The Origin of Matrix and Fracture Mega-Porosity in a Carbonate Bitumen Reservoir, Grosmont Formation, Saleski, Alberta
K.R. Barrett1, J. Hopkins2, K.N. Wilde1, and M.E. Connelly1
1Laricina Energy, Calgary, AB, Canada
2University of Calgary, Calgary, AB, Canada
The Grosmont Formation is an Upper Devonian carbonate succession that is located in northeastern Alberta. It is approximately 120m thick and consists of four shallowing upwards carbonate cycles, designated A,B,C and D in ascending order, that are separated by thin argillaceous markers. The economic interest in the Grosmont lies in the fact that it contains approximately 318 billion barrels of bitumen (AEUB, 2007). 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 of the Grosmont Formation. These zones are up to 12m thick with porosities commonly exceeding 30% and permeabilities that are too high for accurate core measurements but are known to 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, sonic log cycle skipping 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.
Not surprisingly, these mega-porosity zones have been described as caves created by karsting during the pre-Cretaceous erosional event. Data from Laricina’s five well drilling program from last winter, in conjunction with previous well control, provide evidence that is inconsistent with the use of the term ‘cave’ to describe these zones. Unlike caves which are narrow and unpredicatable in their distribution, these mega-porosity zones can be correlated throughout the Saleski area and show minimal thickness variation. It is felt that there is a strong depositional facies control on the development of these mega-porosity units that explains their regional extent.
A three stage diagenetic model is proposed to explain the development of porosity and fracturing in the Grosmont Formation:
1. The Grosmont was dolomitized during the Late Devonian or Mississippian. The degree of dolomitization varied within the Grosmont, favouring strata with high initial porosity and low lime mud and clay content.
2. The Grosmont was uplifted and eroded during the Jurassic which exposed it to the influences of meteoric water. Surface waters percolated down into the Grosmont, preferentially leaching calcite from the strata and enlarging existing pore systems through dissolution. The mega-porosity zones had the highest amount of leaching allowing porosity to exceed 30% and the enlargement of vugs to the extent that they could not support the overburden weight and collapsed into themselves.
3. Sometime after reservoir development, oil migrated into the Grosmont. This preserved the reservoir from the deleterious effects of subsequent porosity reduction. This oil was later degraded to bitumen.
The presentation will incorporate core, CT scan and SEM images along with XRD 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.
AAPG Search and Discovery Article #90075©2008 AAPG Hedberg Conference, Banff, Alberta, Canada