Using depth imaging tools to improve the interpretation of overthrust geology

In the 2019 Silver anniversary of the 3D Seismic Symposium held by the Denver Geophysical Society in Colorado on March 29, 2019, Robert Horine (Devon Energy) presented a case study of depth imaging work done to improve the seismic image over the Beaver Creek field in Fermont County, Wyoming. The case study was co-authored by Marianne Rauch-Davies, and Mike Bradshaw, Jeff Codd and David Kessler (SeismicCity Inc.). The resulting PSDM data over the field has helped improve our understanding of the structure of the producing field. 

The Beaver Creek Field in Fremont County, Wyoming is a classic Rocky Mountain thrust anticline with productive intervals from the Cretaceous down to the Mississippian. The deepest reservoir is the Madison Limestone which was discovered in 1954 and had by 2008 produced ~45 MMBO. In an attempt to boost production Devon Energy in 2008 decided to initiate a CO2 flood. This required drilling several new wells which encountered unexpected structural complexity. At that time the existing seismic data was processed through prestack time migration (PSTM). The PSTM images could not resolve the complexity of the geology. Following a significantly improved imaging using prestack depth migration (PSDM) at the adjacent Big Sand Draw field, a project was launched in 2016 to apply PSDM to the existing 3-D dataset at Beaver Creek.

The challenge in the application of PSDM is the ability to construct the optimal earth model used as input to PSDM. Application of PSDM will result with a reliable seismic image only in one condition – if the appropriate anisotropic model is constructed and used as an input to PSDM. For the complex geology of the Beaver Creek field this means construction of a faulted anisotropic model followed by high fidelity PSDM. Two types of PSDM algorithms are commonly used. The first is a ray based Kirchhoff summation PSDM (KI PSDM). KI PSDM is very flexible and relatively lower in computational cost. To ensure the best PSDM image possible, a much higher computational cost PSDM algorithm, Reverse Time Migration (RTM PSDM) is used as well. RTM PSDM uses the total recorded wavefield to produce a final PSDM image. The preferred way to apply RTM PSDM is by using an algorithm that can migrate the full frequency range, but without generating any computational noise such as numerical dispersion. To achieve this goal, a recursive operator RTM PSDM was used for imaging of the Beaver Creek seismic dataset. Close examination of the PSDM algorithms used shows areas beneath the fault where the RTM PSDM image matches the geology better compared to the ray based KI PSDM.

Application of PSDM is an iterative process where interpretation is done as part of the model building work. Advances in model building technology enable us to construct complex 3-dimensional earth models that result in superior imaging. Advances in implementation of PSDM algorithms enable us to use both ray based and wave based algorithms. This results with seismic images that reveal both the large scale geological features, as well as smaller features such as correct positioning of faults. Utilizing these new advances in processing technology enabled us to optimize the production plans of the Beaver Creek field.

PSDM of the Beaver Creek field. Left: Kirchhoff summation PSDM. Right: High frequency (60Hz) Reverse Time Migration (RTM) PSDM.

The authors would like to thank Devon Energy Corporation and SeismicCity for permission to publish this paper and BPX Energy and Seismic Exchange, Inc. for permission to show the seismic data. Portions of this data are owned by Seismic Exchange Inc. The interpretation is that of Devon Energy Corporation.