Organic geochemistry and controlling factors of alkaline source rocks within sequence stratigraphic framework in the Fengcheng Formation, Mahu sag, Junggar Basin, NW China

Abstract

The Fengcheng Formation (P1 f) source rock, which was deposited in an alkaline water body, is one of the most significant source rocks in the Mahu sag, Junggar Basin. However, its evolution and mechanism of organic matter accumulation are still not understood. Using seismic, core, and log data, P1 f is divided into 3 third-order sequences, i.e., SQ1, SQ2, and SQ3, from bottom to top. Pyrolysis data of core samples shows that SQ1 and SQ3 mainly are characterized by poor to fair source rocks, with Type III to Type II2 kerogen, while SQ2 mainly are characterized by fair to good to excellent source rocks, with Type II2 to Type I kerogen. The biomarker parameters of 40 source rock samples from different sequences suggest that the reducibility of sedimentary water is characterized by SQ2 > SQ1 > SQ3. The organic matter contribution in different sequences is obviously different. SQ1 organic matter contain more bacteria and algae than that of SQ2 and SQ3, and terrigenous higher plants may have contributed to SQ3 organic matter. Lake level changes and water salinity dominate organic matter origin, accumulation, and preservation. During SQ1 deposition, the lake level was the highest, the salinity of sedimentary water was moderate, and the organic matter contained more conventional algae sources. During SQ2 deposition, the lake level was the lowest and the salinity was the highest, and the organic matter contained special algae characterized by C29 regular steroids. The lake level of SQ3 was higher than that of SQ2, the salinity was the lowest, and terrigenous higher plants may have contributed to organic matter during this period.