Salt-Induced Subsidence: Accommodation of Stacking Patterns and Implications for Reservoir Development in Oolitic Ramp Depositional Sequences: Jurassic Smackover Formation, State Line Graben, Louisiana and Arkansas

Paul D. Crevello, Lise Brinton, Thomas M. Smith

The Smackover Formation is often cited as a classic example of a progradational, carbonate ramp developed within a major-ordered sea-level cycle. In the vicinity of the state line trend, however, sedimentation, stratigraphy, and reservoir geometries, though influenced by eustacy, were dominanted by salt-induced tectonism. Regional mapping of Smackover B facies indicates that the ramp is a retrogradational ramp-margin wedge with stratigraphic expansion into salt-withdrawal grabens and thinning over antiformal structures. These strata are not present in the updip (Reynolds) Smackover shelf.

We have examined more than 60 cored wells, hundreds of log suites along the Arkansas-Louisiana state line graben, and present sedimentologic data for a tectonically modified, storm influenced, wave dominated, mixed carbonate-siliciclastic ramp. Shoal carbonates were dominated by shoreline-attached, barrier bar, oolite grainstone and localized, low-energy, siliciclastic sebkha mudstone and sandstone. Upper shoreface ooid grainstone strata are transitional offshore to lower shoreface graded and hummocky cross stratified packstone and sandstone with coral patch reefs. Siliciclastic turbidites dominate strata deposited below storm wavebase.

Syndepositional salt tectonism induced by extension of the underlying LouAnn Salt modified the carbonate ramp within the graben. Subsidence within strike-oriented subbasins accommodated multi-fold thickening and increase in the number of stacked oolite depositional sequences. Offlapping upper shoreface oolite strata occur as subbasin-restricted reservoirs. 'Landward' facies shifts across the subbasin suggest 'subsidence pulses' had to be rapid to account for the vertical facies successions exhibited in core, with relative sea-level changes greater than 20-30 meters. These shifts are not recorded on the margin of the subbasin, yet they mimic high-frequency, sea-level pulses. The absence of erosional unconformities and co-eval shoal facies on the high areas supports our interpretation o variable subsidence resulting in limited accommodation on the 'highs' and increase in accommodation and stacking in the expanding 'lows'. Multiple-datumed well-log correlations, isopachs, and seismic data, have facilitated recognition of salt-induced subsidence on stacking patterns and-reservoir distribution, which has been paramount for successful field exploitation.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995