Murray G. Gilhooly1, John A.W. Weissenberger1
(1) PanCanadian Petroleum Limited, Calgary, AB
ABSTRACT: Deep Gas Reservoirs in Upper Devonian Hydrothermal Dolomites, Modelled in Subsurface and Outcrop, Alberta, Canada
Hydrothermal dolomite bodies form important hydrocarbon reservoirs in the Wabamun Group (Famennian) of Alberta, Canada. An outcrop of Palliser Formation dolomite (equivalent to the subsurface Wabamun) at Three Sisters Pass, near Canmore, Alberta, displays spectacular hydrothermal alteration and porosity creation, described here for the first time.
The muddy carbonate ramp limestones of the Palliser contain a 75 to 100 meter wide domal body of hydrothermal breccia, with partly rotated blocks up to 5 meters long and abundant sparry cement. Underlying stratiform dolomite- at least 2 kilometers wide- displays zebra textures, crackle breccia with bed-parallel sparry cements and matrix dolomite, passing upward into the megabreccia. The contact between the breccia and the host limestone is sharp, with no apparent disruption or offset of overlying beds.
Preliminary fluid inclusion data show homogenization temperatures of 120- 160 oC for associated calcite cements and salinities of 16- 22 weight percent NaCl equivalent- analogous to ancient saddle dolomite (e.g. Davies, 1997). A hot brine bubbling-effervescence model (ibid.) may explain several features of this outcrop, including the domal shape and megabreccia fabric. Tight lime mudstone caprock and laterally extensive, coarse lower Palliser dolomite, implies some stratigraphic control.
Gas reservoirs in the Alberta Devonian resemble the Three Sisters outcrop in the distribution of dolomite, diagenetic fabrics and stratigraphic relationships. Seismic data and outcrop seismic models are also very similar. The Three Sisters outcrop extends the play fairway for Wabamun hydrothermal dolomites, and provides insight into reservoir scale heterogeneity, which affects pool exploitation.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado