Lithofacies and sedimentary sequence of the lower Cambrian Niutitang shale in the upper Yangtze platform, South China

Abstract

The lower Cambrian Niutitang shale in the Upper Yangtze Platform is a hot target for shale gas exploration in China. In this paper, the lithofacies and sedimentary evolution characteristics about this shale formation are studied on the basis of outcrop investigation and experimental measurements. The variations on geochemistry, mineralogy, lithofacies and stratigraphic architecture of this shale are analyzed. This shale is rich in organic matter, with an average TOC content of 5.5%. Quartz and clay mineral dominate the mineral composition, with an average content of 57.3% and 29.8%, respectively. The TOC content correlates positively to quartz content, but negatively to clay mineral content. Trace element ratios of V/Cr, V/(V + Ni) and Ni/Co indicate anoxic to dysoxic conditions prevailed during the Niutitang shale deposition period in Early Cambrian, and moderate to weak degree of restriction are inferred by the covariation of Mo and TOC content. Five lithofacies are identified based on TOC content, mineral composition, texture and fabric: organic-rich siliceous shale, silty siliceous shale, calcareous siliceous shale, argillaceous shale, and silty mixed shale. Four lithofacies associations are interpreted to represent four different depositional facies from shelf margin to abyssal environments. The Niutitang Formation generally shows a shallowing-upward prograding depositional sequence and can be divided into two large shale cycles. From bottom to top, the organic matter content and anoxic degree decreases largely with the sea level fall. The average content of quartz deceases, but the average content of clay mineral increases. Stratigraphic correlation for this shale reveals an obviously decreasing trend of quartz content, but an increasing trend of organic matter and clay mineral contents from slope to basinal facies.