Mechanism of Extraordinarily High Organic Matter Enrichment in High‐Quality Lacustrine Source Rocks: A Case Study of Chang 7 Member in Northern Ordos Basin

ABSTRACT Black shale with extraordinarily high organic matter (EHOM) content is prevalent in the Chang 7 Member of the Middle Triassic Yanchang Formation, Ordos Basin. This interval shows significant potential for hydrocarbon generation; however, mechanisms for EHOM accumulation of these black shales remain highly debated. In order to reconstruct the depositional environment of the basin and investigate the mechanisms behind the EHOM enrichment, a comprehensive analysis encompassing elemental carbon and sulfur, major elements, mineral and microbiological fossils, and biomarkers was conducted on 45 shale samples of the Chang 7 Member from a well located in the north slope break belt of the basin. The results indicate that EHOM of these shales has evolved into the mature stage, with a notable potential for hydrocarbon generation. Their OM has mixed‐source origins, primarily derived from algal matter, with a significant contribution from higher plants. During the deposition of the Chang 7 Member, the prevailing climate was warm and humid, with moderate weathering intensity and relatively low paleosalinity, fresh‐brackish water depositional environment. In semi‐deep to deep lake environments, the organic matter enrichment in the Chang 7 3 sub‐member shale follows a medium‐high palaeoproductivity and anoxic preservation model. During the Chang 7 2 depositional period, water depth decrease leads to the deterioration of preservation conditions, resulting in a significant reduction in the abundance of organic matter. During the deposition of the Chang 7 1 period, the increase in organic matter abundance is mainly attributed to the decrease of terrestrial debris dilution and medium‐high paleoproductivity. The enrichment of EHOM is related to volcanic activity, in conjunction with a warm and humid climate, low salinity level, minor terrigenous detritus input, anoxic conditions, algal blooms, and an elevated paleoproductivity sedimentary environment.