Accumulation Process And Distribution Of Natural Gas In Dengying Formation In West Side Of The Deyang-Anyue Trough

The Cambrian-Precambrian oil and gas resources are the central issues of Chinese three marine cratons' exploration.On the east side of Deyang-Anyue Trough,a large-scale marine gas field,the Anyue gas field,was discovered.However,there has been no breakthrough on the west side,and there may be differences in the process of gas accumulation.The Hanshen-1 well is an unsuccessful exploration well andthe dolomites in the Fourth Member of Dengying Formation are the main reservoir.The author discusses the hydrocarbon accumulation process and the cause of the exploration failure in this well,thus indicating the favorable gas exploration areas on the west side,which is of great significance.Through observation of field outcrop and core as well as analysis via microscopic and Scanning Electron Microscopy(SEM),the author systematically studied the lithology type,reservoir space evolution and main controlling factors of the Fourth Member.The karstprocess of Tongwan movement period I and II formed a large number of dissolved fissues,pores and caves.The analysis of the characteristics,SEM,filling stages of bitumen,observation of inclusions and the measuring of the homogenization&freezing temperatures clarifies that there are two kinds of bitumen and the filling sequence of bitumen and mineral is Dolomite I? Dolomite II?Quartz?Bitumen.Liquid hydrocarbon inclusions with homogenization temperatures less than 110 °C in the first stage(before the oil peak)and the liquid-gas hydrocarbon inclusions between 120 and 160 °C in the second stage(during the oil peak)indicate that there is normal oil charge in HS1 well happened in middle Triassic-early Middle Jurassic.Through spectrum scanning,observation of fluid inclusions,the measuring of the homogenization&freezing temperatures and the analysis of Raman characteristics,we learn that the gas-liquid hydrocarbon inclusions above 170 °C of the third stage(after the oil peak)represent the cracking process of oil,which happened from middle Jurassic and reached their peak in the end of Jurassic.However,the washed and oxidized bitumen and low peak methane and non-hydrocarbon symbiotic prove that the gas accumulation was destroyed.Combined with the conditions of accumulation,we think the trap failed due to foreland tectonic uplift in Yanshan period,which turned an anticline to a ramp.Finally,through the analysis of tectonic movement we found that,the structure returned to an anticline during Himalayan period,but the gas accumulation was over then,Hanshen-1 well did not trap any gas successfully.Combined with reservoir characteristics and accumulation process,the author proposes that the main controlling factors of gas accumulation on the west side of the trough are tectonic evolution condition,reservoircondition,source rock condition and trap condition.It is predicted that the Weiyuan-Ziyang area is the most favorable area for gas accumulation.Four areas including western Emei are the second favorable areas.This thesis makes a detailed analysis of the accumulation process of Hanshen-1 well,and uses a variety of experimental methods to trace the accumulation process.The reasonable explanation for its failure is proposed thus it provides a new thinking and method to study lost wells.