Interpreted sequence stratigraphy and gross packaging of chert and carbonate lithologies in the University Waddell Thirtyone chert reservoir. Thirtyone chert reservoir intervals in University Waddell field are developed in the middle and top of the formation. CHANNELIZED GRAIN-RICH CHERT Maps of individual grain-rich, cycle-base chert flow units. Note highly variable patterns among units in thickness and φh reflecting the control on reservoir architecture of depositional style. 0.5 mm 2.0 cm Grain-rich spiculitic cherts are channelized grain-rich debris flows that display evidence of rapid, high-energy deposition. Marginal facies are highly burrowed, reflecting slower deposition in low-energy, moderately well oxygenated conditions. Laminated muds document interchannel to distal hemipelagic deposition. University Waddell field is developed over a simple anticline that is probably fault controlled along it’s western margin. THIRTYONE STRATIGRAPHY Pragian sequence 2 Pragian sequence 1 Architecture of grain-rich, cycle-base chert reservoir units. Note marked differences in thickness and continuity. 2.0 cm Discovery date: 1949 Average depth: 8,600 ft (2,620 m) Area: 8,700 acres (3,510 hectares) Well spacing: 20-40 acres (8-16 hectares) Top seal: Woodford Formation Bottom seal: Frame Formation (Silurian/Devonian Wristen Gp.) Trap: Anticline Hydrocarbon source: Woodford Formation Producing unit: Thirtyone Formation (Lower Devonian) Reservoir lithology: Chert Oil-water contact: -6,650 ft (2,030 m) subsea elevation Average gross pay: 900 ft (27 m) Average net pay: 100 ft (21 m) Average porosity: 9 percent Average permeability: 1 md Water saturation: 0.37 Residual oil saturation (Sor ): 0.23 Oil gravity: 44 ° API @ 60 ° F Original bottom-hole pressure: 4,200 psia Temperature: 140 ° F (50 ° C) Formation volume factor: 1.73 (at original bottom-hole pressure) Oil viscosity: 0.475 centipoise (at original bottom-hole pressure) Solution gas-oil ratio 1,330 SCF/STB (original) RESERVOIR CHARACTERISTICS AND VOLUMETRICS 2.0 cm 0.5 mm DISTAL CHERT RESERVOIRS University Waddell Field PLATFORM-DERIVED SKELETAL CARBONATE Paired core slab and thin-section photographs of thin-bedded to massive skeletal packstone. These rocks contain crinoids, brachiopods, and other minor skeletal debris. Pore space is occluded by syntaxial cement and carbonate mud. HEMIPELAGIC LAMINATED CHERT Paired core slab and thin-section core slab photographs of laminated chert and lime mudstone. These rocks are hemipelagic deposits that represent the most distal phase of Thirtyone deposition. They contain no porosity. Paired core slab and thin-section photographs of thickly laminated to massive chert. These rocks, which are composed of abundant sponge spicules and carbonate detritus deposited by grain-rich gravity flows, constitute the primary productive facies in distal chert reservoirs. Paired core slab and thin-section photographs of burrowed chert. Laminations, graded bedding, and fluid escape structures are locally common. Although minor spicule-moldic pore space exists, these rocks are mostly nonporous. DISTAL CHANNEL MARGIN BURROWED CHERT CHANNEL MARGIN DISRUPTED LAMINATED CHERT Paired core slab and thin-section photographs of laminated chert with convolute and disrupted laminations caused by soft sediment deformation and burrowing. These rocks, which are nonporous, contain siliceous mud and very fine grained peloid/skeletal debris deposited by episodic transport of fine-grained carbonate platform detritus into a deep-water, outer ramp to slope setting. CHANNEL MARGIN INTERBEDDED CARBONATE AND CHERT Paired core slab and thin-section photographs of nodular chert and limestone formed by burrowing and soft sediment deformation of interbedded and interlaminated chert and carbonate. Minor porosity exists in chert layers; carbonates are nonporous. 0.1 mm 1.0 mm 2.0 cm 0.5 mm Woodford Fm. QAc2339(a)c Upper chert Lower lst. Upper lst. Northwest Southeast Top Frame Fm. Top 31 Fm. A B C Lower chert A A' Channel margin and levee: disrupted laminated chert, nodular chert & limestone, burrowed chert Channel: thick laminated to massive chert Interchannel: laminated chert & limestone QAb7651(b)c Compensation lobes Bifurcating channels with attached lobes Nested channels with levees Compensation lobes Bifurcating channels with attached lobes Nested channels with levees Platform margin Slope-Basin Platform THIRTYONE DEPOSITIONAL MODEL DISTAL SETTING Chert; high-energy turbidite fan deposits Diagenetically overprinted chert & siliceous limestones Pelmatozoan packstone & grainstone Low-energy hemipelagic chert laminates QAc9145c STRUCTURAL SETTING