Introduction
The main ultra-deep marine strata in China are the ancient Precambrian-Lower Paleozoic marine strata with a burial depth of more than 6000 m, and the Upper Paleozoic marine sedimentary strata with a burial depth of more than 6000 m. The relevant oil and gas resources are mainly distributed in the three major cratonic basins of Sichuan, Tarim and Ordos, and their exploration prospects and strategic positions have attracted great attention [1⇓⇓-4]. In recent years, wells ST-1 and ST-3 drilled into the Permian Qixia Formation in the northwest Sichuan Basin (Northwest Sichuan for short) have successively obtained 876 000 m3/d and 418 600 m3/d of high-yield industrial gas flow from ultra-deep reservoirs at 7200-7500 m, indicating a good exploration and development prospect of the Upper Paleozoic marine ultra-deep reservoirs in the west of the Sichuan Basin (hereinafter referred to as West Sichuan).
Aiming at the new domain of ultra-deep gas reservoir research under the background of complex structural evolution in West Sichuan, many scholars have carried out research on the genetic mechanism of the sedimentary reservoirs in the Permian Qixia Formation-Maokou Formation, prediction of the distribution of favorable reservoirs and hydrocarbon accumulation model[5⇓⇓⇓-9]. Most researchers focused on the dolomitization of the Permian Qixia Formation-Maokou Formation reservoirs and the formation and preservation of favorable reser-voirs. In these researches, major and trace elements associated with rare earth elements were used to identify the fluid sources. For example, according to the abnormal conditions of Ce and Eu elements, we can judge whether the diagenetic fluids are in a reduction environment or mixed with hydrothermal fluids and other non-marine source fluids. In terms of isotopic composition, carbon isotopic composition can be used to identify microbial and non-microbial dolomites, and oxygen isotopic composition can be used to characterize the sources of late diagenetic fluids. Due to the influence of multi-stage structural superposition in Northwest Sichuan, in addition to dolomitization, the dolomites of the ultra-deep Permian Qixia Formation buried below 6000 m are affected by hydrocarbon charging, rock fracturing and diagenesis such as dissolution. In addition, the whole hydrocarbon accumulation process under deep burial environment is complex and difficult to recover, so there are fewer discussions on the evolution law of diagenetic fluids and hydrocarbon accumulation model. This is unfavorable for understanding the formation and enrichment law of ultra-deep gas reservoirs in Northwest Sichuan.
Based on the analysis of typical wells in ultra-deep strata in the Shuangyushi region in Northwest Sichuan, taking the dolomitization of the reservoir as the analytic clue of petrology, relying on the study on reservoir diagenesis, this paper focuses on the Permian Qixia Formation, and strives to clarify the fluid evolution process and hydrocarbon accumulation model of ultra-deep gas reservoirs in Northwest Sichuan using comprehensive analysis of petrology, major and trace elements, carbon isotopic composition, oxygen isotopic composition, calcite U-Pb dating and fluid inclusions, so as to promote marine ultra-deep oil and gas exploration in Northwest Sichuan.
1. Geologic setting
Northwest Sichuan starts from Qingchuan-Guangyuan region in the north, and is bounded by the Nanjiang-Cangxi area in the east, Zitong in Mianyang in the south and Aba Tibetan and Qiang Autonomous Prefecture in the west, with an area of about 10 000 km2. Structurally, it is located at the junction of the eastern edge of the Qinghai-Tibet Plateau and the southern edge of the Qinling orogenic belt. Under the influence of the Caledonian tectonic movement, the upper Yangtze platform was uplifted on a large scale, and the Sinian and Cambrian-Silurian had been denuded to varying degrees before the end of the Carboniferous. Since the Indosinian Period, under the influence of multi-stage compressive tectonics, especially the tectonics dominated by the thrust uplift on the eastern edge of the Qinghai-Tibet Plateau during the Late Cenozoic period, complex peripheral thrust structures and fold structures inside the basin in West Sichuan and its adjacent areas have been formed (Fig. 1 ) [9⇓-11]. The study area located at the northwest of the West Sichuan foreland basin is surrounded by several structural zones. Its west and north sides are covered by the Longmenshan-Micangshan fold thrust belt, where several NE-SW trending fault zones are developed (Fig. 1 ) [9⇓-11].
