(Exploration and Development Research Institute of Daqing Oilfield Co., Ltd., Daqing, Heilongjiang 1637 12)
About the author: Zhang, male, engineer, now mainly engaged in sedimentary and sequence research, email: zdz-007 @163.com.
Abstract: Tight sandstone gas belongs to unconventional natural gas and is one of the reserve resources of conventional natural gas. Based on the comprehensive analysis of the gas accumulation conditions of Shahezi Formation in Xujiaweizi fault depression, it is considered that Shahezi Formation has the geological conditions for forming tight sandstone gas reservoirs, with thick source rocks and glutenite, wide distribution, high organic matter content, mature and over-mature, large resources and broad exploration prospects. The preliminary understanding of the reservoir-forming conditions of tight sandstone gas reservoirs in Shahezi Formation is: (1) favorable structural and sedimentary background; (2) Sufficient gas source conditions; (3) Low porosity and low permeability reservoir conditions; (4) Good sealing and storage conditions; (5) Matching migration and dredging conditions; (6) favorable trap conditions; (7) Good source-reservoir-cap combination. Comparing the characteristics of shale gas reservoirs at home and abroad, it is found that Shahezi Formation has the conditions to form mudstone gas reservoirs. With the deepening of research and the breakthrough of exploration technology, the exploration of unconventional gas reservoirs in Shahezi Formation may make great progress.
Key words: Xujiaweizi fault depression; Shahezi Formation; Unconventional oil and gas; Tight sandstone gas; Reservoir forming conditions; Exploration potential
Reservoir forming conditions and exploration potential of tight sandstone gas in Shahezi formation of Xujiaweizi fault depression
Zhang Yangfeng Pingyin Changhai Small
(Exploration and Development Research Institute of Daqing Oilfield Co., Ltd., Daqing 1637 12, China)
Abstract: Tight sandstone gas belongs to unconventional natural gas and is one of the reserve resources of conventional natural gas. The hydrocarbon accumulation conditions of Shahezi Formation in Xujiaweizi fault depression are analyzed. The study shows that the Shahezi Formation has developed geological conditions favorable to the formation of tight sandstone gas. The source rocks and glutenite of Shahezi Formation are thick, widely distributed and rich in organic matter, and the source rocks are mature to over-mature. Therefore, Shahezi Formation is rich in resources and has broad exploration prospects. The preliminary conclusions of tight sandstone gas accumulation conditions in Shahezi Formation are: (1) favorable structural and sedimentary background; (2) abundant gas sources; (3) Low porosity and permeability reservoir; (4) Conducive to protection; (5) Adaptive migration; (6) Favorable traps; (7) Well-generated reservoir-cap combination. At the same time, the geological characteristics of Shahezi Formation are similar to shale gas at home and abroad. Shale gas may be developed in Shahezi Formation. With the deepening of exploration and technological breakthrough, unconventional gas exploration in Shahezi Formation is expected to make significant progress.
Key words: Xujiaweizi rift depression; Shahezi Formation; Non-traditional natural oil and gas; Tight sandstone gas; Hydrocarbon accumulation conditions; Exploration potential
Unconventional oil and gas is an important alternative resource of oil and gas energy in the future. Unconventional oil and gas refers to "all natural gas reservoirs that do not conform to the conventional oil and gas accumulation principle or are special in genesis, composition, occurrence, nature, storage medium and sealing mechanism" [1], including oil shale, shale gas, coalbed methane, natural gas hydrate and tight sandstone gas. This paper mainly discusses the genesis of Shahezi Formation in Xujiaweizi fault depression. Tight sandstone gas refers to unconventional natural gas in sandstone layers with low porosity (< 12%), low permeability (< 1× 10-3 μ m2), low gas saturation (< 60%), high water saturation (> 40%) and slow natural gas flow. At present, the large-scale tight sandstone gas reservoirs developed abroad are mainly deep basin gas reservoirs, which are concentrated in western Canada and the western United States [4]. Huge tight sandstone gas reservoirs have been discovered in Junggar basin, Ordos basin and western Sichuan basin in China [5]. At the same time, tight sandstone gas reservoirs have also been discovered in Songliao, Bohai Bay, Nanxiang, Subei, Jianghan, Tarim and Turpan-Hami basins [6], which is promising in the future. Taking Shahezi Formation in Xujiaweizi fault depression in Songliao Basin as the research object, the reservoir-forming conditions and exploration potential of tight sandstone gas reservoirs are analyzed, which has certain theoretical and guiding significance for the exploration and development of unconventional gas reservoirs in this area in the future.
