4b), setting it apart from sedimentary breccia and diagenetic breccia. In the thin-sections, this kind of breccia has squeezing and crushing characteristics of the surrounding rock. Under the microscope, the breccias are aligned in edges, angular without rounding, and not directionally arranged (Fig. 4d and 4e). This kind of breccia has rich cracks and holes among breccias. The holes have regular edges. The cracks are in the broken breccias and not directional in arrangement. The cracks and holes are partially filled with cements such as anhydrite in late stage. The fault breccia is overall good in oil-bearing.
Fig. 4.
Fig. 4.
Photographs of fault breccia in upper member of Lower Ganchaigou Formation of Yingxi area. (a) Well S3-1, 4 382.14-4 382.44 m, core photo, fault breccia, the surrounding rock breaking into breccia, with cracks and holes partially filled by anhydrite between breccias, Note oil in cracks and holes; (b) Well S3-1, 4 383.04-4 383.29 m core photo, argillaceous lime-dolomite at the bottom, with large dip angle and high angle cracks; (c) Well S43, 3 913.15-3 913.35 m, core photo, fault breccia, argillaceous lime-dolomite breaking into breccia, with cracks and holes partially filled or full-filled by anhydrite between breccias; (d) Well S3-1, 4 374.76 m, photomicrograph of fault breccia, with cracks and holes partially filled by anhydrite between breccias, blue cast slice, polarized light; (e) Well S43, 3913.05 m, photomicrograph of fault breccia, with cracks and holes partially filled by anhydrite between breccias, blue cast slice, polarized light.
2.3.2. Interlayer slip breccia
Interlayer slip breccia can be seen in the core section of the wellbore far away from the fault zone, and is concentrated in the carbonate layer below the thick salt layer. This kind of breccia has both rigid fracturing and plastic crumpling features, different particle sizes, and composition consistent with the surrounding rock (Fig. 5). The breccia section interbeds with plastic mudstone section, and is in normal sedimentary contact with the upper and lower strata. The upper and lower strata have not significant changes in dip angle, and no obvious high- angle fault planes and cracks, but the mudstone section interbedding with it has obvious near-horizontal slip surface and extrusion plastic deformation feature (Fig. 5a, 5b), so this type of tectonic breccia is defined as interlayer slip breccia, which is different from fault breccia formed due to high-angle thrust fault activity and the other two types of breccias mentioned above. The section of this breccia has rich cracks and holes, the holes have regular edges, the cracks are mainly shearing network cracks partially filled with cements such as late stage anhydrite (Fig. 5c-5e). This breccia has overall good oil-bearing.
Fig. 5.
Fig. 5.
Photographs of interlayer slip breccia in upper member of Lower Ganchaigou Formation of Yingxi area. (a) Well S40, 3 146.61- 3 147.53 m, the carbonate rock is crumpled and broken, and the top and bottom strata are flat and plastically deformed, and inter-breccia cracks and holes are developed; (b) Well S40, 3 147.61-3 147.81 m, with the same characteristics as (a); (c) Well S38-4, 3031.49-3 031.79 m, cracks and holes between breccias partially filled by anhydrite; (d) Well S40, 3 150.66 m, fracture network; (e) Well S38-4, 3 733.47 m, cracks and holes between breccias partially filled by anhydrite.
3. Genesis and distribution of Yingxi lacustrine carbonate breccia
3.1. Genesis and distribution of sedimentary breccia
In the sedimentary period of the upper member of Paleogene Lower Gancaigou Formation, the Yingxi area was at the center of the lake basin, under the arid paleoclimate background, the lake basin experienced high-frequency oscillation of rapid freshwater replenishment desalination and slow evaporation and salinization (Fig. 6). In the vertical profile, the sedimentary breccia occurs in the half-cycle of the lake level rising in freshwater rapid recharge stage (Fig. 6). On the plane, through the restoration of the paleo-geomorphology of this period (Fig. 7), the Yingxi area was found to be in a low-amplitude bulge of the depression, and the drilled sedimentary breccia is on the edge of the steep slope zone, and it transits to mudstone deposits in the depression, and grain beach deposits of higher energy in the high part of the uplift. Therefore, the sedimentary breccia in the Yingxi area is the product of early weakly-consolidated carbonate rock in the steep slope zone slumping and redeposition under the effect of short-term high-energy water-reformation (Fig. 8), with overall development characterized by small in scale and limited in spacial distribution.
Fig. 6.
Fig. 6.
Composite column of the core section of upper member of Lower Ganchaigou Formation in Well S41-6-1 of Yingxi area.
Fig. 7.
Fig. 7.
Restoration of paleotopography in different periods of the Lower Ganchaigou Formation in the Yingxi area.
Fig. 8.
Fig. 8.
Developmental model of sedimentary breccia in Yingxi area.
