Geochemical Study On Hydrocarbon Accumulation Of Ordovician Carbonate Rocks In The Ha 6 Block

Geochemistry is one of important methods to systematically study the process of hydrocarbon accumulation,and the key of solving relative geological problems with this method is the combination of geochemical analysis and actual geological backgrounds.This study is just based on this combination to exploringly study the source of hydrocarbons and their accumulation periods in the research area,Ha6 Block.It is mainly manifested in the following aspects.Firstly,the analyses of light hydrocarbons,medium molecular hydrocarbons,conventional biomarkers,the carbon isotope of hydrocarbon gases as well as their combination with geological factors indicate that the source of oil and gas in the Ha6 Block Ordovician carbonate reservoirs is the Lower Ordovician and Cambrian source rock.Secondly,the evolution history of the regional structure,the burial thermal evolution history of the hydrocarbon source rock,the fluorescence properties of reservoir bitumen,the characteristc of the hydrocarbon inclusions and their associated brine inclusions and other geological factors and geochemical analyses demonstrate that the main period of hydrocarbon accumulation is the middle-late Caledonian and the late Hercynian,and the contribution of Himalayan is weak.Thirdly,the current oil and gas reservoirs is mainly the result of recharge of the late Hercynian hydrocarbons and readjustment and redistribution of the early charged.In addition,we put forward some explorative views on the phenomenon of high content of H2S in some reservoir and the distribution characteristics of N2 in research area.From there we establish the accumulation model of Ordovician carbonate reservoirs in Ha6 Block and conclude some factors controlling hydrocarbons distribution of it.Firstly,the favorable space-time configuration relationship of good source rock,tectonic movement and reservoir evolution define that Ha6 Block will be a favorable hydrocarbon accumulation zone.Secondly,the shape of paleostructure and distribution characteristics of regional caprocks determine that the oil density presents the south is lighter than the north.Thirdly,the good seal condition in late Hercynian is a critical factor of the hydrocarbons recharge and readjustment.In addition,we have put forward a new explanation that it is caused by "covered" effect about the phenomenon that some Cambrine-Lower Ordovician source oils has a distribution of Upper Ordovician biomarkers.In summary,the local traps near the "X" shape strike-slip fault composed of Ha601 and Ha803 strike slip belts is the key areas of oil and gas exploration,and in northern Qianshan area some heavy oil will be find above the Ordovician formations but below the Yingshan Group there will be more water reservoirs.The southern blocks such as Jinyue,Repu have a good exploration prospect that some light oil will be find in the O2y and O3l,and in the deeper formation,some condensate oil or gas reservoir will be find.