Abstract
The organic geochemistry and organic carbon structures of gas-producing shale reservoirs were investigated from the Silurian Longmaxi Formation of the Weiyuan Block, Sichuan Basin, by the total organic carbon (TOC) measurement, Rock-Eval pyrolysis, and scanning electron microscopy/transmission electron microscopy (SEM/TEM) image analyses. The purpose of this article is to provide awareness of the nature and complexity in organic carbon structures within overmature shales and to provide an organic geochemical evaluation on the gas-producing reservoirs. Present-day TOC values range from 0.07 to 8.2 wt % (2.17 wt % on average), which primarily reside in type IV kerogen (inert solid bitumen and porous pyrobitumen). From the Rock-Eval pyrolysis data, the shales are found to be overmatured, with an average calculated R (o) value of 2.71%. Most samples have low-pyrolysis S (2) peaks and present-day hydrogen index (HI), with an average of 0.19 mg of hydrocarbons (HC)/g of rock and 20.47 mg of HC/g of TOC, respectively, indicating little present-day hydrocarbon generative potential from the remaining organic matter. SEM and TEM high-resolution (∼0.5 nm) image analysis not only provides a visual appreciation of the shale organic carbon structure but also can provide a semiquantitative method to calculate the carbon layer spacing and size of organic nanopores and "onion-like'' concentric organic particles. SEM and TEM images highlight complex organic carbon structures in overmature shales at the micro- and nanoscale. A large number of turbostratic carbon nanostructures result from hydrocarbon generation and are considered to be the end products of the thermal evolution of organic matter. Confirming their origin and abundance provides a key geological indicator for assessing thermal maturity evolution and identifying promising overmature shale gas exploration targets in analogous basins.