3.2. Genesis and distribution of diagenetic breccia
Flügel defined diagenetic breccia as carbonate with breccia structure formed by the authigenic mineral growth, recrystallization and pressure dissolution during diagenetic period[1]. In other words, the diagenetic breccia is pseudo-breccia, a diagenetic phenomenon. The pseudo-breccia structure of the lacustrine carbonate in the Yingxi area is generally resulted from the growth of authigenic minerals such as anhydrite and salt. On the vertical profile, this kind of breccia often occurs at the top of the half-cycle of the lake level fall when the evaporation and sedimentation of the lake basin was relatively continuous and stable, that was also the depositional period of the gypsum dolomitic flat (Fig. 6). On the plane, pseudo- breccia structure is distributed in the gypsum dolomitic flat facies belt. In Yingxi area, under the sedimentary and diagenetic background of salt lake basin, gypsum-salt minerals generally developed in the sedimentary, quasi-contemporaneous and burial diagenetic periods, but they differ widely in occurrence. During the depositional period, the gypsum-salt minerals mainly formed layered salt rock[25]; during the penecontemporaneous period, the gypsum-salt minerals were generated in the argillaceous lime-dolostone, in euhedral and coarse-crystals and patches; during the burial diagenesis period the gypsum-salt was generated and filled in holes and cracks as cement, and was small in crystal size and low in euhedral degree. According to the characteristics of the Yingxi diagenetic breccia (Fig. 3), the main reason for the formation of breccia-like structure in this area is the crystallization and growth of euhedral coarse crystal salt minerals: i.e. during the penecontemporaneous period, the weakly consolidated argillaceous lime-dolostone was saturated with high-salinity formation water, then the gypsum-salt precipitated, forming authigenic mineral aggregates such as anhydrite and glauberite with a high euhedral degree and coarse crystals; the mineral aggregates, while occupying the original rock space, propped open the weakly consolidated argillaceous calcite dolostone rock to form breccia structure.
3.3. Genesis and distribution of tectonic breccia
Affected by continuous contraction and strike-slip effect of the Kunlun Mountains and the Altyn tagh Mountains during the Himalayan movement, the sedimentary combination of mudstone, carbonate and gypsum-salt in the upper part of the Lower Ganchaigou Formation in the Yingxi area shows dual structural deformation feature[26]. Through the structure interpretation of the seismic data in the Yingxi area (Fig. 9a), the upper thick layer of rock salt of the Lower Ganchaigou Formation in the Yingxi area developed a large-scale thrust fault in the Shizigou area, in the rock salt formation is the near-level slippage section, and as the gypsum-salt had plastic flow to the shallow layer, the fracturing transformed into thrust feature. Under the thrust extrusion and lateral traction of the fault, a secondary fault system formed in the stratum below at the same period. Because the rock below the gypsum salt rock is mainly carbonate rock, followed by mudstone, with higher rigidity, the main body of the fracture system is primarily the two-way thrust with larger angle.
Fig. 9.
Fig. 9.
Seismic interpretation and tectonic breccia development model in the Yingxi area (see
According to the above structural interpretation conclusions, and analysis combined with the development characteristics of two types of tectonic breccia revealed by drilling core and logging data, it is found that the two types of breccia are closely related to the fault activity of the large regional Shizigou Fault, but they are different in stress mechanism and distribution. Based on the difference, the formation model of the Yingxi tectonic breccia was established (Fig. 9b). We suggested that the interlayer slip breccia is concentrated in the carbonate rock section below the slip-off layer of gypsum-salt rocks adjacent to the Shizigou fault, and is formed by the transverse traction shearing and crumpling of the slippage section of the fault. The strata below far from the slip layer were subjected to gradually weakening force and become less fragmented, and the breaking disappears in the plastic deformation of the mudstone layer. The fault breccia is concentrated in the carbonate interval within the influence scope of the secondary fault system below the Shizigou Fault, and is formed by the rigid crushing of the carbonate rock along faults. Laterally, under the gypsum salt rock layer, the interlayer slip breccia is widely developed and can be connected with the fault breccia into big pieces. In the vertical profile, the fault breccia develops more deeply, but its extension range is controlled by the scale of the secondary fault system.
4. Significance of carbonate breccia in petroleum exploration
According to the analysis of lithology, physical properties and productivity data, there are three types of reservoirs in the upper section of the Lower Ganchaigou Formation in the Yingxi area: (1) Fracture-pore reservoir, mainly made up of two kinds of tectonic breccia—fault breccia and interlayer slip breccia, this kind of reservoir, with tectonic breccia fractures and holes as storage space, has a porosity > 8%, permeability of generally greater than 1×103 μm2, and featured by initial high yield and long-term stable production (Fig. 10a). (2) Dissolved pore reservoir, mainly composed of arene lime-dolostone, laminated lime-dolostone and sedimentary breccia, with dissolved pores and intercrystalline pores as storage space, this kind of reservoir has a porosity of 5%-8%, and permeability of (0.01-1.00)×103 μm2, and features low and long-term stable yield (Fig. 10b). (3) Fracture-type reservoirs, made up of largely massive muddy lime-dolostone, with intercrystalline pores and cracks as storage space, this kind of reservoir has a matrix porosity of 3% to 8%, and permeability of non-cracked samples of generally less than 0.01×103 μm2, and features high-yield in the initial stage and rapid decline of productivity (Fig. 10c).
Fig. 10.
Fig. 10.
Comparison of typical testing curves of the upper member of the Lower Ganchaigou Formation in Yingxi area.
Hence it can be seen that the fracture-pore reservoir of the fault breccia and interlayer slip breccia of tectonic genesis is a high and stable yield reservoir in the deep zone of Yingxi area, which is of great significance for the next step of efficient exploration. Based on the genesis and distribution law of these two types of tectonic breccia, it is considered that the carbonate layer beneath the thick layer of gypsum-salt rock is the most favorable zone with interlayer breccia and fault breccia that are high and stable yield reservoirs. This has been proved by high yield wells S38 and S205. The two types of tectonic breccia reservoirs are connected in large pieces on the plane view, forming large-scale high-quality reservoirs in the Yingxi area. In the middle and lower part of the Lower Ganchaigou Formation, interlayer slip breccia is hardly exists, but in the secondary fault concentrated zone, large-scale fault breccia high-yield and stable reservoirs can be found, which is also the major production interval in S20 well area with long-term high and stable production.
