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Evidence for Several Charges of Migrated Gas in Austin Chalk, Eagle Ford, and Buda Reservoirs on the San Marcos Arch


Geochemical data measured on mud-gas samples indicate: (1) high-maturity wet gas generated by Eagle Ford (EF) source-rock (SR) beds migrated laterally and updip to different stratigraphic levels at two wells located on the San Marcos Arch in Gonzalez County; and (2) a distinct dry gas charge (principally isotopically-heavy methane) generated by a different SR is present in Upper Cretaceous and Lower Cretaceous intervals. These gas charges probably influence the GOR of oil in Upper Cretaceous reservoirs and their drive mechanism. Oil and wet gas migrated updip toward the San Marcos Arch via the Buda Formation, and then vertically into the Austin Chalk via faults penetrating the EF Formation: i.e., oil fingerprinting results indicate the Austin Chalk contains a migration mixture of oil generated by local and distant EF SRs. Geochemical data measured on mud-gas samples collected from the Anacacho Formation through the Georgetown Formation at two vertical monitor wells were used to identify the top of one of these gas charges, where an abrupt change occurs in the C isotopic composition of methane, ethane, and propane (which are heavier below that boundary). This feature is present ≈30 ft below the top of the Buda Formation at Well #1. But it occurs at a much higher stratigraphic level (≈180 ft above the base of the Austin Chalk) at Well #2 located ≈7.5 miles NE of the other well (where EF SR beds are ≈700 ft deeper than at Well #1). The boundary in the middle Buda Formation corresponds to two good gas shows at Well #1, where the methane/ethane ratio increases from 5.6-8.0 (in the upper Buda Formation and the overlying Eagle Ford Formation) to 9.8-11.4 at those gas shows. Haworth ratios indicate the presence of wet gas, and fractures are present in the Buda reservoir. The middle Buda apparently is the regional carrier bed through which light oil and wet gas generated by deeper EF SR beds sequentially migrated updip: e.g., a ≈35°API oil sample produced from the Buda reservoir at Well #1 apparently is gas-washed because it is depleted in compounds more volatile than nC11. Another good wet gas show occurs in Well #1 ≈25 ft below the top of the Georgetown Formation, where the mud logger observed dull yellow fluorescence and “asphalt” - probably EF oil that migrated into that zone. A good show of wet gas (identified using Haworth ratios) occurs in the Austin Chalk at Well #2 just below the top of the isotopically-heavy gas charge. Furthermore, in all mud-gas samples collected at Well #1 from the Austin Chalk (and deeper intervals) the C isotopic composition of methane is ≈7.5 per mil heavier than predicted using the C isotopic composition of ethane and propane. This also is true for mud-gas samples collected above and below the boundary marking the appearance of high-maturity gas in the middle Buda Formation. Methane in mud-gas samples obtained at Well #2 is ≈6.0 per mil heavier than expected. An additional very dry gas charge explains these results.