吴起地区马家沟组马五段岩相特征及成藏期次研究OA

In order to provide reliable basis for the exploration and evaluation of natural gas in deep carbon-ate reservoirs,taking the middle and lower members of Majiagou Formation in Wuqi area of the Ordos Basin(M 5-M10)as an example,core,thin section,logging,and structural thermal evolution history simula-tion data were used to study rock types,lithofacies characteristics,natural gas development,and reservoir accumulation stages.The results show that the main rock types are limestone and dolomite rocks.Limestone can be divided into micrite,sand micritic limestone,sand clastic limestone and transitional limestone.The sand clastic limestone can be further subdivided into micritic sand clastic limestone and sparry sand clastic limestone.The transitional limestone mainly includes dolomitic spots dolomitic containning limestone and dolomitic spots dolomitic limestone.Dolomite can be divided into micrite dolomite and powder-fine dolo-mite.The petrographic paleogeography of the main production layers,Ma 55 and Ma 54 are finely delinea-ted.The distribution of Ma 55 and Ma 54 is generally consistent,mainly in the eastern part,while the strata are denuded over a large area in the west.Ma55 mainly develops mud-grained chert and biotite in high-energy shallow and open terrace environments,and Ma54 mainly develops dolomite in a cloudy apron envi-ronment.The source rocks of Majiagou Formation in the study area are mainly kerogen types of type Ⅰ and Ⅱ,and followed by type Ⅲ,and with Ro ranging from 2.0％to 3.0％.The bauxite rock,dark mudstone and gypsum rock in the bottom of the Benxi Formation are stable and have good preservation conditions.The δ13 C1 distribution of gas from Ma55-Ma510 ranges from-40‰to-32‰,and the δ13 C2 distribution ranges from-38‰to-22‰,which shows the mixed characteristics of coal type gas and oil type gas.The distri-bution of natural gas type is related to the distribution of the top horizon of the the Ordovician Majiagou For-mation.Through structural thermal evolution history simulation and inclusion homogenization temperature analysis,it is concluded that there are two main hydrocarbon migration and accumulation processes.The first stage is the migration and accumulation of liquid hydrocarbons from the Middle-Late Jurassic(180～155 Ma),and the second stage is the accumulation period of large amounts of natural gas in the Early Cre-taceous(155～145 Ma),which is the main accumulation period of natural gas in the Ordovician Majiagou Formation.