In addition to the above-mentioned high and stable yield reservoirs of tectonic breccia, although the sedimentary breccia and diagenetic breccia pointed out in this study have smaller scale and poorer oil-bearing properties, they have good indicative significance for searching commercially valuable carbonate reservoirs. The sedimentary breccia marks the slope-paleo-uplift zone transiting to the high-energy water body, suggesting that the paleo-uplift zone nearby develops favorable reservoir of grain beach facies carbonate, which is likely to form dissolved pore reservoir with high storage capacity, and can be stimulated to achieve high productivity. The cyclical and planar distribution of the diagenetic breccia development reflects the source and activity pattern of the high salinity brine in the penecontemporaneous period, revealing the evolution of the salt lake basin entering the shallow water, high salinity and low energy stage. That is of great significance for the prediction of space-time distribution in the facies and carbonate concentrated interval.
5. Conclusions
There are carbonate breccia of three genesises in the upper part of the Lower Ganchaigou Formation of Yingxi area, sedimentary breccia, diagenetic breccia and tectonic breccia. The tectonic breccia is further divided into fault breccia and interlayer slip breccia.
Sedimentary breccia in the Yingxi area is the product of slump and redeposition of early weakly-consolidated carbonate rock affected by short-term high-energy water and other geological processes in the steep slope zone of low-amplitude paleo-uplift in the depression. The crystal growth of euhedral and coarse-grained salt minerals in the weakly consolidated argillaceous carbonate in the penecontemporaneous period propped open and cut the surrounding rock to form breccia-like structure, that is the diagenetic breccia. Both the fault breccia and interlayer breccia are closely related to the fault activity of the large Shizigou Fault. The interlayer slip breccia is caused by the transverse traction shearing and crushing of the slippage of the Shizigou Fault. The fault breccia is the product of rigid breaking of carbonate under the effect of squeezing and thrusting of the secondary faults derived from the Shizigou Fault.
The fracture-pore reservoir of fault breccia and interlayer slip breccia of the tectonic genesis is high and stable yield reservoir in the deep zone of Yingxi area, which is of great significance for the next step exploration. It is believed that the carbonate layer beneath the thick layer of salt rock is the most favorable zone for the interlayer breccia and fault breccia high and stable yield reservoir. The two types of tectonic breccia reservoirs are connected into large pieces on the plane view and large-scale high-quality reservoirs in the Yingxi area. In the middle and lower part of the Lower Ganchaigou Formation, large-scale fault breccia high and stable yield reservoirs may be found in the secondary fault concentrated development zone. Sedimentary breccia and diagenetic breccia have good indicative significance for searching large-scale carbonate reservoirs.
Reference
Carbonate breccias in the lower-middle Ordovician Maggol limestone (Taebacksan Basin, South Korea): Implications for regional tectonism
DOI:10.1007/BF02668072
URL
[Cited within: 1]
Carbonate breccias occur sporadically in the Lower-Middle Ordovician Maggol Limestone exposed in the Taebacksan Basin, South Korea. These carbonate breccias have been previously interpreted as intraformational or fault breccias. Thus, little attention has been focused on tectonic and stratigraphic significance of these breccias. This study, however, indicates that the majority of these breccias are solution-collapse breccias, which are causally linked to paleokarstification.Carbonate facies analysis in conjunction with conodont biostratigraphy suggests that an overall regression toward the top of the Maggol Limestone probably culminated in subaerial exposure of platform carbonates during the early Middle Ordovician. Extensive subaerial exposure of platform carbonates resulted in paleokarst-related solution-collapse breccias in the upper maggol Limestone. This subaerial exposure event is manifested as a major paleokarst unconformity elsewhere beneath the Middle Ordovician sequence, most notably North America and North China. Due to its global extent, the early Middle ordovician paleokarst unconformity ( he Sauk-Tippecanoe sequence boundary ) has been viewed as a product of second-order eustatic sea level drop during the early Middle Ordovician. Although we recognizes a paleokarst breccia zone in the upper Maggol Limestone beneath the Middle Ordovician sequence, the early Middle Ordovician sequence boundary appears to be a conformable transgressive surface or a drowning unconformity, rather than a major paleokarst unconformity. The paleokarst breccia zone in the upper Maggol Limestone is represented by a thinning-upward stack of exposure-capped tidal flat-dominated cycles that are closely associated with multiple occurrences of paleokarst-related solution-collapse breccias. The paleokarst breccia zone in the upper Maggol Limestone was a likely consequence of repeated high-frequency sea level fluctuations of fourth- and fifth-order superimposed on a second-and third-order eustatic fall in sea level that was less than the rate of tectonic subsidence across the platform. It suggests that second- and thirdorder eustatic sea level drop may have been significantly tempered by substantial tectonic subsidence near the end of maggol deposition. The tectonic subsidence in the basin is also evidenced by the occurrence of coeval off-platform lowstand siliciclastic quarzite lenses as well as debris flow carbonate breccias. With the continued tectonic subsidence, subsequent rise in the eustatic cycle caused drowning and deep flooding of carbonate platform, forming a conformable transgressive surface or a drowning unconformity on the top of the paleokarst breccia zone. This tectonic implication contrasts notably with the slowly subsiding carbonate platform model for the Taebacksan Basin as previously intepreted. Here we propose that the Taebacksan Basin evolved from a slowly subsiding carbonate platform to a rapidly subsiding intracontinental rift basin during the early Middle Ordovician. This study also provides a good example that the falling part of the eustatic sea-level cycle may not produce a significant event at all in a rapidly subsiding basin where the rate of eustatic fall always remained lower than the rate of subsidence.
