Characteristics and resource potential of source rocks in the Linhe Depression, Hetao Basin

The Linhe Depression in the Hetao Basin represents an important area for potential breakthroughs in hydrocarbon exploration of medium-and small-sized basins and ultra-deep strata in China. Although the Jilantai and Bayan oilfields have been discovered in recent years, multiple exploratory wells in this depression have failed to deliver expected results. The failure analysis suggests that insufficient oil sources represent a critical constraint on hydrocarbon exploration in the depression. Based on the latest exploration achievements and laboratory data, we systematically investigate the characteristics of source rocks therein and calculate their resource potential using an integrated approach that combines geological assessment, basin simulation, and resource prediction. The research results indicate that the Linhe Depression in the Hetao Basin experienced an evolutionary pattern during the Meso-Cenozoic characterized by multi-stage tectonic subsidence and staged migration of subsidence centers, with the Naoxi sub-sag acting as a long-term inherited subsidence center. The depression contains four suites of source rocks: source rocks of the Guyang, Wulate, Linhe, and Wuyuan formations. The source rocks of the Linhe Formation, among others, predominate, characterized by substantial thicknesses, extensive distribution, high organic matter abundance, and favorable kerogen types. Furthermore, these source rocks exhibit low activation energy for hydrocarbon generation, offering advantages including early maturity, early hydrocarbon expulsion, a wide oil window, and high efficiency of organic matter conversion. These characteristics and advantages are closely related to the deposition of the source rocks in a highly reducing, saline lacustrine sedimentary environment and the enrichment of algae and sulfur within organic matter. Basin simulation results identify the Pliocene-Quaternary as the major oil generation period of source rocks in the Linhe Depression, accounting for 83. 1% of the total oil generated. This result aligns well with the late-stage tectonic activity and contributes to the formation of a rapid late-stage hydrocarbon accumulation model. Hydrocarbon resource evaluation indicates that the Linhe Depression has a total amount of generated oil of 31. 19 × 10⁹ t and petroleum resources of 2. 299 × 10⁹ t. Given that the current cumulative proven reserves account for merely 1. 2% of the total petroleum resources, the Linhe Depression holds considerable potential for hydrocarbon exploration. Since the oil generation centers are concentrated in the Naoxi sub-sag and the central fault zone, future exploration efforts in the depression should focus on tectono-lithologic composite traps within the Linhe Formation in the central fault zone (including the Xinglong, Nalinhu, and Ulan Buh structural zones), as well as lithologic traps and shale oil in the Linhe and Wuyuan formations in the sub-sag.