AAPG Annual Convention and Exhibition

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Migration and Retention of Hydrocarbons in Barnett Shale


Understanding the retention versus expulsion of hydrocarbons in shale resource plays is clearly of fundamental importance. This certainly is the case for the Barnett Shale, since the secondary cracking of retained oil into gas exerts a primary control of economic viability. Here we report a quantitative evaluation of migration and retention within and from the Barnett Shale using a core of oil window maturity. Combining petrologic techniques, i.e. Attenuated Total Reflection Fourier Transform Infra-red Spectroscopy (ATR-FTIR), normal and Scanning Electron Microscopy (SEM), with organic geochemistry methods, i.e. Rock-Eval pyrolysis, thermovaporization-GC and pyrolysis-GC, we have analyzed 99 core samples from an in total 150 ft thick sequence of Barnett Shale in the Mesquite#1 well, Hamilton County, Texas. The Tmax values are on average 450 °C, but show abnormally low values (430 – 440 °C) in intervals where ‘oil crossover’ occurs, an effect describing the free oil content (S1) exceeding more than 100 mg/g TOC. Applying an empirical formula (S1= 0.32*TOC + 0.06*Quartz – 0.0068, R=0.91) we concluded that the oil crossover effect in the discussed interval might be related to quartz content. This was also qualitatively confirmed by the presence of fluorescing oil in the axial chamber of silicified sponge spicules. Thermovaporization-GC revealed that the main carbon number of alkanes gradually increases from 8 to 13 in approaching the oil crossover interval, a phenomenon which reflects migration fractionation resulting in the accumulation or retention of heavy hydrocarbons. This migration-fractionation-retention model is further confirmed by comparing retained amounts of long chain alkanes with mass balances calculated from pyrolysis-GC yields, utilising an immature Barnett Shale sample. Furthermore we have calculated the amount and the composition of hydrocarbons that expelled out of the source rock. Our results show that the most abundant normal alkane for the expelled oil is n-C8, accompanying with the highest expulsion efficiency of 84%. The implications for shale oil and shale gas occurrence will be presented.