Calibration of surface seismic attributes to natural fractures using horizontal image logs, Mississippian Lime, Osage County, Oklahoma

Abstract

Summary The Mississippi Lime, located in parts of Oklahoma, Kansas, Arkansas and Missouri is one of the most recent unconventional plays, and is characterized by tight limestone, fractured chert, and high-porosity tripolitic chert sweet spots. Exploited since the early 1920s, this formation has been rejuvenated by the advent of horizontal drilling, hydraulic fracturing, and efficient water disposal into the deeper karsted Arbuckle Formation. In the absence of wide-azimuth seismic data, the present-day orientation of maximum horizontal stress can be determined from image logs. Fracture density is controlled by three factors: strain, lithology, and bed thickness. Only a limited number of publications have been reported on the use of image logs measured in horizontal wells and their calibration with seismic data. We characterize the fractures seen in these horizontal wells by their orientation and whether they are layer bound or through-going, open or closed. These measurements are then correlated to the surface seismic attributes k1 and k2 principal curvature and acoustic impedance inversion to estimate fracture density, orientation, and lithology at the Mississippian Lime objective throughout the survey. Core measurements of the tight and tripolitic chert are used in our lithology estimation. Correlating production from horizontal wells to surface seismic attributes and impedance is problematic and requires a hypothesis that completion is constant along the length of the well. As this play develops into a true resource play, we anticipate the correlation of production data to seismic attributes. The Mid-Continent Mississippian Lime Play Mississippian lime reservoirs in northern Oklahoma and southern Kansas are contained in stratigraphic traps resulting from thin alternating layers of sealing tight limestone, fractured chert, and its diagenetically altered form, tripolite. Detrital and in-situ cherts and tripolites resulted from localized subareal exposure and post-erosional redeposition during the Pennsylvanian. As a result the reservoir is often compartmentalized, and characterized by high porosity (2-48%) and low permeability. Observations by Matson et al. (2011) provide evidence that these reservoirs exhibit multiple porosity systems. Lithologically controlled fracture swarms in brittle reservoir rock provide the majority of the porosity in the reservoir. Matrix porosity is dominated by a chert and lime bed boundary system and consists of isolated pores that must be connected with fracture porosity streaks through horizontal hydraulic fracture stimulation to enhance hydrocarbon production. Without the presence of conductive natural fractures matrix permeability is greatly reduced. Ericsson et al. (1998) postulated that horizontal drilling can intersect multiple fracture swarms while traditional vertical drilling may miss all the fracture swarms completely. Therefore, strategies to delineate areas and layers with a high density of natural conductive fractures must be developed, as well as locally specific horizontal completion techniques, for commercial success in this play. In the absence of dense horizontal well control through the Mississippian Lime in the Osage County region, fracture density prediction in the reservoir is still somewhat unpredictable. In this study, a method examining the relationship observed between natural fractures observed in image logs from two horizontal wells and k1 and k2 curvature derived from 3D seismic data is utilized to test the correlation between surface seismic attributes and fracture density. Fracture Prediction Using Surface Seismic Attributes