Unconformity Characteristics of Kunbei Fault Terrace Belt and Its Influence Factors on Petroleum Migration in the Southwestern Qaidam Basin, <scp>NW</scp> China

ABSTRACT Hydrocarbon migration is a critical process controlling petroleum accumulation. Understanding the types and spatial distribution of hydrocarbon carrier systems is essential for deciphering migration pathways. This study integrates structural analysis of unconformities, core observations and geochemical tests to compare migration pathways of oils from different charging episodes and to elucidate the control mechanism of unconformities on hydrocarbon migration. The results indicate that: (1) The bedrock unconformity in the Kunbei fault terrace zone serves as the main conduit for lateral oil migration in the hanging wall of the fault. Its binary structure, consisting of ‘basal conglomerate + semi‐weathered rock’, and variations in thickness spatially influence the transport capacity; (2) Reservoir bitumen exhibits a C 29 sterane predominance, low gammacerane index (&lt; 0.3) and relatively low sterane maturity parameters, indicating a low‐maturity origin. In contrast, crude oil shows C 27 sterane predominance, higher gammacerane index and higher maturity parameters, suggesting distinct sources; (3) Two phases of accumulation are identified: bitumen formed during the first phase (early N 1 ) and is distributed near the unconformity, whereas crude oil formed during the second phase (late N 2 ), initially migrating laterally along the unconformity before being vertically adjusted through secondary faults and ultimately accumulating in bedrock, E 1 + 2 and E 3 1 formations. These findings provide theoretical guidance regarding the role of unconformities as critical carrier systems in controlling hydrocarbon migration and accumulation.