The Origin of matrix and Fracture mega-Porosity in a Carbonate Bitumen Reservoir, Grosmont Formation, Saleski, Alberta

Paper Number: 2008-344

Authors: Kent Barrett, Ken Wilde, Marnie Connelly, - Laricina Energy Ltd., and
John Hopkins – University of Calgary

Source: Presented at the World Heavy Oil Congress March 10 – 12, 2008

Copyright: Copyright 2008, Petroleum Society of CIM

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This paper describes a three stage diagenetic model to explain the development of porosity and fracturing in the Grosmont Formation.

The Grosmont Formation is an Upper Devonian carbonate succession that is located in northeastern Alberta. It is approximately 120m thick and consists of four shallow marine carbonate cycles, designated A, B, C and D in ascending order. The economic interest in the Grosmont lies in the fact that it contains approximately 318 billion barrels of bitumen. At Saleski, the upper two cycles of Grosmont are bitumen bearing and have up to 40m of net pay.

One remarkable aspect of the Grosmont geology in the Saleski area is the presence of several mega-porosity zones within the C and D cycles. These zones are up to 12m thick with porosities exceeding 25% and permeabilities that are too high for accurate core measurements but are known to occasionally exceed 10 darcies. Visual inspection of the core reveals a massive bed of bitumen with imbedded angular dolomite breccia fragments. The main petrophysical responses of these zones include caliper log breakout, anomalously low sonic velocities and high porosity values recorded by the neutron and density porosity logs. They offer little resistance while drilling and often more bitumen than rock is recovered during coring.

The presentation will incorporate core, CT scan and SEM (scanning electron microscope) images along with XRD ( x-ray de-fraction) data to make the case for the diagenetic model. Core photos from a recent Laricina well will also be used to illustrate the sequence of events that led to the evolution of a Grosmont C bioturbated facies into a mega-porosity zone.

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