N QAc2336(b)c - 6250 - 630 - 6350 - 640 - 6450 - 650 - 650 Block B-25 Block 31 161 A A ' B B' - 620 H.H. No.14 1 mi 0 0 1 km C.I. 50 ft Cored well Well Well with digital logs THIRTYONE RESERVOIR ARCHITECTURE DISTAL SETTINGS 500 ft 100 m 0 0 30 m ft 100 Porosity zones 8800 9000 9200 8800 9000 9200 Lower porosity interval Middle porosity interval Upper porosity interval QAc1286(b)c GR NPHI GR PHI GR PHI GR PHI GR PHI GR Southeast Northwest B' B Depth (ft) Depth (ft) PHI Depth (ft) Depth (ft) Depth (ft) Depth (ft) 8800 9000 9200 8800 9000 9200 8800 9000 9000 9200 C Oil completions Oil recompletion opportunities Injection recompletion opportunities Depth ft m 0 100 30 0 3000 ft 900 m 0.25 mm 1.0 cm SUMMARY OF HETEROGENEITY IN DISTAL THIRTYONE RESERVOIRS In contrast to proximal Thirtyone chert reservoirs where there is a single, continuous porous chert reservoir, distal chert reservoirs contain numerous separate and discontinuous stacked porous chert units. In distal reservoirs, lack of continuity is the primary contributing factor to heterogeneity and low recovery efficiency. The distribution of porous chert in distal settings is a function of sediment geometries associated with submarine fan and debris-flow deposition. Episodic downslope transport of siliceous spiculitic sediment along the margins of the carbonate platform has resulted in vertical segregated and laterally discontinuous chert reservoir intervals. These deposits are interbedded with and grade laterally into lower energy mud-rich sediments that typically have low porosity and permeability. Although these muddy rocks are not flow barriers, they do act as baffles to flow and impact recovery efficiency. Detailed correlation and mapping of individual porous chert layers is critical for establishing a reservoir framework that can serve as a basis for defining recompletion and infill drilling targets. Such an approach has led to the identification of several drilling and recompletion prospects in Waddell field. CONCLUSIONS Devonian chert reservoirs in West Texas contain a large remaining oil resource that is a target for more efficient exploitation techniques based on a better understanding of the geological controls on heterogeneity. Because these controls differ systematically between chert reservoirs developed in updip, proximal settings and downdip, distal settings, it is crucial that both regional and local geologic controls be examined and integrated into modern reservoir characterization and exploitation efforts. The Three Bar and University Waddell fields provide fundamental models of the styles of heterogeneity expected in proximal and distal settings, respectively. FLOW UNIT CONTINUITY QAc6231(a)c B' B CI 2 ft 1 mi 0 0 1 km Well locations Well with digital logs Petrophysical analysis QAc3512c P.S.L. Block B-25 University Lands Block 31 THICKNESS (ft) 6–8 4–6 >10 8–10 2–4 10 11 3 2 1 QAc2347c P.S.L. Block B-25 B' B QAc1284c B' B P.S.L. Block B-25 CI 5 ft Well locations Well with digital logs Petrophysical analysis THICKNESS (ft) 15–20 10–15 >20 5–10 THICKNESS (ft) 8–12 4–8 >16 12–16 N 10 11 3 2 1 10 11 3 2 1 CI 0.5 phi•h 1 mi 0 0 1 km Well locations Well with digital logs Petrophysical analysis QAc6083c P.S.L. Block B-25 University Lands Block 31 B' B 1.0 2.0 0.9 0 10 11 3 2 1 P.S.L. Block B-25 University Lands Block 31 1 . 5 1 . 0 0 . 5 0 . 5 1 . 0 B' B QAc1246c P.S.L. Block B-25 University Lands Block 31 2.0 3.0 1.0 1.0 1.0 2.0 B' B QAc3960c CI 0.5 ft phi•h 1 mi 0 0 1 km Well locations Well with digital logs Petrophysical analysis CI 0.5 ft phi•h 1 mi 0 0 1 km Well locations Well with digital logs Petrophysical analysis N 10 11 3 2 1 10 11 3 2 1 1 mi 0 0 1 km N N CI 4 ft 1 mi 0 0 1 km Well locations Well with digital logs Petrophysical analysis University Lands Block 31 University Lands Block 31 1 . 0 N N Pragian sequence 2 Pragian sequence 1 30 φ 0 N C B A GR Woodford Formation Frame Fm. Thirtyone Formation 8500 8700 8600 9200 9400 9300 Thick laminated to massive chert Burrowed chert Disrupted laminated chert Nodular chert/limestone Laminated chert/limestone Skeletal packstone Completion RESERVOIR FACIES Typical vertical section in distal Thirtyone reservoirs. Highest porosity is associated with high-energy deposits of spiculitic chert that are interbedded with lower energy, mud-rich facies. QAc4149(b)c University Waddell 161 THIRTYONE TYPE LOG CHEVRON U.S.A. T e x a s “ H . H . ” N o . 1 4 QAb9005(b)c Major reservoir interval 0 100 100 200 GR 0.3 0 DPHI 0.30 0 NPHI Depth (ft) 9000 8900 8800 8700 8600 8500 Frame Formation 9500 9400 9300 9200 9100 Woodford Formation Thirtyone Formation C A B Wink Formation 2.0 cm