Research paperComposition and pore characteristics of black shales from the Ediacaran Lantian Formation in the Yangtze Block, South China

- Composition and pore characteristics of Ediacaran Lantian shales were investigated.
- Lantian shales are highly over-mature with high TOC content up to 12%.
- They are dominated by quartz, notable for abundant pyrite, highly varied carbonates.
- Bulk porosity and micro-mesopore volume show good, positive correlation with TOC.
- Relationships between minerals and porosity may also be related to the OM content.

The Ediacaran period was critical in the evolution of the biosphere and ocean in the history of the Earth. Shales rich in organic matter (OM) are well developed for this period in the Yangtze Block, China, and have recently been related to shale gas exploration in South China. To date, detailed characterisation of the Ediacaran shales is not available. The present work sets out a detailed investigation of the composition and pore characteristics of samples from a shallow drill core in the Ediacaran Lantian Formation in the Lower Yangtze area, and the effects of shale composition on the pore growth are discussed.The 90-m-thick Lantian black shales are highly over-mature (equivalent vitrinite reflectance (EVRo) value approximately 4.0%) with a total organic carbon (TOC) content up to 12%. They are dominated by quartz (33-72%), generally with low clay or feldspar content (<20%) and highly varied carbonate content (up to 60%). The Lantian shales are also notable for their high pyrite content (up to 19%), suggesting a depositional environment of an anoxic water body rich in sulphates; greatly enriched OM is facilitated in deep-water environments.Bulk porosities of the shales range from 1.0% to 7.9%, and are positively correlated with TOC content. High-magnification scanning electron microscopy (SEM) images support the crucial importance of OM content on pore development. Generally, pore volume shows excellent, positive correlation with TOC content for micropores, good correlation for mesopores, and relatively poor correlation for macropores. Two samples with mostly high TOC content showed very low macropore volume and medium mesopore volume. These results suggest a close relationship between OM with relatively small pores, and enhanced compaction effect on large pores in highly OM-enriched shales.