Diversity and distribution pattern of high-quality dolostone reservoirs within sequence framework of the Xiaoerbulake carbonate ramp in Bachu area, Tarim Basin, NW China

The deep to ultra-deep Cambrian strata in the Tarim Basin are widely developed with dolostones, characterized by considerable thickness and an excellent source–reservoir–caprock assemblage, possessing abundant oil and gas resources. This study focuses on the petrology of the carbonate ramp dolostones to clarify the lithofacies associations of the Lower Cambrian Xiaoerbulake Formation. By integrating the sequence stratigraphic framework with multi-scale sedimentary cycle analysis, the types, spatial distribution patterns, and controlling factors of high-quality dolostone reservoirs were systematically investigated. Three types of dolostone were identified: dolograinstone (D1), microbial dolostone (D2), and crystalline dolostone (D3). The pore types of these dolostones are dominated by fabric-selective intergranular pores, intragranular pores, and microbial-related framework pores, exhibiting a notable correlation with lithofacies. The Xiaoerbulake Formation is stratigraphically divided into two transgressive–regressive 3^rd-order sequences and seven 4^th-order subsequences. Five sets of high-quality reservoirs developed vertically, primarily occurring within the 4^th-order regressive cycles. At the meter-scale, a strong vertical cyclic stacking pattern of pores was observed for microbial mound–shoal complexes. Additionally, penecontemporaneous karstification-related dolostones, characterized by excellent quality and considerable thickness, were initially proposed as being controlled by the boundary of 4^th-order to 3^rd-order sequences, affecting approximately 60 m below the 3^rd-order sequence boundary. Four types of high-quality reservoir lithofacies associations were identified, along with a proposed model that elucidates their distribution patterns within inner ramp facies of the Bachu area. In the carbonate ramp, the development of large-scale, high-quality reservoirs is controlled by a combination of factors, including sedimentary microfacies, micropaleogeomorphy, sea-level fluctuations, and early dolomitization. The microbial mound–shoal complex within the inner ramp, the lagoon-margin shoal, and the restricted supratidal flat located at the micropaleogeomorphic heights provide a geological foundation for the formation of high-quality reservoirs. This is primarily attributed to their conducive exposure and dissolution by meteoric water, forming abundant fabric-selective pores at the penecontemporaneous stage.