Fig. 1. Location of the study area and distribution of sampled wells [11]. |
After the "filling and leveling" by clastic rocks of the Liangshan Formation in the Middle Permian in Northwest Sichuan, the overall distribution of the Qixia Formation became stable, with carbonate platform to gentle slope deposits, which can be divided into the first member of Qixia Formation (Qi-1 Member) and the second member of Qixia Formation (Qi-2 Member) from bottom to top. The Qi-1 Member features open platform facies, consisting of mainly dark gray argillaceous limestone and micritic limestone. The Qi-2 Member is platform margin facies, including mainly light gray beach dolomite, leopard porphyry limestone and sparite limestone [7].
Several sets of source rocks are developed in Northwest Sichuan, and the hydrocarbon accumulation conditions are superior. However, located at the junction of the Longmen Mountain and the Sichuan Basin, the Northwest Sichuan has experienced multiple stages of tectonic movement and with well-developed faults. Similarly, oil and gas reservoirs there have experienced multiple stages of charging, adjustment and transformation. It is necessary to accurately determine the natural gas source, reservoir diagenesis and hydrocarbon accumulation process of the Permian Qixia Formation gas reservoir in Northwest Sichuan, and clarify the key factors of gas reservoir formation and establish corresponding hydrocarbon accumulation model, providing a basis for ultra-deep oil and gas exploration and development in areas with stronger structural transformation in China.
2. Samples and experimental method
Four coring wells (ST-3, ST-8, ST-9 and ST-10) were selected in Northwest Sichuan (Fig. 1 ). Samples of Well ST-3 were taken from 7433.5-7494.62 m depth, those of Well ST-8 were taken from 7312.30-7430.78 m depth, those of Well ST-9 were taken from 7704-7759 m depth, and those of Well ST-10 were taken from 7427-7560 m depth. We analyzed the petrologic characteristics (by thin section identification, cathodoluminescence analysis) and fluid inclusion characteristics for matrix dolomite and calcite/ dolomite filling in fractures and karst caves of the Qixia Formation. Twenty fine-grained, fine-medium-grained and powder-fine-grained dolomites were selected, and grinded to less than 0.075 mm (200 mesh) with disc crusher and an agate mortar according to the standard of Methods for Chemical Analysis of Silicate Rocks (GB/T14506.29— 2010)[12], and tested by the rare earth elements and carbon and oxygen isotopic compositions experiments.
Cathodoluminescence analysis was completed in the School of Earth Science and Technology, Southwest Petroleum University, and carried out under a CL8200 MK5 cathodoluminescence instrument (combined with a Leica microscope). The test was set at a beam voltage of 9 kV and a beam current of 300 μA. The observations of ordinary and cast thin sections were completed in the State Key Laboratory of Reservoir Geology and Development Engineering, Chengdu University of Technology. After being stained by alizarin red, the thin sections were observed and photographed under a Nikon E600 Pol + polarizing microscope and photographic system. The carbon and oxygen isotopic composition tests were completed in the professional comprehensive laboratory of School of Materials, Chemistry and Chemical Engineering, Chengdu University of Technology. McCrea orthophosphoric acid method was adopted by the instrument of Finnigan MAT253 gas isotope mass spectrometer. Both the carbon and oxygen isotopic compositions adopted PDB standard, and the accuracies of measured carbon and oxygen isotopic compositions are ±0.1‰ and ±0.2‰, respectively. The test of rare earth elements was completed in the Key Laboratory of Environmental Change and Surface Processes in the Qinghai-Tibet Plateau, Chinese Academy of Sciences. The test instrument was Thermo X7 series inductively coupled plasma mass spectrometer (ICP-MS). The experimental results were standardized with North American shales to eliminate the odd-even effect of elements. The fluid inclusion test was completed in the School of Geosciences, Chengdu University of Technology by the instrument of LINKAM THMS600 cold-hot platform.
3. Petrologic characteristics and diagenetic sequences
3.1. Petrologic characteristics
The dolomite of the Qixia Formation in the study area is chiefly developed in the Qi-2 Member. The main matrix rock types in this member are granular limestone, fine- medium-grained allomorphic dolomite, fine-grained euhedral dolomite with straight surfaces, fine-grained subhedral-euhedral dolomite with straight surfaces, coarse- grained saddle dolomite with curved surfaces. The primary cement type is coarse-grained saddle dolomite with curved surfaces (Fig. 2 ). The specific characteristics are as follows.
