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Uniting Petrophysics and Stratigraphy to Decipher Classified Facies From a Pre-Stack 3-D Inversion: Wolfcamp and Spraberry, Howard County, Midland Basin

Abstract

Geologically-constrained pre-stack 3D seismic inversion can potentially predict reservoir properties ahead of the drill bit. Using a 3D seismic dataset from the Midland Basin, we integrate pre-stack seismic inversion with petrophysics and sequence stratigraphy to derive reservoir facies architecture and corresponding geomechanics. This multi-disciplinary collaboration results in seismic volumes of classified lithofacies and geomechanical properties that assist Wolfberry exploitation, and is testimony to achieving full value from 3D seismic data in an unconventional play. The Wolfcamp and Spraberry (Wolfberry) sections lie in toe-of-slope depositional positions within the Midland Basin. Production is from organic-rich, transgressive basinal mudstones interbedded with siliciclastic and carbonate debrite and turbidite beds. Optimal completions practices rely on fully understanding the lateral and vertical distribution of the desired mudrock source beds. Pre-stack inversion allows capturing of shear wave velocity information, otherwise lost in the offset domain of post-stack inversion. Pre-stack inversion provides P and S impedance (Ip and Is) which in turn yield elastic constants Lambda and Mu. Petrophysically-defined, wireline log-derived facies classifications can be cross-plotted to define Lambda-Rho, Mu-Rho (LMR) regions. Comparing Lambda-Mu-Rho (LMR) cross plots calculated from the well log data with similar data derived from inverted seismic impedances helped delineate facies families, or depositional trends within the 3D seismic. Wolfberry lithofacies and geomechanical properties were thus highlighted from inversion results and mapped into the sequence-scale basin architecture. Geomechanical parameters were defined from the inversion results at a scale useful for completion engineers. This up-scaling of the petrophysical solution yields vertical resolution of the inversion comparable with the scale of mechanical stratigraphy that controls well completions. Using a properly processed seismic dataset as input, pre-stack seismic inversion deliverables (Ip and Is) were used in conjunction with petrophysics to define facies and geomechanical parameters critical to efficient exploitation. Because of its superior areal extent and degree of spatial sampling, 3D seismic data can be extensively mined for reservoir properties and geomodel characteristics to become the canvas for a multidisciplinary compilation for any given unconventional play.