From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
Bureau Seminar, March 3, 2006
Three-Dimensional Seismic Forward Modeling of a Sinuous Slope Channel, Brushy Canyon Formation, West Texas
Although a seismic interpretation reflects some aspects of the subsurface geology, the scaling of seismic images means that they rarely show the full geological picture. One of the first steps to delineating a reservoir is the interpretation of subsurface stratigraphy and depositional elements using seismic images. Exploration seismic data are typically acquired with peak frequencies varying from 30 to 60 Hz, giving average vertical stratigraphic resolution of between ~23 m (30 Hz) and 11 m (60 Hz) in siliciclastic sediments.
Many stratigraphic architectural elements are typically at higher resolution, and their interpretability on seismic profiles is limited. The ability to compare outcrops directly with seismic objects has the potential to bridge this scale-gap. Seismic forward modeling of outcrops helps to address this problem, enabling the examination of known stratigraphy and depositional elements at various scales.
We discuss the construction of a fully three-dimensional geologic model, based on the Beacon Channel outcrop of the Brushy Canyon Formation of West Texas, which is interpreted as a sinuous slope channel. High-resolution 3D outcrop profiles were acquired using a ground-based lidar scanner. Stratigraphic surfaces and architectural elements investigated using traditional field methods were interpreted directly onto the digital data, similar to seismic interpretation, providing a truly 3D outcrop interpretation. These outcrop-constrained stratigraphic data were used to construct a 3D zone-based reservoir model. Petrophysical properties for producing analog reservoirs of similar depositional geometry were assigned to lithofacies zones, with the use of property transforms, allowing an accurate representation of intra-zone facies asymmetry. Seismic forward modeling of the reservoir model was carried out at several peak frequencies, allowing a direct comparison between outcrop architecture and its seismic image at varying resolutions. The results are also compared with other channelized outcrops along a slope to basin-floor transect through the Brushy Canyon Formation.