Ordovician Pingliang Formation of Southern Ordos Basin

Microbial carbonate reservoir is one of the research hotspots in petroleum geology in recent years. Microbial structure plays an important role in the formation of reservoir space. A large number of microbial carbonate rocks are developed in Ordos Basin. However, there is no report on Ordovician Microbial Carbonates in the southern Ordos Basin. In this paper, the field observation and thin slice observation are carried out in the field outcrops of Chenlu Renjiawan in the southern Erdos basin. The petrological characteristics and reservoir space characteristics of the Microbial Carbonates are described. The sedimentary environment of the Microbial Carbonates is synthetically analyzed, and the relationship between the pore evolution and the diagenesis process of the Microbial Carbonates is discussed. The study suggests that: Four types of rock containing bioclastic mudstone limestone, gravel limestone, sand-clastic limestone, and algal limestone are developed in Ordovician Pingliang Formation in the southern Ordos Basin. Microbial carbonate is mainly developed in algal limestones. Most of them are thrombolites, followed by stromatolites and dentrolite. The depositional period of the Pingliang Formation in the field outcrop of Chenlu Renjiawan belongs to platform margin facies to platform foreslope facies, while the thrombolites and the stromatolites are mainly developed in the relatively low energy environment in platform margin facies. The reefs, shoals and microbial carbonate rocks in the Pingliang formation of southern Erdos are often symbiotic. There are various types of reservoir space in microbial carbonate reservoirs, mainly including grape-lace structure pores, intergranular dissolved pores of thrombolites, cavernous pores, interlaminar pores and pane pores. The formation of microbial carbonate reservoir is controlled by many factors such as microbial structure, diagenesis and sedimentary microfacies. The reservoir space is affected by the microbiological structure. At the penecontemporaneous stage, the reservoir is transformed by precipitation corrosion, followed by a supergene corrosion modification. However, after the cementation, the pores decrease or disappear. The sedimentary microfacies control the type of reservoir and its distribution characteristics in general. The microbial carbonate reservoirs in the Ordovician system in the southern Ordos basin may exist, and the microbial carbonate reservoir will be an important type of oil and gas exploration in the future.