Discovery of solid bitumen in the Ordovician pre-salt reservoirs of the Ordos Basin and its implications for natural gas accumulation
Recently,with the significant breakthrough and new progress of natural gas exploration,the Ordovician pre-salt reservoirs in the Ordos Basin have become an important new field of natural gas exploration.However,the critical issues related to hydrocarbon accumulation in this field have not been understood yet,such as the origin and resource potential of natural gas.In this paper,core observation and microscopic analysis are carried out with the 7th Submember of the 5th Member of Ordovician Majiagou Formation and the 4th Member of Majiagou Formation(hereinafter referred to as Ma 57 Submember-Ma 4 Member)in the central and eastern Ordos basin as the research object.It shows the existence of solid bitumen in the pre-salt reservoir and clarifies the occurrence and geochemical characteristics of solid bitumen.And combined with natural gas geochemical analysis,the origin and resource potential of natural gas in pre-salt reservoirs(hereinafter referred to as pre-salt natural gas)are discussed.And the following research results are obtained.First,there is a large amount of solid bitumen in the Ordovician pre-salt reservoirs.Solid bitumen is observed by transmitted light microscope in the form of flowers,flakes,clumps,thin films or strips in pores,fractures,sutures,and bitumen inclusions of dolomite and limestone.The mass percentage of solid bitumen ranges from 0.07%to 0.55%(average 0.17%),the equivalent vitrinite reflectance ranges from 1.75%to 1.97%,and the carbon isotope value ranges from-28.4‰ to-27.7‰.Second,the drying coefficient of pre-salt natural gas ranges from 0.95 to 1.00,making it a typical dry gas.The carbon isotope values of methane,ethane and propane in natural gas range from-44.8‰ to-32.4‰,from-31.1‰ to-22.6‰,and from-28.8‰ to-19.7‰,respectively,presenting the abnormal characteristics of negative drift of methane carbon isotopes and positive drift of ethane and propane carbon isotopes.Third,both solid bitumen and natural gas in the Ordovician pre-salt reservoirs are the products of oil cracking.The oil came from the Majiagou Formation or the older strata,and the oil cracking happened in the middle and late stages of Early Cretaceous.Fourth,the pre-salt natural gas is reworked by varying degrees of thermochemical sulfate reduction.And in the Wushenqi area,the reworking degree is higher,and the H2S content of pre-salt natural gas is high,with the highest value up to 13.31%.In conclusion,the predicted natural gas reserves in the Ma 57 Submember-Ma 4 Member reservoir are approximately 1.00×1012 m3,indicating great exploration potential of the Ordovician pre-salt natural gas in the Ordos Basin.
Ordos BasinOrdovicianPre-saltSolid bitumenOrigin of natural gasOil cracking
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