ABSTRACT: Diagenetic Pressure Seal Formation in the Deep Tuscaloosa Trend, Gulf Coast: I. Textural, Compactional, and Chemical Constraints

WEEDMAN, SUZANNE D., SUSAN L. BRANTLEY, RYOJI SHIRAKI, THOMAS ROSS, and ARTHUR W. ROSE, Pennsylvania State University, University Park, PA

A pore fluid pressure differential of >20 MPa (3000 psi) is maintained by a pressure seal within the deep lower Tuscaloosa Formation (Moore-Sams and Morganza fields). Unusually high sandstone porosity is observed within 50 m (above and below) of the seal and has been documented as secondary. Petrographic study of the sandstones in the vicinity of the pressure seal at ~5.6 km depth documents a correlation between the mechanisms of formation of the pressure seal and of secondary porosity and requires that the seal formation post-dates cement and framework-grain dissolution. Additionally, a packing analysis suggests that the seal may have formed at <5 km depth and be tens of millions of years old.

High porosity has been partially destroyed above the seal by late-stage compaction accompanying development of secondary porosity but is preserved below the seal by overpressured fluids. This secondary compaction is manifested as sutured contacts between quartz grains, crushed lithic grains, and significant fracturing and re-orientation of framework grains, suggesting that both cement dissolution and rock fragment crushing are necessary to initiate a secondary compaction seal.

We have run SOLMNEQ.88 using pore fluid chemistry from Morganza to investigate the possible scenarios for carbonate precipitation and dissolution during updip fluid flow. If such fluids were responsible for carbonate precipitation and dissolution in these sandstones, then depth dependent variations in Pco2 or organic acid concentrations are necessary. Pco2 and organic acid concentration required to explain textural evidence are well within the expected ranges.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)