--> Thermal Maturity Assessment for Source Rock and Producible Fluid in the Eagle Ford Shale

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Thermal Maturity Assessment for Source Rock and Producible Fluid in the Eagle Ford Shale

Abstract

To better understand variability of original kerogen quality, source rock thermal maturation, and key controls on producible fluid quality, we have carried out organic geochemical analyses on hundreds of source rock extracts and three produced crude oil samples from the Upper Cretaceous Eagle Ford Formation, southwest Texas. The studied samples were taken from eight wells located in several counties where the organic matter of the Eagle Ford reached thermal maturities levels ranging from the early oil generation to wet gas window stage (equivalent vitrinite reflectance from 0.4 % to 1.3 %). SARA data show that saturate fraction in residual and produced oil increases with thermal maturity. This shows that the thermal maturity of the source rock is the main control to the oil mobility. Defining the spatial distribution of thermally mature shale is crucial for estimation of productive areas in unconventional hydrocarbon resource assessment. Therefore, multiple maturity parameters (Tmax from Rock-Eval pyrolysis, gas chromatographic spectra of rock extracts and produced oils) were evaluated in this study. Those geochemical thermal maturity parameters were further compared with the bitumen reflectance values measured by previous Eagle Ford shale researchers. This systematic work refined spatial and regional variations in thermal maturity and suggested that the accuracy of Tmax could be interfered by indigenous bitumen in immature rocks and mineral matrix in high-mature rocks (equivalent vitrinite reflectance >1.2%). Bitumen reflectance values from Eagle Ford rocks underestimate thermal maturity in early mature areas. Sterane and extended hopane biomarker ratios from rock extracts and produced oil are enhanced by sulfurisation in organo-sulfur rich facies, which overestimates thermal maturity for areas in the early oil generation window. Ts / Tm (tricyclic terpanes) ratio is not affected by sulfurization and is suitable for thermal maturity assessment for Eagle Ford rocks and produced oils in the oil window. The methylphenanthrene index (MPI-1) (a thermal maturity parameter calculated from aromatic compounds) is reliable for thermal maturity assessment from mature to gas condensate stages.