Distribution characteristics and oil mobility thresholds in lacustrine shale reservoir: Insights from N2 adsorption experiments on samples prior to and following hydrocarbon extraction

Abstract

The pore structure and oil content of shales have an important influence on the oil mobility and enrichment. In this study, the lacustrine shale samples from the Qingshankou Formation (Q1) of Songliao Basin were selected. TOC, pyrolysis, XRD and nitrogen adsorption were performed on the original and extracted shale samples. Then the influence of mineral composition and organic matter (OM) on the development of nano-scale pore, the oil phase states and mobility were analyzed. The Q1 shale samples can be sub-divided into three types according to the isotherm characteristics. Type A samples are characterized by high kerogen content, with oil mainly existing in the free phase state. Type B samples are characterized by medium kerogen content, oil mainly exists in the absorbed phase state. Type C samples are characterized by low kerogen content, with trace oil found in the absorbed phase state. Nano-scale organic pores are well developed in the Q1 Formation. Oil is primarily found in the pore spaces with diameters less than 10 nm, this being the pore size threshold for mobile shale oil. When TOCo > 2.0 wt% and EOM >1.0 wt%, Q1 Formation shale oil mobility is high, resulting in prospective drilling targets.