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Triassic doig formation sand bodies in the Peace River area of western Canada : depositional and structural models, and the impact of diagenesis on reservoir properties Harris, Richard Gordon


Middle Triassic Doig reservoirs in the Fireweed, Buick Creek, Cache Creek and West Stoddart fields (94-A-13 to Twp.86, Rge.18) of northeastern British Columbia consist of deltaic and shoreline sands encased in shelf and offshore mudstones and siltstones. The reservoirs comprise a series of northwest and northeast trending elongate sand bodies that lie along a south-southeast depositional trend analogous to the position of the original Doig shoreline. Sediments of the Doig Formation are divisible into two facies associations and ten lithofacies representing deposition in shelf to offshore, and deltaic and inter-deltaic environments. Hydrocarbon producing intervals consist of clean, very fine to fine grained, sub-lithic to quartz arenites, inter-bedded sandstones and bioclastic detritus, and disseminated bioclasts. Effective porosity is primarily inter-granular in the sandstone facies with significant moldic and intra-granular porosity developed in the coquina facies at the West Stoddart and Cache Creek fields. Average porosities range from 6.5 - 9.5% for sandstone lithofacies and 4.9 - 8.6% for coquina lithofacies. Pore occluding cements are mainly calcite in the northwest part of the reservoir trend to dolomite and anhydrite in the southeast. The sedimentology and facies architecture of recent discoveries at Cache Creek and West Stoddart contrast with those identified in previous studies of Doig reservoirs at Buick Creek in northeastern British Columbia and Sinclair in west central Alberta. A three-dimensional facies model of the Doig Formation at the Cache Creek, West Stoddart and Fireweed fields depicts sandstone deposition in a deltaic environment as distributary channel fills and slumped delta front deposits. Data for the Buick Creek field confirms and extends the incised shoreface model to include fluvial or tidal channels deposited laterally continuous in the same systems tract as the shoreface sands. Sand bodies for all four fields were deposited contemporaneously as the Doig shoreline prograded over mudstones and siltstones of the Doig shelf. Seismic and well data reveal tectonic control on the position and orientation of the Doig reservoir trend. Synsedimentary growth faults control the location and geometry of thick sand bodies along the reservoir trend in the Fireweed area. Diagenetic controls along the reservoir trend include the precipitation of calcite in the near surface and shallow burial realm, fracturing of cemented horizons prior to extensive burial and dissolution of cements and framework grains in three distinct episodes. Multiple dissolution phases created moldic, vuggy and inter-granular secondary porosity. Reservoir quality and production from Doig Formation sand bodies in the Peace River area of Western Canada are significantly impacted by the preservation of inter-granular porosity and fracturing related to the distribution of early calcite cements. The early calcite cements were sourced from bioclastic debris and calcareous mudstones distributed during sand body deposition. The formation of open fractures during early diagenesis enhanced both the secondary pore network and the permeability of the West Stoddart and Cache Creek Doig pools. Sand bodies with only minor interstitial calcite have extensive porosity loss by compaction and precipitation of authigenic quartz. A fairway for Doig sand body exploration is constrained by production and core analysis data, structural and depositional models, thermal maturity data and diagenesis.

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