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 propert...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.展开更多
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 petrolo...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.展开更多
基金financially supported by the State Key Lab of Oil and Gas Reservoir Geology and Exploitationfunded by the National Natural Science of China(grant No.41572130)
文摘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.
基金supported by the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation Foundation (Grant No. PLC201101)the National Natural Science Foundation of China (Grant Nos. 41172119 and 41272130)
文摘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.
