Impact of petrographic characteristics on reservoir quality of tight sandstone reservoirs in coal‐bearing strata: A case study in Lower Permian Shihezi Formation in northern Ordos Basin, China

Abstract

The lower Shihezi Formation in the Sulige gas field is the main gas exploitation target in the Ordos Basin and is crucial in finding high-quality reservoirs for further exploration.Using X-ray diffraction, cathodoluminescence (CL), and scanning electron microscope (SEM) techniques, the eighth member of lower Shihezi Formation (He8) sandstones are characterized as fine-to coarse-grained quartzarenite and litharenite with good sorting, which are controlled by different sources and depositional faces. Four types of sandstones are recognized according to varied petrographic characteristics, among which fine-grained sandstones (type I) has the poorest reservoir quality and coarse-grained sandstones with chlorite coating (type II) are the best reservoirs. The complicated diagenetic process mainly includes intense compaction, two-stage dissolution, quartz and calcite cementation, which is closely associated with the thermal evolution of coals. The increased ductile fragments account for low reservoir quality in type I sandstones. Due to the absence of feldspar, the influence of other detrital components on the reservoir quality has increased. Sandstones with abundant volcanic fragments (type II) usually formed grain-coating chlorite, which inhibit tight compaction and quartz cementations.Quartz overgrowth originated from feldspar dissolution as well as pressure dissolution of detrital quartz are frequently observed in medium-to coarse-grained quartzarenite (type III). The high-quality reservoir exits when the ductile grains content is about 4%, according to the negative relationship between quartz cement and ductile content.Although there are a lot of dissolution pores in medium-to coarse-grained litharenite (type IV), the reservoir quality did not improve much due to the diagenetic minerals filling in the secondary pores. K E Y W O R D S coal-bearing strata, diagenesis, Ordos Basin, petrographic characteristics, quartz cementation, tight gas reservoir 1 | INTRODUCTION Tight sandstone reservoirs in coal-bearing strata are gradually receiving much interest because of their large hydrocarbon reserves worldwide (Zou et al., 2012). In China, the production of gas in coal buried strata has reached 902 × 10 8 m 3 , accounting for 61.5% of the total production (Zou et al., 2019). Therefore, it is very significant to focus on factors affecting production in coal buried tight sandstones.

Impact of coal evolution on formation of tight sandstone reservoirs: A case study in the Daniudi gas field, Ordos Basin, China

“…As a contrast, sandstones in pattern I were far away from the coal layers, and the diagenetic fluid was weakly acidic to weakly alkaline, leading to more early calcite cements remained. This was also conducive to the formation of early chlorite (Liu et al, 2021), of which Fe 2+ and Mg 2+ was provided by the dissolution of volcanic fragment. This is why there were more chlorites in the Xiashihezi Formation.…”

“…This is why there were more chlorites in the Xiashihezi Formation. Sandstones in pattern III were formed in the environment of low water energy with more matrix and ductile fragments (Liu et al, 2021). Dissolution in model III was also weak because of large amounts of matrix that blocked intergranular pores inhibiting the entry of diagenetic fluid.…”

“…Ordos Basin is a multicycle craton located in western China, which can be divided into six tectonic units (Liu et al, 2021), including Tianhuan depression, Weibei Uplift, Western thrust belt, Jinxi Fault-fold Belt, Yishan slope and Yimeng Uplift (Figure 1). The Daniudi gas field is situated in the northeast of Yishan Slope, which is characterized as a westward dipping monocline with few structures and faults developed (Xu, 2017).…”

“…As an important unconventional resource, tight sandstone gas has attracted more and more attention (Cao et al, 2014;Lai et al, 2022a;Zou et al, 2012Zou et al, , 2019. Generally, many tight sandstones gas reservoirs in the world are often associated with coal-bearing strata, such as the Xiashihezi and Shanxi Formation of Upper Permian in Ordos Basin, China (Li et al, 2019), the Xujiahe Formation of Upper Triassic in Sichuan Basin, China, and the Mesaverde in San Juan Basin in America (Dai et al, 2012;Lai et al, 2018Lai et al, , 2021Liu et al, 2021). Therefore, it is of great significance to clarify the formation mechanism and high-quality reservoir distribution of tight sandstone reservoirs in coal measures, which is conducive to further production and development.…”