Characteristics, Genesis, and Hydrocarbon Aspects of Lacustrine Siderite-Rich Marl (Early Jurassic): A Case Study in the Dachanggou Basin, Xinjiang, Northwest China

Abstract

Thinly bedded marls are widely developed in the oil shale-bearing series of the Lower Jurassic Badaowan Formation in the Dachanggou Basin (Northwest China). In this study, the characteristics, genesis, and hydrocarbon prospects of marls in a lacustrine organic-rich shale series were analyzed via X-ray diffractometry, element geochemistry, carbon and oxygen isotope analysis, total organic carbon content analysis, rock pyrolysis, and sedimentary analysis. The results show that the main carbonate component of marl is siderite, which is mainly present in areas with lithologic changes at the top and bottom of organic shale, and that the development of a thick siderite-rich marl is closely related to the presence of the thick oil shale. The elemental indicators of the sedimentary environment show that the low initial organic matter abundance and low clastic input were conducive to the formation of siderite and that the water body in the marl development period provided an oxic–suboxic environment, although the formation of siderite was still disturbed by oxygen in the relatively reduced bottom sediments. The carbonate carbon isotopes in the marl are clearly positive, indicating that δ13C-rich carbonate provided by methanogens decomposing sedimentary organic matter contributed to the siderite formation. These siderite-rich marls formed in a relatively restricted environment; i.e., they were deposited close to a rich organic layer, with a sufficient methanogenic metabolite CO2 content and a small amount of terrigenous clastic input. These factors reached an appropriate balance, thus forming a narrow zone (partial shallow lake and delta front facies). Although the hydrocarbon generation potential of the marl is low relative to that of other source rocks, the thin marl that is widespread in continental shallow-water organic-rich sedimentary systems can improve the fracturing ability of shale reservoirs, which is of far-reaching significance for the development of shale oil and gas in continental shallow-water sedimentary systems.