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

•Gases released by step wise ball-mill crushing of Barnett Shale were analyzed.•Both thermal maturity and gas desorption affects gas chemistry of rocks.•CH4 dominated free gas is mainly stored in larger, connected pores.•Gases released from>250μm particle are the most representative of produced gas.•Isotopic fractionation in gas released from rocks is caused by gas transport.