Print this pageProductive Wrench Grabens Imaged from 3-D Seismic Tiger Ridge Field, Blaine and Hill Counties, Montana
The Tiger Ridge field was discovered in 1966 and has produced over 288 BCF from fault traps created by gravitational sliding off of the Bearpaw Uplift during the Eocene (48-58 mybp). The primary reservoir is the marine shoreface facies of the Upper Cretaceous (Campanian) Eagle Sandstone, which has porosities ranging from 18-30% and permeabilities ranging between 50 and 250 md in the field area. Production occurs in fault block ranging in size from <100 acres to over 2000 acres, with individual wells producing between 50 mmcfg and 13 bcfg. Before 2006, exploration for these fault blocks was done using subsurface mapping and an extensive 2D seismic grid. This mapping had identified several large tear faults that separate the field into areas where the local faulting has a similar style and grain. These tear faults often bifurcate and create large grabens that have been considered non-prospective because the 2D data does not allow features within in them to be imaged and mapped with confidence. In early 2006 the first 3D survey in the field was acquired by Devon Energy. The Cricket 3D survey was designed to image the faults that control the gas accumulations and allow for continued development in the field. Using the 3D data, an area previously identified as a graben was found to contain several tilted fault blocks that provide a favorable trapping configuration for Eagle gas accumulations. Although in many areas the grabens are narrow and only a few hundred feet across, in this particular area the graben-bounding faults are more widely spaced, with the graben being several thousand feet across. Individual fault blocks within the graben that contain Eagle Sandstone cover about 35 acres and are rotated and tilted in a consistent direction. The graben-bounding faults exhibit both normal and reverse throw along their trace, indicative of lateral offset. Despite having minimal throw in some areas, these faults effectively separate fault blocks within the graben from productive fault blocks outside of the graben. This allows Eagle gas wells separated by only a few hundred feet and at a similar structural elevation to produce from separate accumulations with no interference across the graben-bounding faults. Since acquiring and interpreting the Cricket 3D survey, Devon has acquired two additional 3D surveys in the Tiger Ridge field and is continuing to identify and exploit accumulations that would not have been found using only 2D seismic data.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009