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Structural Style and Fracture Development in a Tight-Gas Sandstone Reservoir from the Canadian Foothills

Fothergill, Patrick 1; Andjelkovic, Dragan 1; Murray, Lloyd 2; Vadnai, Steve 2; MacKay, Paul 3
1 Schlumberger, Calgary, AB, Canada.
2 Devon Canada Corporation, Calgary, AB, Canada.
3 General Reef Corporation, Calgary, AB, Canada.

Sandstone reservoirs in the Canadian Foothills are often tight, with production rates linked directly to the presence of open natural fractures. Understanding the orientation and distribution of fractures down the wellbore, and how they relate to the structure, is often the key to determining what makes a successful gas producer in this area. Although seismic sections can give a good indication of the regional setting, they are often difficult to interpret in these structurally complex areas, and they lack the resolution to identify fractures. For these reasons, the best way to carry out detailed structural modeling and quantitative fracture analysis on a wellbore scale is to use borehole images.

In this study, advanced borehole image analysis was carried out for nine wells from a fractured, tight-gas sandstone reservoir in NE British Columbia. The reservoir intervals are within the lower Cretaceous Cadomin Formation and the Jurassic to lower Cretaceous Nikanassin Formation. The borehole image interpretations have been combined with outcrop studies and established structural models to produce detailed single well cross-sections, relating fracture development to structural style. Stereonet analysis has been used to identity fracture trends within individual wells and over the entire study area.

In the better producing wells, the results show well formed hangingwall anticlines and footwall synclines, developed above and below relatively low angle, SW dipping thrusts. The interlimb angles are often tight, with stratigraphic thinning along the frontlimb of the anticline. In some cases, the trend of the structure identified from the wellbore (WNW) is at an angle to the regional trend (NW) interpreted from the seismic data. This may have led to some reservoir compartmentalization. In all of the wells, two significant fracture trends were observed: a primary NNE striking fracture set, transverse to the structure; and a secondary NW striking fracture set, longitudinal to the structure. The NW striking fracture set was best developed in the fold pairs above and below faults identified in the wellbore. Although the greatest fracture density occurs in the core of the folds, the analysis also identified additional targets in less immediately obvious horizons away from the highly deformed zones.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009