Lithofacies paleogeographic characteristics of the 5^th sub-member of Ma 5 of Ordovician Majiagou Formation in the northeastern Ordos Basin

The 5th sub-member of Ma 5 of Ordovician Majiagou Formation in the northeastern Ordos Basin is one of the key target layers for natural gas exploration. However, the understanding of its sedimentary characteristics remains insufficient, which restricts further exploration planning. Therefore, a systematic study on the lithofacies paleogeography of the 5th sub-member of Ma 5 of Ordovician Majiagou Formation in the northeastern Ordos Basin was conducted using integrated data from drilling, core samples, and thin sections, aiming to predict the distribution of favorable reservoir zones. The research results indicate that: 1) the 5th sub-member of Ma 5 of Ordovician Majiagou Formation in the northeastern Ordos Basin develops microfacies such as grain banks, microbial mounds, dolomitic lagoons, and calcareous lagoons; 2)the paleogeographic pattern of alternating uplifts and sags controlled the sedimentary differentiation of the 5th sub-member of Ma 5 of Ordovician Majiagou Formation, with grain banks distributed in a ring-like pattern around the central paleo-uplift, while microbial mounds and dolomitic lagoons are distributed along the high paleo-topographic areas in the eastern basin, and calcareous lagoons are mainly found in the sag areas; 3) grain banks and microbial mounds developed pre-existing pores due to grain accumulation or microbial frameworks, while dolomitic lagoons developed pores due to bioturbation and early dolomitization. These three microfacies are considered favorable reservoir facies zones for the 5th sub-member of Ma 5 of Ordovician Majiagou Formation. It is concluded that the Wushenqi-Wuqi area and the Mizhi area are favorable reservoir distribution zones for the 5th sub-member of Ma 5 of Ordovician Majiagou Formation in the northeastern Ordos Basin, and merit further exploration.