Exploration and Development Assessment of Cretaceous Reservoirs, Ladyfern Area, N.W. Alberta and N.E. British Columbia

Introduction

Cretaceous reservoirs in the Ladyfern area host initial marketable gas reserves of more than 3 TCF, and recoverable oil reserves of greater than 20 million barrels (Map 1). There is excellent potential for new discoveries, and for adding reserves within established pools, through systematic application of fundamental regional geology.

Recent exploration at Ladyfern has been driven by a huge discovery in the Slave Point Formation in early 2000. Many companies have established land positions, more than 100 deep wells have been drilled or are licensed, new seismic has been acquired, and new pipelines are delivering large gas volumes. As early as 2002, however, Slave Point production from the Ladyfern Field peaked and began to decline. New discoveries are required to support the massive new infrastructure, and to respond to ever-increasing North American gas demand. Slave Point exploration continues, but explorationists must look to other targets to sustain production volumes in the future.

Petrel Robertson Consulting Ltd. has completed a regional, multidisciplinary exploration and development study of Cretaceous reservoirs in the area covering Twp. 85 93, Rge. 1 22W6 (Fig. 1, Map 1). It adjoins Petrel's existing regional studies at Buick Creek, Silver/Dahl, and Bivouac, and builds upon the regional geological framework established by these studies. The Ladyfern area encompasses opportunities ranging from long-established pools which still hold development potential, to wide-open exploration fairways. Over much of the area, deeper targets have been the focus of exploration drilling, so there has been relatively little systematic exploration of the Lower Cretaceous.

At Ladyfern, Petrel has assessed Spirit River Formation reservoirs, in addition to the Bluesky, Gething, and Buick Creek reservoirs addressed in previous studies.

Background to the Area

The Ladyfern area encompasses flat to rolling topography, with predominantly forested to swampy terrain. To the southwest, it includes the northerly reaches of the more settled and developed Peace River Block. Much of the area is accessible in winter only, although a number of local permanent roads service existing petroleum operations, particularly on the B.C. side.

First Nations land claims can be an issue when considering ground access, particularly in B.C. Although established reservations are small, road blockades and other disruptions periodically occur in response to local disputes. In Alberta, the Chinchaga Wildland Provincial Park lands are not available for petroleum exploration activities (Map 1).

Oil and gas exploration has taken place in the Ladyfern area since the 1950's, driven initially by large Cretaceous gas discoveries at Buick Creek, and Triassic gas and oil on the B.C. side. Deep Devonian gas became a legitimate exploration target in the 1980's and 1990's, with significant discoveries at Hamburg, Cranberry, and Chinchaga on the Alberta side. The Ladyfern Slave Point discovery in 2000 re-invigorated the Devonian exploration effort, and pushed it westward into British Columbia.

As a result, gas pipelines and production facilities are relatively common in the western half of the Ladyfern study area. East of Range 12W6, pipeline access is focused around two or three major trunk lines.

Geologic Setting

Petrel Robertson has incorporated publicly-available data as of December 2002, but databases were updated in 2003 to include results from significant new wells. Cumulative production data on the production maps is current to early 2004. Regional maps are drawn at a 1:150,000 scale to encompass the entire study area on one map sheet.

The Ladyfern database includes 3952 wells (see Appendix 2 - tops database CD). All exploration wells were used, but selected wells were omitted in heavily developed areas, in order to keep the project at a manageable size. To establish a regional stratigraphic framework, we built 16 regional cross-sections at the Bluesky to Triassic level, and 6 regional cross-sections at the Peace River to Spirit River level, in a north-south and east-west grid pattern (Map 1; enclosed cross-sections). The Bluesky-Triassic sections were tied directly to existing cross-section grids in the Buick Creek, Silver/Dahl, and Bivouac project areas. Borehole logs from each well were compared to the regional cross-section grid, in order to interpret stratigraphic tops and thus regional map units. Well logs were also used to calculate values for net sandstone isopach maps, using a 75% clean gamma ray cutoff to determine net sands (except for the Spirit River, where a 21% porosity cutoff in clean sandstone was used). Logger KB values (from the GeoScout database) were used to construct structure maps. Where these were not available, the Government KB was used instead.

We logged cores from 153 wells, including representative cores from producing field areas, and most available cores from less-drilled exploration areas (Map 1). We judged this control to be sufficient to properly characterize reservoir characteristics and stratigraphic relationships. Each core was described in detail and a graphic core log plotted in AppleCore format (Appendix 1). We took numerous core photographs to illustrate reservoir characteristics (Appendix 3). Cores were sampled for petrographic analysis; selected photographs in Appendix 4 illustrate the petrography of each stratigraphic unit. Appendix 5 contains spreadsheets summarizing petrographic characteristics for each of the 99 thin sections examined.

Seismic modeling was undertaken with the goal of identifying seismic expressions of mapped stratigraphic features. We review this work in the Seismic Interpretation chapter.

Ward Hydrodynamics Ltd. assembled hydrogeological data, including: drillstem tests, production (buildup and AOF) tests, and gas, water, and oil analyses. Ward's interpretations and data are presented in Appendix 6. We have integrated these data with production information and geological interpretations in the Hydrodynamics and Production chapter.

With these geological, hydrogeological, and geophysical interpretations in hand, we conclude the report by summarizing regional exploration and development potential for each Cretaceous reservoir at Ladyfern. Productive examples are presented to highlight structural and stratigraphic controls.

Methodology

Lower Cretaceous rocks in northeastern British Columbia and adjacent Alberta were deposited near the western edge of the Western Canada Sedimentary Basin. During Late Jurassic and Cretaceous time, the collision of discrete landmasses termed allochthonous terranes with the western edge of the North American craton caused mountain building to occur. Stott (1984) distinguished two major orogenic episodes - the Columbian Orogeny in latest Jurassic and Early Cretaceous time, and the Laramide Orogeny in Late Cretaceous to Cenozoic time. During each episode, rapidly-subsiding foredeeps formed between the orogenic belt and the stable craton, and were filled with thick clastic successions shed from the rising orogen.

Stott (1984) delineated three major clastic wedges, corresponding to two phases of the Columbian Orogeny and the Laramide Orogeny. This study addresses strata from both Columbian clastic wedges: the Upper Jurassic - lowermost Cretaceous wedge contains Fernie and Buick Creek strata, while the second (Lower Cretaceous) Columbian wedge contains Gething, Bluesky, and Spirit River strata. A profound unconformity separating the two wedges can be mapped throughout the entire basin.

The Ladyfern study area lies at the eastern edge of the westerly foredeep, as shown by pronounced thickening of the Fernie-"Nordegg" and Bluesky-Gething sections to the southwest (Map 2, 5). Regional Jurassic / Cretaceous stratigraphy at Ladyfern ties well with the stratigraphic schemes developed in previous Petrel studies. The Hay River Fault Zone, a southwest-northeast trending Proterozoic structural belt, traverses the northern part of Ladyfern. PRCL (1997) reviewed evidence for reactivation of the HRFZ throughout the Phanerozoic, and showed that differential block faulting associated with the Zone influenced depositional and erosional patterns during the Late Jurassic and Early Cretaceous.



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Leslie Sears
Petrel Robertson Consulting Ltd.
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lsears@petrelrob.com
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