Organic biofacies of Jurassic source rocks in northern Qaidam Basin, NW China

Organic petrological microscopic observation indicates that algae, cutinites, and sporophytes are the main organic macerals of Jurassic mudstone source rocks in the northern Qaidam Basin. According to the occurrence and abundance of these three types of macerals in rocks as well as their relative contents, the hydrocarbon-generating bio-facies are divided into four types: A (algae-bed), B (dense transport-deposit algae), C (sparse transport-deposit algae), and D (algae-bits). From type-A to type-D rocks, the organic matter abundance gradually decreases, and the quality becomes poor. The type-A source rock has the best generative potential, with TOC higher than 4.0% and organic matter types as II1-II2. It occurs only in parts (with ca. several tens of meters) of Lower Jurassic and the Seventh Member of Middle Jurassic strata, implying the heterogeneity of source rocks. Lagoon environments of lake-deltaic sedimentary systems are good for its deposition.