Division of fine-grained rocks and selection of “sweet sections” in the oldest continental shale in China: Taking the coexisting combination of tight and shale oil in the Permian Junggar Basin

Abstract The Permian formation in the Junggar Basin contains the oldest continental shale in China and has abundant petroleum resources. The lithofacies and lithology of the shale strata in the Lucaogou Formation, which are characterized by two sets of coexisting combinations of tight oil and shale oil in the Jimsar Sag, eastern Junggar Basin, are studied via experiments to explore their petrophysics, geochemistry, reservoir pores, and oil-bearing properties. The results show that (1) the Lucaogou Formation in the Jimsar Sag consists of shallow-deep lacustrine sediments and can be divided into six stratigraphic units vertically; 2) fine-grained sediments in the sag composed of shale, dolomite, and siltstone can be specifically divided into carbonate mudstone (CM), siliceous mudstone (SM), carbonate rock (CC), carbonate sandstone (CS), and siliceous sandstone (SS); (3) CM and SM are good source rocks for shale oil, and P 2 l 2 and P 2 l 5 are the “sweet sections” for shale oil; CC, CS, and SS are reservoirs of tight oil, and P 2 l 1 and P 2 l 4 are the “sweet sections” for tight oil. Finally, definitions are discussed and proposed for tight oil and shale oil.