Occurrence states and potential influencing factors of shale oil in the Permian Fengcheng Formation of Mahu Sag, Junggar Basin

Taking the Permian Fengcheng Formation of the Mahu Sag of the Junggar Basin as an example, a systematic study of shale oil was carried out for the occurrence states and potential influencing factors by integrating various data including cores, well logging, X-ray diffraction, argon ion polishing and field emission-scanning electron microscopy (FE-SEM) observations, high-pressure mercury injection (HPMI), rock pyrolysis, and two-dimensional nuclear magnetic resonance (NMR) experiments. The shale oil of the Fengcheng Formation in the Mahu Sag mainly has two modes of occurrence:film-like adsorbed oil and infilled free oil. The main constrains for shale oil occurrence state include lithofacies association, mineral composition and reservoir space. Four lithofacies associations namely dolomitic shale, sandy shale-dolomitic siltstone, dolomite rock containing alkaline mineral-argillaceous siltstone, and siliceous dolomite rock-dolomitic siltstone were recognized in the shale reservoirs of the Fengcheng Formation. The mineral composition, reservoir space, micro-pore structure and shale oil occurrence states of different lithofacies appeared to be quite different. The contents of quartz, pyrite and organic matter were positively correlated with thermal cracking hydrocarbon (S2), and the contents of feldspar and dolomite showed a weak positive correlation with free hydrocarbon (S1). The free oil was mainly stored in mesopores and large pores, such as secondary dissolution pores and residual intergranular pores. The absorbed oil was mainly stored in mesopores and small pores, such as organic pores, intercrystallite pores and the surface of mineral particles. The mineral composition and pore-throat structure were dominant factors controlling the occurrence state of shale oil.