Petrographic and Hydrodynamic Evaluation for Belly River Formation. Peco-west Pembina Area, West Central Alberta
Location: T45-54, R11-19W5
Strata: Belly River/Brazeau
Year of Study: 1994
Location: T45-49, R11-17W5
Petrel Robertson has partially updated its previously completed Belly River evaluations in the Peco-Edson region of west-central Alberta, with the major aims being to determine the main geological controls on current pools and to develop new prospect and fairway leads. Analysis of log, core and drillstem test data from over 1000 wells has gone into the assessment, and several leads have been determined.
Belly River reservoirs were deposited as narrow, linear channel fills within a sluggish, muddy, anastomosed fluvial network. Productive units are restricted to the coarsest (medium-grained and coarser) deposits associated with channel fills.
These productive channel fills are located at a variety of stratigraphic levels, which makes mapping of individual trends difficult. Trend definition has been facilitated, in part, by the application of slice mapping techniques. Seventeen ten-metre thick slices were mapped from a coal datum upwards; it should be noted that commercial oil production is derived from as shallow a zone as slice 29 (i.e. 300 metres above the coal datum).
Oil pools have been created by two major means:
- stratigraphic pinchouts, which are concentrated mainly in the east; and
- fault traps, caused by segmentation of elongate channel fills by shallow faults, primarily in the west.
There is a significant hydrodynamic influence on pooling and prospectivity. In the east, areally small (</= 1 section), prolific light oil pools with little or no gas (e.g. Pembina DDD and Cyn-Pem C pools) are located where sandstone pinchouts perch upon small structures surrounded by dynamic aquifers, in which water movement is strongly down dip (i.e. to the west). Several other potential traps of this type may exist in the northeastern portion of the study area. These pools appear to have an affinity with present-day rivers, reflecting the hydrodynamic continuity from surface to subsurface.
Further west, all significant porosity is charged with underpressured hydrocarbons, and gas caps or high GOR oil pools are common. In this setting, large pools or pool constellations are present (e.g. Peco) with very large oil columns. There are several potential Peco-like targets present in the study area, including Owl River (45-15W5), south of Peco at 45-14W5, Fickle lake (51-19W5) and Brazeau River (48-13W5), as well as southwest and northeast extensions of the main Peco fairway within slice 2. The best potential rock properties are found within slices 8 and 9 (porosities up to 13%; permeabilities up to 87mD), at 6-15-45-14W5M where no tests have been done and few wells are located.
The general underpressuring of the Belly River zone combined with an emphasis on deep, high-pressure gas drilling, suggests that much formations damage has occurred due to mud invasion. Several bypassed wells have been noted, based on drill stem test chart analysis, but this total must be viewed as a minimum estimate due to the general absence of tests and cores in the Belly River Zone, especially in the wells removed from the main Peco trend in 46-16W5, 47-15W5 and 47-14W5.
Results from a previous structural and stratigraphic investigation of the Belly River Formation in the Peco-West Pembina area were merged with a petrographic and hydrodynamic study. The resulting project indicates the presence of a largely bypassed "Deep Basin" style of oil play in much of west-central Alberta.
Belly River reservoirs are characterized by a complex diagenetic history within a predominantly fresh to brackish water regime. Compaction, several phases of cementation and mineral dissolution are all processes which have influenced porosity evolution. Most porosity is secondary in nature, and the most porous and productive zones are related to late fracturing associated with Laramide tectonics.
Reservoirs are not sensitive to formation damage by clay minerals, which are a relatively insignificant constituent. Migration of chlorite rims may cause some pore throat blockage. Iron-bearing minerals, predominantly ferroan calcite, are a potentially greater source of damage if the reservoirs are subjected to acid treatments. The greatest cause of formation damage appears to have been by mud invasion into the reservoir due to the employment of overbalanced muds used in the course of drilling deeper targets.
The Belly River is a regionally underpressured unit with relatively high pressure aquifers in the shallower, eastern region and less pressured, hydrocarbon-bearing zones in the deeper parts of the basin in the west. Groundwater flow is generally westward and water salinities and oil gravities show systematic trends, with more brackish waters and higher oil gravities in the west. Progressive westward underpressuring is due to post-orogenic uplift and erosion which resulted in more sediment stripping in the west.
Many bypassed zones have been identified within the Belly River, and extensions of several known pools may exist. The geological setting of the Peco-West Pembina area extends along depositional and structural strike to the northwest and southeast. In these settings, other Belly River accumulations are likely to be encountered.For more information contact:
Petrel Robertson Consulting Ltd.
500, 736- 8th Avenue S.W.
Phone: (403) 218-1618
Fax: (403) 262-9135