Characterization of crude oil charging of Xiaoerbulake dolomite in the Tarim basin: Insights from bitumen distribution

Understanding hydrocarbon charging mechanisms is essential for reservoir quality evaluation. In this study, a systematic investigation of micro-nano scale oil charging characteristics in the Lower Cambrian Xiaoerbulake Formation of the Tarim Basin was performed via mercury intrusion capillary pressure (MICP) on samples of four types of dolomitic lithofacies before and after bitumen removal. Additionally, to determine the pore structure and space that was occupied by the bitumen, a series of observations using both optical and field-emission scanning electron microscopy (FE-SEM), along with micro-computed tomography (micro-CT) were conducted. A combination of these micro-CT and MICP techniques enabled us to determine how bitumen is distributed in micro-nano pore space of the bulk sample. Imaging results show that three types of pores (intergranular pores, intercrystalline pores and intragranular pores) and microfractures are developed within these lithofacies and confirmed that bitumen has occupied pores >20 nm in diameters. Furthermore, a relationship between pore-throat threshold and charging force was established under the reservoir conditions. It was found that the oil charging force and pore-throat threshold are different for each lithofacies with the microbial dolomites demonstrating the highest charging force and smallest pore throat threshold; as a consequence, they are the most favorable lithofacies for the accumulation and migration of oil and gas which should be considered for future exploration efforts in dolomitic reservoirs.