Genesis of Heavy Oils and Hydrocarbon Accumulation Process in Chepaizi Uplift (NW Junggar Basin)

Abstract

This study investigates the distribution and genesis of heavy oil in the Chepaizi Uplift by analyzing various aspects, including physical properties (density, viscosity, wax content, sulfur content, water content, and total acid number), molecular and bulk characteristics, hydrogeochemical data of formation water (total dissolved solids, pH values, and hydrochemical types), geothermal data, and microthermometry of fluid inclusions. The research identifies biodegradation as the dominant factor increasing oil viscosity with oxidation exacerbating this process. Conversely, water washing and diffusion have minimal impact on the oil viscosity increase, and the formation of heavy oil from low maturity source rocks is unlikely. Regional variations in viscosity increase factors are observed, with Eastern Chepaizi showing higher biodegradation due to lower mineralization, poor formation water types, and favorable temperatures and pH conditions compared to Western Chepaizi and the Hongche Fault Belt. Fluid inclusion microthermometry and biomarker characteristics indicated that the lower strata (C-J) of the Eastern Chepaizi experienced two hydrocarbon charging stages, corresponding to an early stage of heavy oil and a later stage of light oil charging, whereas Western Chepaizi had a single stage. The western region of Western Chepaizi and the central region of the Hongche Fault Belt are prime targets for light oil exploration. The hinge line of Chepaizi Uplift requires certain thermal recovery methods for extraction due to the high density and viscosity of crude oil. These zones reflect the varying degrees of secondary alteration processes that have affected crude oil in the study area. These findings hold significant guidance for future exploration and deployment of heavy oil resources in this region and serve as a reference for studying the genesis of heavy oil in other complex geological settings.