Abstract

The Jean Marie Member of the Upper Devonian Redknife Formation is a gas-saturated, dominantly limestone unit with minor dolostone that occurs throughout a widespread area in the subsurface of northwestern Alberta, northeastern British Columbia, and the southern Northwest Territories. In British Columbia this unit is estimated to most likely contain approximately 10 TCF of gas in place and 6.5 TCF of marketable gas.

This paper discusses the depositional facies framework, evolution, and reservoir architecture of the Jean Marie Member in the July Lake area of extreme northeastern British Columbia, based on the description and investigation of 18 cores from vertical wells drilled in the early 1990s. The Jean Marie in these wells varies from 11.7 to 21.2 m thick and consists of the upper portion of one Transgressive-Regressive (T-R) cycle and two other complete, overlying T-R cycles. The basal 3 to 4 m of the Jean Marie consists of brachiopod-bearing lime wackestones that were deposited on an areally extensive carbonate ramp. These carbonates were deposited during the regional progradation of limestones out over deeper-water shales of the underlying Fort Simpson Formation. The base of the second T-R cycle marks the change from regional progradation to the start of a more aggradational phase of sedimentation. The second T-R cycle attains a maximum cumulative thickness of 16.5 m and consists of coalescing patch reef deposits. The patch reefs are speculatively interpreted to be only 50 to 100 m across to begin with, but expanded in certain directions by lateral growth during progradation. The uppermost T-R cycle consists of either a continuation of these patch reef facies, the nucleation of Amphipora shoals on underlying T-R cycle 2 patch reefs or intervening, detrital stromatoporoid and coral-bearing foreslope deposits and deeper-water branching coral-bearing limestones. Unlike the second T-R cycle, foreslope facies of T-R cycle 3 consist of more fragmental carbonates with grain-support (pellet packstone) matrices. Progradation during this interval caused further coalescence of patch reefs and Amphipora shoals, and the appreciable infilling of low-lying areas between these reefs and shoals. This resulted in lower depositional relief at the top rather than at the base of this T-R cycle. Preferential partial dolomitization and dissolution of these deposits produced permeable strata between the patch reefs and/or Amphipora shoals.

Reservoir facies consist of two main types. Firstly, and most importantly, are lime framestones that make up the patch reefs of the second and third T-R cycles, consisting of in situ platy stromatoporoids and pendent Renalcis that grew downward into shelter cavities beneath the platy stromatoporoids. Porosity occurs mainly in molds or partial molds of Renalcis that are connected by cm-scale, non-planar fractures. Secondly are somewhat less fractured limestones and partially dolomitized limestones of the detrital stromatoporoid-coral foreslope facies that occur as prograding aprons around the patch reefs in the upper T-R cycle. Pores in this facies are predominantly micropores or coalesced micropores, of dissolution origin. The degree of calcite dissolution is generally greater in samples with a higher matrix replacement dolomite content.

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