Variable depositional environments and organic matter enrichment of Early Cambrian shales in the Middle Yangtze region, South China

Abstract

Organic-rich shales of the Lower Cambrian Shuijingtuo Formation (SJT) in the Middle Yangtze region (South China) represent an important target for shale gas exploration and development in China. The lower section of the SJT mainly consisting of organic-rich black shale deposits has been regarded to be a favorable target for shale gas exploration and received substantial attentions, while the upper section of the SJT primarily consisting of limestone, silty shale and black shale deposits has been rarely studied. Most recently, a major breakthrough of shale gas exploration and development has been made in the upper section shale deposits. In order to better understand the redox evolution of sedimentary environment throughout the whole Early Cambrian time and organic matter (OM) enrichment mechanism of the SJT shale deposits, geochemical investigations, including analyses of total organic carbon (TOC), as well as major and trace element compositions, were conducted on the upper section of the SJT shales from Well EYY1. Our results showed that the upper section of the SJT shale deposits was accumulated during the Cambrian Stage 3. The TOC values of the studied SJT shale samples ranged from 0.34 to 3.65 wt%. The lower part of the upper section of the SJT shale, which exhibited relatively high TOC contents (∼2.0 wt%), was deposited under anoxic bottom-water conditions during the early Cambrian Stage 3, while the upper part, showing relatively low TOC content (1.0 wt%), was deposited under suboxic bottom-water conditions with intermittent episodes of anoxic bottom-water conditions during the middle–late Cambrian Stage 3. The palaeo-productivity of the SJT shale was estimated to be generally low, and displayed little variation. The OM enrichment mechanism of the SJT shale deposits underwent dynamic changes due to relative sea-level changes in the Yangtze Ocean. The OM enrichment of the early and late Cambrian Stage 3 shale deposits was primarily controlled by reducing conditions, while that of the middle Cambrian Stage 3 shale deposits may have been influenced by episodic upwelling activities.