Reservoir characteristics and geological implications of marine sandstone on the periphery of Awati Sag, Tarim Basin, China: Case study of Upper Ordovician-Lower Silurian Kepingtage Formation

Abstract

The marine-origin source rocks on the periphery of the Awati Sag in the western Tarim Basin hold significant potential for hydrocarbon resources. However, the developmental level of marine sandstone reservoirs remains a key factor limiting the hydrocarbon exploration across the Keping thrust nappe belt along the western margin of the Awati Sag. This study focuses on the Upper Ordovician-Lower Silurian Kepingtage Formation sandstone through a multi-factor comprehensive approach, integrating data from outcrops, drilling, seismic surveys, and experimental analyses. Is the study clarifies that the Kepingtage Formation in the western margin of the Awati Sag is predominantly characterized by an early tide-dominated sedimentary system transitioning into late littoral deposition, resulting in tight sandstone reservoirs of sufficient thickness. The Kepingtage Formation sandstone is mainly composed of lithic sandstone, followed by lithic quartz sandstone. It exhibits low compositional maturity but high textural maturity, with well-developed intergranular pores and structural fractures. Porosity generally generally ranges from 6% to 10%. Class-IV reservoirs dominate, followed by Class-III and a few Class-II reservoirs. Reservoir quality is mainly controlled by sedimentary microfacies and structural compression. The reservoir rocks of sufficient scales of the delta and foreshore sandstone are more developed in the backward-breaking zone of the thrust nappe belt and the western margin slope of the Awati Sag. In contrast, the northwestern margin slope of the Awati Sag tends to develop fault-block oil and gas reservoirs as well as updip pinch-out sandstone oil and gas reservoirs. The favorable play spans an area of approximately 4320 km², with the predicted oil and gas resources of 78.17 × 10⁶ t and 707.6 × 10⁹ m³, respectively. The deep to ultra-deep low-structural-compression zone at the front of the Keping thrust nappe structure are identified as strategic favorable field of exploration for structural-lithological oil and gas reservoirs.