Exploring for Deep Basin Gas Resources in the Western Canadian Sedimentary Basin: A Case Study of the Cutbank Ridge Cadomin Field
Brian Tuffs, James Wood, and Dan Potocki
EnCana Corporation, Calgary, Alberta, Canada
The Cadomin Formation in the Cutbank Ridge area is the north-westerly extension of the giant Elmworth Cretaceous deep basin gas system first described by Masters (1979). Recognition of this northwesterly extension of the proven unconventional Cretaceous Deep Basin hydrocarbon system has allowed EnCana to focus on cost controls, execution efficiency and sweet spot identification, which, coupled with robust commodity prices, has allowed for commercial development of this previously uneconomic gas accumulation.
The study area is located in northeast British Columbia and adjacent areas in west-central Alberta in the Western Canadian Sedimentary Basin (Fig. 1).
The lower Cretaceous Cadomin formation can be regionally correlated on well logs, core and outcrop from the Canada/US border in the south, to the Rocky Mountain foothills of northeast British Columbia in the north (to as far north as 56 degrees 30’ N; Mclean, 1977). – an overall extent of approximately 1000 kms by up to 100 km west-east. In the subsurface the Cadomin is bounded to the west by the Rocky Mountain Foothills and reaches an easterly depositional limit defined by the Fox Creek escarpment (Fig. 2).
Structurally, the area is bounded to the west/southwest by the thrusted units of the Rocky Mountain Foothills. Compressional structural deformation mainly took place during the Late cretaceous-early Tertiary, however onset of deformation began with the Late Jurassic – earliest Cretaceous Columbian orogeny (Bell and Babcock 1986). Subsequent Laramide deformation, uplift and erosion has removed in excess of 2500m of strata in western Alberta and adjacent NE British Columbia (Hitchon, 1984).
The Cadomin Formation is a laterally persisitent unit, generally between 5-40m thick, which extends the length of the southern Rocky Mountain foothills in Canada and extends into the subsurface in Alberta and British Columbia. Deposition of the Cadomin reflects the dispersal of coarse-clastics in the foreland basin in response to the tectonic events and hiatuses associated with the Columbian orogeny – it forms the lowermost preserved Cretaceous deposit and directly overlies the regionally extensive basal Cretaceous unconformtity. (Leckie and Cheel, 1997). The Cadomin Formation is difficult to age date due to its conglomeratic composition however a few shaley interbeds have revealed Barremian ages (Cant, 1996). Maximum thickness in the subsurface of approximately 170m is reached in the area southwest of Monias (T81, R24w6). Overall thickening to the west is considered to be due to increased subsidence and accommodation associated with development of the foredeep (Fig. 2).
In the subsurface, the Cadomin comprises a series of compound braid plains with sediment being regionally derived from the west and transported generally northwards within the confines of the Spirit River drainage system. Discrete stacked fluvial channel systems are preserved further to the east adjacent to the Fox Creek escarpment which bounds the easterly depositional extent of preserved Cadomin (Varley 1984).
The Cadomin Formation in the Cutbank Ridge area ranges from 5 – 10 m thick at the Alberta/BC border to a maximum of approximately 40 m within the Cutbank Ridge area of British Columbia. Lithologically the Cadomin in the Cutbank Ridge area is predominantly composed of interbedded chert pebble conglomerate and lithic sandstone. Examination of core and outcrop indicates conglomerate beds vary from several metres to layers only one pebble thick. Within the conglomerates, medium grained lithic sandstones form discontinuous lenses that are decimetres to metres thick and extend laterally from a few metres up to 150 m. Clasts are predominantly composed of chert with minor quartzite clasts present. Clast sizes range from 5-100mm and clasts typically display a high degree of rounding and variable sorting.
Cretaceous Deep Basin Gas System
The Cadomin formation forms the oldest stratigraphic unit of the Cretaceous deep basin system. Cutbank Ridge, in common with most Cadomin deep basin fields, is underpressured, however regionally a continuum occurs that links shallow, underpressured units with deep supranormally pressured zones (Putman and Ward, 2001). Cretaceous and Tertiary foreland basin filling resulted in deep burial and early hydrocarbon charging of Cretaceous reservoirs, whereas post-Laramide erosion has resulted in the removal of in excess of 2500m of sediment (Hitchon 1984). Areas of maximum uplift are located within the British Columbia portion of the deep basin.
