The fourth member of the Ordovician Majiagou Formation(Ma 4 Member)in the Ordos Basin is a set of carbonates that was formed during the transgression period when the deposition of the Majiagou Formation lasted the longest.It is a set of sedimentary formation of carbonate rocks under the depositional background of epicontinental marine platform,and has been the key strata of high-sulfur gas exploration for many years.To further clarify the accumulation characteristics and exploration potential of the high-sulfur gas in the Ma 4 Member in the Ordos Basin,this paper redetermines the paleotectonic pattern based on seismic,well logging,drilling and other data.In addition,the source-reservoir-caprock assemblages and their relationships are investigated systematically,and the accumulation pattern of the Ma 4 Member high-sulfur gas is clarified.The following results are obtained.First,the paleotectonic pattern during the deposition of the Ma 4 Member was high in the east and low in the west in a typical sedimentary environment of large epicontinental carbonate gentle slope.Second,for the first time,a source rock layer primarily composed of microbial carbonate rocks was discovered in the Ma 4 Member,with an average organic carbon content of 1％and a thickness of 2-6 m.Third,the Ma 4 Member dolomite reservoirs are controlled by high-frequency sequence cycles,exhibiting multiple thin layers vertically and continuous distribution laterally.Fourth,a quasi-continuous tight carbonate gas reservoir is developed in the Ma 4 Member with marine source rocks for hydrocarbon supply,layer controlled dolomite as reservoir,and continuous thick gypsum-salt rock for sealing,and it is characterized by vertical migration,near-source accumulation,and enrichment at the structural high and fault zone.It is preliminarily confirmed that its favorable gas-bearing area is 1.8 × 104 km2.In conclusion,a new idea that marine source rocks are developed in the Ma 4 Member for hydrocarbon supply is put forward,and it is confirmed that low-abundance marine source rocks can lead to large-scale hydrocarbon accumulation.What's more,the Ma 4 Member high-sulfur gas reservoir is expected to become an important strategic replacement for deep marine oil and gas exploration in the Ordos Basin,and the exploration direction of the Majiagou Formation tight carbonate gas reservoirs in the Ordos Basin is pointed out.
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