Complex Deformation Models from the Debolt Gas Fields of the N.E. British Columbia Foothills, as seen on 2D and 3D Seismic

The interpretation of seismic data in the foothills is typically model driven in areas where the quality of this seismic data is moderate to poor in quality. The deformation model used by the interpreter will have a major impact on locating wells on the structures and calculating in-place reserves.

In structural geology terms we can define two end member deformation models: fault bend folds and detachment folds. A fault bend fold forms in the hanging wal1 of a fault above the footwall ramp or the hanging wall ramp. The displacement on the fault is constant across the structure. A detachment fold also forms in the hanging wall of a fault. In this case, the displacement on the fault dies out into the folded beds above the end of the fault. The mechanism of deformation for these two models varies considerably between them and is fundamental in understanding the internal bed geometry. When the seismic data is good the interpretation of the deformation models is not difficult; when the data is only poor or moderate it becomes much more difficult. In this case, a greater understanding is needed of the structural style of the area. This can be gained by the careful evaluation of the surface geology, well data, dipmeter data and regional seismic lines. To truly understand which deformation model is most appropriate to use in the interpretation, it may even become necessary to examine data sets from many miles away from the prospect

The Debolt gas fields of N.E. British Columbia are at latitude 52.25° and longitude 123.00°, approximately 100 miles N.W. of Fort St. John, British Columbia. They produce gas from the Mississippian, Debolt formation in four fields: Pocketknife, Sikanni, Buckinghorse and Grassy. These fields have from 3 - 25 Bet (85 - 700 10⁶ m³) of recoverable gas per well and produce at rates of 10 - 30 mmcf/d (280 - 850 10³ m³/d). Typically this gas has less than 1% H₂S and commands a premium price because of its strategic position on the Westcoast pipeline system.

This area of the foothills is an interesting mixture of good to poor quality seismic data. It is further complicated by having examples from both types of deformation model, fault bend folds and detachment fields. By using 2D seismic data (with thanks to TGS - Calibre Geophysical Company and Anderson Exploration Ltd.) and a 3D seismic data set (with thanks to CGG Geophysics Canada, a division of CGG Canada Ltd.) it is possible to use analogues from the good data area to interpret the poorer data.

On the regional seismic data structures are seen as a series of narrow en-echelon anomalies that have their long axis lying in the NNW-SSE direction. These structures are not typical fault bend folds. They exhibit several characteristics, such as disharmonic folding, forelimb thickening, inclined fold axis and back thrusts, that complicate the interpretation of the moderate to poor quality seismic data. It would appear that they have a large detachment folding component within them and interpreting them as a purely fault bend fold can lead to problems. For example, using a constant time isochron to "ghost in" the Debolt event from the better imaged Permo/Penn event can lead to overly optimistic reserves calculations and poor placement of well locations on these structures.

By using 3D seismic data, details of the time structure within the Pocketknife gas field can be interpreted. In this field the deformation model is further complicated by structural changes in the third dimension. On true strike lines, available from the 3D seismic data, a lateral ramp can be interpreted that cuts up the stratigraphic section of the south of the gas well c-70-E 94-G-7.

At the b-60-E 94-0-7 well, to the south of c-70-E-94-0-7 the Debolt event is lower by 60 milliseconds. This is enough to put the reservoir below the gas water line for the field. This explains why a good gas well is offset by a wet well, although the older 2D seismic data would indicate that the Permo/Penn at the wet well was higher on the time structure map. Once again the problem for the interpreter has arisen when he used the better imaged Permo/Penn event to ghost in the poorer imaged Debolt event using a constant time isochron.

Drilling successful oil and gas wells in overthrust belts, like the foothills of Alberta and British Columbia is a challenging experience. It requires a high level of expertise and good interdisciplinary management. An important part of this is the understanding of deformation models and how they relate to seismic data acquisition, processing and interpretation of structures in overthrust belts.