Geochemistry of oils and condensates from the lower Eagle Ford Formation, south Texas. Part 5: Light hydrocarbons

Abstract

There is a rich history of using light hydrocarbon distributions to characterize source and thermal maturity based on observations made on source rocks and petroleum fluids produced from conventional reservoirs. In this study, a suite of lower Eagle Ford oils and condensates was used to determine the accuracy and reliability of established light hydrocarbon parameters in an unconventional petroleum system. These samples have well defined organic source facies, generative temperatures, and extent of thermal cracking. They are largely free of the influence of long-distance migration, multiple charging events, reservoir alteration processes that commonly occur in conventional accumulations and free of contributions from other reservoirs along faults induced by fracturing.All established light hydrocarbon thermal maturity parameters are influenced to varying degrees by source facies. Ratios of i-C4/n-C4 and quaternary/tertiary dimethylpentanes linearly correlate with %Ro (R2 > 0.8) with negligible sensitivity to differences in source facies. The Isoheptane and C6–C7 branched/(branched + cyclic) hydrocarbon ratios correlate with %Ro (R2 > 0.8) but also are influenced by source facies. Some established C7 parameters are affected by thermal cracking: i-C5/n-C5 and toluene/n-C7 are the most sensitive ratios to the onset of intense hydrocarbon cracking (∼1.35 %Ro) and do not appear to be significantly influenced by source facies, C6–C7 branched/(branched + normal) hydrocarbons and methylcyclohexane/n-C7 both increase in response to cracking conditions but are significantly influenced by source facies, which is likely related to the presence of reactive clays. A revised formula for Ctemp is proposed: Ctemp = 161.9 + 29.4•(ln[2,4-DMP/2,3-DMP]). The revised Ctemp formula is applicable for oils/condensates with 2,4-DMP/2,3-DMP >0.25, expanding the practical limit from ∼120 to ∼160 °C, and yields Ctemp values consistent with the maximum exposure temperature with an accuracy of ∼ ±5 °C for most samples, excluding those that have been altered by evaporation. The Mango (1997) Ctemp formula should be used for lower maturity oils having DMP/2,3-DMP values < 0.25. Sample integrity is critical as many of the light hydrocarbon parameters are significantly altered by evaporative loss.