Study on gas yields and generation kinetics of a type I kerogen sample by open and confined pyrolysis

Abstract

A shale sample collected from the Neoproterozoic Xiamaling Formation in Xiahuayuan region, North China, was studied by a newly developed apparatus, Multiple-Gas Container Open Pyrolysis Devise, and data were compared with those from confined gold tube pyrolysis. This device is mainly composed by a pyrolysis heater and 15 gas containers, a whole heating process was divided into 15 time/temperature segments, gases generated in different temperature segments with carrier gas were temporarily conserved in their gas container correspondingly. When pyrolysis completed, gases in each container were introduced to a gas chromatography for analysis. Sample was heated from room temperature to 650 °C. Because oil generated from sample was removed by carrier gas immediately from heating area, oil cracking did not play an important role in gas generation. Maximum yield of methane is only 28.4% of that from confined gold tube pyrolysis. Trend of methane yield from open method is similar to that from confined method in EASY%Ro range from 0.58% to 1.80%, indicating that oil cracking in this maturity range is minor, but at higher temperature, gas yields produced in confined pyrolysis increased rapidly, much higher than those from closed method. Activation energies of methane generation from open method ranges from 53 to 72 kcal/mol, central value of C1, C2, C3 decline with their carbon number, showing a clear generating order that the gas with larger carbon number will be generated earlier. δ 13C of methane from open method is similar to that from confined method in EASY%Ro range from 0.58% to 2.2%; But in later pyrolysis stage, it is much heavier than that from confined method. δ 13C deviation of methane between open and confined methods provides a possible way to calculate proportion of the gas from oil cracking. This open pyrolysis device offers a convenient and efficient way to simulate gas generation potential and acquire kinetics parameters, especially for the source rock from open or semi open gas producing strata.