1 geological survey
Xujiaweizi fault depression is located in the north of Songliao Basin, with a total area of 5300km2, and the whole fault depression is distributed in the northwest direction. The west is separated from the central paleouplift belt by faults, and the east is in a slope transition with Shangjia-Chaoyanggou uplift belt. It is a compound dustpan-shaped fault depression controlled by three faults: Xu Xi fault (north-south segment), Xu Zhong fault and Xu Dong fault (Figure 1a, B), and it is generally a structure of breaking in the west and overtaking in the east. Controlled by regional structure and sedimentation, upper Jurassic Huoshiling Formation, lower Cretaceous Shahezi Formation, Yingcheng Formation, Denglouku Formation and Quantou Formation developed from bottom to top [7 ~ 12]. Among them, Shahezi Formation is divided into Sha-1 member and Sha-2 member, and the lithology is mainly a set of black and gray-black mudstone, gray medium sandstone, fine sandstone and conglomerate, sandwiched with coal seams (Figure 1c). The sedimentary environment is mainly lacustrine facies and fan delta sediments, and the thickness of residual strata is 400 ~ 2,800 m m. Due to the deep burial depth of Shahezi Formation in Xujiaweizi fault depression (over 3,000 m) and dense glutenite reservoir (porosity less than 6%, permeability less than 0. 1× 10-3μm2), the research work is less. The predecessors thought that Shahezi Formation was the main source rock with low exploration degree and unclear glutenite development characteristics. At present, most of the drilled wells are located at the edge of the fault depression, but basically all of them show gas, and the maximum daily production is after the single well pressure exceeds 5× 104m3, which shows that the exploration prospect of tight sandstone gas in Shahezi Formation is good.
Figure 1 Structural Features and Lithologic Histogram of Xujiaweizi Fault Depression
2 Shahezi Formation tight sandstone gas accumulation conditions
2. 1 favorable tectonic and sedimentary background
Shahezi Formation is in a period of intense fault depression, controlled by three faults, namely Xu Xi Fault, Xu Zhong Fault and Xu Dong Fault, and two depressions, Xu Xi and Xu Dong, are formed, which become the center of sedimentary subsidence in this period, and extremely thick Shahezi Formation strata are developed, with local thickness of 2,800 meters (Figure 2), gravel thickness of 0-700 meters (Figure 3) and dark mudstone thickness of 0- 165438. Sedimentary facies such as fan delta, nearshore subaqueous fan and alluvial fan are mainly developed at the edge of the depression (Figure 5). Due to its proximity to provenance, rapid sedimentation rate, complex composition of clastic materials and poor sorting, clastic sediments can rapidly advance into lake sediments to form superimposed source rocks and reservoir rocks, and its structural and sedimentary background is conducive to the formation of tight sandstone gas reservoirs.
Fig. 2 Residual stratum thickness in Shahe sub-group map.
Fig. 3 Residual glutenite thickness map of Shahezi Formation
2.2 favorable gas source conditions
Sufficient gas source is the basis for forming favorable gas reservoirs. Shahezi Formation itself is the most important source rock in Xujiaweizi fault depression. Source rocks are widely distributed and thick. The thickness of dark mudstone revealed by earthquake is 0 ~ 1 100 m, and the thickness of coal seam revealed by drilling is 0 ~150 m. The organic carbon content of dark mudstone is 0.22% ~ 5.48%, with an average of 65,438 0.57%, and the "A" content of chloroform asphalt is 0.0066. The average value is 0.04965438 0%, and the maturity is 65438 0.02 ~ 4.666465476. The organic carbon content in coal seam is 9.6545438. The kerogen type is mainly Type III, accounting for 55.3%, followed by Type II, accounting for 3 1.8%, and Type I is the least, accounting for/. According to previous studies [13, 14], the threshold depth of oil generation of type I kerogen source rocks in Songliao basin is about 1 100 m, and that of type II kerogen source rocks is1200 ~1300 m. From the above geochemical indicators, it can be seen that according to the study on the gas-generating intensity of the source rocks of Shahezi Formation in Xu Shen 1 Well, it is found that [15], the gas-generating intensity of dark mudstone is 38× 108m3/km2, and that of coal seam is 100× 108m3/km2. Well Xu Shen 1 is located in the middle of Shengping-Xingcheng anticline structure, and the sedimentary thickness of dark mudstone is relatively thin. According to the prediction of basement thickness and sedimentary facies distribution, the maximum thickness of dark mudstone in Shahezi Formation in the middle of Xujiaweizi fault depression is about 1 100m, so the maximum gas generating intensity of dark mudstone in Shahezi Formation in Xujiaweizi fault depression can reach 220× 108 m3/km2, which is enough to meet the gas source conditions for gas field formation.
