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Characterization of a Carbonate Mudrock Reservoir by Integrating Sequence Stratigraphy, Sonic Previous HitVelocityNext Hit and Macro to Micropore Architecture


The Mid-Continent Mississippian age limestone is a proven and valuable carbonate mudstone reservoir in Oklahoma and Kansas. Vertical wells have been producing oil and gas for over 50 years, but recent horizontal activity has highlighted the need to understand and predict the internal reservoir architecture. Using the high resolution sequence stratigraphic framework as the foundation, laboratory measured sonic Previous HitvelocityNext Hit response, porosity, and permeability, coupled with characterization of the macro- to nanopore architecture using 2-D and 3-D image data are integrated to enhance the predictability of key reservoir facies with similar petrophysical properties within the same paleo-basin. Detailed facies analysis from five cores located in north-central Oklahoma and southern Kansas indicate deposition occurred on a regionally pervasive, distally steepened carbonate ramp with Previous HitvariableNext Hit siliciclastic input. Facies stack into shoaling upward packages of weakly calcareous mudstones to wackestones overlain by progressively higher energy skeletal packstone to grainstone facies capped, in some areas, by tidal flat facies. Enhanced porosity or permeability due to significant alterations resulting from subaerial exposure and hydrothermal dolomitization are also incorporated into the analysis as applicable. The span of facies and depositional environments results in a wide range of porosity (<1% to >40%), permeability (<0.001md to >170md), and sonic Previous HitvelocityTop response (6500-2000m/sec (Vp)). Observed pores are mostly oblong to oval, and include intercrystalline, interparticle, intraparticle, vuggy/moldic, and matrix pores. Pores and pore throats span from meso- (4mm-62.5 μm) to nanopore (1μm-1nm) size. Acoustic response data show an inverse relationship with porosity and a distinct pattern specific to the Mid-Continent basin that can be used as the foundation for predicting porosity in other Mid-Continent Mississippian age carbonates. The sequence stratigraphic hierarchy of shoaling upward packages observed in core at the 3rd, 4th, and 5th order scale are roughly correlated to wireline logs at the third and fourth order with the 5th order being used to assist in predicting facies with enhanced porosity and permeability. Deviations from basin trends are explained by the pore architecture and differences in diagenetic alterations. Interpreting the data set using an integrated, holistic approach, allows for enhanced predictability of facies and development intervals.