Pore Structure of Cambrian Shales from the Sichuan Basin in China: Implications to Gas Storage and Migration Pathways
Feng Yang and Zhengfu Ning
China University of Petroleum, Beijing, China
The microstructure of black siliceous shale from the lower Cambrian Niutitang Formation, Sichuan Basin in China was investigated by the combination of field emission scanning electron microscope (FE-SEM) and argon ion beam milling. The nanometer- to micrometer-scaled pore systems of shales are an important control on gas storage and fluid migration. In this paper, the organic porosity in shale samples within oil and gas window has been investigated, and the formation mechanism and diagenetic evolution of nanopores have been researched.
FE-SEM reveals five pore types that are classified as follows: organic nanopores, pores in clay minerals, nanopores of framework minerals, intragranular pores in microfossils, and microfractures. Numerous organic nanopores are observed in shales in the gas window, whereas microfractures can be seen within the organic matter of shales in oil window. Microfractures in oil window shales could be attributed to pressure buildup in the organic matter when incompressible liquid hydrocarbons are generated. Pores in clay minerals are always associated with the framework of clay flakes, and develop around rigid mineral grains because the pressure shadows of mineral grains protect pores from collapse. The smectite-illite transformation contributes significantly to the formation to secondary pores. Nanopores of rock framework are probably related to dissolution by acidic fluids from hydrocarbon generation. While various pore types provide a storage space, microfractures could provide space as well as form permeability pathways for gas migration.
AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013