Three-Dimensional In Situ Stress-Field Simulations of Shale Gas Formations:A Case Study of the 5th Member of the Xujiahe Formation in the Xinchang Gas Field,West Sichuan Depression

This work established a geological model for the 5th member of the Xujiahe Formation（X5 member） in the Xinchang gas field of the West Sichuan Depression based on the lithological, structural and depositional properties, as well as logging and well completion data and drill-core observations. Rock mechanical parameters were calculated according to rock mechanic experiments and rock mechanic interpretations from logging data. We also calculated the magnitudes and orientations of the in situ stresses based on acoustic emission tests, differential strain tests, fracturing behaviour and logging interpretations as well as anisotropy logging tests, borehole-breakout measurements and well-log data. Additionally, the present stress field of the X5 member was simulated using finite element numerical（FEM） simulation methods. The numerical simulation results indicate that the distributions of lithology and fractures are key factors that influence the present stress field. The stress field in the study area is discontinuous as a result of fractures and faults in the central and eastern areas. Stress is concentrated at the end sections and bends of faults, but dissipates with distance away from both sides of the faults. A longitudinal profile clearly demonstrates the zonality and continuity of the stress field and an increase with depth. The differential stress distribution is relatively uniform; however, large deviations occur in fracture zones.

Big Gas Field Found in West Sichuan Depression

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Cenozoic structural deformation and expression of the “Tan-Lu Fault Zone” in the West Sag of Liaohe Depression, Bohaiwan basin province, China

Based on the interpretation of 3D seismic data and structural mapping we analyzed the geometry and kinematics of the fault system and validated the expression of the"Tan-Lu Fracture Zone"in the West Sag of Liaohe Depression,Bohaiwan basin province.The Cenozoic structural deformation within the West Sag of Liaohe Depression can be divided into extensional structure system and dextral structure system.The extensional system is constituted by numerous NNE-NE trending Paleogene normal faults,where the Taian-Dawa fault(F1)is the master boundary fault(MBF)dominating the deposition during Paleogene so that the sag shows a complex half-graben with"boundary fault in the east and overlap in the west".The dextral system is constituted by 2–3 dextral basement faults in NNE-NE trending(F2,F3,F4)and associated structure,and the time of structural action started in Oligocene and continued to Quarternary so that some associated secondary faults of the dextral system cut off the Neogene and Quaternary.Under the influence of the position and attitude of NNE-NE trending basement strike-slip faults,the central north part and the south part of the West Sag show obviously different structural features.The former appears to be a complex"graben"structure limited by the reversed strike-slip fault in the west and bounded by the inverted normal fault in the east,the latter remains the complex half-graben structure with"boundary fault in the east and overlap in the west",and the graben was mildly reconstructed by one or two normal strike-slip faults.The dextral system within the West Sag is the element of the west branch fault of the Tan-Lu Fracture Zone,which is a deep fracture zone extending along the east of the Liaodongwan Gulf.The deep fracture zone branches off into two separate faults within the Liaohe Depression.The east branch goes through from northern part of the Liaodongwan Gulf to the East Sag of Liaohe Depression and links with the Denghua-Mishan Fault near Shenyang,and the west branch passes from northern part of the Liaodongwan Gulf to the West Sag and Damintun Sag of Liaohe Depression and links with the Yilan-Yitong Fault.The principal displacement zone of the west branch of the Tan-Lu Fracture Zone cuts off the master extensional fault(F1)within the West Sag of Liaohe Depression and induces many cover faults in EW trending within the Neogene and Quaternary.

Diagenetic fluid evolution and water-rock interaction model of carbonate cements in sandstone: An example from the reservoir sandstone of the Fourth Member of the Xujiahe Formation of the Xiaoquan-Fenggu area, Sichuan Province, China

Carbonate cement is the most abundant cement type in the Fourth Member of the Xujiahe Formation in the Xiaoquan-Fenggu area of the West Sichuan Depression. Here we use a systematic analysis of carbonate cement petrology, mineralogy, carbon and oxygen isotope ratios and enclosure homogenization temperatures to study the precipitation mechanism, pore fluid evolu- tion, and distribution of different types of carbonate cement in reservoir sand in the study area. Crystalline calcite has relatively heavy carbon and oxygen isotope ratios （δ13C = 2.14%o, 8180 = -5.77‰）, and was precipitated early. It was precipitated di- rectly from supersaturated alkaline fluid under normal temperature and pressure conditions. At the time of precipitation, the fluid oxygen isotope ratio was very light, mainly showing the characteristics of a mixed meteoric water-seawater fluid（ δ180 = -3‰）, which shows that the fluid during precipitation was influenced by both meteoric water and seawater. The calcite cement that fills in the secondary pores has relatively lighter carbon and oxygen isotope ratios （δ13C = -2.36%0, 8180 = -15.68‰）. This cement was precipitated late, mainly during the Middle and Late Jurassic. An important material source for this carbonate cement was the feldspar corrosion process that involved organic matter. The Ca2＋, Fe3＋ and Mg2＋ ions released by the clay mineral transformation process were also important source materials. Because of water-rock interactions during the buri- al process, the oxygen isotope ratio of the fluid significantly increased during precipitation, by about 3‰. The dolomite ce- ments in calcarenaceous sandstone that was precipitated during the Middle Jurassic have heavier carbon and oxygen isotope ratios, which are similar to those of carbonate debris in the sandstone （δ13C = 1.93%o, δ180 = -6.11‰）, demonstrating that the two are from the same source that had a heavier oxygen isotope ratio （δ180 of about 2.2‰）. The differences in fluid oxygen isotope ratios during cement precipitation reflect the influences of different water-rock interaction systems or different wa- ter-rock interaction strengths. This is the main reason why the sandstone containing many rigid particles （lithic quartz sand- stone） has a relatively negative carbon isotope ratio and why the precipitation fluid in calcarenaceous sandstone has a relatively heavier oxygen isotope ratio.