Depositionally and Diagenetically Controlled Reservoir Heterogeneity at Jordan Field

Abstract

Summary The San Andres reservoir in the University Lands Jordan field has produced68 MMSTB [11 X 106 stock-tank M3] of the 182 MMSTB [29 × 106 stock-tank M3] Oforiginal oil place (OOIP); an estimated 44 MMSTB [7 × 106 stock-tank M3] ofmobile oil remains in this reservoir. The reservoir is divided into four flowunits on the basis of depositional textures and subsequent diageneticoverprint. The locus of highest production corresponds to a trend of productioncorresponds to a trend of high reservoir storage capacity in Zone C, which hasbeen affected by a permeability-increasing, carbonate-leaching diageneticalteration. This locus crosscuts structure. An area of low productioncorresponds to an area of highest reservoir storage capacity in Zone B, whichhas not been affected by this diagenetic alteration. This low-production areais in an updip position but has been inefficiently swept by the waterflood. Introduction Jordan field is located on the eastern margin of the Central basin platformin the Permain Basin of west Texas (Fig. 1) on the Ector/ Crane county line. The field is part of a five-field complex that produces oil from a combinedstructural and stratigraphic trap on the eastern flank of a broad, asymmetricantincline. Discovered in 1937, Jordan field produces oil from a Permian(Guadalupian) produces oil from a Permian (Guadalupian) San Andres formationreservoir at a depth of about 3,500 ft [ 1070 m]. Following peripheralwaterflooding in 1968, a program peripheral waterflooding in 1968, a program ofinfill drilling, well deepening, and conversion of producing wells towater-injection wells began in 1969. By 1971, the University Lands part of thefield was on a modified five-spot waterflood with a producing well spacing ofroughly 40 acres [ 16 ha] per well (Fig. 2). The two Jordan field units on University Lands have produced 68 MMSTB [11 x106 stock-tank M3] of the 182 MMSTB [29 × 10 stock-tank M ] OOIP, and anestimated 44 MMSTB [7 × 10 stock-tank M] of model oil remains in the UniversityLands part of the reservoir. This high remaining mobile-oil resource prompted acombined geological and engineering study of the Jordan field San Andresreservoir on University Lands to develop strategies for recovering the oil. Reservoir Description Lithologic description of the San Andres reservoir at Jordan field is basedon examination of seven cores from within two University Lands units and isaugmented by the study of two Jordan field cores from an area immediately westof the University Lands boundary and 14 cores from the East Penwell/San AndresUnit, which offsets Penwell/San Andres Unit, which offsets Jordan field to thenorth. The reservoir is composed of thoroughly dolomitized carbonate rockscemented by sulfates. Depositional Facies. The San Andres formation at Jordan field is composed ofa 400-ft [120-m] -thick upward-shoaling sequence of rocks deposited asshallow-water ramp sediments. For this discussion, the description ofdepositional facies is divided two parts: rocks deposited as tidal-flatsediments and rocks deposited as open-marine sediments. Tidal-Flat Depositional Facies. Tidal-flat facies are pisolite packstone andmudstone. Pisofite packstone is composed of symmetri- Pisofite packstone iscomposed of symmetri- cal and asymmetrical pisolites generally 0.008 to 0.15in. [O.2 to 4 mm] in diameter with fine-grained muddy matrix. Pisolitescommonly have a fitted fabric. This facies is characterized by abundantcaliche, fenes- trae, and desiccation and sheet cracks. Lo- cally, the pisolitepackstone facies contains karst collapse breccias generally less than 3 ft [less than 1 m] thick. This facies is generally nonporous, but locally is bothporous and permeable. porous and permeable. The presence of caliche, collapsebreccia, and desiccation features indicates periodic subaerial exposure, andthe pisolite pack-stone facies is interpreted as having been pack-stone faciesis interpreted as having been deposited in an interidal or supratidalenvironment. Mudstone is composed of cream-colored, generally massive dolomite, althoughsome mudstone is faintly laminated. Stromatolitic laminae are present but rare. Mudstone contains dolomite crystals generally smaller than 0.0008 in.[0.02 mm]but lacks fossils, suggesting that it was deposited in a hyper-salineenvironment in which stromatolites could survive but marine invertebrates wereexcluded. The absence of fossils and the close association with the pisolitepackstone facies suggest deposition in hypersaline ponds on a tidal flatisolated from and ponds on a tidal flat isolated from and probably landward ofthe open-marine probably landward of the open-marine depositionalenvironment. Tidal-flat facies are interbedded with three intervals of siliciclastic siltthe bay be correlated regionally with gamma ray logs. Tidal-flat facies areseparated from subjacent open-marine facies by an interval of greenish-grayorganic-rich shale that may be correlated throughout Jordan field with gammaray logs. Open-Marine Depositional Facies. Subtidal facies are pelletpackstone/grainstone and bioherms composed of bryozoans, algae, and corals withassociated flanking facies of skeletal grainstone. JPT P. 1304