LITHOFACIES AND ITS CONTROLS ON THE ORGANIC MATTER-HOSTED PORES IN THE CAMBRIAN GAS-RICH NIUTITANG SHALE IN UPPER YANGTZ PLATE, CHINA

The organic-rich shale in the Lower Cambrian Niutitang Formation，southern China has great gas resources potential, but no commercial gas production has achieved yet. The EYY1 well is the first well located at the eastern edge of the upper Yangtze plate which obtains success in commercial gas production of the Niutitang Formation , providing a rare case of examining gas enrichment mechanism of Niutitang shale. Using core analysis, thin sections observation and X-ray diffraction analysis of EYY1 samples, the Niutitang shale is classified into three organic-rich lithofacies: calcareous shale (CS), siliceous shale (SS) and transitional shale (TS). The characteristics of organic matter (OM) and OM-hosted pore in different lithofacies were examined by scanning electron microscope (SEM) with argon ion polishing sample treating, accompanied by energy spectrum test and SEM image analysis. The results show that: (1) Shale lithofacies control the organic matter type and development degree of organic matter-hosted pores (OMPs). (2) CS contains plenty of depositional organic matter (DOM), among which nano-micron sized OMPs are poorly developed and are easily subject to compact, with its surface porosity ranges from 3.05% to 9.08%, and an average of 4.88%. Siliceous and transitional shale contain more migrated organic matter (MOM), which possess more sponge-like nano-sized pores with surface porosities range of 10.7%~31.5%, averagely 14.46%. The pore development and gas content of siliceous shale are higher than transitional and calcareous shale. (3) Organic matter abundance (TOC), type, and mineral composition also has important constrains on OMPs development. TOC 4 wt% might be the balance threshold of high pore pressure support against strata compaction. The OMPs associated with or adjacent to clay minerals in siliceous shale are more developed than those associated with or adjacent to other minerals, which attributes to clay minerals have positive effects on the adsorption of organic matter and the formation and preservation of OMPs.