Main controlling factors of organic-matter enrichment in the Ordovician-Silurian marine organic-rich mudrock in the Yangtze Block, South China

The Ordovician-Silurian organic-rich black shales located in the Wufeng-Longmaxi Formation in the southeastern part of the Upper Yangtze region are among the most important source rocks for shale gas in China. Given the importance of these sources, this study was initiated to determine the dominant factors controlling organic-matter enrichment in this region through the integration of sedimentology, petrology and geochemistry. Based on the sedimentary fabric features observed in thin section, six major mudrock lithofacies were identified in the Wufeng-Guanyinqiao-Longmaxi Formations black shale: (1) indistinctly laminated siltstone; (2) indistinctly laminated mudrock; (3) parallel-laminated mudrock; (4) nonparallel-laminated mudrock; (5) lenticular mudrock; and (6) normal-graded mudrock. Based on changes in the characteristics of lithofacies, trace-element ratios and the sizes of pyrite framboids, six kinds of mudrock facies were identified in the three formations. First, the indistinctly-laminated mudrock of the Wufeng Formation and the lower part of the Lower Longmaxi Formation that was deposited in a continuous suspension condition with pervasively anoxic and quiet water columns; second, the parallel-laminated mudrock of the upper part of the Lower Longmaxi Formation that was deposited from episodic suspension under an anoxic water column with weak fluctuation; third, the nonparallel-laminated mudrock of the lower part of the Upper Longmaxi Formation that was deposited in debris flow conditions with oxygen-rich and fluctuating water columns (graded mudrock deposited through event sedimentation in a suspension environment with a low-density, oxygen-rich bottom and turbidity currents also occurred in this stage); and finally, the indistinctly-laminated siltstones of the Guanyinqiao Formation and the upper part of the Upper Longmaxi Formation that were deposited in debris flow conditions with oxic and fluctuating water columns. The study then incorporated major- and trace-element analysis, global and regional sea-level fluctuations and regional stratigraphic characteristics to develop the depositional model. The results of the study indicate that the accumulation of organic matter in the middle region of the Sichuan Basin was predominantly controlled by regional tectonic activity.