Fig. 2. Rock types of Qixia Formation in Northwest Sichuan. (a) Granular limestone; Qi-2 Member; 7456.17 m; Well ST3-21; ×2.5(-). (b) Fine-medium-grained allomorphic dolomite; Qi-2 Member; 7759.00 m; Well ST9-2; ×2.5(-). (c) Fine-grained euhedral dolomite with straight surfaces; Qi-2 Member; 7759.14 m; Well ST9-1; ×2.5(-). (d) Fine-grained subhedral-euhedral dolomite with straight surfaces; Qi-2 Member; 7750.80 m; Well ST9-6; ×2.5(-). (e) Coarse-grained saddle dolomite with curved surfaces; Qi-2 Member; 7704.50 m; Well ST9-10; ×2.5(-). (f) Coarse-grained saddle dolomite cement with curved surfaces and associated calcite; Qi-2 Member; 7721.30 m; Well ST9-9; ×2.5(-). |
(1) Granular limestone (Fig. 2 a). The original texture is basically well preserved and can be associated with a small amount of micrites. The granular limestone represents the product of direct precipitation of seawater. A small amount of sporadic euhedral-subhedral dolomites can be seen in some granular limestones, which may indicate weak dolomitization product. It’s also found some symbiotic fine-medium-grained dolomites at a low dolomitization degree, and local recrystallization, which may be the product of early diagenesis such as penecontemporaneous process.
(2) Fine-medium-grained allomorphic dolomite (Fig. 2 b). The overall crystal size range is more extensive, about 0.1-0.6 mm. Under a single polarizer, a single crystal is curved and anhedral with dirty crystal surfaces and unobvious ring band, and the crystals are mainly in line contact. Under an orthogonal polarizer, it presents uniform extinction. This kind of rock may indicate transformation by buried dolomitization again after penecontemporaneous dolomitization.
(3) Fine-grained euhedral dolomite with straight surfaces (Fig. 2 c). The overall crystal size is 0.1-0.4 mm, mostly classified into powder crystal. Under a single polarizer, a single crystal is euhedral-subhedral with dirty crystal surfaces, and often has a "foggy center and bright edge" texture. Under an orthogonal polarizer, it presents uniform extinction.
(4) Fine-grained euhedral-subhedral dolomite with straight surfaces (Fig. 2 d). The overall crystal size is 0.3-0.5 mm, mainly fine crystals. Under a single polarizer, a single crystal is euhedral-subhedral with straight surfaces, with local dirty crystal surfaces, and the crystals are mainly in line contact. Under an orthogonal polarizer, it presents uniform extinction. Under cathodoluminescence, the ring band is obvious.
The above two types of dolomites are of typical burial origin. The former was developed at the early stage of burial transformation, so it was weakly affected by burial transformation, not crystallized into good euhedral crystal form, which shows the same transitional feature with fine-medium-grained allomorphic dolomite. The latter is the product of complete transformation by burial dolomitization. It has a larger crystal size and better crystal form than the former.
(5) Coarse-grained saddle dolomite with curved surfaces (Fig. 2 e). The overall crystal size can reach the centimeter level. Under a single polarizer, a single crystal surface is curved or stepped, with dirty crystal surfaces and an unobvious "foggy center and bright edge" texture. And there are abundant micro-cracks inside the crystals. Under an orthogonal polarizer presents wavy extinction, and the crystals are mainly in curved surface contact.
(6) Coarse-grained saddle dolomite cement with curved surfaces (Fig. 2 f). It may be associated with calcite with typical twin lamellae. On the whole, the crystal surface of the dolomite is relatively clean, and the "foggy center and bright edge" texture is not apparent.
The matrix and cement of the two kinds of coarse- grained dolomites may be related to hydrothermal activity.
