Improving Fold and Thrust Belt Imaging, Wyoming Thrust Belt, Wyoming - A Case Study
Authors: Feeley Missy , Parry Mervyn , Becker Thomas , Feldman Howard , Boothe David 
Published: May 2016

The LaBarge area of the Wyoming Range is located along the western edge of the Green River Basin in westcentral Wyoming. ExxonMobil's predominantly gas and minor oil production from the LaBarge area began in 1953 from the Cretaceous Frontier Formation with additional production from the Mississippian Madison Formation in the mid 1980's. Methane gas production from these main producing intervals currently exceeds 200 Mcf/d from ExxonMobil leases. In 2004 ExxonMobil undertook a 4 year effort focused on improving seismic imaging over the extensive LaBarge structure with the primary objective to improve understanding of the complex structural framework and fault geometries that control productivity trends and to identify new opportunities on the existing leases.

The existing 3D seismic data consisted of four separate legacy 3D datasets (vintages 1999 - 2005) which proved inadequate to unravel the complex structural history. Improvement in the seismic imaging was undertaken in a two phased approach - 1) new data acquisition in mountainous terrain to the west along the Darby Thrust, and 2) a full pre-stack merge and migration of the 4 existing 3D seismic datasets with the new acquisition.

New data acquisition in 2005 and 2006 in the high altitude, forested, rugged mountains west of the LaBarge 3D was undertaken on predominantly public land administered by the U.S. Forest Service and the U.S. Bureau of Land Management, a popular destination for hiking, camping, and other activities. The terrain required predominantly helicopter - portable operations with shot-hole drilling requiring 70% heliportable drilling for dynamite sources. Significant safety challenges during acquisition included steep, rugged terrain, tree dead fall on heavily forested slopes, weather extremes and significant elevation changes. The survey was completed without a Lost Time Incident (LTI).

During Phase 2 of the seismic imaging improvement effort the seven diverse 3D surveys were then merged. The seven surveys spanned varying topography ranging from elevations of 6,500ft to 10,800ft. Factors such as wide variations in near surface velocities and a combination of vibroseis and dynamite sources contributed to the generation of a wide range of seismic noise types throughout the merged survey area. In all, 8–10 applications of noise attenuation are applied with flows and parameters customized on a survey by survey basis.

A Kirchhoff PSTM across the full pre-stack merged dataset significantly improved the imaging of the steeper dips and provided sharper fault planes. Improved fidelity provided by the prestack imaging significantly improved interpretation of geometric relationships critical for assessing new exploration opportunities.

This presentation will focus on specific learnings from the 4 year effort in 3 principal areas - 1) data acquisition in rugged mountainous terrains, 2) noise attenuation, and 3) velocity model building and merging of legacy datasets in complex terrains. The business impact of the improved imaging on the structural interpretation and subsequent drilling program will also be discussed.