Pyrolysis constraints on the generation mechanism of shale gas.

The three types of low mature organic-rich samples from US Green River shale (Type I kerogen), Woodford shale (Type II kerogen), and China Pearl River Mouth Basin( Type III kerogen) were chosen as the initial samples for pyrolysis simulation on hydrocarbon generation. A pyrolysis system of sealed gold tubes in high pressure vessels was employed to conduct pyrolysis experiments at two heating rates of 20 °C/h and 2 °C/h under a constant confining pressure of 24. 1 MPa. The yields of gaseous hydrocarbons (C1-5) and liquid hydrocarbons (C6-14 and C14+), and the carbon isotopic compositions of gaseous hydrocarbons (C1-C4) were measured to reveal the generation mechanism of shale gas. The results show that the organic matter conversion from organic-rich shales to oil and gas can be divided into three stages: oil generation, gas generation from oil secondary cracking and gaseous hydrocarbons cracking. At the stage of oil generation by the thermal decomposition of primary kerogen, the yields of liquid hydrocarbons increase rapidly with the increase of temperature, and reach the peak values at approximately 0. 9%- 1.1% Ro, whereas gaseous hydrocarbons show low yields. At the stage of oil secondary cracking, gaseous hydrocarbons increase rapidly in the yields associated with the decreases in the yield of liquid hydrocarbons. At the stage of gaseous hydrocarbons cracking, the gas yields increase slightly, whereas gas is getting drier and enriches in CH4. The carbon isotopes of gaseous hydrocarbons vary from heavier to lighter with the increase of temperature, and reach the lightest values before the end of oil cracking, and then become heavier. The partial reversals of carbon isotopic compositions of gaseous hydrocarbons at the stage of oil cracking is observed for Type I shale, i. e., δ13C1<δ13C2<δ13nC4. The results also suggest that shale gas is mainly derived from kerogen primary cracking and oil and gas secondary cracking, and shale gas content is constrained by the organic matter type and thermal maturity, and the Type I shale has the highest gas yield. The partial reversal distributions of carbon isotopic compositions of gaseous hydrocarbons in shale gas could be caused by oil secondary cracking, or the mixing of gaseous hydrocarbons from different stages of thermal maturation. [ABSTRACT FROM AUTHOR]