| The Ordovician natural gas resources are abundant in the central and eastern Ordos Basin.With the further development of deep gas exploration,considerable natural gas resources have been discovered in the Ordovician subsalt strata,but there is still a great controversy about the origin of subsalt natural gas and the conditions of source rock.In view of this problem,this paper systematically discusses the origin and source of natural gas in Majiagou Formation and the control of source rock development conditions on natural gas based on the experimental results of natural gas composition and isotopes,total organic carbon and rock pyrolysis analysis of source rock,saturated hydrocarbon chromatography-mass spectrometry analysis,kerogen macerals and carbon isotopes,organic petrology of source rock,etc.The natural gas subsalt in the study area is divided into three type:A,B and C.Type A gas is oil-formed gas,which is distributed in the inside of the study area under salt,with a wide range and far away from the boundary of the weathering crust;type B gas is coal-formed gas,which is mainly distributed in the north of the study area near the weathering crust;type C gas is a mixed source gas mainly composed of oil-formed gas,which is mainly located in Wuqi-Huachi area.The organic macerals of the subsalt source rock samples of Majiagou Formation of Ordovician are mainly algae,which are in the stage of high-over maturity.The overall organic matter abundance is less than 0.25%,among which the average TOC content of Om56,Om58,Om510,Om3,Om2 and Om1 member is relatively high.According to the basic principle of hydrocarbon generation and expulsion,the lower limit of TOC of the source rock of hydrocarbon expulsion carbonate is about 0.1%.In the discovered gypsum development area,the subsalt gas reservoir of well longtan-1 is far away from the weathering crust,which belongs to the self-generation and self-reservoir type.The carbon isotopic composition of the natural gas ethane is mainly the result of TSR reaction.The natural gas in the subsalt gas reservoir of well block 99 is the lateral migration and accumulation of natural gas from Carboniferous system,which is a typical new-generation and ancient-reservoir type.The subsalt gas reservoir of well block 30 is a mixed source.On the whole,the source rock samples of Majiagou Formation belong to low organic matter abundance,sapropelic organic matter and high over mature source rock samples,reaching a large amount of gas generation stage.At present,the natural gas production of single well in subsalt is under the dual control of Carboniferous and Ordovician source rock samples.The closer to the weathering crust,the greater the contribution of Carboniferous source rock samples is,and the inner story is mainly the self-generation and self-reservoir of Ordovician. |
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