Fig. 4 Thickness map of residual dark mudstone in Shahezi Formation
Fig. 5 Sedimentary map of the second member of Shahezi Formation
2.3 Reservoir conditions
The main reservoirs of tight gas reservoirs in Shahezi Formation are fan delta glutenite reservoir and nearshore subaqueous fan glutenite reservoir. Gravel particles are mainly in line contact and point contact, with strong compactness. Cements are mainly volcanic rocks and carbonates, and the cementation types are pore cementation and basement cementation. The thickness of single layer varies greatly, generally between 1 ~ 10m, and the maximum thickness exceeds 40m. Because it is close to oil source, the reservoir is sandwiched between source rocks laterally, which is easy to accumulate oil and gas. At present, the research shows that the total thickness of glutenite reservoir is between 0-700 meters, accounting for 0-40% of the total thickness of Shahezi Formation. Physical property analysis shows that the reservoir has the characteristics of low porosity, low permeability and strong diagenesis. Porosity is mostly less than 10% (mainly between 2% and 6%), and permeability is less than 0. 1× 10-3μm2. Core observation and microscopic characteristics show that there are still some secondary pores in the glutenite of Shahezi Formation, which have the reservoir conditions for forming gas reservoirs.
2.4 sealed preservation conditions
The Shahezi Formation itself has developed thick source rocks, which can be used as both source rocks and effective regional caprocks. There are two main sealing mechanisms. One is lithologic sealing. The source rock itself is dense and can be used as a good caprock. Gravel is deeply buried and the reservoir is dense. Because of the dense lithology, the rock itself has high breakthrough pressure, which hinders the migration of natural gas. The second type is pressure sealing. At present, the pressure coefficients of production intervals in Shahezi Formation are all greater than 1, which belongs to overpressure and has good pressure sealing conditions, so it is difficult for natural gas to migrate upward. Although the existence of fractures and cracks will cause gas loss, sufficient gas sources can be continuously supplied to form a dynamic balance. At the same time, in areas with severe reconstruction in the later stage, the early natural gas reservoirs were redistributed and adjusted in the later stage, and gathered in new traps to form new gas reservoirs.
2.5 Migration and Guidance Conditions
Shahezi Formation has dense reservoirs, and there are two kinds of natural gas migration. In the first case, the migration of natural gas occurred before the densification of glutenite, at which time the diagenesis was not strong, and a large number of primary pores and secondary pores developed, forming natural gas enrichment. In the second case, the migration of natural gas occurs after the densification of glutenite. At this time, due to the strong transformation of diagenesis, the physical properties of glutenite decreased sharply, and natural gas migrated along the faults or fracture systems formed by tectonic activities, forming natural gas enrichment. From the current research, the large-scale migration of natural gas should occur after the densification of glutenite. In terms of structural characteristics, the Shahezi Formation has strong tectonic activity, and the Xu Xi and Xu Zhong faults that control the development of Xujiaweizi fault depression are strong. At the same time, the development of associated faults connecting source rocks and reservoirs provides a channel for natural gas to migrate along unconformity surfaces and large faults to high parts of the structure. Many traps, such as stratigraphic overlap, upward dip and pinch-out, are developed on the edge of fault depression, which is beneficial to natural gas accumulation and accumulation.
The source rocks of Shahezi Formation in Xujiaweizi fault depression began to gas rapidly before 1 100 Ma, and there were two significant gas-producing peaks before 95Ma and 75Ma. The gas accumulation period of deep volcanic rocks in Xingcheng area is before 60 ~ 103 Ma, that is, after the deposition of the main caprock Denglouku Formation, which is beneficial to the preservation of deep gas [16]. As a result, large volcanic lithologic gas reservoirs in Ying-1 member (such as Changde Cave gas reservoir), strata overlapping gas reservoirs in Denglouku Formation (such as Shen Wei 5 gas reservoir) and updip pinch-out gas reservoirs (such as Fangshen 80 1 gas reservoir and Shen Wei 50 1 gas reservoir) [1] were formed. Because the natural gas generated by Shahezi Formation can migrate for a long distance, gas reservoirs can be formed in Yingcheng Formation and Denglouku Formation, or they can migrate and accumulate in the gravel of Shahezi Formation through the connection of faults and fractures to form gas reservoirs.
2.6 trap conditions
The gas reservoir of Shahezi Formation is mainly formed after reservoir densification, and the glutenite reservoir interacts frequently with source rocks. Under the transformation of late tectonic activities, coincidence traps with paleostructure as the background were mainly formed, such as structural-lithologic, structural-diagenetic and fault-lithologic traps.
2.7 source-reservoir-cap combination
Within Shahezi Formation, the source rocks are both source rocks and caprocks, and the adsorbed gas can accumulate on dark mudstone and fracture surface, which can be used as a "self-generating and self-storing" reservoir. At the same time, natural gas can be accumulated in the glutenite interacting with nearby source rocks, or it can be accumulated in the glutenite at the edge of the fault depression through long-distance migration, forming gas reservoirs with "near source and other storage" and "far source and other storage".