3.2. Diagenetic sequences and features
Based on the previous research on burial history and diagenesis [10], as well as the characteristics of petrologic assemblages, we analyzed the diagenetic sequence of the Qixia Formation, and concluded the following diagenetic sequence (Fig. 3 ) and diagenetic characteristics (Fig. 4 ): micritization→seafloor cementation→penecontemporaneous dolomitization (Fig. 4 a) →shallow buried dolomitization (Fig. 2 b)→compaction and pressure dissolution→buried dolomitization (Figs. 2c and 4d)→recrystallization (Figs. 2d and 4e)→Stage 1 fracturing and hydrothermal action (coarse-grained saddle dolomite with curved surfaces) (Figs. 2e and 4f)→Stage I oil charging and residual asphalt (Fig. 4 b)→Stage 1 buried dissolution (Fig. 4 c)→Stage 2 fracturing and calcite vein filling (Fig. 4 g, 4h)→Stage II hydrocarbon charging and residual asphalt (Fig. 4 g, 4h)→Stage 2 burial dissolution (Fig. 4 g).
Fig. 3. Burial history, geothermal history, reservoir diagenetic sequence (a) and hydrocarbon accumulation history (b) of Qixia Formation in Northwest Sichuan. |
Fig. 4. Diagenetic characteristics of Qixia Formation reservoir in Northwest Sichuan. (a) Dolomite with low dolomitization degree associated with limestone, recrystallized by dolomitization in penecontemporaneous period; 7759.14 m; Well ST9-1; ×2.5(+). (b) Stage 1 asphalt in the cracks of fine-medium-grained allomorphic dolomite; 7758.7 m; Well ST9-3; ×5(+). (c) Fine-grained subeuhedral-euhedral dolomite with straight surfaces with edges dissolved into harbor shape, Stage 1 burial dissolution and burial dolomitization; 7748.4 m; Well ST9-7; ×2.5(+). (d) Fine-grained subeuhedral-euhedral dolomite with straight surfaces, presenting cathodoluminescent ring band formed in burial process; 7755.2 m; Well ST9-4B; ×2.5 (cathodeluminescence). (e) Stage 1 asphalt in the cracks of recrystallized fine-grained subeuhedral-euhedral dolomite with straight surfaces; 7748.4 m; Well ST9-7; ×2.5 (-). (f) Stage 1 asphalt associated with coarse-grained saddle dolomite with curved surfaces formed by hydrothermal fluid; 7704.5 m; Well ST9-10; ×5(-). (g) Stage 1 asphalt occurred between coarse-grained saddle dolomite particles with curved surfaces, asphalt veins cut by late calcite veins, vugs filled with late stage calcite with twin striation; 7721.3 m; Well ST9-9; ×2.5(-). (h) Stage 2 asphalt, harbors formed by Stage 2 burial dissolution, and coarse-grained saddle dolomite with curved surfaces and coarse-grained calcite with twin striation in vugs; 7721.3 m; Well ST9-9; ×2.5(-). |
The Qixia Formation in Northwest Sichuan experienced two stages of dolomitization, one stage of hydrothermal process, and two stages of hydrocarbon charging and associated burial dissolution. Stage 1 asphalt is mainly distributed along sutures or penetrates the micro cracks in the fine-medium-grained dolomite particles (Fig. 4 b, 4e, 4f), or along the intercrystalline pores or intercrystalline solution pores of hydrothermal coarse- grained saddle dolomite, which is related to Stage 1 burial dissolution (Fig. 4 c). This reflects that Stage I hydrocarbon charging occurred after hydrothermal saddle dolomite, which may be related to Stage I charging after the Lower Permian source rock entered the stage of large-scale oil generation at the end of the Triassic [13]. Stage 2 asphalt is more minor, mainly distributed on the edge of late-stage calcite veins or intercrystalline solution pores. It can be seen that late-stage calcite veins penetrate the veinlets containing Stage 1 asphalt. Stage 2 asphalt is related to Stage 2 burial dissolution (Fig. 4 g, 4h), and may correspond to Stage II large-scale hydrocarbon charging after the Lower Permian source rock entered the stage of massive condensate generation from the Late Jurassic to the Early Cretaceous [13] (Fig. 3 ).
