Sedimentary facies characteristics and distribution patterns of the sub-member Ma 5₁ in the Majiagou Formation, eastern Yanchang exploration block, Ordos Basin

This study focuses on the fifth sub-member of the Majiagou Formation, specifically the first sub-member（referred to as "sub-member Ma5₁"）in the eastern Yanchang exploration area. Utilizing the outcomes from the meticulous delineation of smaller stratigraphic units, combined with core observations, thin-section analysis, single-well log sedimentary microfacies interpretation, and comparative inter-well section analysis, we conducted an in-depth examination of the sedimentary microfacies types and their planar distribution within the four minor layers of the sub-member Ma 5₁⁴ in the study area. This analysis aims to elucidate the distributional and evolutionary patterns of sedimentary microfacies across each minor layer. The results show that the sub-member Ma 5₁ in the eastern Yanchang exploration area is overall a restricted platform sub-facies deposit, mainly developing mud dolomitic flat, dolomitic flat, lime flat, and muddy flat sedimentary microfacies. The sub-layer Ma 5₁⁴ is characterized by the development of dolomitic flat, mud dolomitic flat, and muddy flat. Dolomitic flats are predominantly found in the northwest and central parts of the study area, while mud dolomitic flat cover the rest majority of the region. Muddy flats are notably restricted to the southwest portion of the study area. During the deposition of the sub-layer Ma 5₁³, the expansion of dolomitic flat in the central part of the study area and muddy flat in the south has increased compared to the sub-layer Ma 5₁⁴, attributed to enhanced evaporation in some local areas and an increase in terrigenous muddy input. During the sedimentation of the sub-layer Ma 5₁², a relative increase in water depth in the western and southwestern parts of the study area resulted in a significant reduction in the extent of dolomitic flat in the west and muddy flat in the southwest. Concurrently, lime flat emerged in some local areas of the southwest and southeast. In the sub-layer Ma 5₁¹, mud dolomitic and muddy flat predominantly occurred in the western and central parts of the study area, while dolomitic flat was mainly found in the eastern part. Additionally, lime flat developed in some northern local areas. The research results can provide scientific reference for the control mechanism of sedimentary microfacies on strongly heterogeneous carbonate reservoirs and the prediction of favorable areas for favorable reservoir distribution in the Yanchang exploration area.