Gas evolution during kerogen pyrolysis of Estonian Kukersite shale in confined gold tube system

Pyrolysis of Kukersite kerogen was conducted in gold capsules, with the yield and stable carbon isotopic (delta C-13) values of selected gas components (C-1, C-2, C-3, i-C-4, n-C-4, i-C-5, n-C-5, CO2) and liquid hydrocarbons (C-6-C-14) separately measured to investigate the primary versus secondary mechanisms of gas hydrocarbon generation from overmature source rocks. With increasing pyrolysis temperature over the range 336-600 degrees C (and especially >430 degrees C) the progressive cracking of hydrocarbons led to increasing yields of low molecular weight gases, particularly CH4 and CO2. The increase determined for each of the C-1-C-5 hydrocarbons was in the order C-5 > C-4 > C-3 > C-2 > C-1 below 408 degrees C, but showed the inverse order of C-1 > C-2 > C-3 > C-4 > C-5 at > 420 degrees C. The yields (well reflected by traditional lnC(2)/C-3 versus lnC(1)/C-2 relationships) and stable isotopic profiles (e.g., delta C-13(2)-delta C-13(3) versus lnC(2)/C-3 plots) showed four distinct stages to the thermal evolution of the gas hydrocarbons: (1) During the first stage (final temperatures of 336-360 degrees C and with heating rate of 2 degrees C/min) kerogen cracked mostly into C3+, with just a small amount of C-2 and minimal C-1; (2) the second stage (360-408 degrees C) showed an increased production of lower molecular weight gases, particularly methane but also ethane and propane and the consistency of corresponding delta C-13(2) and delta C-13(3) values suggests they were produced in similar abundances; (3) the third stage (432-528 degrees C) was attributed to oil cracking as there were significant increases in the yields of both ethane and methane (cf. propane) and greater differences between delta C-13(2) and delta C-13(3); (4) a continued increase in methane during the fourth stage (552-600 degrees C) was attributed to cracking of C-2, since no C3+ precursors survived to these pyrolysis temperatures. Methane (304 mg/g OC) was detected in much higher abundance than all other gases including CO2 at the final pyrolysis temperature of 600 degrees C, with initial kerogen cracking, secondary oil cracking and even the cracking of C-2-C-3 gases all contributing to its production. (C) 2013 Elsevier Ltd. All rights reserved.