4. Fluid development
4.1. Rare earth elements
The total amount of rare earth elements in the samples from the study area is 19.211×10-6-89.476×10-6, with an average of 40.812×10-6. Except for one limestone sample whose total amount of rare earth elements is as high as 89.476×10-6, other samples are basically in the range of normal marine carbonate rocks (1×10-6-50×10-6). The standardized curves of rare earth elements are generally inclined left. The ratio of light rare earth elements to heavy rare earth elements is 0.185-0.552, with an average of 0.370, indicating the loss of light rare earth elements and the enrichment of heavy rare earth elements. The overall δCe value is 0.085-3.545, of which most samples show negative abnormal Ce content, indicating that dolomitization occurred in a slightly alkaline low-temperature and oxidizing environment. Only a few samples show positive abnormal Ce content, which may be affected by hydrothermal fluid. The δEu value is between 0.153 and 7.688, with an average of 2.425. The positive abnormality of the Eu content in many samples indicates that hydrothermal transformation is obvious to the samples in the study area, that is, it is greatly affected by the burial process. Especially for the coarse-grained saddle dolomite with curved surfaces and fine-medium-grained allomorphic dolomite, whose abnormal δEu values may be 3-7, with typical hydrothermal origin. However, the δEu values of two fine-grained euhedral dolomite samples with straight surfaces are about 1, which may indicate that they are basically from seawater, that is, such types of rocks are basically penecontemporaneous products.
4.2. Carbon and oxygen isotopic compositions
Carbon and oxygen isotopic compositions can reflect the salinity and temperature during rock formation, to judge the source of dolomitized fluids and the genesis of dolomite. Taking global Permian System as a benchmark, the δ13C values are 1‰-6‰, and the δ18O values are -6‰-0 [14]. For the samples of the study area, δ13C values are -2.24‰- 3.37‰, with an average of 1.44‰; δ18O values are from -9.24‰ to -3.00‰, and the average value is -6.36‰. They have the characteristics of "double negative deviation" for δ13C and δ18O values. The negative deviation of δ13C value indicates an obvious organic carbon source in the diagenetic process, while the negative deviation of δ18O value may indicate the transformation of non-marine fluids, such as deep fluids and hydrothermal fluids during burial process. Combined with relevant sedimentary background and petrologic characteristics[15⇓-17], it is considered that this abnormal index comes from the formation of pores in carbonate rocks under the leaching influence of atmospheric fresh water in the early stage of diagenesis. During burial diagenesis, the pores were filled with cements formed by microbial degradation of hydrocarbons [18]. As a result, the feature of "double negative deviation" was formed.
4.3. Temperature measurement of fluid inclusions and U-Pb dating
In the Qixia Formation in Northwest Sichuan, brine inclusions associated with hydrocarbon inclusions are mainly distributed in coarse-grained calcite and dolomite cements in fractures and karst caves. The measurement results of homogenization temperatures of gas-liquid two-phase brine inclusions show that the Qixia Formation has two stages of inclusion homogenization temperature peaks (Fig. 3 a), reflecting two stages of oil and gas charging events. In deep carbonate reservoirs, due to very high temperature and pressure, carbonate minerals are more prone to suffer from lattice deformation and fracture, resulting in re-equilibration of fluid inclusions[19-20]. For example, the re-equilibration of inclusions caused by stretching may increase the homogenization temperature of inclusions, and keep the temperature of freezing point unchanged. The leakage of inclusions caused by brittle deformation may increase the homogenization temperature and freezing point temperature of inclusions. Therefore, for deep carbonate inclusions, we recommend using the lowest homogenization temperature as the true capture temperature of inclusions. Combined with previous research results and the analysis of the burial history and geothermal evolutionary history of source rocks, it is considered that the Qixia Formation has two stages of hydrocarbon charging. Stage I hydrocarbon charging (Fig. 3 a, 3b) time was 207-216 Ma, mainly in the Late Triassic, which was a stage of massive hydrocarbon charging. At this time, the Permian source rock became mature and began to generate oil to charge the reservoir of the Qixia Formation. The corresponding lowest homogenization temperature of Stage 1 gas-liquid two-phase brine inclusions is about 120-130 °C. Stage II hydrocarbon charging (Fig. 3 a, 3b) time was 143-150 Ma, corresponding to the Late Jurassic-Early Cretaceous. At this time, the Permian source rock was highly mature, and began to provide condensate oil and gas to the reservoir of the Qixia Formation. The corresponding lowest homogenization temperature of stage 2 gas-liquid two- phase brine inclusions is about 160.5-170.0 °C.
