Sequence stratigraphic analysis of superimposed coal measure gas-bearing system in Daning-Jixian block, eastern margin of Ordos Basin, China

The identification of superimposed gas-bearing systems in coal measures is the basis for expediting the optimization of coal measure gas co-production. Through the analysis of drill cores and log data of Upper Carboniferous Benxi Formation to the member 8 of Middle Permian Lower Shihezi Formation in Daning-Jixian block, eastern margin of Ordos Basin, four distinct superimposed coal measure gas-bearing systems were identified, and their formation mechanism was discussed from the sequence stratigraphic perspective. Type I system mainly contains multiple coal seams, shales and sandstone layers. Type II system is dominated by multiple coal seams and shales. Type III is characterized by multiple sandstone layers, and type IV system is dominated by limestones and mudstones. In general, the gas-bearing systems deposited in barrier-lagoon are type II, those deposited in carbonate tidal flats are type IV, and those deposited in the delta front are types I and III. The marine mudstone, acting as a key layer near the maximum flooding surface, exhibits very low permeability, which is the main factor contributing to the formation of superimposed gas-bearing systems. The sedimentary environment plays a significant role in controlling the distribution of gas-bearing systems. Notably, the vertical gas-bearing systems in the south-western region, where delta front and lagoon facies overlap, are more complex than those in the north-eastern delta front facies.