Mineralogical and petrographic characteristics of the Ordovician-Silurian Wufeng-Longmaxi Shale in the Sichuan Basin and implications for depositional conditions and diagenesis of black shales

Abstract

Depositional and diagenetic processes play important roles in controlling the fabric and petrophysical properties of black shales. In this study, an outcrop section of the Wufeng-Longmaxi Shale was studied through sedimentologic facies analysis, petrographic observations (both transmitted- and reflected-light microscopy and scanning electron microscopy), and X-ray diffraction analysis to investigate how depositional and diagenetic processes controlled the accumulation and evolution of organic-rich black shales. The studied Wufeng-Longmaxi Shale consists of over-mature black shales, with an equivalent vitrinite reflectance of 3.07%. Organic matter is dominated by pyrobitumen and graptolite, the former of which hosts abundant organic pores. Minerals mainly consist of quartz (average 42 wt %), carbonates (average 37.4 wt %), and clay minerals (average 15 wt %). Eighty percent of quartz is of biogenic origin, which occurs as siliceous fossils and microcrystalline quartz. Microcrystalline quartz forms a rigid framework and preserves organic pores from compaction. Clay minerals comprise dominantly (95 wt %) illite and mixed-layer illite/smectite. The vertical variations in lithofacies and quartz and carbonate mineral content exhibit three cycles, likely corresponding to three transgressive-regressive cycles. Within each cycle, quartz content first increases to reach a maximum at the maximum flooding surface, and then decreases. Although long being considered as deposited under quiescent and anoxic conditions, sedimentologic and petrographic analyses in this study indicate that the deposition of the Wufeng Formation and lower Longmaxi Formation was subject to intermittent reworking by bottom currents and bottom-water dysoxia. Meiofauna capable of tolerating low oxygen content could have lived in surficial sediments and disrupted the original fabric of sediments formed by bottom currents. Mineralogical and petrographic analyses in this study provide new insights into the accumulation mechanism of the organic-rich black shales of the Wufeng-Longmaxi Formations, calling for a critical reappraisal of other black shales that have conventionally been considered as deposited under mostly quiet and anoxic conditions. • Mineralogical and petrographic characteristics were studied. • Three stratigraphic cycles were identified. • Depositional conditions were not persistently quiescent and anoxic. • Intermittent reworking by bottom currents and bottom-water dysoxia existed.