Origin and evolution of gypsum dolomite as a favorable reservoir in the Ordos Basin, China

Gypsum dolomite is the most favorable natural gas reservoir in M51 of the Paleozoic Majiagou Formation in the middle eastern Ordos basin while its properties are mostly affected by dissolution. On the basis of past studies, the composition and genesis of gypsum dolomite and geopetal structure of M51 were determined by core data, cathodoluminescence and geochemical analyses. Thin section and physical property studies show that the numerous cast holes and other secondary pores from gypsum dissolution form the main reservoir space. In addition, a large number of geopetal structures are found in gypsum dolomite. Cathodoluminescence and elemental analysis show that the upper part of the geopetal structure is filled with calcite formed during the burial period; while the lower clastic dolomite is filled with clastic dolomite that has similar geochemical characteristics with the penecontemporaneous mud-powder dolomite outside the geopetal structure, but are obviously different from the fine-mesocrystalline dolomite formed in the deep burial period. The evolution of geopetal structural is an important component of gypsum dolomite as a favorable reservoir. Governed by the principle of chemical and ionic equilibrium, dissolution and filling alternate and repeat throughout the different diagenetic stages, demonstrating that chemical instability is the intrinsic cause of gypsum dissolution, the key to the formation of gypsum dolomite reservoir. © 2021, Editorial Office of Earth Science Frontiers. All right reserved.