tFigure Captions

tSummary

tReservoir & source rock

tReservoir development

tBibliography

tBiography

tAcknowledgments

tFigure Captions

tSummary

tReservoir & source rock

tReservoir development

tBibliography

tBiography

tAcknowledgments

tFigure Captions

tSummary

tReservoir & source rock

tReservoir development

tBibliography

tBiography

tAcknowledgments

tFigure Captions

tSummary

tReservoir & source rock

tReservoir development

tBibliography

tBiography

tAcknowledgments

Figure Captions

Figure 1. Index map showing location of Double A Wells Field, Polk County, Texas.

Figure 2. Interpretation of 3-D seismic line across Double A Wells Field. The canyon was formed in late Woodbine – Eagle Ford time as indicated by truncated seismic reflections along its upper wall. Note the superposed Midway-Wilcox canyon.

Figure 3. Diagrammatic dip section showing Woodbine - Eagle Ford stratigraphic relations and location of Woodbine fields. Note the clinoform beds in the Eagle Ford shale that reflect seaward growth of the slope.

Figure 4. Core depiction and porosity-permeability plots of upper Woodbine sandstones in the “sweet spot” of the field. Modified after George Bolger, PetroTech Associates, who analyzed the core for Black Stone Energy.

Figure 5. Thin section photograph of dissolution porosity (blue) in fine-grained Woodbine sandstone gas reservoir at 13,946 ft in Double A Wells field, Polk County, Texas. Porosity 21%, permeability 931 md. Photograph by George Bolger, PetroTech Associates.

Summary

Double A Wells Field, a gas condensate giant located 75 miles northeast of Houston, Texas (Figure 1), will eventually produce close to 1/2 TrillionCFG and 20 MillionBC. The gas is trapped in lenticular sandstones that pinchout updip or have reduced permeability along thinned edges. Buried near 14,000 ft, the slightly overpressured (0.7 psi/ft gradient) fine-grained quartz sandstones have unusually good reservoir quality (see Figure 5), up to 23% porosity and 1 (one) darcy permeability.

Hopefully, the geological model presented here, combined with 3-D seismic data, will lead to the discovery of other Woodbine sandstone "sweet spots" containing future giants waiting to be found in this lightly explored trend.

Reservoir and Source Rock

The sandstones are the terminal fill of a submarine canyon, initially 700-800 ft deep and 4 ½ mi wide (Figure 2), which probably resulted from gravitational failure of oversteepened, progradational shales, seen as clinoform reflections on dip seismic lines, above the buried Sligo Lower Cretaceous (LK) shelf margin (Figure 3). The clinoform sequence is overridden by landward, onlapping deposits of a major marine transgression (late Cenomanian-Turonian) consisting of the Woodbine sandstone sequence, thin Rapides shale, and Austin chalk. As shown in Figure 3, the Woodbine sandstones, comprising the basal member of the transgression, are considered to be contemporaneous with and connected to producing sandstones of updip fields.

Sandstones of the submarine fan are encased in organic rich source rocks including interfingering and underlying Eagle Ford shale, overlying Rapides shale, and basal Austin chalk. The organic content level of the shales has been established by previous workers, and the basal Austin chalk lime mudstone is a well-known producer from fractured reservoirs in the area (Brookland Field). The source rocks at Double A Wells Field are presently discharging gas at a temperature of 325-350°F.

Reservoir Development and Performance

The exceptional reservoir quality of multiple sandstones at Double A wells Field (Figure 4) involves the early emplacement of pore filling calcite cement after incipient quartz overgrowths, followed by cement dissolution by acidic waters during deeper burial. Note the calcite cement (orange) undergoing dissolution in the center of Figure 5. Acidic waters containing dissolved C0₂ (carbonic acid) are postulated from two sources; 1) organic rich proximal source rocks and 2) inorganic chemical reactions of kaolinite with other minerals in very thick, hotter shale sections downdip, connected to field reservoirs by sandstone "pipelines." The latter inorganic source is favored to supply the greatest volume of acidic waters needed for cement removal. Both the organic and inorganic sources have been documented by geochemists.

After cement dissolution, the restored, born-again "surrogate" pores, mimicking early primary porosity, were invaded by oil (left as part of the black stain around grains in Figure 5) then replaced by gas with increasing burial temperature. Since Double A Wells Field was discovered in 1986, it has produced more than 305 Billion CFG and 15.5 Million BC. The gas contains 4 to 6% C0₂.

Selected Bibliography

Foss, Deane C., 1979, Depositional Environments of Woodbine Sandstones, Polk County, Texas: GCAGS Trans., V. 29, p. 83‑93.

Franks, S.G. and Forester, R.W., 1984, Relationships among Secondary Porosity, Pore-Fluid Chemistry and Carbon Dioxide, Texas Gulf Coast: AAPG Memoir 34, p. 63‑79.

Schmidt, Volkmar and McDonald, David A., 1979, The Role of Secondary Porosity in the Course of Sandstone Diagenesis: SEPM Special Pub., No 26, p. 175‑207.

Siemers, C.T., 1978, Submarine Fan Deposition of the Woodbine‑Eagle Ford Interval (Upper Cretaceous), Tyler County, Texas: GCAGS Trans., V. 28, p. 493‑533.

Smith, J.T. and Ehrenberg, S.N., 1989, Correlation of Carbon Dioxide Abundance with Temperature in Clastic Hydrocarbon Reservoirs: Relationship to Inorganic Chemical Equilibrium: Marine and Petroleum, V. 6, p. 129‑135.

Stricklin, Fred L., 1998, Double A Wells Field ‑ A Prolific Sleeper in the Downdip Woodbine Trend in Texas: GCAGS Trans. (Extended Abstract), V. 48, p. 563‑564.

Stricklin, Fred L., 2002, Evolution and High Dissolution Porosity of Woodbine Sandstones in a Slope Submarine Fan, Double A Wells Field, Polk County, Texas – A deep Water Gulf of Mexico Model Onshore: HGS Bulletin, v. 44, no. 7, p. 19, 21, 23.

Biographical Sketch

Fred L. Stricklin, Jr. graduated from Louisiana State University just after the middle of the last century with a Ph.D. in Geology. After working 21 years with Shell as a research and exploration geologist, achieving the rank of Senior Staff Geologist, Fred left Shell during the "Great Boom" of the 1970's to become an Independent Geologist. For the past 21 years, he has worked primarily conducting exploration analyses of Texas oil and gas trends for sale to industry under his company names of Exploration Trend Analyses and Wilcox Exploration Enterprises.

Acknowledgments

I wish to gratefully thank Dr. John T. Smith, retired "Professor Emeritus" formerly of Shell Development Company, who spent 35 years working on geochemical problems related to oil and gas exploration. John patiently helped me to understand the role of circulating subsurface fluids involved in pore preservation and destruction. Appreciation is also extended Dr. Michael Lloyd, likewise retired from Shell and now President of Roxanna Petroleum, who assisted in the same capacity. Many thanks are also due Tom Carter, CEO of Black Stone Energy, the company that discovered Double A Wells Field, for permission to use data presented in the this presentation.