Organic matter origin and accumulation in tuffaceous shale of the lower Permian Lucaogou Formation, Jimsar Sag

The Jimsar Sag is an important shale oil exploration target area in the Junggar Basin, northwestern China. The lower Permian Lucaogou Formation, with a thickness of 200–300 m, is composed of thin tuffite, tuffaceous dolomite, mudstone and siltstone, and is the primary exploration target. A comprehensive analysis is conducted on the bulk and molecular geochemical characteristics of the tuffaceous shale, to define the organic matter origin, depositional environment and organic matter accumulation. The key features of the tuffaceous shale extracts are short-to long-chain n-alkanes, a range of Pr/Ph ratio values (0.80–1.42), a relatively high content of C29 steranes and a moderate content of gammacerane, showing that the redox state ranged from oxic to anoxic and that the water salinity conditions varied from fresh to brackish. And the high content of cyanobacteria is the primary cause of the high abundance of β-carotane and C29 sterane. It appears that volcanic ash did not change the redox condition of depositional environment. Considering that volcanic ash-bearing sediments usually have relatively high β-carotane and C29 sterane content, and previous studies have pointed out that volcanic ash can stimulate phytoplankton and promote productivity in modern sediments, we could speculated volcanic ash may release mineral ions such as iron, and then stimulated cyanobacteria, which was the main cause of organic matter accumulation in the tuffaceous shale of the Jimsar Sag.