Coarse carbonate breccias as a result of water-wave cyclic loading (uppermost Jurassic-South-East Basin, France)
DOI:10.1046/j.1365-3091.2001.00395.x
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In the uppermost Jurassic of the central part of the South-East Basin of France, an association of lime mudstone beds, calcarenite beds and coarse carbonate breccia bodies form an informal stratigraphical unit called the 'Barre Tithonique'. In the 'Barre Tithonique', gradual transitions from lime mudstone or calcarenite to breccia show different stages of deformation leading to progressive brecciation of the original lithologies. The study of the breccia facies, and the observed gradual transitions as a whole, document a new early diagenetic process in carbonate environments, resulting from water-wave and seabed interaction. Water-wave induced brecciation and its abundance in the 'Barre Tithonique' indicate that sea–seabed interaction was significant. Comparison with modern studies of the mechanics of wave–seabed interaction suggests that water depth was less than 200 m. It is demonstrated that sedimentary features such as channel-like structures, previously interpreted as being the result of erosion and deposition of mud-flows, were in fact produced by wave-induced, in situ reworking of lime mud, without any significant unidirectional flow or gravity induced displacement.
Storm deposits and storm-generated coarse carbonate breccias on a pelagic outer shelf (South-East Basin, France)
DOI:10.1046/j.1365-3091.2001.00358.x
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<P>Uppermost Jurassic limestones of the South-East Basin (France) are organized into four facies associations that were deposited in four distinct zones: (1) peritidal lagoonal limestones; (2) bioclastic and reefal limestones; (3) pelagic lime mudstones; (4) lime mudstones/calcarenites/coarse breccias. Calcarenite deposits of zone 4 exhibit sedimentary structures that are diagnostic of deposition under wave-induced combined flow. In subzone 4a, both vertical and lateral transitions from lime mudstone/calcarenite to breccia indicate in situ brecciation under wave-cyclic loading. Breccias were produced by heterogeneous liquefaction of material previously deposited on the sea floor. Deposits in subzone 4a record relatively long periods (>400 kyr) of sedimentation below wave base, alternating with periods of deposition under wave-induced currents and periods of in situ deformation. In this zone, storm waves were attenuated by wave ediment interaction, and wave energy was absorbed by the deformation of soft sediment. With reference to present-day wave attenuation, water depths in this zone ranged between 50 and 80 m. Landwards of the attenuation zone, in zone 3, storm waves were reduced to fair-weather wave heights. Storm wave base was not horizontal and became shallower landwards. As a consequence, water depth and wave energy were not linearly related. On a small area of the seaward edge of subzone 4a, cobbles were removed by traction currents and redeposited in subzone 4b. There, they formed a 100-m-thick wedge, which prograded over 3 km and was built up by the stacking of 5- to 20-m-thick cross-stratified sets of coarse breccia. This wedge records the transport and redeposition of cobbles by a high-velocity unidirectional component of a combined flow. The increase in flow velocity in a restricted area is proposed to result from flow concentration in a channel-like structure of the downwelling in the gulf formed by the basin. In more distal subzone 4c, the hydrodynamic effect of wave-induced currents was quasi-permanent, and brecciation by wave ediment interaction occurred only episodically. This indicates that, seawards of the attenuation zone, hydrodynamic storm wave base was deeper than mechanical storm wave base. Uppermost Jurassic carbonates were deposited and soft-sediment deformed on a hurricane-dominated ramp of very gentle slope and characterized by a zone of storm wave degeneration, located seawards of a zone of sedimentation below wave base.</P>
The South Pyrenean Eocene carbonate megabreccias revisited: New interpretation based on evidence from the Pamplona Basin
DOI:10.1016/S0037-0738(99)00004-4
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The South Pyrenean Foreland Basin contains numerous units of Eocene carbonate megabreccias intercalated with siliciclastic turbidites and derived by resedimentation of shallow-marine carbonate platforms. Previous studies were limited mainly to the foreland eastern part, known as the Jaca Basin. The present study from the Pamplona Basin, a western part of the foreland trough, sheds new light on the origin and regional significance of these South Pyrenean Eocene carbonate megabreccias (SPECMs). The number of the SPECM units in the foreland basin is higher than previously recognized and their age is somewhat older than originally assumed. The SPECM units appear to occur as time-stratigraphic clusters, which can be correlated with the relative sea-level lowstands and linked with phases of tectonic activity. The megabreccias were derived from a carbonate-platform system hosted by the foreland basin's southern (passive) margin. The episodic instability and mass wasting were triggered by phases of structural steepening (forebulge uplift) accompanied by high-magnitude earthquakes, with the former causing platform emergence, increased load stresses and excess pore-water pressure in the carbonate ramp. The SPECM deposits were emplaced by cohesive debris flows evolving into high-density turbidite currents. An ideal SPECM unit consists of (1) an immature, homogeneous debrite in the proximal part; (2) a differentiated, bipartite debrite and turbidite in the medial part; and (3) an incomplete, base-missing debrite overlain by turbidite, or a turbidite alone, in the distal part. The debrite component volumetrically predominates in the SPECM units, and the original terms `megaturbidite' and `seismoturbidite' thus seem to be inappropriate for these deposits.
