Reservoir Characteristics Research From Member 1 Of The Shanxi Formation To Member 8 Of The Shihezi Formation Of The Yanchi Area In The Western Ordos Basin

The Sulige Gas Field is a large gas field that was recently discovered in China. However, with the development of the gas field, oil and gas resource reserve successors to the field in the Ordos Basin have become insufficient. The Yanchi area, which is located in the western part of the basin and adjacent to the field, has become a new area for exploration. Member 1 of the Shanxi Formation and Member 8 of the Shihezi Formation in the Upper Paleozoic are the main reservoirs in the Yanchi and Sulige areas, with differing capacities. Based on the theoretical systems of multiple disciplines, in this paper the author uses a variety of testing, analysis and statistical methods to conduct a comparative study into Member 1 of the Shanxi Formation and Member 8 (upper and lower) of the Lower Shihezi Formation in the Upper Paleozoic in terms of sedimentary microfacies, reservoir petrology, pore throat microstructure, diagenesis type, characteristics of reservoir physical properties and heterogeneity. Finally, this paper discusses the main factors that influence the reservoir's physical properties and provides a comprehensive evaluation of the reservoirs.The Shihezi and Shanxi formations of the Yanchi area in the western Ordos Basin are consistent in provenance and dominated by Proterozoic khondalite, TTG gneisses and products of volcanic and magmatic activities in the Devonian-Carboniferous period. The tectonic setting of the source area is complex, with active continental margins, passive continental margins and continental island arcs coexisting. The provenance mainly includes the Alxa archicontinent on the northwestern edge and the western section of the Yinshan archicontinent on the northern edge of the basin, forming a mixed source area in the range from Yanchi to Jiyuan. This paper defines, for the first time, the pattern of the mixed source area as a fusiform shape with two pointed ends and a wide middle part.Member 1 of the Shanxi Formation has a complete dual structure, with a delta deposit of a meandering river developed. Member 8 of the Shihezi Formation is characterized by sand body overlay, with a delta deposit of a braided river. Both members have a NW-SE trend. Sandstone reservoirs in the Yanchi area are mainly distributed in Member 8 of the Shihezi Formation where the braided river delta developed. Some sandstone is also found in Member 1 of the Shanxi Formation where the meandering river developed, but significantly less than that in Member 8 of the Shihezi Formation in terms of layers. Member 1 of the Shanxi Formation and Member 8 of the Shihezi Formation have substantially the same rock types, with medium to good particle sizes that are sub-angular in roundness. However, some differences also exist. The quartz in Member 8 of the Shihezi Formation is higher in content, compositional maturity, lithic fragment maturity and resistance to compaction than that in Member 1 of the Shanxi Formation. Therefore, the former has some primary porosity reserved.Reservoir spaces mainly exist in the form of intergranular pores, intercrystalline pores, dissolution pores and other composite pores. Very few or no fissures are to be found. The reservoir components of Member 1 of the Shanxi Formation have lower maturity and mainly include quartz sandstone and lithic quartz sandstone, with a high content of lithic fragments and fillings; kaolinite, hydromica, carbonate, and siliceous content are commonly found. The reservoir components of Member 8 of the Shihezi Fonnation have higher maturity and mainly include quartz sandstone and lithic quartz sandstone, with a low content of lithic fragments and fillings and moderately developed primary pores; kaolinite, hydromica, and siliceous content are commonly found. Based on analysis of relevant data regarding physical properties, the two members are pore-type reservoirs with ultra low porosity and permeability. Five samples from the target members in the research area were put to a constant velocity mercury injection test. The test results indicate that the samples are of medium-large pore size and micro-fine throat type, and that the radius and morphology distribution of pores contribute substantially to their permeability. CT scanning results reveal that short-extension micro-cracks exist in the samples.Both constructive diagenesis and destructive diagenesis exist and their main types include compaction and pressure-solution, cementation and filling, metasomatic alteration and dissolution. The main stages of diagenetic evolution include stages B to C of late diagenesis. However, the intensity of diagenesis varies for sandstone in different horizon, region, lithology and other conditions. The quartz sandstone exhibits the characteristics of low to moderate compaction, moderate to high cementation, and low dissolution. Lithic sandstone exhibits the characteristics of moderate to high compaction, moderate to high cementation, and high dissolution. The lithic quartz exhibits diagenesis in between that of the above two rock types. Taking into account various factors including lithology, pore structure and physical properties, the diagenesis of Member 1 of the Shanxi Formation and Member 8 of the Shihezi Fonnation in the Yanchi area can be divided into four types of diagenetic facies. The main diagenetic facies of a typical well in the longitudinal direction are given as examples and a distribution map of diagenetic facies in the research area is outlined to provide a basis for a comprehensive evaluation of the reservoirs.The physical properties of parent rocks in the source area determine the types of the reservoir rocks in the research area and their compositional maturity. The tectonic setting and provenance influence the spatial distribution of sand bodies and further influence the division of sedimentary facies. Sedimentation controls particle size, detrital composition, and sand body contact types for the reservoirs, and further influences the intensity of diagenesis from before rocks were formed up to the early diagenesis stage. Therefore, sedimentation is the most important factor in controlling the physical properties of the reservoirs. Middle to late diagenesis further strengthens the heterogeneity of the physical properties of the reservoirs. Therefore, provenance, sedimentation and diagenesis are the main reference factors when evaluating the characteristics of the reservoirs at the two locations in the research area. Taking into account the lithology, physical properties, pore structure, characteristics of capillary pressure curves, intensity of diagenesis, type and characteristics of diagenetic facies and other factors, the reservoirs in the research area can be classified into four classes from good to poor. Favorable areas of the reservoirs in Member 1 of the Shanxi Fonnation and Member 8 of the Shihezi Formation in the research area were screened and a forecast map of favorable exploration areas has been drawn up for Member 1 of the Shanxi Formation, Lower Submember 8 and Upper Submember 8 of the Shihezi Formation.