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The Silurian hydrocarbon accumulation research in the southeastern Sichuan Basin lacks evidence of the hydrocarbon accumulation and destruction stages in reservoirs. Bitumen and calcite veins that were developed in the Shiniulan Formation had recorded the information of the fluid evolution in reservoirs and have important implications for fluid activities and hydrocarbon accumulation processes in the Dingshan area, southeast Sichuan Basin. Based on analysis of petrographic, trace element, fluid inclusion and laser Raman spectroscopy of the calcite veins, this research determines the timing and origin of the paleo-fluids formed the calcite veins; recovers the paleo-pressure of the methane inclusions, and reveals the petroleum system evolution and controlling factors of the Shiniulan Formation. The results indicate that two phases of calcite veins were developed in the Shiniulan Formation, and oil charging happened before the formation of the two phases of the calcite veins. The first phase of calcite veins were formed at ~127 Ma under relative reduction conditions, with the forming fluid sourcing from the diagenetic fluid of the same strata, which show brown-red cathode luminescence colors. The second phase calcite veins were formed at ~83 Ma under relative oxidation conditions, showing brown-yellow cathode luminescence colors. Oil generation peak of the Longmaxi source rocks, which were the main source of the Shiniulan reservoirs, was from ~190 Ma to ~150 Ma. Oil to gas cracking happened at ~135 Ma. The trapping pressure of methane inclusions is 51.77-57.46 MPa, and the pore fluid pressure coefficient is 1.26-1.40. Since the Yanshanian period, a series of tectonic movements have taken place in the Dingshan area, which uplifts and lead to erosion and fracturing of the Dingshan area is accounts for the destruction of the Shiniulan gas reservoirs.

Based on the understanding of the basic geological conditions of shale gas from the Upper Ordovician Wufeng Formation to the Lower Silurian Longmaxi Formation in Dingshan area of the southeastern Sichuan Basin, the occurrence, migration mode and intensity of natural gas in the shale gas system were discussed according to the results of drilling, logging, fracturing test, laboratory test and structural interpretation, moreover, a shale gas enrichment mode suitable for the study area was proposed. In shale gas systems, high-angle fractures extend for a short distance, while horizontal fractures such as bedding-parallel detachment and interlayer bedding fractures extend for a long distance. As a result, the permeability in the bedding direction is much greater than that in the vertical direction. These characteristics together determined that natural gas in shale gas system was usually transported horizontally, and sometimes both horizontally and vertically. For the Qiyueshan fault zone, the burial depth of shale reservoir and the distance to Qiyueshan fault zone are the major constrains for shale gas enrichment in the Wufeng-Longmaxi formations in the study area. The Qiyueshan fault zone is a basin-controlling fault zone with fractures and associated fractures developed, and is a shale gas escaping area. In the shallow-buried area in the southeastern part, high-angle fractures were relatively developed, by which horizontal fractures were opened. The proportion of free gas was less than 60%. A normal pressure system indicates a semi-retention area for shale gas. In the middle-to deep-buried area in the northwest, high-angle fractures were relatively undeveloped, and horizontal fractures were closed. The vertical and horizontal migrations were blocked, and most of shale gas was trapped in shale reservoirs. The proportion of free gas is more than 60%. A high-to ultra-high pressure system indicates a shale gas accumulation area.

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Tectonic fractures are a significant factor for production of the Longmaxi formation in Dingshan area. The analyse of the distribution and development degree of tectonic fractures is the main research method in present-day. Based on the data of field outcrops and borehole core, the new method-binary method is put forward by rock fracture and energy value to decide tectonic fracture development. According to the method the fitting formula is offered between the relation of the value of rock fracture, energy and the fracture density. Compared the fracture density predictive value and the observed value of 15 wells, all wells relative error less than 25%. The prediction results are accurate. According to the fitting formula of fracture development and distribution in the study area are obtained. The results show that the fracture density is high as a whole of the Longmaxi formation in Dingshan area. Whole shows by south east to gradually reduce the trend of the north west. Supported the future of the oil and gas exploration to provide geological basis and guidance in Dingshan area.