The carbonate gravity flow sediments in the juncture of Yunnan, Guizhou and Guangxi
A sea floor miner expansion which occurred from Dong-Wu Orogeny to Indian-China stage in the juncture of Yunnan, Guizhou and Guangxi resulted in a paleolandscape characterized by the continental margin seas On the slope foot zones which were located along the boundary between the carbonate platforms and the continental margin sea basin or on the edges of separated carbonate platforms in the continental margin sea trench, carbonate debric flow was extremely developed; most feeding channels were filled with resedimentary graval-grained debris limestones; the branch channels---with carbonate grained rock flow sediments; on the carbonate platform or in the deep water areas near the reef block edges there are some limestone debris turbidites of the A or B sequences. In the open seas the limestone debris turbidites of the C sequence are widely distributed.The carbonate gravity flow sediments may be used as an important facies markers in determing such ancient basin edges as those of carbonate platform adjasent to sediments of this kind, carbonate continental shelves and carbonate reef blocks.
Deep water carbonate debris flow in the middle Ordovician Pingliang formation of Fuping, Shaanxi
DOI:10.11743/ogg19820105
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A chaotically brecciated limestone is found in the Pingliang formationof the Middle Ordovician in Fuping area,Shaanxi province.It occurs asbedded or sheet strata in,and in sharp contact with,the foliated and thinlytabular limestones.The fragments of it are highly varied in size and chao-tically distributed,with the giant being 1 7m or larger.This limestone islack of stratification and sorting,in which floating boulders and blocks arescattered in the matrix of finer clasts.These features show that it is of deepwater carbonate sediments of debris.This limestone is dominantly composedof breccias of inner basin source.These clasts,formed by gravity slidingof consolidated or semiconsolidated sedimentary layers on slope of deepwater basin,are transported by gravity and have accumulated on the mar-gin of the basin floor.In the Middle Ordovician,the Fuping area was located on the northernmargin of the deep water basin,where the maximum water depth was nearlyone thousand metres.After the deposition of the Early Ordovician Majia-gou formation,this area was subsided to form a deep water basin on thecontinental margin,by the tension fracturing of the back-arc basin on thenorthern side of the Qinling and Qilian Mountains.
Deep water carbonate brecclas in the Nanpan river area during the Permian and Triassic periods
The carbonate breccias exist widely in the Nanpan river area during the Permian and Triassic periods. According to their petrological chara cteristics and relations with surrounding rocks, we can regard them as deep water carbonate rocks of the debris flow and turbidity current sediments. In this area Dong Wu movement resulted in a lot of faulttrough basins controlled by faults in different directions and obvious difference in lithofacies and topography between the fault-trough and the platform, which was very helpful for gravity flow. carbonate to form. The patterns of the carbonate breccias can reflect the paleotectonic geomorphic environment at that time.
Preliminary study on Triassic and late Paleoxoic reef facies in Yunnan-Guizhou-Guangxi region and their Petroleum prospect
Re-discussion on the origin of the rudstone in Middle Permian Qixia Formation along Lower Yangtze River of Anhui Province
DOI:10.1007/s11783-011-0280-z
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The Middle Permian Qixia Formation along Lower Yangtze River in Anhui province is very thick, with high organic content and well-preserved condition, the well-developed rudstone is also the key attention object. But about the origin of the rudstone has different viewpoints. Undoubtedly, this is an imperfection for reconstruction of the Permian paleogeography and oil as exploration along Lower Yangtze River area. Therefore, it is significant to ascertain the origin and sedimentary facies of the rudstone in the Qixia Formation. According to the colour, content, appearance, size and roundness, writers differentiate 4 types of rudstone. Summarizing international understanding of the origin of rudstone and previous study of this area, combining with this study, in-depth discussion has been made on the origin of the rudstone. The result showing that the rudstone is formed in slope. In addition, because of the difference of gradient, it respectively developed different gravel shapes, gravel and matrix assemblages, and microfacies' characters. Clinothem facies of the rudstone can be further divided into upper slope, middle slope and lower slope.
