Research On Natural Gas Accumulation In The Middle Permian Maokou Formation In Northwestern Sichuan

There are more than ten wells with high production of commercial hydrocarbon flow in Maokou Formation of the middle Permian in northwestern Sichuan Basin,which showed bright prospect in the exploration of gas hydrate.However,the northwest Sichuan is located in the confluence between Micangshan foreland thrust fold belt and Longmenshan nappe structural belt,which caused natural gas accumulation was complicate,and was difficult to compare the carbonate oil and gas source in the environment where was deeply buried.Aimed at revealing the rule of natural gas accumulation,this paper particularly analyzed the geochemical characteristics of the related source rock of gas accumulation in Maokou Formation,and the source of natural gas in the Maokou Formation of northwestern Sichuan Basin in detail on the base of the theory of petroleum geology,oil and gas geochemistry,oil and gas geological theory and so on.On the other hand,combined with the method of field investigation,core sampling and laboratory analysis,this paper discussed the geochemical characteristics of source rocks and the sources of gas in Maokou Formation in detail,and on the base of analysis of accumulation condition,we summarized the main controlling factors of gas accumulation.The following conclusions are drawn below.(1)There were 6 sets of effective sources rocks developed in Permian and below strata in northwestern Sichuan,and four sets were related to Maokou gas accumulation,respectively,the lower Cambrian Qiongzhusi Formation,the lower Silurian Longmaxi Formation,the middle Permian Maokou Fomation and the upper Permian Longtan Formation.Among those four sets of sources rocks,the lower Cambrian Qiongzhusi Formation was with the highest quality,followed by the lower Silurian Longmaxi Formation,the middle Permian Maokou Formation and the upper Permian Longtan Formation.(2)The maturity of the natural gas in Maokou Formation was all mature or over mature,with different types and sources in different tectonic zones.The natural gas in the Hewanchang field was dominated by high mature and over mature oil-type gas,which was mainly from the Silurian source rocks.The genetic type of natural gas in Kuangshanliang field and Wujiaba field were mainly oil type gas and at mature phase,which came from Maokou source rock.The natural gas in the Shuangyushi structure was mainly composed of high mature coal-type gas,which was from the upper Permian Longtan Formation.(2)The natural gas in Jiulongshan structure was composed from high mature oil-type gas as well as high mature coal-type gas,its natural gas was not only derived from Maokou Formation source rocks,but also from the upper Permian Longtan Formation.The reservoir bitumen of Maokou Formation is mainly derived from the source rocks of the Maokou Formation.(3)There were four main areas of distribution of the fluid inclusions homogenization temperature in Maokou Formation,indicating that the hydrocarbon accumulation in this area might include at least four stages.The first-period of filling was from 218Ma to 206Ma,which was in late Triassic,and the corresponding temperature of the saline inclusions was from 60 centigrade to 90 centigrade.The second-period filling was from 206Ma to 145Ma,which was from early Jurassic to late Jurassic,and the corresponding temperature of the saline inclusions was from 90 centigrade to 140 centigrade.The third-period filling was from 145Ma to 65Ma,which was from early Cretaceous to late Cretaceous,and the corresponding temperature of the saline inclusions was from 140 centigrade to 180 centigrade.The final period of migration began at 65Ma,which was from Tertiary,and the corresponding temperature of the saline inclusions was higher than 180 centigrade.Abundant oil and gas was filling in the reservoirs from late Triassic to middle Jurassic.(4)The oil and gas reservoirs of Maokou Formation in the study area are mainly affected by reservoir conditions,effective traps and preservation conditions.The development of high quality reservoirs is the key factor for gas accumulation.Effective traps control the type and scale of gas reservoirs,and preservation is a necessary condition for the formation of gas reservoirs.