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.

The tectonic evolution process of Paleozoic paleo-uplifts in the north and south of the Ordos Basin is still not quite clear and this restricts the oil and gas exploration and the study on sedimentary filling evolution process in the basin. Based on the latest drilling, outcrop and seismic data, the study characterizes and defines the Qingyang and Wushenqi paleo-uplifts by means of seismic identification of reflector contact relationship and shore facies. The results show that the two paleo uplifts have experienced six major stages of evolution, namely the gradual submersion in the Cambrian, uplifting and denudation from the Cambrian to the Ordovician, stabilization of the Qingyang paleo-uplift and rapid subsidence of the Wushenqi paleo-uplift after the Yeli-Liangjiashan period, the compression, denudation and finalization of the paleo-uplifts during the Late Caledonian, the extinction of the paleo-uplifts in Carboniferous-Permian, and the tectonic tilting and finalization after the Permian. The two paleo-uplifts were similar during the early stages and differentiated during late stages with the south uplift rising and north uplift subsiding. The two paleo-uplifts together formed an alternative rising-subsiding framework that had obviously controlled the sedimentary reservoirs of the Cambrian system. The paleo uplifts are gentle in the east and steep in the west. The east is more favorable for the large scale development of dolomite reservoirs. During the process of Cambrian lithofacies paleogeography evolution, both paleouplifts controlled the lithofacies paleogeography framework but with the Qingyang paleouplift working more continuously on the uplift-depression pattern and sedimentation. The Huaiyuan tectonic movement in the late Cambrian led to the uplift and denudation of the Ordos Basin, resulting in a widespread development of karstification at the top of the Cambrian.