Abstract

The aim of the work is to determine the volume balance of gas generated in shale gas systems in both the oil and the gas windows. For this purpose, immature as well as mature source rocks were selected in order to carry out experimental simulation of kerogen cracking in a closed pyrolysis system. First, the compositional kinetic model previously calibrated on three kerogens (Behar et al., 2008a, 2010) was applied to immature Barnett and Posidonia shale kerogens, respectively Mississippian and Lower Jurassic in age. Results show that gas generated in the oil window under geological conditions is mainly wet gas with a maximum yield of gases of 30 mg/g C for Barnett and 25 mg/g C for Posidonia kerogens. Subsequently, the mature kerogens were artificially matured at 375°C/24 h in order to mature the kerogens to the beginning of the gas window and remove all generated products by solvent extraction. The goal is to create postoil generative kerogen in order to measure generated products from mature kerogen only. These oil-free mature kerogens were heated to higher temperatures for evaluation of late gas generation from postoil kerogen. Results show that the late gas is almost 100% dry, and its maximum yield is 80 mg/g C for the Barnett and 75 mg/g C for the Posidonia kerogens. These values do not correspond to the maximum kerogen late gas potential because in laboratory conditions gas production does not reach a plateau even at the highest thermal severity (550°C/24 h). In terms of kinetic modeling, the distribution of the activation energies for late gas generation from kerogen is between 55 and 62 kcal/mol with a frequency factor at 10¹² s⁻¹. When these data are extrapolated to geological conditions, most of the late gas is generated in the vitrinite reflectance (R_o) range between 1.5% and 3.0%. A tentative simulation of gas generation from secondary cracking of the retained compounds after the main phase of primary expulsion demonstrates that, although between 1.5% and 2% of wet gas is produced, the overall gas dryness in the R_o range 1.5-3.0% is between 85 and 90 molar %. A tentative gas balance (scf/ton of rock) under geological conditions was calculated for an initial source rock of 2% TOC. It predicts that 20 scf/ton of wet gas are generated from residual oil cracking and gas present after the main phase of oil expulsion, whereas 60 scf/ton of rock of dry gas are produced from the thermal evolution of the residual insoluble organic matter.