Main reservoir controlling factors and diagenetic evolution of the Xiao'erbulak Formation of Tarim Basin, NW China: A case study of Well KPN1 in Kalpin area

Abstract

The Cambrian subsalt dolomite in the Kalpin area of the Tarim Basin is an important reserve growth point and strategic replacement area. However, there is a lack of clear understanding regarding the formation mechanism of high-quality reservoirs in this region, which has hindered oil and gas exploration. This study aims to address this knowledge gap by providing a comprehensive description of the rock types and characteristics of the Xiao'erbulak Formation. Microscope observations, geochemical analyses, and interpretations of well logging data from Well KPN1 were used in this analysis. The Xiao'erbulak Formation can be divided into four members, arranged from bottom to top. The relatively high manganese (Mn) content (87–137.7 ppm), oxygen isotope composition (δ18O) with an average value of −6.37‰, and strontium isotope ratio (87Sr/86Sr) with an average value of 0.7109 indicate that dolomitization of the Xiao'erbulak Formation likely occurred during the penecontemporaneous-shallow burial period. The early formation of dolomite contributed to increased reservoir porosity and resistance to compaction during deep burial, which laid the foundation for reservoir formation. The carbon isotope composition (δ13C) of the Xiao 1 and Xiao 2 members exhibit frequent zigzag curves, indicating recurring progression/regression processes. The subsequent development of granular beach facies played a crucial role in reservoir formation in the Xiao 2 Member. Tectonic fractures and penecontemporaneous karstification controlled the reservoir characteristics of the Xiao 3 Member. Furthermore, this study provides an analysis of the diagenetic evolution model of Well KPN1 and examines the impact of diagenetic transformations on reservoir quality. The systematic analysis of downhole data from Well KPN1 serves as a foundational reference for comparative studies with other drilling sites in the area. It also offers valuable guidance for future exploration and deployment strategies in the northwest Tarim Basin.