Thermal Maturity and Kerogen Type of Miocene Sands, Offshore Vermilion Parish, Louisiana

Mark A. Pasley, Ray E. Ferrell, Roger Sassen

Representative lithofacies of the Miocene (Burdigalian) Robulus L zone from the Texaco 6 well, Vermilion Block 31, offshore Louisiana, were sampled and subjected to mean vitrinite reflectance measurements and programmed pyrolysis. Vitrinite reflectance and pyrolysis T_max results are consistent; they indicate organic matter in the Robulus L sands is thermally immature to marginally mature with respect to oil generation. Based on low hydrogen indices (HI) and high oxygen indices (OI) measured during pyrolysis, kerogen in the Robulus L interval is type III with no meaningful oil source potential. Maceral analysis supports this assertion. Crude oil in these reservoirs has migrated from deeper and more mature source rocks.

Total organic carbon in the interval ranges from 0.26% in the sand lithofacies to 0.99% in the interbedded sand and shale lithofacies. Because of the predominance of recycled organic matter in the coarse size fractions, finer size fractions (< 10 µm) had significantly lower T_max values than the coarse fraction (> 10 µm) and whole-rock samples.

The presence of type III kerogen at a pregenerative stage of maturity suggests this reservoir interval is a possible source of carboxylic acids. The carbonate dissolution observed in the Robulus L sands may have been caused by this organic acid production. Hence, organic matter deposited in association with reservoir facies may be important in porosity modification and should be considered in studies concerned with geochemical mass balances.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.