Characteristics Of Groundwater And Hydrocarbon Distribution In Continental Formations In The Middle Of Western Sichuan Depression

The Western Sichuan Depression is located in the northwest of Sichuan Basin. It is a foreland continental basin of Longmen Shan nappe structure since later Triassic. Three gas bearing systems have been deveoped in clastic sandstones from Xujiahe formation of upper Triassic to Qianfoya formation, Shangshaximiao formation, Suining formation and Penglaizhen formation in middle-upper Jurassic. Each gas system produces groundwater more or less, and the relationship between gas and groundwater is complex. Based on the conclusion of structural geology background in the middle of Western Sichuan Depression, analyzed the characteristics of the groundwater chemical and hydrodynamics both in the mid-shallow formations in Jurassic and the deep formations in upper Triassic, explored the connection of fluids between them combined with transporting systems here. In-depth analysis has been done in exploring the interconnections between characteristics of groundwater and distribution of gas in the key gas fields. It helps to recover the process of migration and accumulation and forecast the favorable exploration districts both in deep formations and mid-shallow formations, and to provide favorable proposal for future exploration in research districts. It has important theoretical and practical significance. The main contents of this dissertation are as follows:(l)There are obvious differences in the features of chemical characteristics of groundwater between mid-shallow formations and deep formations. The chemistry characteristics of groundwater in the deep formations of Xujiahe formation mainly present high sanity, Cl-concentration, salinization coefficient. But the sanity, Cl-concentration, salinization coefficient of groundwater is low in the mid-shallow formations in Jurassic. The reason mainly concerns with that the groundwater upwells to Jurassic formations along the faults and combine with the groundwater in Jurassic formations around the faults.(2)According to the analysis of the Novak water chemistry coefficient, the oxygen and hydrogen isotopes of present groundwater, carbon and oxygen of calcite cement, the Jurassic groundwater obviously suffered invasion of the deep Xujiahe formation groundwater along the faults belts,which presents the features of combination groundwater even deep groundwater of Xujiahe formation in local areas. All show that the close connection between the deep groundwater with mid-shallow groundwater.(3)Overpressure is very common in the continental formations in research area. The reasons of developing overpressure in Xujiahe formation are rapid deposition, tectonic compression and hydrocarbon-generating pressurization. All the reasons are the same but deep liquids filling is the different reason that lead to the overpressure in the Jurassic formations. Because of the opening of the South Longquanshan fault, Luodai district shows normal pressure.(4) The gas of Jurassic reservoir is mainly from the deep Xujiahe formation. The location and scale of gas reservoir in the mid-shallow formations remind on the faults which connected with deep Xujiahe formation and mid-shallow Jurassic formations or not and the location and distribution of reservoir in vertical. Such as the Xinchang gas field,Majing gas field and Xindu-Luodai gas field.(5) The gas of Jurassic reservoir is mainly from the deep Xujiahe formation. The model of gas accumulation in the mid-shallow reservoir is gas enrich locally due to the faults connection between deep Xujiahe formation and mid-shallow Jurassic formation. Xujiahe gas reservoir has the characteristics of continuous type for its tight reserviors.(6) Xinchang tectonic belt is the most favorable gas accumulation area of the deep Xujiahe formation. Xiaoquan, Xinchang, Hexingchang in the Xinchang tectonic belt and Majing-Shifang, Guanghan-Jintang, Xindu-Luodai in the Chengdu depression lie closely to the faults in the anticline and incline are the most favorable exploration districts for the mid-shallow reservoir. Foreland of Longmen Mountain tectonic belt and the southern part of Longquan Mountain are not favorable for forming gas fields.