Sandstone Diagenesis above and below a Pressure Seal--Tuscaloosa Trend, Louisiana
WEEDMAN, SUZANNE D., WOLFGANG ALBRECHT, and SUSAN L. BRANTLEY, Pennsylvania State University, University Park, PA
Significant diagenetic differences are observed in the Lower Tuscaloosa sandstone sampled above and below a pressure seal. Additionally, textural evidence suggests that the high porosity (>20%) in some of the sandstones is secondary.
Samples are classified into two groups: "normal" (<12%) and high porosity (12-24%). The "normal" porosity sandstones, situated predominantly above the seal, are cemented by either quartz or calcite. At some grain-quartz overgrowth contacts, dissolution of overgrowths and portions of grains is concurrent with precipitation of calcite; some calcite is partially replaced by saddle dolomite. Pore-lining chlorite is present but rare in the "normal" porosity sandstones.
The high porosity sandstones, situated predominantly below the seal, retain remnants of quartz and carbonate cements; virtually all pores are lined with chlorite. High porosity is attributed primarily to dissolution of carbonate cement. Evidence for previously existing carbonate cement includes: remnant cement, rhomb-shaped porosity, and partially dissolved quartz grains whose surfaces are similar to grain surfaces of the calcite-cemented sandstones. The characteristic robust pore-lining chlorite growth is attributed to accelerated dissolution of volcanic rock fragments with the increase in porosity and permeability after carbonate dissolution.
We suggest that the pressure seal plays two roles in creating and preserving high porosity: (1) the seal impedes fluid flow between two zones of different fluid chemistry and therefore different paragenesis in rocks of essentially the same age and depositional environment, and (2) the seal creates a compartment in which fluid pressures support the rock, inhibiting further compaction after the dissolution of the cement.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)