Depositional and non-depositional carbonate breccias, Chiantla Quadrangle, Guatemala
DOI:10.1130/0016-7606(1969)80[429:DANCBC]2.0.CO;2 URL [Cited within: 1]
Descriptive field classification of sedimentary and diagenetic breccia fabrics in carbonate rocks
Origin of structurally controlled hydrothermal dolomite in epigenetic karst system during shallow burial: An example from Middle Permian Maokou Formation, central Sichuan Basin, SW China
DOI:10.1016/S1876-3804(16)30117-3
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Based on the outcrops, drilling cores, thin sections observation and geochemical analysis, studies were done on the genesis of dolomite in the Middle Permian of central Sichuan Basin. Mosaic fine-crystalline dolomite and subhedral-euhedral siliceous fine-crystalline dolomite can be identified microscopically, which have different geochemical characteristics. Genetic analysis indicates insufficient mantle-derived fluids and marine-derived fluids entered karst system through basement faults, and then were mixed with previously-preserved crust-derived fluids in varied degrees during early Longtan period; in the relatively porous karst system, carbonate sands were dolomitized to subhedral-euhedral dolomites; insufficient mantle-derived hydrothermal fluids were mixed with previously-preserved crust-derived fluids, with coincidence reconstruction on the regions where there were preexisting karst systems but short of hydrothermal fluids, resulting in precursor limestone breccias and host rocks dolomitized to form anhedral mosaic dolomite; during late Longtan period, the overlying sediments sealed the epigenetic karst systems, and the insufficient deep mantle-derived fluids were mixed with previous fluids within the karst systems, with coincidence reconstruction on the relatively porous dolomites, while the tight anhedral mosaic fine-crystalline dolomites didn't change much, forming two types of dolomites with different petrological and geochemical characteristics. Thus, the formation of preferable dolomite reservoir is possibly related to inland facies-controlled epigenetic karst, and therefore the exploration of Maokou Formation in Middle Permian should pay more attention to the karst-related reservoirs.
Origin, characteristics and significance of collapsed-paleocave systems in Sinian to Permian carbonate strata in Central Sichuan Basin, SW China
DOI:10.1016/S1876-3804(14)60060-4
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Stimulation principles, construction techniques, equipment requirements and technical features of liquid carbon dioxide fracturing were summarized, and the existing problems and development trend of this technology were discussed. Compared with the conventional hydraulic fracturing technology, it has several advantages including high flowback, small damage in reservoir, outstanding stimulation effect and so on. There are five main problems existing in this technology: friction of liquid carbon dioxide is very high; liquid carbon dioxide has an extremely low viscosity, poor proppant carrying ability and a large amount of fluid loss, thus behaves poorly in fracturing; the phase behavior of carbon dioxide is very complex in the process of fracturing, it is hard to realize accurate prediction and control for phase transition of carbon dioxide; fracturing equipments, especially blenders have obvious defects and should be improved further; computational methods for operation parameters in liquid carbon dioxide fracturing is still lacking. Supercritical carbon dioxide fracturing technology succeeds almost all the advantages of traditional liquid carbon dioxide fracturing technology, and has a better stimulation effect, smaller pump pressure and fewer requirements for blenders, thus is the trend in carbon dioxide fracturing.
Impact breccias in carbonate rocks, Sierra Madera, Texas
DOI:10.1130/0016-7606(1971)82[1009:IBICRS]2.0.CO;2
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ABSTRACT Two main types of deformational breccia occur in the Sierra Madera cryptoexplosion structure: monolithologic breccias composed of shattered rock of a single lithology and mixed breccias composed of rocks of several lithologies. Monolithologic breccias generally show no mineralogic signs of shock deformation, but a few samples are shatter-coned in a manner suggesting simultaneous formation of breccias and shatter cones. Mixed breccias, forming irregular, cross-cutting bodies, consistently contain moderately to highly shocked material, with mineralogic evidence of shock pressures of 50 kb to more than 200 kb, which, with evidence from the structural geometry of Sierra Madera and orientation of shatter cones, indicate an impact origin of the breccias. The mode of occurrence of the breccias, petrographic characteristics, and association with shock features are shared by breccias in many other cryptoexplosion structures in both carbonate and crystalline rock terranes, suggesting that such breccias have a common origin.
Carbonate dilation breccias: Examples from the damage zone to the Dent Fault, northwest England
DOI:10.1130/B25568.1
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The obliqu-reverse Dent Fault, northwest England, throws Carboniferous limestone units in the footwall against mudston-dominated lower Paleozoic rocks in the hanging wall. The fault zone cuts the kilometer-wide steep limb of a precursory forced monocline. However, individual fault strands comprise centimeter-scale cataclasite cores fringed in the footwall carbonates by damage zones, some meters to tens of meters wide, composed of random-fabric dilation breccias. Breccia texture and microstructure, revealed by stained thin sections and peels, imply rapid coseismic fragmentation and then interseismic resealing by void-filling cements. The cements varied in composition through time from calcite to dolomite and then to ferroan calcite. Pervasive dolomitization of the protolith is common in the breccia zones. A key observation is that each volume of dilation breccia shows only limited refracture. This tendency to singl-phase brecciation suggests that cementation caused reseal-hardening of breccia with respect to intact protolith. Breccia thickness and refracture are greatest at jogs in the Dent Fault, but breccia distribution suggests that damage also accumulated in fault walls and at propagating fault tips. Dilation breccias are a common but poorly documented product of brittle deformation of limestone. Their reseal histories can provide valuable general clues to fault zone evolution.
Geological conditions and distributional features of large-scale carbonate reservoirs onshore China
DOI:10.1016/S1876-3804(12)60010-X
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Based on well cores and thin section observations of more than 300 wells from major exploration target areas and formations in the Tarim, Sichuan and Ordos Basins, combined with seismic, well logging and testing data, the types and characteristics of carbonate reservoirs as well as the geologic conditions for their extensive development are analyzed systematically, and their distribution features are summarized. All varieties of marine carbonate reservoirs are developed in China, including three types of large-scale effective reservoirs, which are (1) depositional reef-shoal and dolomite reservoirs, (2) epigenetic dissolution-percolation reservoirs and (3) deep burial-hydrothermal altered reservoirs. Besides sedimentary facies, paleoclimate and paleogeomorphy, other factors controlling the development of deep large-scale effective reservoirs include interstratal and intrastratal dissolution-percolation and burial dolomitization which can be impacted by hydrothermal processes. Large effective reservoirs in deep carbonate rocks are distributed along unconformities and hiatuses in sedimentation, while reservoirs of epigenetic dissolution-percolation type extend from paleohigh uplift zones to lower slope reliefs. The reservoirs are widely distributed in stratified planar forms, and are superposed by multi-stage karstification processes vertically and have obvious heterogeneity controls. Burial dolomitization is restricted by primary sedimentary facies, and can form extensive effective reservoirs in deep layers in layered or stratified shapes. Hydrothermal related reservoirs are always distributed along deep, large faults, forming effective reservoirs in the form of a bead string in vertical direction and band-rod horizontally, which are not restricted by burial depth.
