Abstract: Consolidation History of Gulf Coast Lower Tertiary Sandstone Reservoirs--Porosity Formation and Preservation

R. G. Loucks, D. G. Bebout, W. E. Galloway

Porosity and permeability of Gulf Coast lower Tertiary sandstone reservoirs vary considerably. Reservoir quality is controlled by depositional environment, mineralogic composition, and consolidation history (compaction, cementation, and leaching). In general, shallow reservoirs consist of primary porosity that is reduced by compaction and cementation, whereas deeper reservoirs result from late subsurface leaching.

Frio sandstones have the following idealized consolidation history.

Near-surface to shallow subsurface compaction and cementation stage (0 to ± 4,000 ft or ± 1,200 m):
Starts with early feldspar leaching and replacement by calcite, followed by precipitation of poikilotopic, pore-fill calcite cement, clay coats and rims, feldspar overgrowths, and initial quartz overgrowths. Sand is compacted until arrested by cementation. Reservoir porosity is reduced from 40% to approximately 25%.

Moderate subsurface cementation stage (± 4,000 to ± 8,000 ft or 1,200 to 2,400 m):
Consists of general precipitation of quartz overgrowths, localized welding by massive quartz overgrowths, and development of sparry pore-fill calcite cement. Porosity commonly is reduced to 10%.

Moderate subsurface leaching stage (± 8,000 to 11,000 ft or 2,400 to 3,350 m):
Results in massive leaching of feldspars, volcanic and carbonate rock fragments, and calcite cements. Continued leaching may resurrect porosities to as high as 30%.

Deep subsurface cementation stage (± 11,000 ft or 3,350 m):
Involves reduction of leached porosity by precipitation of pore-fill kaolinite and iron-rich carbonate cements; resulting porosities depend on the amount of this late cement.

This rock-consolidation history can be modified by residence time in each burial stage, thermal gradient, pore-fluid changes, and mineralogic differences.

Deep Frio production is not from single primary porosity between grains, as in shallow reservoirs, but production depends on formation and preservation of late, secondary leached porosity.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas