Empirical relationship between gas composition and thermal maturity in Eagle Ford Shale, south Texas

Organic-rich and carbonate-rich Eagle Ford Shale is a self-sourced oil and gas reservoir with little alteration of gas chemistry as might be affected by petroleum expulsion and migration. As such it provides an ideal natural laboratory to quantify the compositional variation of gases generated from oil-prone type II kerogen during thermal maturation. The chemical composition of the gas released from rock crushing was conducted and integrated with Rock-Eval pyrolysis to define the empirical relationship between gas compositional parameters and thermal maturity in this study. From 10 wells in the Eagle Ford Shale in south Texas, we collected 74 core samples having a range of thermal maturity (the measured maximum temperature [T-max] values of hydrocarbons generated in Rock-Eval pyrolysis range from 427 degrees C to 494 degrees C [800 degrees F to 921 degrees F], and the calculated equivalent vitrinite reflectance (Roe) values range from 0.51% to 1.73% based on Tmax values). Total organic carbon content ranges from 0.3% to 8.53%, with an average of 3.12% (standard deviation of 1.77%). Burial depth is from 2989.6 to 13,827.3 ft (911.2 to 4214.6 m). Our results showed that gas composition in the Eagle Ford Shale is mainly controlled by thermal maturity, and three stages of gas generation are identified based on the C-1 and C-2 concentrations of the gases released by rock crushing from Eagle Ford Shale core samples. The three stages of gas generation correspond to the following processes of organic matter conversion: (1) kerogen and bitumen thermal cracking to crude oil, (2) bitumen and heavy crude oil thermal cracking to light oil, and (3) light oil cracking to gas. Methane-rich gas and an abundance of branched butane and pentane are generated in light oil cracking to gas, resulting in high C-1/C-2, C-1/(C-2+ C-3), i-C-4/n-C-4, and i-C-5/n-C-5 ratios. Increased cracking of normal alkanes such as n-butane and n-pentane occurs in the light oil cracking to gas. Empirical equations between gas compositional parameters and thermal maturity (T-max or Roe) are obtained for oil-prone type II. TheC(1), C-2, C-1/C-2, C-1/C-2 + C-3, and i-C-4/n-C-4 ratios are the five best parameters for determining thermal maturity with an exponentially derived R-2 value of 0.74. The composition of gas produced from the Eagle Ford Shale following hydraulic fracturing is used to validate the empirical equations. Calculated thermal regime for the oil production based on the produced gas is located at the peak of oil generation and the beginning of light oil cracking to gas, corresponding to Tmax from 454 degrees C to 464 degrees C (849 degrees F to 867 degrees F) or at an Roe ranging from 1.01% to 1.19%. Empirical equations provide a basis for interpretation of mud gas logging data and produced gas composition.