Geologic characteristics of oil and gas reservoirs in old lower Paleozoic and Sinian Carbonate rocks
Correlated with statistics and analysis of the characteristic parameters from eight Ordovician carbonate reservoirs in eight oil and gas fields in north American, the features and the main controlling factors of the Lower Paleozoic and Sinian carbonate reservoirs from some oil and gas fields in China are studied. In North American, the Ordovician sedimentary environments mostly were circular tide flats to shallow shelves and the carbonate reservoirs can be briefly divide into four types of lithofacies, including mudstone,wackestone, packstone and grainstone, in which intercrystal pores, pores related with breccia and dissolved pores are common while in China, the evolution of Lower Paleozoic and Sinian carbonate reservoirs are more complicated and primary pores are mostly damaged out. The both results of reservoir analysis to North American and China have shown that both of kast reservoirs and dolostone reservoirs are all effective ones. When burial depth are more than 3000m, dolostone is the dominant type of reservoir rock. As increasing depth, dolostone can keep good reservoir capability, which is a elementary trend. The Lower Paleozoic and Sinian carbonate rock are commonly greatly deep-buried in many basins in China so that the dolostone reservoirs are a domain worthy to pay an attention especially.
N1—E1+2 fractured reservoir evaluation and control factors of petroleum accumulation in Shizigou Structure, Qaidam Basin
Analysis of hydrocarbon accumulation period in Paleogene reservoirs, Shizigou oilfield of Qaidam Basin
It is proved by practice of exploration that the Shizigou Paleogene reservoir contains abundant hydrocarbon resources, but deep oil-gas exploration didn't make a big breakthrough, and the major reason is that, a poor understanding of hydrocarbon accumulation stages restrains the accurate research about accumulation process. In this paper, through the microscopic observation, fluorescence analysis and homogenization temperature and salinity test of fluid inclusions in Paleogene reservoirs of study area, the hydrocarbon charging history of Paleogene reservoir have been discussed. The result shows that there are two major stages. By integrating with the study on the burial and thermal history, the first change occurs in middle Pliocene(12-10 Ma bp) and the entrapped fluid inclusions are mainly consisted of liquid hydrocarbon and gas-liquid hydrocarbon with buff-yellow and yellow-green fluorescent light. The second time occurs in the final stage of Pliocene (4-2 Ma bp) and the entrapped fluid inclusions are mainly consisted of gas-liquid hydrocarbon with yellow-green and green fluorescent light. In addition, the reservoirs in study area also have the characteristics of multi-stage charging and continual charging, but major fluid injection time is in the later period on the whole. Therefore, the scale of later hydrocarbon accumulation is larger.
Reservoir characteristics and controlling factors of deep diamictite in Yingxi area, Qaidam Basin
DOI:10.3969/j.issn.1673-8926.2015.05.035
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Based on the classification method of diamictite proposed by Zhang Xionghua, the lithology of deep diamictite in Yingxi area of Qaidam Basin was classified simply into three types: carbonatite, mudstone and sandstone.The data of thin section, whole rock mineral analysis, nuclear magnetic resonance and core porosity were used to study the reservoir characteristics of deep diamictite in Yingxi area. The main rock type of the reservoir is micrite limestone(dolomite), and the minerals include terrigenous minerals, carbonate minerals and clay minerals. The porosity ranges from 0.58% to 8.58%, with the average value of 2.69%, and the permeability ranges from 0.010 m D to 0.501 m D, with the average value of 0.053 m D. The reservoir has the characteristics of low porosity and low permeability,and the correlation between the two parameters is poor. The reservoir spaces mainly include intergranular pores and fractures, with fewer dissolved pores. The content of carbonate minerals and compaction are the main controlling factors for reservoir properties.
