Characteristics of Pore Structures and Controlling Factors of the Lower Paleozoic Marine Shales in the Western Region of Middle Yangtze, Central China
The marine shales of the Lower Cambrian Formation and the Upper Ordovician to the Lower Silurian Formations in the western region of middle Yangtze of central China are shale gas potential intervals. Shale gas is storaged as both free gas and adsorbed gas in the macro fractures and nanometer-scale pores, and the micro pore structures in shales are complex, which are different from pores in the conventional reservoirs. This study is focused on the pore structures characteristics analyses and the discussions on the major factors controlling the pore structures forming in the Lower Cambrian Formation and the Upper Ordovician to the Lower Silurian Formations using the mercury injection capillary pressure and N2 adsorption methods to measure the pore structures. The results are summarized to four points: (1) The main inorganic mineral components of the shales are quarts and clay, and the total content of quarts plus feldspar ranges from 29% to 90.68%, the content of clay ranges from 9.25% to 69.54%, but little calcite and pyrite content; (2) The porosities of the marine shales range from 0.49% to 5.4%, and the total organic matter contents (TOC) range from 1.1% to 5.5%, with the vitrinite reflectance (Ro) varying 2.5% to 4.0%; (3) The pore structures in the study area can be divided into two types: macro fractures and nanopores. The macro fractures are mainly micron-scale micro pores and micro fractures. The nanopores are mainly micropores (<2nm) and mesopores (2~50nm), and the specific surface area of the nanopores ranges from 9.67 m2/g to 34.54 m2/g, suggesting that micropores and mesopores mostly contribute to the specific surface area and pore volume; (4) TOC are the main internal factors controlling the specific surface area and pore volume. And the specific surface has a positive correlation with inorganic minerals, quarts content, but a negative correlation with clay content. It can be inferred that the organic pores in the Lower Paleozoic marine shales of the western area of middle Yangtze gradually increase as the degree of organic matter maturity of the marine shales enhances, because the specific surface area and pore volume of the marine shales increases with thermal maturity.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013