Depositional Controls and Environments of the Upper Frio Formation: Southeastern Louisiana Subsurface

MARLIN, DONALD J., Louisiana State University, Baton Rouge, LA

The subsurface upper Frio Formation (late Oligocene) consists of interbedded carbonate and siliciclastic facies. Synthesis of induction-electric logs and fragmented conventional core data show that resistive, low porosity carbonates are key to understanding environment of deposition, sandstone facies geometry, and hydrocarbon distribution.

The southeastern Louisiana Oligocene shelf received little clastic influx during Frio Formation time. Periodically, the resistive, carbonate facies formed a semi-continuous lineation of banks at the paleo shelf-edge at or about 30 degrees N latitude. The banks acted as barriers to siliciclastic deposition with sediments being deflected westward behind the banks and funnelled basinward through breaks in the banks at the shelf edge. Petrographic study of the carbonates commonly reveals lepidocyclinid-nummulitid biomicrites that show signs of early porosity occlusion and minor burial diagenesis. They range in composition from pure limestone to impure, iron-rich dolomitic sandstones.

Several cycles of carbonate-shale-sandstone are noted in the upper Frio Formation driven by relative sea-level change. Carbonate banks grew during transgressive periods and were periodically inhibited during early regressive siltation events or water-deepening below the photic range. Inhibitive growth periods are defined by clay-rich and hemi-pelagic facies overlying carbonates. Subsequent lowering of sea-level initiating progradational pulses of fine- to medium-grained sands that were funnelled around carbonate banks. Sandstone distribution patterns imply funnelling aspects and paleo-bathymetric bank relief while sand lenses within shales imply progradation. The next sea-level rise initiated conditions advocating renewed bank growth as depicted by carbonate facies overlying sandstone .

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)