In order to further determine the accurate time of hydrocarbon accumulation, on the basis of the diagenetic sequence, we tested the U-Pb ages of calcite veins developed before stages I and II hydrocarbon charging (Fig. 5 ). The results show that the U-Pb age of calcite veins before Stage I hydrocarbon charging is (221±11) Ma, indicating that the hydrocarbon charging occurred during the Late Triassic; the U-Pb age of calcite veins before Stage II hydrocarbon charging is (143.6±6.9) Ma, indicating that the hydrocarbon charging occurred during the Early Cretaceous. This is basically consistent with the measurement results of inclusions.
Fig. 5. U-Pb isotopic age of Qixia Formation in Northwest Sichuan. (a) The U-Pb of calcite veins developed before Stage I hydrocarbon charging is dated to 221 Ma. (b) The U-Pb of calcite veins developed before Stage II hydrocarbon charging is dated to 143 Ma. |
To sum up, there are at least two stages of hydrocarbon charging in the Qixia Formation of the Middle Permian in Northwest Sichuan. According to the dating results of inclusions and calcite U-Pb, combined with the evolution history of hydrocarbon generation of source rocks, it can be seen that the two stages of hydrocarbon charging are mainly related to crude oil and light oil and gas. At present, the natural gas in the Qixia Formation is crude oil cracked gas. It is dry gas with a drying coefficient greater than 0.997, low C2H content and almost no C3H8 and C4H10[21]. This is related to the further burial, maturity and high oil cracking degree in the Middle-Late Cretaceous after oil charging.
5. Hydrocarbon accumulation model
The carbon isotopic composition of ethane in the natural gas of the Qixia Formation in Northwest Sichuan is mainly from -28.5‰ to -25.6‰, showing the characteristics of oil-type gas. The carbon isotopic composition of methane is relatively heavy, mainly from -31.1‰ to -29.27‰. On the basis of the empirical formula: δ13C1= 25.55lgRo-40.78, the calculated Ro value of natural gas in the Qixia Formation is between 2.21% and 2.89%[22], which is in the over mature stage, indicating that the gas reservoir has experienced high-temperature evolution. Through the comparison of hydrocarbon isotopic composition and light hydrocarbon composition of natural gas, reservoir asphalt and biomarkers of regional source rocks, previous researchers found that the natural gas of the Qixia Formation in Northwest Sichuan is the gas from oil cracking, and originated from both mudstone of the Cambrian Qiongzhusi Formation and marl of the Maokou Formation in the Middle Permian [23⇓-25]. Middle Permian carbonate rock, Upper Permian argillaceous source rock and coal source rock of the Longtan Formation are widely developed in the Permian in Northwest Sichuan. The source rocks and reservoirs are interbedded and distributed in isochronous or heteropic pattern, forming a high-quality source-reservoir-seal combination with self-generation and self-storage characteristics [22⇓⇓-25]. Therefore, the Permian natural gas presents self-geneartion characteristics and mixed sources. It can be seen from Fig. 6 that the carbon isotopic compositions of methane and ethane in the natural gas of the Qixia Formation and the Maokou Formation of Shuangyushi structure are obviously different in distribution range from the typical oil cracking gas of the Longwangmiao Formation in Gaoshiti-Moxi region and the Carboniferous in East Sichuan, but similar to the natural gas in the Permian Changxing Formation-Triassic Feixianguan Formation in Puguang, Yuanba and Longgang regions. The natural gas in these regions is dominated by self-generated and self-stored natural gas in the Permian. The natural gas of the Qixia Formation and the Maokou Formation in Jiulongshan structure is similar, mainly self-generated and self-stored natural gas in Permian. Gao et al. found that the reservoir asphalt of the Qixia Formation has a cognate relationship with the source rocks of the Maokou Formation and the Qixia Formation in the study area through the comparison of biomarker compounds from the source rocks with the reservoir extracts of the Qixia Formation in Shuangyushi structure [26]. This also proves that the natural gas of the Qixia Formation should be the product of the oil generated by the Permian source rock, which was early accumulated and late cracked into gas. Although the carbon isotopic composition