3 Evaluation of exploration potential of tight sandstone gas in Shahezi Formation
3. 1 resource estimation
Many scholars have deeply studied the hydrocarbon expulsion coefficient of shale. Li Mingcheng [18] obtained the hydrocarbon expulsion rate (hydrocarbon expulsion coefficient) of 70% ~ 80% through the simulation calculation of Paleogene source rocks in various depressions in Bohai Bay Basin. Song [19] calculated the relationship between the maturity of upper Paleozoic coal and rock and hydrocarbon expulsion coefficient in Shengli oil province (table 1), which reflected that the hydrocarbon expulsion rate of coal and rock was different in different evolution stages, and the higher the evolution degree, the more hydrocarbon expulsion systems were. The maturity of Shahezi Formation is 1.02 ~ 4. 16, with an average of 2.89, and its hydrocarbon expulsion coefficient should be above 80%. See Table 2 for gas production and gas displacement of mudstone and coal seam. According to the accumulation coefficient of 2% ~ 3%, the natural gas resources of Shahezi Formation are (4072 ~ 6 108) × 108m3, and the natural gas may migrate to the nearby Shahezi Formation sandstone to form gas reservoirs. At the same time, referring to the data in Table 1, it can be seen that the gas adsorption capacity of mudstone and coal seam of Shahezi Formation is relatively large, exceeding 20m3/d, so there may be abundant natural gas resources in mudstone of Shahezi Formation.
Table 1 Calculation Table of Relationship between Coal Maturity and Hydrocarbon Expulsion Coefficient [19]
Table 2 Natural gas generation, displacement and resources of Shahezi Formation in Xujiaweizi fault depression
Table 3 Comparison of natural gas characteristics between Shahezi Formation and China tight sandstone in Xujiaweizi fault depression
3.2 Evaluation of exploration potential
Some progress has been made in the exploration and development of tight sandstone gas reservoirs at home and abroad. There are 23 basins with tight sandstone gas reservoirs in the United States, and their exploration and development have achieved great success through the improvement of three-dimensional seismic technology, well pattern encryption and vertical well layered fracturing [20]. The exploration and development of tight sandstone gas in China are mainly concentrated in Sichuan Basin, Ordos Basin and Turpan-Hami Basin. Comparing their characteristics, it can be found (Table 3) that the tight sandstone of Shahezi Formation in Xujiaweizi fault depression is similar to it, and some conditions are even superior, such as gas-bearing property and source rock thickness. Therefore, the tight glutenite of Shahezi Formation has the potential to form large-scale reserves.
At the same time, the thick mudstone and coal seam of Shahezi Formation have the possibility of adsorbing natural gas accumulation. The formation of mudstone natural gas reservoir is the result of a large amount of natural gas staying in source rocks. The functions of natural gas reservoir formation, storage, sealing, migration, accumulation and preservation are all completed in the same set of mudstone strata, so the controlling factors of reservoir formation are relatively simple, and huge resources can be formed [2 1]. Compared with American basins that have successfully exploited shale gas (Table 4), the TOC and Ro of Shahezi Formation are higher, and the thickness of source rocks is greater. Therefore, Shahezi Formation has the geological conditions to develop mudstone natural gas.
Table 4 Comparison between Shahezi Formation in Xujiaweizi Fault Depression and shale gas reservoirs at home and abroad
sequential
(Foreign and western Sichuan data are quoted from reference [2 1])
At present, Shahezi Formation has carried out old well review, fracturing design and other related work, and further research work is gradually being carried out. Therefore, with the deepening of research and the continuous progress of exploration technology, unconventional gas exploration in Shahezi Formation may make great progress.
4 preliminary understanding
Through the above analysis, the following preliminary understandings are obtained: First, Shahezi Formation in Xujiaweizi fault depression has the geological conditions to form tight sandstone gas, with large resources and broad exploration prospects. Second, the conditions of tight sandstone gas accumulation in Shahezi Formation: (1) favorable structural and sedimentary background; (2) Sufficient gas source conditions; (3) Low porosity and low permeability reservoir conditions; (4) Good sealing and storage conditions; (5) Matching migration and dredging conditions; (6) favorable trap conditions; (7) Good source-reservoir-cap combination. Third, the dark mudstone of Shahezi Formation is thick, the coal seam is developed, and TOC and Ro are high. Comparing the characteristics of shale gas reservoirs at home and abroad, it is possible to form mudstone gas reservoirs. With the deepening of research and the progress of exploration technology, shale gas reservoir exploration may make progress.
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