Characteristics of Connected Pores and Evaluation of Shale Oil Mobility in the Qianjiang Formation, Qianjiang Sag, Jianghan Basin, China

The connectivity of shale pores and the occurrence of movable oil in shales have long been the focus of research. In this study, samples from wells BX7 and BYY2 in the Eq34-10 cyclothem of Qianjiang Formation in the Qianjiang depression, were analyzed. A double mercury injection method was used to distinguish between invalid and effective connected pores. The pore characteristics for occurrence of retained hydrocarbons and movable shale oil were identified by comparing pore changes in low temperature nitrogen adsorption and high pressure mercury injection experiments before and after extraction and the change in the mercury injection amounts in the pores between two separate mercury injections. The results show that less than 50% of the total connected pores in the Eq34-10 cyclothem samples are effective. The development of effective connected pores affects the mobility of shale oil but varies with different lithofacies. The main factor limiting shale oil mobility in Well BX7 is the presence of pores with throat sizes less than 15 nm. In Well BYY2, residual mercury in injection testing of lamellar dolomitic mudstone facies was mainly concentrated in pores with throats of 10–200 nm, and in bulk argillaceous dolomite facies, it was mainly concentrated at 60–300 nm. The throats of hydrocarbon-retaining pores can be 5 nm or even smaller, but pores with movable shale oil in the well were found to have throat sizes greater than 40 nm. Excluding the influence of differences in wettability, the movability of shale oil is mainly affected by differences in lithofacies, the degree of pore deformation caused by diagenesis, the complexity of pore structures, and the connectivity of pore throats. Dissolution and reprecipitation of halite also inhibit the mobility of shale oil.