The main geological factors controlling the accumulation and yield of marine‐facies shale gas reservoirs are the focus of the current shale gas exploration and development research. In this study, the Wufeng‐Longmaxi Formation in the Dingshan area of southeast Sichuan was investigated. Shale cores underwent laboratory testing, which included the evaluation of total organic carbon (TOC), vitrinite reflectance (Ro), whole‐rock X‐ray diffraction (XRD), pore permeability, and imaging through field emission scanning electron microscopy (FE‐SEM). Based on the results of natural gamma ray spectrum logging, conventional logging, imaging logging, and seismic coherence properties, the exploration and development potential of shale gas in the Dingshan area have been discussed comprehensively. The results showed that (1) layer No. 4 (WF2‐LM4) of the Wufeng‐Longmaxi Formation has a Th/U ratio <2 and a Th/K ratio of 3.5‐12. Graptolites and pyrite are relatively abundant in the shale core, indicating sub‐high‐energy and low‐energy marine‐facies anoxic reducing environments. (2) The organic matter is mainly I‐type kerogen with a small amount of II1‐type kerogen. There is a good correlation among TOC, Ro, gas content, and brittle minerals; the fracturing property (brittleness) is 57.3%. Organic and inorganic pores are moderately developed. A higher pressure coefficient is correlated with the increase in porosity and the decrease in permeability. (3) The DY1 well of the shale gas reservoir was affected by natural defects and important late‐stage double destructive effects, and it is poorly preserved. The DY2 well is located far from the Qiyueshan Fault. Large faults are absent, and upward fractures in the Longmaxi Formation are poorly developed. The well is affected by low tectonic deformation intensity, and it is well preserved. (4) The Dingshan area is located at the junction of the two sedimentary centers of Jiaoshiba and Changning. The thickness of the high‐quality shale interval (WF2‐LM4) is relatively small, which may be an important reason for the unstable production of shale gas thus far. Based on the systematic analysis of the geological factors controlling high‐yield shale gas enrichment in the Dingshan area, and the comparative analysis with the surrounding typical exploration areas, the geological understanding of marine shale gas enrichment in southern China has been improved. Therefore, this study can provide a useful reference for shale gas exploration and further development. [ABSTRACT FROM AUTHOR]

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Copyright of Petroleum Geology & Recovery Efficiency is the property of Petroleum Geology & Recovery Efficiency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Shale gas has currently attracted much attention during oil and gas exploration and development. Fractures in shale have an important influence on the enrichment and preservation of shale gas. This work studied the developmental period and formation mechanism of tectonic fractures in the Longmaxi Formation shale in the Dingshan area of southeastern Sichuan Basin based on extensive observations of outcrops and cores, rock acoustic emission (Kaiser) experiments, homogenization temperature of fracture fill inclusions, apatite fission track, thermal burial history. The research shows that the fracture types of the Longmaxi Formation include tectonic fractures, diagenetic fractures and horizontal slip fractures. The main types are tectonic high‐angle shear and horizontal slip fractures, with small openings, large spacing, low densities, and high degrees of filling. Six dominant directions of the fractures after correction by plane included NWW, nearly SN, NNW, NEE, nearly EW and NW. The analysis of field fracture stage and fracture system of the borehole suggests that the fractures in the Longmaxi Formation could be paired with two sets of plane X‐shaped conjugate shear fractures, i.e., profile X‐shaped conjugate shear fractures and extension fractures. The combination of qualitative geological analysis and quantitative experimental testing techniques indicates that the tectonic fractures in the Longmaxi Formation have undergone three periods of tectonic movement, namely mid‐late Yanshanian movement (82–71.1 Ma), late Yanshanian and middle Himalaya movements (71.1–22.3 Ma), and the late Himalayan movement (22.3–0 Ma). The middle‐late period of the Yanshanian movement and end of the Yanshanian movement‐middle period of the Himalayan movement were the main fracture‐forming periods. The fractures were mostly filled with minerals, such as calcite and siliceous. The homogenization temperature of fracture fill inclusions was high, and the paleo‐stress value was large; the tectonic movement from the late to present period was mainly a slight transformation and superposition of existing fractures and tectonic systems. Based on the principle of tectonic analysis and theory of geomechanics, we clarified the mechanism of the fractures in the Longmaxi Formation, and established the genetic model of the Longmaxi Formation. The research on the qualitative and quantitative techniques of the fracture‐phase study could be effectively used to analyze the causes of the marine shale gas fractures in the Sichuan Basin. The research findings and results provide important references and technical support for further exploration and development of marine shale gas in South China. [ABSTRACT FROM AUTHOR]