Quantifying Seismic Volumetric Error Using Outcrop-Based 3-D Forward Seismic Modeling

Janson, Xavier, Hongliu Zeng, Charlie Kerans, Fred Wang, Sergey Fomel, The University of Texas at Austin, Austin, TX

The increasing volume, resolution, and quality of seismic data, as well as more reliable interpretation techniques for extracting geological information from the data, have led to the use of seismic data as a primary tool for building reservoir models and forecasting reser­voir volumes at all stages of the reservoir’s life. This evolution has progressively eclipsed outcrops as a source of geological information to analyze and quantify stratigraphic archi­tecture of reservoirs and to assess uncertainties. Outcrop studies remain the only source of almost continuous linked scales of observation of unambiguous geology. Accurate 3D geo­logical models are built rapidly from outcrops using laser-scanning technology. We calcu­late a 3D synthetic seismogram using an outcrop-based 3D geocellular model of carbonate reservoir analogs in order to quantify the volumetric error generated when calculating vol­ume solely on the basis of seismic data with sparse depth control. The volumetric error intrinsic in the seismic methods is dependent on the seismic frequency, but other sources of error include velocity model uncertainties, migration errors, and horizon picking. Seismograms were generated for two Permian carbonate outcrops in West Texas where car­bonate toes of slope redeposited grainstone and ramp-crest grainstone high-energy shoals are exposed. We estimate the seismic volumetric error to be approximately 450%, 200%, and 160% for perfectly depth-migrated data at 20 Hz, 40 Hz, and 80 Hz peak frequency, respectively. Depending on the frequency content and the velocity model and inversion tech­nique used, seismic inversion significantly reduces volumetric errors.