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.

To clarify the relationship between sand body superposition type and oil-water distribution of the Chang 4+5² from Yanchang Formation in the Nan 212 well block of the study area, the sedimentary microfacies type, vertical superposition and lateral contact style of the Chang 4+5² were analyzed through core observations and logging interpretations, and the oil-bearing characteristics of different types of sand body superposition were discussed in combination with well test data. The results indicate that the Chang 4+5² in the Nan 212 well block of the study area was deposited in a deltaplain sedimentary environment, that primarily generates distributary channels and interchannel. On the basis of vertical superposition, sand body types may be separated into massive, segmented interbedded, thin interbedded and thin sand bodies, and according to horizontal contact relationships, it can be divided into isolated, vertical superposition, horizontal overlapping, and cutting superposition sand bodies. Thin interbedded and segmented interbedded sand bodies make up most of Chang 4+5² vertical superposition relationship sand body, and segmented interbedded sand bodies are better at holding oil than thin interbedded sand bodies. The horizontal contact relationship of Chang 4+5^2-1 sand body consists mostly of vertical superposition, cutting superposition, and horizontal overlap sand body, with cutting superposition sand body having the highest oil-bearing property. The Chang 4+5^2-2 mainly develops a horizontal overlap and an isolated sand body with a relatively worse oil-containing property.

The Sedimentary features of Benxi Formation of the upper Carboniferous in Shenmu-Jiaxian area and the distribution pattern of sedimentary microfacies and evolution of the Section One and Section Two were studied in depth based on the comprehensive analysis of the rock type, rock texture, sedimentary structure, rock electric and the thickness and the exhibition features of sandstone, mudstone, limestone, coal seam and ratio of sand and stratum of 1500 wells. The results show that the delta and barrier coastal sedimentation systems dominate the study area. The delta system is primarily composed of delta front subfacies, which are further divided into underwater diversion channel and diversion bay microfacies. The barrier coastal system is divided into three subfacies: barrier island, tidal flat, and lagoon, which are further subdivided into various microfacies, such as sand flats, mud flats, and coastal marshes. Lagoon-barrier islands are developed in the west of the study area in Section Two and Section One of the Benxi Formation and the barrier islands distribution are sporadic. In the eastern part of the study area, tidal flat deposits is the main Sedimentary subfacies in Section Two of the Benxi Formation, which include mud flat and sand flat mainly, and only a small scale delt front developed in the north of the study area. But in section one of the Benxi Formation, the tidal flat evolved into delta front which with a distinct north-south zoning features. The sedimentary facie is delta controlled by river in north of Shuangshan-Shuang 99 well, and the sedimentary facie is tidal modification delta in southwest of Shuangshan-Double 99 well line. Because of the high degree of tidal modification, the rock of modification delta in southwest are coarse granularity and higher content of rock debris and cement, therefore, the reservoir is more physical property and higher natural gas production because it had higher content of intergranular hole, debris dissolution hole and intercrystalline pore.