Summary

East Velma West Block, Sims Sand Unit (EVWBSSU), located in Stephens County, OK, is currently beingflooded with CO2. A detailed, quantitative, reservoirdescription was constructed for a simulation model thatwill aid in the evaluation and operation of the CO2 flood. The Sims sandstone member of the Upper-Mississippian/Lower Pennsylvanian Springer formation was deposited in a deltaic environment. Four deltaic facies are present: the delta plain, proximal and distal delta-front, present: the delta plain, proximal and distal delta-front, and abandonment phase. The delta-plain facies forms ashale barrier to vertical fluid flow between the abandonment and proximal delta-front sandstones. The proximaldelta-front facies forms the bulk of the reservoir. Porosity (16%) and permeability (70 md) in this facies are high Porosity (16%) and permeability (70 md) in this facies are high because of minor clay and cement content. The proximaldelta-front facies grades laterally downstructure into the distal delta-front facies. Authigenic clays and carbonateand silica cement have reduce porosity (14%) and permeability (30 md) in the distal delta-front sandstones. permeability (30 md) in the distal delta-front sandstones. A reservoir description was built from the geologic analysis for a four-component, three-dimensional (3D)full-field model. Reservoir zonation was based on the arrangement of deltaic facies. Porosity from gamma-ray/densitylogs was calibrated to core porosity to determine porosity in uncored wells. A linear correlation between core porosity in uncored wells. A linear correlation between core porosity and permeability was established for each facies. porosity and permeability was established for each facies. Permeability and gamma ray cutoffs were statistically Permeability and gamma ray cutoffs were statistically determined to define net pay. Vertical-to-horizontal permeability ratios were determined from measurements on permeability ratios were determined from measurements on oriented core plugs. Computer isopach maps of gross pay, net pay, porosity, and horizontal and verticalpermeability were generated for individual reservoir zones and permeability were generated for individual reservoir zones and then entered into the reservoir model.

Introduction

ARCO Oil and Gas Co. is conducting an enhanced-oil-recoveryprogram on EVWBSSU in Stephens County, program on EVWBSSU in Stephens County, OK. Continuous injection Of CO2 into the updip portion of the reservoir began in Sept. 1983. EVWBSSU wasdiscovered in 1949 with an estimated 220 × 10 STB[34 958 × 10 m3] of original oil in place (OOIP). Primary depletion was by gravity drainage. Crestal gas Primary depletion was by gravity drainage. Crestal gas injection for pressure maintenance began in 1962 andwater injection began in 1972. Approximately 32% ofOOIP was recovered by these methods of production. Success Of CO2 flood depends on distribution ofreservoir lithology, porosity, and permeability. Distributionand continuity Of sandstone reservoirs are controlled bydepositional environments. Clay and cement contentdetermine the subsurface distribution of porosity andpermeability. Thus the purpose of this study was to analyze permeability. Thus the purpose of this study was to analyze mineral composition and to determine the depositionalsetting of Sims sandstone. On the basis of this geologicanalysis, a reservoir description was constructed for asimulation model.

Stratigraphy

The Upper-Mississippian/Lower-Pennsylvanian Springerformation consists of approximately 2,500 ft [7670 m] ofalternating sandstone and shale. The Goddard formationconformably underlies the Springer formation, which isunconformably overlain by Morrowan-Age rocks.

Four subsurface Springer sandstone members-indescending order, the Markham, Aldridge, Humphreys, andSims-have been defined by drilling in the westernArdmore basin. The Sims sandstone member, ranging inapparent gross thickness from 300 to 600 ft [91.4 to 182.9m], is the major oil-producing horizon within the Velmareservoir. It consists of three sandstone units: indescending order, C1, C2, and C3. Sandstones are white to buff, fine-grained, and subrounded to subangular. They gradebasinward into clay-rich sandstones and finally into shales. Approximately 250 ft [77.1 m] of shale overlies the Simssandstone member.

Geologic Setting

The Sims member of the Springer formation was depositedon moderately subsiding shelves in theAnadarko-Ardmore basin, which is bounded by the Arbuckle upliftto the north, by the Wichita-Criner uplift to the south, and by the Ouachita foldbelt to the east (Fig. 1).

The EVWBSSU is part of the Velma reservoir, locatedin the northwestern part of the Ardmore basin. TheEVWBSSU is approximately 3.5 miles [5.6 km] long and0.75 miles [1.2 kin] wide (Fig. 2). This reservoir strikesin a southeast-northwest direction. It dips steeply (30 to60* [0.52 to 1.04 rad]) to the northeast. To the south, the EVWBSSU is bounded by an erosional unconformityand, to the north, by poor reservoir quality and a water/oilcontact (WOC).

Folding and faulting of the Velma reservoir, whichoccurred in Early Pennsylvanian times during collision ofNorth American, South American, and Africancontinental masses, is postdepositional.

JPT

P. 1420

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