Chemical and isotopic composition of gases released by crush methods from organic rich mudrocks

Abstract

We report on the composition of mudrock gases released under vacuum by ball-mill rock crushing and pressure induced fracturing. Nine core samples from organic rich Barnett Shale were used in this study. TOC content varies from 3.3–7.9%; thermal maturity varies from 0.58–2.07%Ro.Our results show that both thermal maturity and gas desorption contributes to changes in the CH4/CO2 ratio of gases released during rock crushing. CH4/CO2 ratios of these gases are lower at low thermal maturities and higher at high thermal maturities because more CH4 rich gas is generated at higher maturity levels. CH4/CO2 ratios decrease with longer rock crushing times because of the increase in the CO2 rich gas contribution. However, no obvious compositional fractionation occurs among C1, C2 and C3 of crushed-rock gas and C1/C2 and C2/C3 ratios remain nearly constant during crushing although these ratios are greatly increased overall when the level of thermal maturity is high. Gas geochemical parameters (C1/C2, C2/C3, and i-C4/n-C4) of released gas are good indicators of thermal maturation of organic rich shales. The CH4/CO2 ratio is a function of selectivity, partition coefficients and (possibly but less likely) sorption kinetics between CH4 and CO2 molecules in shales.Trends in released gas yield and gas chemistry during rock crushing relate to gas storage states and pore connectivity. The δ13C1, δ13C2 and δ13C3 values of gas released from particles of coarser size (> 250μm) are similar to values of gas produced from Barnett shales after hydraulic fracturing. CH4 dominated gas appears to be stored in larger connected pores and is therefore released during the initial stages of crushing. The carbon isotope values of methane, ethane and propane are heavier in the more thermally mature samples, suggesting that this released gas is representative of the gas chemistry of subsurface rocks. Retrieval of gas chemistry data from existing core samples provides information of great relevance for understanding deep shale gas reservoirs.