Paleostructure Restoration And Its Control On Gas Accumulation In Eastern Slope Of The Western Sichuan Depression

The eastern slope of the middle part of the Western Sichuan Depression is a secondary structure in the east of the Western Sichuan Depression.It is an oil and gas structural unit in the Sichuan Basin.There are 5 3D seismic exploration blocks such as Hexingchang-Gaomiaozi,Fenggu,Yongtai,Zhongjiang-Huilong and Zhixinchang-Shiquanchang field.It is great significance to correctly understand the morphology of the east slope of the Western Sichuan Depression and its controlling effect on the formation of oil and gas reservoirs in Shaximiao group.This paper is based on the hydrocarbon generation history of hydrocarbon source rocks and fluid inclusion analysis to determine the reservoir forming time and period.Through analyzing the data of drilling and logging,the sonic velocity analysis method was used by Himalayan movement respectively.Using 2DMove software to make the structural history section.Adopted the method of equal thickness map to restore the contour map of the paleo tectonic surface when JS11,JS14,JS23,JS24-1,JS31-2,JS33-1,JS33-2 and JS33-3 are formed.Using this map to analyze the control of tectonic evolution of Shaximiao group on migration,accumulation and adjustment of natural gas.The history of strata subsidence controls the hydrocarbon expulsion time of source rocks.Since the late Indosinian,the strata have been buried deeply,and T3x5 source rocks have reached the threshold of hydrocarbon generation at 145Ma(Late Jurassic),and began to produce hydrocarbon.Hydrocarbon source rocks enter the peak of hydrocarbon generation at 105 Ma and stop hydrocarbon generation at 72 Ma.The results of fluid inclusion system reveal that there are three stages of hydrocarbon accumulation in the study area.The first stage is from 141 Ma to 128 Ma,and the reservoir may have been destroyed.Second reservoir forming time is from 105 Ma to 88 Ma.The reservoir formation time of the third stage is from 83 Ma to 68 Ma.The peak time of hydrocarbon generation in source rocks is from 105 Ma to 72 Ma.The Western Sichuan Depression experienced the movement of the county,the late Indosinian movement,the Yanshan movement and the Himalayan movement.During the Yanshan tectonic movement,the compressional stress occurred in the East and West,forming the embryonic form of the North-South tectonic belt.Performance of Xiaoquan-Fenggu is fast subsidence.Xinchang-Shiquan field area formed some fault block structure.There are some "Y" faults in the strata and these faults formed anticline structure traps.At the early stage of Yanshan movement,the deep faults such as F1-1,F2,F3,F4 and F8 play an important role in the migration of natural gas from the Xujiahe formation to the upper Shaximiao group.The calculation results of the demuded amounts and shows that the average thickness which restored by Himalayan movement is over 1000 m.The thickness of erosion has decreased from south to north.The thickness of eroded strata is close to 1500 m where near the fault zone area in Zhixinchang.In Zhongjiang-Huilong region,the erosion is smaller.Thickness of erosion decreased from west Gaomiao to east Fenggu region.The reservoir forming pressure in the study area is mainly the formation pressure,and the evolution process of the formation pressure determines the evolution of the reservoir forming dynamics.During the period of Yanshan,there are three episodic hydrocarbon expulsion processes.From early to late stage,formation pressure changed from high pressure to low pressure.Tectonic compression and uplift erosion resulted in the abnormal high pressure in the Shaximiao group.The tectonic high pressure controls the hydrocarbon accumulation dynamics,and the gas gathers to the high point under the formation pressure.The high point of the structure are Hexingchang anticline,Zhongjiang anticline and Huilong-Fenggu anticline in the period of hydrocarbon accumulation.These large scale paleo tectonic ridges have been formed before the reservoir forming period.It controls natural gas migration path distribution.During the period of hydrocarbon accumulation,oil and gas migrated from the lower part to the higher part and gathered to the high point of structure to form the structural lithologic gas reservoir.The structure of the study area was reversed in the Himalayan period,and the southern region was strongly uplifted.The south is higher than the north.The distribution of the original gas reservoir has been adjusted by the change of the tectonic uplift and tectonic configuration.