Geochemical characteristics and depositional conditions of the Lower-Middle Jurassic source rocks in the Southern margin of the Junggar Basin, NW China

Abstract

The Lower-Middle Jurassic source rocks are identified as the primary contributors along the southern margin of the Junggar Basin. To elucidate their geochemical characteristics, a comprehensive and unprecedented collection of deep Basin (>4500 m) source rock samples was undertaken in the region. A systematic analysis of 124 samples was performed, including studies on organic carbon, Rock-Eval, and organic petrology. Additionally, Gas Chromatography-Mass Spectrometry (GC-MS) analysis was conducted on 21 selected samples. The findings reveal that the Lower-Middle Jurassic source rocks were deposited in a weakly oxidizing to reducing freshwater environment, predominantly comprising liptinite, and vitrinite organic macerals, with a high abundance of Types III and II organic matter. Maturity assessments indicate a varied reservoir formation potential along the southern margin of the Junggar Basin, with the western part generally exhibiting lower maturity levels compared to the central and eastern regions. This suggests that condensate gas reservoirs are more likely to form in the central and eastern areas, while the western part is more favorable for oil reservoir formation. These results underscore the “source control” characteristics of the southern margin of the Junggar Basin.