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Expulsion Efficiencies and Timing of Oil Generation from Different Source-Rock Facies in the Eagle Ford Group and Boquillas Formation as Determined by Hydrous Pyrolysis

Lewan, Michael D.*1; Little, Jessica 2; Formolo, Michael J.2
(1) U.S. Geological Survey, Denver, CO.
(2) Department of Geosciences, University of Tulsa, Tulsa, OK.

Four lithofacies of Upper Cretaceous immature oil-prone source rocks in the Eagle Ford Gp. and Boquillas Fm. were characterized by hydrous pyrolysis. Two of the lithofacies in the Eagle Ford Gp. were collected from recent road cuts west of Dallas, TX. One consists of medium-gray calcareous shale (26.9 wt% carbonate) with a total organic carbon (TOC) content of 4.1 wt% and a hydrogen index (HI) of 709 mg S2/g TOC. The other facies consists of medium-gray chalky marlstone (61.5 wt% carbonate) with a content of 5.0 wt% and a HI of 639 mg S2/g TOC. Organic-sulfur contents of these rocks indicate that both of these facies contain low-sulfur Type-II kerogen. Another lithofacies from the South Bosque Fm. of the Eagle Ford Gp. was collected in a stream cut west of Waco, TX. It consists of medium-gray chalky marlstone (58.1 wt% carbonate) with intercalated limestone lenses at 10- to 30-cm intervals. The marlstone sample representing this lithofacies has a TOC of 5.7 wt% and a HI of 614 mg S2/g TOC. Organic-sulfur contents of the rock indicate that this facies contains low-sulfur Type-II kerogen. The fourth lithofacies is represented by two samples from the Boquillas Fm. near the Texas-Mexico border. One sample is from a stream cut west of Langtry, TX and the other is from a road cut northwest of Comstock, TX. Both samples are dark-gray dense marlstone (56-58 wt% carbonate) with 4.2 to 5.6 wt% TOC and HI values of 605 to 722 mg S2/g TOC. Organic-sulfur contents of these rocks indicate that this facies contains moderate-sulfur contents that approach being Type-IIS kerogen. Based on hydrous pyrolysis, this lithofacies will generate expelled oils higher in sulfur and at lower thermal maturities than the other lithofacies sampled in the Waco and Dallas areas. The implication is that, although Type-IIS kerogen commonly occurs in carbonate-rich source rocks, not all carbonate-rich source rocks contain Type-IIS kerogen. Yields of expelled oil from hydrous pyrolysis indicate that chalky lithofacies with high porosities in the Eagle Ford Gp. of the Dallas and Waco areas expel less of their generated oil than those more dense lithofacies with lower porosities in the Boquillas Fm. near the Texas-Mexican border. The yield differences are not only in quantities of expelled oil, but also in the expelled-oil trends with increasing temperature. An important implication is that physical properties of a source rock can influence the amount of oil expelled and retained.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California