of ethane in the natural gas of the Devonian Guanwushan Formation is similar to that of the Qixia Formation and the Maokou Formation in Well ST-3, the carbon isotopic composition of methane in the Devonian System is obviously lighter, indicating that the natural gas of the Devonian System may not be from the same source as the natural gas of the Qixia Formation and the Maokou Formation. The carbon isotopic compositions of methane and ethane in the Devonian System in Well ST-3 are very similar to those of the Sinian natural gas in the Anyue gas field, and are similar to those of methane in the Cambrian Longwangmiao Formation in the Anyue gas field and Permian natural gas in Central Sichuan, but are quite different in ethane. This may indicate that the natural gas in the Devonian Guanwushan Formation was not from in-situ cracking of Cambrian oil, but late migration of highly cracked gas from deep Sinian reservoir through faults. Similarly, the characteristics of the carbon isotopic compositions of methane and ethane in the Permian natural gas in Kuangshanliang region are similar to those of the Sinian natural gas in the Anyue gas field. It is inferred that it is also from the Sinian paleo gas reservoir after vertical migration and adjustment. The above analysis and comparison of gas sources show that the natural gas of the Qixia Formation and the Maokou Formation in Northwest Sichuan should be mainly generated and stored in the Permian System. In the stable structural regions far away from faults, the Permian reservoir is mainly self-generated and self-stored, such as the Permian natural gas in the Jiulongshan region. Although the Shuangyushi structure has relatively developed faults, it is mainly characterized by layered structural deformation under the influence of two sets of detachment layers of Lower Triassic and Lower Cambrian. The faults are mainly concentrated in the Permian System [27], and most of them have not penetrated the deep Sinian System (Fig. 7 a-7c). Therefore, the Permian natural gas in the Shuangyushi region is also dominated by self-generated and self-stored gas in the Permian System. In the regions close to the piedmont zone with more faults, the deep Sinian natural gas migrates upward through faults and accumulates, such as in the Permian Kuangshanliang and Hewanchang regions. Well ST-3 is closer to the piedmont fault zone than Well ST-1, where deep and large faults penetrate the deep Sinian system. Therefore, the natural gas of the Guanwushan Formation in Well ST-3 comes from the cracked gas of the deep Sinian System, and the carbon isotopic compositions of methane and ethane are consistent with the characteristics of the Sinian natural gas in the Anyue area.
Fig. 6. Carbon isotope characteristics of methane and ethane and gas source identification of natural gas in Sichuan Basin. Data of Sinian-Cambrian gas and Longgang gas in Anyue and Weiyuan gas fields are from Reference [28]. Data of Puguang and Yuanba gas fields are from Reference [29]. Data of Xujiahe Formation gas are from Reference [30]. Some data of Northwest Sichuan, Shuangyushi and Jiulongshan regions are from References [21,24,28]. |
Fig. 7. Profile of hydrocarbon accumulation in Shuangyushi structure-Zitong syncline-Jiulongshan structure in Northwest Sichuan (see |
To sum up, the hydrocarbon accumulation model of the Qixia Formation in Northwest Sichuan is characterized by "multiple hydrocarbon sources, composite transportation, differential hydrocarbon accumulation and late finalization" (Fig. 7 ). The accumulation process is roughly divided into three stages:
(1) Late Triassic. During the Late Triassic period, the marine mixed marl of the Middle Permian Qixia Formation and Maokou Formation in Northwest Sichuan had entered its mature stage, and the source rock of the Qiongzhusi Formation had reached its mature-high mature stage, but without developed faults, the crude oil mainly migrated laterally along the unconformity surface and the reservoir. The crude oil generated by the Qiongzhusi Formation migrated laterally into the traps on the Sinian platform margin and accumulated into ancient oil reservoirs. And at the same time, the Shuangyushi-Zitong-Jiulongshan region was a regional structure high where crude oil generated by the Permian source rocks on both sides converged. The Stage I oil charging occurred, forming Permian ancient oil reservoirs.