The characteristics and major factors controlling on the E3 2 dolomite reservoirs in saline lacustrine basin in the Yingxi area of Qaidam Basin
DOI:10.11764/j.issn.1672-1926.2016.12.005
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In recent years,important breakthrough progress and discovery has been made in exploration of petroleum from deep reservoirs of E_3~2 in western Qaidam Basin.Based on core observation,petrographic and structural microscopic characteristics analysis,and geochemical analysis,the characteristics and major factors controlling the development of dolomite reservoirs in the Yingxi area in Qaidam Basin are studied.Dolomicrite was the main effective reservoir rock of E_3~2 in the Yingxi region,whose physical characteristics are "low porosity-extra low permeability".The reservoir spaces of the dolomite are mainly intergranular pore,but including some micro-cracks.Oil and gas can be produced stably in the Yingxi Oilfield because most oil and gas are preserved in these "small size-large quantity" pores.Research results of the geochemical characteristics of reservoir show that the dolomite is penecontemporaneous metasomatic type which formed in the salty conditions.Main evidence includes:(1)micro pore structure characteristics,(2)high Ca/Mg molar rati-0 of no ideal components,(3)low content of "Mn" element geochemical characteristics,(4) "carbon negative-oxygen positive" isotope geochemistry and the low temperature characteristics reflected by it,(5)low degree of order structure which is caused by the rapid nucleation and crystallization unstable environment.The shrinkage intergranular pores were formed by ion exchange in the crystal lattice in the special salty environment,and its pore size is controlled by the abundance of Mg~(2+) at that time.Therefore,the physical property of the reservoir is indirectly controlled by the paleo salinity.These intergranular pores have a strong ability to resist compaction and they can still be preserved in the deep,ultra deep strata.The origin model of dolomite determines its wide distribution in the plane.So the research findings expanded a new exploration field about lacustrine dolomite in western Qaidam whether in the plane distribution or in depth.
Geological features and exploration fields of tight oil in the Cenozoic of western Qaidam Basin, NW China
DOI:10.1016/S1876-3804(17)30024-1
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Using a large amount of drilling and experimental analysis data, this paper evaluates four potential fields of tight oil exploration in western Qaidam Basin from comprehensive analysis of geological conditions such as sedimentary environments, source rock evaluations, reservoir characteristics, and source-reservoir relationships. Influenced by continuous uplift of Tibet Plateau since Paleogene, the sedimentary environment of the western Qaidam Basin exibits three characteristics:(1) a paleo-topographic configuration consisted of inherited slopes, depressions and paleohighs;(2) frequent alternation of relative humid and arid paleoclimate; and(3) oscillation of salinity and level of the paleo-lake water. Preferential paleo-environment resulted in two sets of large-scale source rocks with high efficiency and two types of large-scale tight reservoir rocks(siliclastic and carbonate), deposited during the late Paleogene to early Neogene. The above source and reservoir rocks form favorable spatial relationships which can be classified into three categories: symbiotic, inter and lateral. Based on sedimentary environments and reservoir types, tight oil resource in western Qaidam Basin can be divided into four types, corresponding to four exploration fields: salty lacustrine carbonate tight oil, shallow lake beach-bar sandstone tight oil, delta-front-sandstone tight oil and deep lake gravity-flow-sandstone tight oil. The temporal and spatial distribution of tight oil has characteristics of layer concentration, strong regularity and large favorable area, in which the saline lacustrine carbonate and shallow lake beach-bar sandstone tight oil are the best exploration targets in the western Qaidam Basin.
Palaeo-salinity and its sedimentary response to the Cenozoic salt water lacustrine deposition in Qaidam Basin
The salt-water lacustrine deposition was developed during the Cenozoic in Qaidam Basin,but its salinity and sedimentary response have not been known.Based on boron and clay mineral data,palaeosalinities of the Cenozoic in Qaidam Basin were reconstructed by Couch formula,which testified that:(1) The Cenozoic sediments belonged to the salt-water lacustrine deposition with the maximum salinity over 20‰.(2) The zones with different palaeosalinities had different sedimentary responses,that is,the palaeosalinity values of terrigenous clastics supplying areas were commonly less than 12‰,while those of shore-shallow lake ranged from 10‰ to 18‰,and those of semi-deep lake exceeded 18‰.Under the control of salt-water with middle to high salinity,the salt water lacustrine deposition in Qaidam Basin has the following characteristics:single layer is thin usually with 1-3 m thickness,sand layer and mud layer are frequently interbedded,delta sedimentary facies belt is relatively narrow,fine grained sediments are distributed in a wide range,and typical lacustrine carbonate rocks,gypsum and terrestrial fine detritus are mixed.
Origin and developing model of rock salt: A case study of Lower Ganchaigou Formation of Paleogene in the west of Yingxiong ridge, Qaidam Basin
DOI:10.7623/syxb201701006
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The Paleogene Lower Ganchaigou Formation in the west of Yingxiong ridge,Qaidam Basin is originated from saline lacustrine-basin sedimentation with the development of multiphase rock salt(layered halite),of which the origin and development mode is a hot issue to be solved at present.The core data reveal that the complete salt-formation single-phase cycle is an evaporation sedimentary sequence.The lithological sequence assemblage characteristics of prospecting wells show that the lacustrine basin experienced three evolutionary stages,i.e.,semi-saline,salinizing and saline lake.Through the petrography analysis,homogenization temperature test and composition research of the inclusions as well as the analyses of the sulfur,carbon and oxygen isotopes of salt-bearing strata,it is explicitly put forward that the rock salt was generated through low-temperature underwater concentration and crystallization,and formed in the confined continental environment with intense evaporation;the material source was carried by terrestrial surface water.Two kinds of salt forming modes were developed in the midlate saline stage and euryhalinous lake stage,and each mode is divided into three evolutionary stages,i.e.,initial saline stage,saline stage and salt forming stage.The comparison and analysis of joint wells indicate that five rock-salt concentration development periods existed,when the saline lake experienced three evolutionary processes,i.e.,the initial stage,peak stage and shrinking stage.The saline lake center had the maximum thickness with limitations in plane distribution,and multiple secondary salt depression centers were formed due to the control of paleo-terrain.The vertical development of multiphase salt rock was caused by the frequent seasonal fluctuation of lake level in terrigenous lake basin.
