Hondo Evaporites in the Grosmont Heavy Oil Carbonate Reservoir, Alberta, Canada
The Upper Devonian Grosmont shelf complex is the world’s largest heavy oil deposit hosted in carbonates, with an estimated 318 billion barrels of bitumen in place. The Grosmont reservoir is not yet under production. Reservoir evaluation includes constraining the nature of deposition of evaporites and their spatial distribution.
The Grosmont complex is subdivided into four shallowing-upward cycles: Lower Grosmont, Grosmont1, Grosmont2, and Grosmont3. The Hondo Formation is an evaporitic sub-unit within the Grosmont and replaces part of the four cycles in some areas of the complex, especially at the top.
This study involves core examination,facies description, isotope analysis,and log interpretation. Detailed petrographic analyses of 19 cores, and cuttings from 75 wells reveal that ‘primary’ evaporites are present in 4 cores, and evaporites occur in cuttings of 22 wells. The thickest continuous evaporite core section is 25m long and contains five lithofacies: laminated anhydrite in dolomudstone, mosaic anhydrite with algal mats, enterolithic anhydrite, nodular anhydrite, and synsedimentary breccias. Most of these lithofacies appear to be marine-subaqueous ‘primary’deposits, and many of the microtextures suggest original formation as gypsum. Additionally, diagenetic ‘secondary’ anhydrite fills fractures and molds, both within the Hondo and in the adjacent strata. Halite is conspicuously absent today, but halite may have been deposited in at least in parts of the region as suggested by isolated molds of halite hoppers and questionable syndepositional breccias.
The composition of the brines that formed the ‘primary’ sulfates is interpreted using isotope analyses. Most 87Sr/86Sr values of anhydrite vary between the range of 0.70806 and 0.70833 (n = 12), which are within the known range of Frasnian seawater. Moreover, data from δ34S vary from 23.7 to 27.9 permil CDT and δ18O from 14.3 to 16.6 permil SMOW, supporting that the saline brines were of marine parentage.
It appears that the Hondo evaporites were deposited in an oblong area that measured about 40x150 km. It is not clear, however, whether this area was one large ‘salina’ or envelopes a series of relatively small brine ponds on the much larger Grosmont shelf.In the eastern part of the study area, the Hondo appears to be dissolved and replaced by solution-collapse breccias; the significance of these findings for heavy-oil trapping and reservoir exploitation are under further investigation
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009