Hydrocarbon Migration and Accumulation in Unconventional Shale Play from the Upper Paleogene Xiaganchaigou Formation in Ganchaigou Area, Western Qaidam Basin, China

Abstract

Shale oil is generally considered to be petroleum generated from organic-rich mudstones and retained in place, or having migrated over a short distance and stored in adjacent organic-lean interbeds. However, in our study on the origin of petroleum in the Upper Xiaganchaigou Formation (E₃²), Ganchaigou area, western Qaidam Basin, China, we found that the E₃² shale reservoirs have experienced large-scale and long-distance hydrocarbon migration and accumulation. This study utilizes organic geochemistry and basin modeling to characterize the maturity of crude oil and source rocks within the E₃² shale reservoir. The results indicate that crude oil found in the immature to low-mature zone exhibits mature to overmature characteristics in organic geochemical and physical properties. In the strata within the oil window, the biomarker parameters and their absolute concentrations in crude oil still show significant differences from those of in-situ source rocks, indicating that the maturity of oil samples is notably higher than the in-situ source rock. Additionally, petroleum system modeling suggests that hydrocarbons generated solely from the in-situ source rocks cannot account for the high gas-oil ratio (GOR) fluids observed in actual production. Therefore, it is inferred that the E₃² unconventional shale plays in the Ganchaigou area have undergone large-scale hydrocarbon migration and accumulation. Besides the contribution of hydrocarbons in-situ, the shale plays also have contributions from the deeper and more mature oil and gas from the down-dip direction. Lateral hydrocarbon migration distances are estimated to exceed 5 km, with vertical migration distances surpassing 1 km based on the relationship of source kitchen and reservoirs.

Keywords

Qaidam Basin / Upper Xiaganchaigou Formation / shale oil / maturity / migration and accumulation / petroleum geology

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