ABSTRACT: Organic Geochemistry of Lower Tertiary Shales of Southern Louisiana: Regional Distribution of Source Potential

Elizabeth W. Chinn, Gary A. Cole, Michael J. Gibbons, Roger Sassen, R. J. Drozd

Outer continental shelf shales of the Paleogene/Eocene Wilcox Group and middle Eocene Sparta Formation are potential source rocks for hydrocarbons in southern Louisiana. Eight hundred and thirty-seven shale samples (Sparta, 219) Wilcox, 618) from 24 wells across south Louisiana were analyzed using Rock-Eval pyrolysis; 24 organic-rich samples were analyzed using pyrolysis-gas chromatography and gas chromatography-mass spectrometry (GC-MS). Regional trends in organic geochemistry are related to two main factors: sedimentary facies transition from proximal deltaic to shelf/slope environments, and thermal maturity. According to geothermal gradient maps, thermal effects increase from east to west across southern Louisiana and from north to south as the lower Tertiary section i buried to greater than 30000 ft. The Wilcox, and to a lesser extent the Sparta, are major regressive deltaic sequences whose primary deltaic depocenters are located in subsurface Texas. Volumetrically, smaller deltas prograded beyond the ancient Cretaceous shelf edge by as much as 50 mi across southern Louisiana. Source potential in both rock units becomes more oil prone from west to east across south Louisiana, with an apparent maximum in the shelf/slope flexure zone in south central Louisiana. In this area, Sparta Formation shales show good to excellent potential in shales overlain by shelf-edge barrier sands. Wilcox source potential is fair to good in shales deposited in submarine fan environments. Source units are interpreted to be a product of transgressive episodes occurring withi primarily regressive sequences. The close proximity of source units to the shelf-edge suggest that they may represent conditions deeper in the Eocene Gulf of Mexico basin.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990