Organic Pore Structures and Origin in the Lower Paleozoic Marine Shales of Jiaoshiba Region of Sichuan Basin, China

Abstract

Shale gas plays have been found in the Lower Paleozoic Marine Shales of Jiaoshiba region in Sichuan Basin, China. The black marine shales from the Upper Ordovician Wufeng Formation and the lower part of Lower Silurian Longmaxi Formation are the gas reservoirs. The volume content of methane in the shale gas is more than 98%, and the stable carbon isotopic value is about −30‰ PDB. The thickness of high gas-bearing shale section is 38m including 6-m-thick Wufeng Formation shale and 32-m-thick Longmaxi Formation shale. For Wufeng Formation shale, mean total organic carbon (TOC) is 4.48% with a range of 4.01% to 5.89%. For Longmaxi Formation shale, mean TOC is 3.36% with a range of 1.97% to 5.51%. The organic matter type of the shale is typeI, and the confirmed organic matter maturities (Ro) are between 2.5% and 3.0%. The values of total porosities of helium pycnometry for Wufeng Formation and Longmaxi Formation shale range from 4.05% to 7.08% and 2.78% to 6.39%, with average values of 5.10% and 4.78%, respectively. The organic pores of 5 to 50 nanometers in diameter are rich in the shale samples from Wufeng Formation and Longmaxi Formation investigated using argon ion polishing-field emission scanning electron microscope. It has been found that there existed significant differences in the organic pore structure of the shale between Wufeng Formation and Longmaxi Formation by the statistical analysis of organic pore shape, pore size, pore-size distribution and organic plane porosity compared to organic matter surface. For the Wufeng Formation shale, the organic pores are mostly irregular in shape, perfectly connected, and the pore-sizes mainly distribute in 10–30nm. The organic plane porosity ranges between 5.0% and 20.0% with an average value of 9.4%, and the organic porosity averages about 1.0%. For the Longmaxi Formation shale, the shapes of organic pores are suborbicular and oval, the pore-sizes are larger and concentrate in 10–50nm, and the micron-size organic pores can be observed on the organic matter surface. The organic plane porosity ranges from 4.0% to 35.0% with an average value of 12.7%, and the organic porosity averages about 1.2%. The reason for the significant differences in organic pore structures between Wufeng Formation and Longmaxi Formation shales is studied and considered to be associated with some differences of microscopic distribution of residual liquid hydrocarbon, kerogen property, and mineral composition of the shales.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015