ABSTRACT Thick, aggradational sequences of shelf and distal shoreface sandstones are prolific hydrocarbon reservoirs in the deep, downdip part of the Frio Formation fronting the Greta/Carancahua shorezone system. Near Corpus Christi, Texas, geopressured shelf reservoirs have produced more than 190 bcf of gas just in the Corpus Channel and Encinal Channel fields; within these fields, two thinly bedded shelf-sandstone units (K2 and K8 reservoirs) have produced 26 and 38 bcf of gas respectively. Cross sections and maps show that shelf sandstones extend basinward from the distal shoreface toes of barrier-island and beach-ridge sandstone bodies. Shelf sequences are typically upward-coarsening, although upward-fining and heterogeneous sequences also occur. Conventional cores reveal that shelf sequences consist of bioturbated muddy sandstone and sandy mudstone thinly interbedded with planar laminated, sparsely burrowed, and occasionally low-angle cross-laminated or ripple-laminated fine to very fine sandstone. Associated burrow-homogenized siltstone to very fine sandstone sequences are from 1.5 to 6 m (5 to 20 ft) thick. Scattered thin zones contain locally derived mudstone clasts, macerated plant fragments, or shell debris. Individual shelf sandstone bodies commonly exceed 30 m (100 ft) in thickness, particularly where expanded on the downthrown side of major growth faults. In plan view, shelf sandstones are irregular sheets covering areas of several hundreds of square kilometers. Sandstone percent maps reveal subparallel, discontinuous, strike-oriented buildups lying seaward of the contemporary shoreface sandstone unit. These shore-parallel belts are typically interconnected and attached to the shoreface sand body by one or more dip-oriented channel-like axes. The geometry and internal features of the shelf sandstones document their deposition and longshore reworking by storm processes. Focused bottom return flow swept sand from the shoreface onto the inner shelf where it was distributed alongshore by wind-forced geostrophic currents. Between the infrequent, high-energy storm events, a diverse shelf infauna reworked the sediments. Although a turbidity current origin has commonly been suggested for similar shelf sequences in the Frio and other stratigraphic units, the geometry, trend, setting, and internal sedimentary structures are incompatible with this depositional process. Reservoir characteristics of Frio shelf sandstones are controlled by original pore properties inherited from the depositional environment and by subsequent diagenetic processes. The reservoirs are composed of moderate to well sorted, fine to very fine quartzose lithic arkoses that contain abundant glauconite pellets. Volcanic rock fragments dominate the lithic component, although some mudstone fragments are present in all samples. Cements are mainly quartz and feldspar overgrowths with minor amounts of sparry calcite, kaolinite, and clay coats; porosity also is reduced by deformed rock fragments. Optimum reservoir porosities range from 16 to 30 percent and average about 23 percent. Permeabilities for the same intervals range from 0 to 1200 md; however, such extremes are rare and values of tens to hundreds of millidarcies are most common. Reservoir quality improves at depth by dissolution of feldspar grains. This secondary porosity accounts for nearly half of the total porosity. Subregional megascopic heterogeneities occur as alternating zones of higher and lower permeability oriented parallel to depositional strike and spaced a few kilometers apart. Within a field, lateral and vertical variations in pore properties primarily depend on facies sequences and degree of burrowing. High and intermediate permeabilities coincide with ripple cross laminations and plane parallel laminations respectively, whereas low permeabilities coincide with bioturbated strata.

Abstract The subsurface hydrologic system of large, actively filling sedimentary basins includes meteoric, compactional, and thermobaric regimes. Boundaries between the regimes and their contained flow systems evolve as basin filling proceeds. As a result, sands are continuously flushed by a succession of fluids of varying origins and chemistries. Careful examination of the existing hydrologic setting and a reconstruction of generalized hydrologic history and its relationship to observed diagenetic features within a depositional sequence may serve to validate interpretive diagenetic models and may explain or predict paragenetic relationships, regional diagenetic variations within the same depositional episode, and differences in diagenetic products in different episodes in the same or similar basins. The Frio/Catahoula Formations of the Texas Coastal Plain provide an example that both illustrates the coexistence of hydrologic regimes and relates associations of diagenetic features with existing regimes or with the important mixing zones that occur between regimes. Hydrocarbon geochemistries indicate meteoric flushing to depths approaching 2000 m. The deeper geopressured section is coincident with a thermobaric regime in which clay dewatering recharges the hydrologically restricted portions of the basin fill.

Abstract 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.

Abstract Reservoir quality trends in Lower Tertiary sandstones along the Texas Gulf Coast are a product of regional variations in intensity of diagenesis. The major controls on diagenesis were detrital mineralogy and regional geothermal gradient. Porosity and permeability in sandstones shallower than 3350 m (11,000 ft) are generally adequate for hydrocarbon production, whereas reservoir quality in deeper sandstones in the onshore Lower Tertiary section is highly variable. Many of these sandstone reservoirs have permeability values of less than 1 millidarcy (md), but in a few areas permeability values are higher than 1000 md. Wilcox sandstones are poorly to moderately sorted, fine-grained, quartzose lithic arkoses, becoming more quartz-rich from the upper to the lower Texas Gulf Coast. Most rock fragments are metamorphic or volcanic in origin. Wilcox sandstones exhibit no systematic regional reservoir quality trends. Along the lower and parts of the middle and upper Texas Gulf Coast, deep Wilcox sandstones are tight, but in other parts of the middle and upper Texas Gulf Coast, porosity exists at depth. Vicksburg sandstones are poorly sorted, fine-grained lithic arkoses. Rock fragments are mainly volcanic clasts with lesser carbonate and minor metamorphic clasts. The deep Vicksburg Formation has low-quality reservoirs. Frio sandstones range from poorly sorted, fine-grained, feldspathic litharenites to lithic arkoses along the lower Texas Gulf Coast to poorly sorted, fine-grained, quartzose lithic arkoses to subarkoses along the upper Texas Gulf Coast. Volcanic and carbonate rock fragments are common in the lower Texas Gulf Coast and decrease in abundance up the coast. Frio sandstones show a systematic improvement in reservoir quality from the lower to the upper Texas Gulf Coast that is related to grain composition and geothermal gradient. Reservoir quality trends in Tertiary sandstones have been substantiated by acoustic log analysis. In spite of variations in composition, Lower Tertiary sandstones exhibit similar diagenetic sequences generalized as follows: Surface-to-shallow-subsurface diagenesis (0 to 1200 m ±; 0 to 4000 ft ±) began with the formation of clay coats on framework grains, dissolution of feldspar, and replacement of feldspar by calcite. Minor amounts of kaolinite, feldspar overgrowths, and Fe-poor calcite was locally precipitated. Porosity was commonly reduced by compaction and cementation from an estimated original 40% to less than 30%. Intermediate subsurface diagenesis (1200 to 3400 m ±; 4000 to 11,000 ft ±) involved dissolution of eary carbonate cements and subsequent cementation by quartz overgrowths and later by carbonate cement. Cementation commonly reduced porosity to 10% or less, but this trend could be reversed by later dissolution of feldspar grains, rock fragments, and carbonate cements. Restoration of porosity to more than 30% occurred, but some porosity was later reduced by kaolinite, Fe-rich dolomite, and ankerite cementation. Deep subsurface diagenesis (>3400 m ±; >11,000 ft ±) was a continuation of late Fe-rich and Fe-poor carbonate cement precipitation. Plagioclase was albitized during this stage. Differences in intensity of diagenetic events and depths at which they first occurred correspond to the chemical and mechanical stability of the original detrital mineralogy and to regional variations in geothermal gradient.