Regionally, sonic transit time and vitrinite reflectance data from shales above the Cadomin both show changes in gradient at approximately 2000 m depth. This suggests that, at the time of maximum burial about 50 million years ago, high rates of hydrocarbon generation led to over-pressured conditions and cessation of porosity reduction by compaction. Subsequent uplift and erosion of overburden resulted in under-pressured conditions that characterize most of the Deep Basin today.
Initial reservoir pressures of most gas pools close to the Deep Basin edge are tied to elevation via gas columns that terminate at up-dip regional water systems. Gas pools below 2000 m are isolated from regional water systems and reservoir pressures are commonly tied to depth along a well-defined 12.2 kPa/m (0.54 psi/ft) pressure-depth gradient. This gradient is interpreted to reflect a pseudo-steady state condition with gas migration upward and outward through the shale-dominated Deep Basin balancing the rates of gas input at depth and gas loss at the upper edge. Interbedded Cretaceous coals are considered to be gas prone source rocks for much of the Cretaceous section.
The Cadomin Formation first received attention as an exploration target during the mid 1970’s and was a part of the massive Elmworth gas discovery by Canadian Hunter in 1976 (Gies, 1984). The Elmworth field has produced in excess of 1.28 Tcf to date from approximately 300 wells producing from multiple stacked Cretaceous reservoirs (including the Cadomin). Significant Cadomin gas production occurred prior to the Cutbank Ridge discovery in western Alberta at the Elmworth and Sinclair fields, however prior to pilot production initiated by EnCana in 2000/2001 there was no commercial gas production from the Cadomin in NE British Columbia. Historically, industry had drilled approximately 250 vertical wells that penetrated the Cadomin Formation in the Cutbank Ridge area of which 20 had flowed gas with no water recovery on drillstem tests. Overall along the Cadomin subsurface trend there are roughly 6000 penetrations – a subset of approximately 2000 wells was selected for petrophysical calculation of water saturations to define the regional Cadomin deep basin gas system. Regional mapping, core and thin section work along with completion data and reservoir modeling resulted in the drilling of a three well vertical pilot in 2001 to test the commercial viability of lower permeability reservoirs in the thicker Cadomin section within British Columbia. Vertical wells were fracture stimulated and came on at initial rates of approximately 1 MMcfd declining to 350 Mcfd after 6 months. These vertical wells showed a steep initial decline with subsequent decline rates flattening to become less than 10% per year. No formation water has been produced from these wells since production began in May of 2002. Based on detailed reservoir models which suggested significant lateral heterogeneities along with micro fracturing, horizontal wells were first drilled in 2002 and formed the basis of subsequent economic models for land capture. EnCana captured the bulk of the available land position during 2003 and has since drilled over 80 wells (including development, step-out and exploratory) and is currently producing approximately 50 MMcfd of gas with no formation water (water gas ratio of 2.5 bbl/MMcf). Moving to larger scale programs and concentrating on capital efficiency has reduced drilling costs by over 20% to date.
Estimates of original gas-in-place have been rigorously defined by log-derived porosities calibrated to full diameter core porosity analyzed at reservoir conditions. Additionally, petrophysically derived water saturation (Sw) has been tuned to core-based Sw determined from capillary pressure analyses of rock samples and to preserved oil based core Sw. Continuous gas saturation (10% Sg) is demonstrated to exist at porosities as low as 1%.
The Cadomin Formation in the Cutbank Ridge area is the north-westerly extension of the Elmworth Cretaceous deep basin gas system described by Masters (1979). In common with many other Cretaceous reservoirs within Alberta and British Columbia the Cadomin Formation at Cutbank Ridge is comprised of a downdip underpressured gas system with an updip, normally pressured water system. Detailed stratigraphic and structural analysis (including 3D seismic data), illustrate regional reservoir continuity across this updip transition. Locally the Cadomin gas accumulation was defined based on several hundred historical well logs, many with core and drillstem test data, along with the regional interpretation of several thousand Cadomin penetrations. Recognition of this unconventional hydrocarbon system has allowed EnCana to focus on sweet spot identification, cost controls and execution efficiency, which, coupled with robust commodity prices, has allowed for commercial development of this previously uneconomic gas accumulation. Currently the Cutbank Ridge field is producing 50 MMcfd of sweet, dry gas from the Cadomin with no formation water (water/gas ratios of approximately 2.5 bbl/MMcf). Continued drilling and investment is planned to double this rate to approximately 100 MMcfd by exit 2005 and future growth is expected to continue to economically exploit this multi Tcf resource.
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