| The Western Sichuan Depression is located in northwest part of Sichuan Basin. It is a foreland basin, since later Triassic. The geological characteristics of Xujiahe Formation are deep buried, overpressure and later structures, in the middle of Western Sichuan Depression. The top and bottom gas systems are formed in upper Triassic. There are many palaeostructural traps and palaeo and present structural superimposed traps in Xujiahe Formation. The hydrocarbon source rocks of upper Triassic are characterized by large thickness, wide distribution, high abundance, high abundance, ideal type, great hydrocarbon generating strength and long-term hydrocarbon supply. The caprocks are characterized by high breakthrough pressure and stable distribution. Because of the superior reservoir forming condition, the Xujiahe Formation is favorable for forming medium to large scale gas fields. Because of the general densification reservoirs and complicated gas-water relationship, the gas accumulation mechanism and gas-water distribution are confused to petroleum geologists. In this study, geofluids are the mainline. Based on the physical properties, water-rock interaction, hydrogeochemistry and hydrodynamics of Xujiahe Formation reservoirs, gas accumulation mechanism is researched. Combined with structural characteristics, the favorable targets of gas reservoirs in Xujiahe Formation are proposed for future exploration. It has important theoretical and practical significance. The main contents of this dissertation are as follows:(1) The main pore type of Xujiahe Formation reservoirs is dissolution intergranuler pore, and the second is dissolution intragranular pore. The pore structure, porosity and permeability of Xujiahe Formation reservoirs are tested. The results show that the pore structure of T3x2is poor and porosity and permeability of T3x2is low. Due to microcrack developed, the phenomenon of low porosity high permeability exists in T3x2and T3x4reservoirs. The fluid inclusions in authigenic Quartz and calcite cement most formed at the end of Late Triassic and from Middle Jurassic to the end of early Cretaceous. And these periods are the rapid phases of reservoir tightening.(2) The hydrogeochemistry characteristics of the Xujiahe Formation water are very complicated. From above to below,4hydrogeochemical units are divided. They are freshening water belt of the mudstone compaction-released water and dehydration of clay minerals water (belt A), the belt main is infiltration-concentrated water, following is leakage-concentrate water (belt B), freshening water belt of the mudstone compaction-released water and dehydration of clay minerals water (belt C), the deep leakage-concentrate water belt (belt D). In lateral direction, hydrogeochemistry characteristics are zoning. The salinity of T3x2formation water in Chengdu sag is low; while, the salinity of T3x2formation water in Anxian-Yazihe-Dayi tectonic area, Xiaoquan-Fenggu tectonic area and Zhixinchang-Longbaoliang tectonic area are high.(3) Based on the research of fluid anion and cation of inclusions, the carbon, oxygen and hydrogen isotopes of paleo and present formation water, the origin of formation water is contemporaneous formation water affected by paleo-atmospheric water and deep f hydrothermal, in the formation water evolution process.(4) The overpressure is very common in Xujiahe Formation in Western Sichuan Depression. The pressure coefficient is from1.09to2.30. And the pressure coefficient of bottom part is less than that of top part. The reasons of overpressure are rapid sedimentary, tectonic compression and hydrocarbon-generating prcssurization.(5) From middle Yanshanian, Anxian-Yazihe-Dayi tectonic area, Xiaoquan-Fenggu tectonic area and Zhixinchang-Longbaoliang tectonic area Xujiahe Formation reservoirs are cross-formational flow areas, which are favorable for forming gas fields.(6) A model of the Xujiahe Formation gas accumulation is presented. Gas accumulation is synchronization with sandstone densification in earlier stage; gas accumulation restarts after cracks formed in later stage. In the process of sandstone densification, tectonic movements, mudstone compaction-released water and authigenic minerals formation can result in gas leakage.(7) Xiaoquan-Xinchang-Hexingchang areas are the most favorable for forming gas fields. Dayi area, Yazihe area, most part of Chengdu sag, Fenggu area, south part of Zitong sag are favorable for forming gas fields. Because of infiltration area of meteoric water in the front thrust system of Longmen Mountain, the gas preservation is massively deformed. Thus, it is not favorable for forming gas fields in the front thrust system of Longmen Mountain. |
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