(2) Late Jurassic-Early Cretaceous. Since the end of the Late Jurassic, most Middle Permian source rocks had entered their high mature stage and generated abundant light oil and gas. The Stage II hydrocarbon charging occurred, and hydrocarbon recharged the structural-lithologic traps of the Qixia Formation. Since the Early Cretaceous, the temperature of the Permian reservoir had exceeded 160 °C (Fig. 3 b). With the increase of burial depth, the reservoir temperature gradually increased, and the oil in early ancient reservoirs began to crack into considerable oil-cracking gas. During this period, the reservoir of the Qixia Formation reached the middle diagenetic stage, the Shuangyushi structure continued to uplift, and oil- cracked gas and kerogen-cracked gas continuously mixed in the Shuangyushi region and accumulated into ancient gas reservoirs. The ancient Sinian oil reservoirs underlying the Shuangyushi structure also underwent in-situ cracking to a high degree, forming ancient gas reservoirs similar to those in the Anyue gas field.
(3) Late Cretaceous to Tertiary. During the Yanshanian-Himalayan period, due to the compression and thrust of the Longmen Mountain, the piedmont belt was lifted and tilted, and Shuangyushi structure and Jiulongshan structure were finalized. As the Shuangyushi structure was close to the Longmen Mountain piedmont, more faults that complicated the original large ancient structures and formed trap groups dominated by fault traps. Local ancient gas reservoirs were reconstructed, and most of them remained stable. In the regions near the piedmont belt, faults were large and penetrated deep strata, resulting in massive loss and vertical adjustment of natural gas. Gas from the deep Sinian ancient gas reservoirs migrated upward through faults and accumulated in some areas. For example, the Permian in Kuangshanliang and the Guanwushan Formation in Well ST-3 have typical Sinian oil-cracking gas. In the main part of Shuangyushi structure and its east, with minor faults that did not penetrate the Sinian gas reservoirs, natural gas mainly came from the Permian. The Jiulongshan structure was formed and finalized during that stage, but faults were not developed. Permian and Sinian natural gas migrated to structural highs and accumulated into independent reservoir systems. As the Permian System in the Shuangyushi-Zitong-Jiulongshan regions developed ancient structural highs and gas reservoirs, they underwent structural reconstruction and fault transformation since the Yanshanian-Himalayan period. However, they still have the characteristics of large-scale hydrocarbon accumulation and larger exploration potential.
6. Conclusions
The dolomite reservoir of the Qixia Formation in Northwest Sichuan has experienced the following sequential stages: micritization, seafloor cementation, penecontemporaneous dolomitization and shallow buried dolomitization, compaction and pressure dissolution, buried dolomitization, recrystallization, Stage 1 fracturing and hydrothermal action (coarse-grained saddle dolomite with curved surfaces), Stage I oil charging and residual asphalt, Stage 1 buried dissolution, Stage 2 fracturing and calcite vein filling, Stage II hydrocarbon charging and residual asphalt, Stage 2 burial dissolution. The fluid sources include seawater, atmospheric fresh water, hydrocarbon fluid and deep hydrothermal fluid.
Hydrocarbon accumulation in the Qixia Formation has two stages of hydrocarbon charging and one stage of cracked gas accumulation. Stage I oil charging was the accumulation stage of ancient oil reservoirs in early structures at 207-216 Ma, corresponding to the Late Triassic. Stage II oil and gas charging took place when a large amount of hydrocarbon was generated by the Permian source rock at 143-150 Ma, corresponding to the Late Jurassic-Early Cretaceous. Then during the Middle-Late Cretaceous, early oil cracked to gas in large scale and charged into reservoirs. This is the last charging stage when present dry gas reservoirs were developed. In the uplifting stage since the end of the Late Cretaceous, the gas reservoirs might undergo reconstruction and transformation.
The hydrocarbon accumulation model of the Qixia Formation in Northwest Sichuan is characterized by "multiple hydrocarbon sources, composite transportation, differential hydrocarbon accumulation and late finalization". The Permian natural gas in the study area is mainly composed of oil-cracked gas generated by the Permian marine mixed source rock, and locally deep Sinian oil- cracked gas. The connectivity between faults and the underlying Cambrian gas source is the key factor determining the difference between Cambrian and Permian gas sources. The large-scale development of ancient stable preservation conditions, although complicatedly transformed by late faults, is the key to the large-scale accumulation of natural gas.