Paragenesis and Formation Mechanism of the Dolomite-Mottled Limestone Reservoir of Ordovician Ma4 Member, Ordos Basin

Despite the discovery of high-producing natural gas reservoirs in the low-permeability dolomite-mottled limestone (DML) reservoir of the fourth Member (Ma4) of the Majiagou Formation in the Ordos Basin, the current understanding of the processes responsible for reservoir formation are still superficial, which extremely restricts the effectiveness of deep petroleum exploration and development in the basin. Therefore, this study analyzed the paragenesis process of the DML reservoir through systematic petrographic and geochemical measurements. The DML consists of burrows and matrix. The burrows are mainly filled with dolomite with a small amount of micrite, calcite cement, and solid bitumen. The matrix mainly consists of wakestone or mudstone. The DML has experienced multiple diagenetic events, including seepage-reflux dolomitization, compaction, calcite cement CaI cementation, micrite recrystallization, dissolution, hydrocarbon charging, calcite cement CaII cementation, and dolomite progressive recrystallization. Dolomitization is critical to the DML reservoir formation. The pore created by dolomitization is the hydrocarbon-migrated pathway and storage space. Due to the difference in Mg2+-rich fluid supply, the degree of dolomitization decreases from west to east, which causes the difference in diagenetic evolution of the western and eastern parts of the study area. The high dolomitization degree led to strong anti-compaction ability in the west, contrary to the east. Thus, the reservoir quality of the west is better than the east.