Characteristics and primary controlling factors of limestone reservoirs in the Carboniferous Benxi Formation, east-central Ordos Basin

In the Ordos Basin, a significant breakthrough was recently achieved in the hydrocarbon exploration of tight limestones in the Permian Taiyuan Formation. The Carboniferous Benxi Formation, underlying the Taiyuan Formation, also contains limestones. However, these limestones remain poorly understood in terms of lithology, the spatiotemporal distribution of sedimentary facies, and the existence of reservoirs. This has restricted the emplacement of hydrocarbon exploratory wells. Using the latest drilling and outcrop data, we systematically analyze the lithology of limestones in the Benxi Formation in the east-central Ordos Basin, as well as the types and spatiotemporal distributions of their sedimentary facies. Furthermore, we explore the primary factors controlling the occurrence of limestone reservoirs in the formation. The results indicate that the Benxi Formation exhibits diverse types of limestones, including sparry bioclastic limestones, micritic bioclastic limestones, bioclast-bearing micrites, and micrites, all of which occur primarily in the 2nd member of the Benxi Formation (also referred to as the Ben 2 Member). These limestones mainly feature tidal flat and bioherm subfacies, with the latter being discovered in this member for the first time. The sedimentary facies generally show a C-shaped distribution pattern with an eastward opening. Bioherms are present in the coastal area and gradually transition to low-energy limestone or marl flats eastward. The limestones in the Benxi Formation primarily contain dissolution pores and microfractures, and the reservoirs are significantly governed by sedimentary facies and lithology. Specifically, reservoirs of bioclastic limestones and micritic bioclastic limestones are identified in the bioherm and bioclastic shoal facies zones, with dissolution and fracturing acting as core diagenesis that plays a constructive role in reservoir development. In sum, the Ben 2 Member is considered a favorable interval for the occurrence of tight limestone reservoirs. This provides a valuable reference for hydrocarbon exploration of tight limestones in the Benxi Formation within the Ordos Basin.