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

摘要

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 %R_o. Our results show that both thermal maturity and gas desorption contributes to changes in the CH_4/CO_2 ratio of gases released during rock crushing. CH_4/CO_2 ratios of these gases are lower at low thermal maturities and higher at high thermal maturities because more CH_4 rich gas is generated at higher maturity levels. CH_4/CO_2 ratios decrease with longer rock crushing times because of the increase in the CO_2 rich gas contribution. However, no obvious compositional fractionation occurs among C_1, C_2 and C_3 of crushed-rock gas and C_1/C_2 and C_2/C_3 ratios remain nearly constant during crushing although these ratios are greatly increased overall when the level of thermal maturity is high. Gas geochemical parameters (C_1/ C_2, C_2/C_3, and i-C_4-C_4) of released gas are good indicators of thermal maturation of organic rich shales. The CH_4/CO_2 ratio is a function of selectivity, partition coefficients and (possibly but less likely) sorption kinetics between CH_4 and CO_2 molecules in shales. Trends in released gas yield and gas chemistry during rock crushing relate to gas storage states and pore connectivity. The δ~(13)C_1, δ~(13)C_2 and δ~(13)C_3 values of gas released from particles of coarser size (> 250 lm) are similar to values of gas produced from Barnett shales after hydraulic fracturing. CH_4 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.