Reservoir-forming by lateral supply of hydrocarbon: A new understanding of the formation of Ordovician gas reservoirs under gypsolyte in the Ordos Basin

There developed thick gypsum-salt strata in the Lower Paleozoic Ordovician in the Ordos Basin, particularly gypsum-salt in the 6th segment of the 5th member of Majiagou Fm (O1m56) in a wide distribution, serving as a good regional cap. Recent drilling and relevant studies have proven that the hydrocarbon-generating capacity of the marine-facies hydrocarbon source rocks under the Ordovician salt layers in the eastern Basin is poor on the whole, making it difficult to form ”self-generating and self-preserving type” commercial gas reservoirs under the salt layer in Mizhi salt subsag. However, further research on reservoir-forming conditions under the gypsolyte on the west side of the salt subsag indicates that the O1m57 – O1m510 strata near the east side of the palaeo-uplift contact directly with the Upper Paleozoic coal measure source rocks, forming a “hydrocarbon supply window”; the Yanshan Movement caused the tectonic inversion of the basin main body, resulting in the tectonic framework “high in the east and low in the west”, which is conductive to the further migration of natural gas generated by the Upper Paleozoic coal measures to the updip high position of the east side along the O1m57 – O1m510 carrier beds after entering the dolomite reservoir under the gypsolyte through the “hydrocarbon–supply window”. In addition, the facies changes in the dolomite rocks under the gypsum-salt provide favorable barrier condition for the regional gathering of natural gas. Therefore, it is concluded through comprehensive analysis that reservoirs may form beneath the Ordovician gypsolith in the central region of the basin on the west side of the salt subsag with hydrocarbon supplied from the Upper Paleozoic source rocks of coal measures, which is expected to open up a new situation of natural gas exploration under the Ordovician gypsum-salt layer in the Ordos Basin.