Influence of volcanic activities on redox chemistry changes linked to the enhancement of the ancient Sinian source rocks in the Yangtze craton

Abstract

Volcanic eruptions and related submarine hydrothermal activities were very extensive during the Precambrian periods in the Yangtze craton in southern China. How these geological events affected redox chemistry changes linked to the enhancement of mudstone/shale hydrocarbon source rocks in ancient strata is an important concern for oil and gas exploration in the Precambrian strata. Samples were collected from two Sinian (Ediacaran) outcrops in the northern Sichuan Basin, the Zhaiziyan and Yanhe outcrops, which revealed Doushantuo Formation (Z1ds) shales approximately 30.5 m in thickness, and shales in the third member of the Deying Formation (Z2dn3) approximately 30.0 m in thickness. TOC contents of the black carbonaceous shales in the Doushantuo Formation of the Zhaiziyan outcrop are mostly greater than 5.0% (up to 12.9%). Positive correlations were observed between TOC contents and trace and rare earth element concentrations in the shales: samples with high concentrations of trace elements such as V, Ni, Cu, Zr, Rb, and Pb and total rare earth element (∑REE) concentrations had relatively high TOC contents. The shales are rich in volcanic ash interlayers and thin siliceous layers and have positive Eu anomalies, indicating that volcanic activities and related hydrothermal activities significantly impacted the development of black shale source rocks. In the rifting background of the Sinian cratonal marginal, extensive volcanic activities brought rich nutrients to the seawater, which may have promoted the breeding of cyanobacteria and multicellular algae. At the same time, the volcanic activities released large amounts of H2S and SO2, which could cause the seawater to become a euxinic environment in which the bottom seawater was sulfurized and anoxic. Massive amounts of organic matters formed in the surface seawater entered the anoxic bottom seawater and were well preserved in the shale sediments. Therefore, under the influence of volcanic activities, ancient Sinian organic-rich high-quality source rocks were widely developed and had a large hydrocarbon supply potential for the Sinian dolomite reservoirs.