Structural Style and Fracture Development in a Tight-Gas Sandstone Reservoir from the Canadian Foothills*
Patrick Fothergill1, Dragan Andjelkovic1, Lloyd Murray2, Steve Vadnai2, Paul MacKay3
Search and Discovery Article #40457 (2009)
Posted September 30, 2009
*Adapted from expanded abstract at AAPG Convention, Denver, Colorado, June 7-10, 2009
1Schlumberger, Calgary, AB, Canada (mailto: [email protected])
2Devon Canada Corporation, Calgary, AB, Canada
3General Reef Corporation, Calgary, AB, Canada
Deep, clastic gas reservoirs in the foothills region of NE British Columbia and Western Alberta (Figure 1) are often located in the Jurassic to lower Cretaceous Nikanassin and Cadomin formations. These formations can have very low porosities and permeabilities, and typically only produce gas at commercial rates if they contain a network of open natural fractures. Determining the orientation and distribution of fractures down the wellbore is therefore critical for deciding which zones to complete. Furthermore, understanding the relationship between the natural fractures and the structure can be important for planning new well locations and targeting the most productive areas of a field.
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.
The nine wells considered in this study were drilled with an oil-base mud (OBM) and logged using a combination of micro-resistivity and ultrasonic borehole images. The micro-resistivity tool, in general, responds well to the small-scale porosity changes associated with bed-boundaries or internal stratification. The ultrasonic tool is extremely sensitive to cracks or openings in the borehole wall (Figure 2), and is therefore particularly suitable for fracture identification. When combined, these tools can be used to carry out detailed structural analysis and to identify, characterize and quantify natural and drilling induced fractures.
However, the borehole information is very localized and may not always reflect the structure away from the well. For this reason, 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 (Figure 3). In addition, 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 (Figure 4). 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.