The Paleo-Fluid Potential In Upper Triassic Of Middle West-Sichuan Sag

The tight clastic rock gas pools in the West Sichuan sag is an important production baseof natural gas of Sichuan Basin, in which the potential volume of gas is about 18000～25000×10~8m~3.However, the identified and estimated reserves, which mainly concentrate inthe Jurassic shallow and mesopelagic layers, are 2300×10~8m~3, only to occupy the totalresources 10.7%.The Xu jiahe Formation in upper Triassic, whose average buried depth is 3000m,belongs to deep exploration in the western depression of Sichuan Basin. With the comprehensivebreakthrough and entire production of exploration activity in shallow and mesopelagic layers,the strategic condition of sustainable development and increase in the western depression ofSichuan basin urgently require to finding large scale breakthrough of exploration activity in deep upperTriassic, in order to make it the backup of gas reservoir in the Jurassic shallow and mesopelagiclayers.Based on the work of predecessors and pattern of present structure, we adopt multiplepalaeotectonic research methods to remold the structural evolution of west-Sichuan sag.Through basin modeling, we studied the paleo-fluid potential in the critical period, thedynamic condition of hydrocarbon emplacement in trap structure of reservoiring period andintensified knowledge of accumulation regular of oil and gas in the western Sichuan forelandbasin. The studies mainly include:①Based on the work of predecessors, we studied the palaeotectonic pattern in thecritical period of the Xu jiahe Formation of middle west-Sichuan sag, using the methods of geologicand basin modelling and so on. The studies show that, in the period of Xu jiahepost-sedimentation～post-Suining, the structural feature of the middle west-Sichuan sag at section Xu 2is mainly half structure, but the Xu jiahe formation in Xiaoquan, Xinchang and He xinchang area ismainly areal slope. In the end of Jurassic, the structural feature of the Xu jiahe formation in Xiaoquan,Xinchang and He xinchang area is mainly geniculate southern slope. And in the end ofCretaceous Period, Xujiahe formation takes shape of uplift.②There are two-step palaeohydrodynamic field in the Xu jiahe formation: in the periodof the Xu jiahe post-sedimentation and post-Jurassic, the streams at section Xu 2 migrate fromwest to east and the streams at section Xu 4 migrate from south to north. In the end ofCretaceous and Xi shah Period, the groundwater at section Xu 2 and 4 migrates to Xiaoquan,Xinchang and He xinchang area. ③The studies of oil and gas potential at section Xu 2 and 4 of middle west-Sichuan sagshow that, in the period of the Xu jiahe post-sedimentation and in post-Jurassic, the fluidpotential at section Xu 2 migrates from west to east, south and north, and Xiaoquan, Xinchangand He xinchang area are situated at migratory slope, and the fluid potential at section Xu 4mainly migrates from south to north. In the end of Cretaceous and Xi shan Period, the fluidpotential at section Xu 2 and 4 migrates to Xiaoquan, Xinchang and He xinchang area.④We analyzed the accumulated hydrocarbon generation and expulsion stage, match ofgeologic feature such as structure of accumulative phase and porous evolution and so on, alsowe studied the developmental feature of dominant migration pathway in the process ofaccumulation in the Xu jiahe formation.