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.