Reservoir quality of lower Teritary sandstone reservoirs along the Texas Gulf Coast is controlled by sandstone depositional environment, mineralogical composition, and consolidation history (compaction, cementation, and leaching). In general, shallow reservoirs have 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 feet ±) starts with early feldspar leaching and replacement by calcite followed by precipitation of poikilotopic pore-filling 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 percent to approximately 25 percent.

Moderate subsurface cementation stage (4,000 to 8,000 feet ±) consists of general precipitation of quartz overgrowths, localized welding by massive quartz overgrowths, and development of sparry pore-fill calcite cement. Porosity is commonly reduced to 10 percent.

Moderate subsurface leaching stage (8,000 to 11,000 feet ±) results in massive leaching of feldspars, volcanic and carbonate rock fragments, and calcite cements. Continued leaching may resurrect porosities to as high as 30 percent.

Deep subsurface cementation stage (>1,000 feet ±) involves reduction of leached porosity by precipitation of pore-filling 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 mineralogical differences. Deep Frio production, then, is not from simple primary porosity between grains, as in shallow reservoirs, but is from secondary leached porosity.

FROM TRANS. GCAGS, V. 27, P. 109-120.

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