Central Alberta Foothills

Outcrop and Subsurface Study of the Upper Cretaceous Chungo Member and Lower Brazeau (Belly River) Formation, Central Alberta Foothills


Summary:
Location: T38-50, R11-26W5
Strata: Chungo/Belly River
Year of Study: 1987


Introduction

A field and subsurface investigation of the Brazeau Formation (Belly River equivalent) of the central Alberta Foothills has revealed several geological trends and manifestations which possess great significance with respect to exploration for Mesozoic targets within both the western Plains and central Foothills.

Due to lithostratigraphic complications, internal correlations within the Brazeau Formation are not possible on a regional basis. Thus, most emphasis was placed on outcrop sections with basal contacts which could be correlated, via a grid of cross-sections using outcrop and well data, into the subsurface to the east. Five surfaces were correlated on well logs: the top of the Cardium Zone, two intra-Wapiabi markers, the top of the Chungo Member of the Wapiabi Formation, and the top of a widespread coaly shaley zone found above the basal Belly River sandstone. The study area is located within the Foothills structural province and is dominated by northwest-striking folds and thrust faults. Northeast-striking strike-slip faults exposed at the surface may blend into a regional fracture set in the subsurface to the east.

The major potential reservoir bodies are Chungo marine sandstones and fluvial channel fills of the Brazeau Formation. Chungo sandstones, grossly equivalent to the productive Chinook sandstone in the Kakwa region, display a regional shale out within the disturbed belt to the east and south. These sandstones were deposited within a shelf setting. To the west, the Chungo incorporates increasing amounts of conglomerate in the uppermost portion in the form of channel fills which rest on a scoured surface cut into underlying marine sandstones. The major potential reservoir bodies located within the Brazeau Formation are lenticular, conglomerate-filled fluvial channels. Individual conglomerate channel fills are up to 18 metres thick and 100-200 metres wide. Most channel fills display northeast-oriented paleocurrents, although there is a subsidiary, but significant, southwesterly-directed paleocurrent component. These trends are reflected by the mapped trends located in the subsurface to the east. Several conglomerate-filled channels were noted along depositional strike which could not be followed into the eastern subsurface due to the paucity of well control.

Brazeau and Chungo coarse clastics were deposited in response to the Laramide Orogeny. The coarsening-upwards nature of Chungo sandstones, dominantly hummocky and swaley cross-stratification, eastern shale out, and western appearance of conglomerate-filled channels all indicate that the bulk of sandstone deposition was restricted to the foredeep, located in front of the rising orogenic belt. Eastward migration of deformation through time caused uplift and erosion of marine sandstones by fluvial channels. Brazeau strata display traits indicative of fluvial deposition in a molasse setting. This is borne out by the geometry and texture of sandstones, their deposition near, and in response to, a disturbed belt, and their encasement within dominantly muddy and coaly sediments. The great lateral and vertical complexity of sandstone, the overall dominance of mudstone in the sedimentary package, and the absence of marine indicators suggest deposition within anastomosed channels far removed from the marine basin.

Only one well has produced from the Brazeau-Chungo zone: 10-34-49-21W5M. It produced over 10,000 bbls of oil, and is significant in that it is located upon a structure west of those from which Belly River oil is obtained at Peco-West Pembina. Thus, migration and entrapment has occurred in the Brazeau strata of the Foothills, thereby making this area, which has historically been explored for Paleozoic targets, prospective for Upper Cretaceous oil production.

Several factors need to be considered in exploration for Brazeau reservoirs. Firstly, grain-supported conglomerates, which contain abundant primary porosity at the surface, make the best reservoirs. This lithofacies tends to occur within channel fills which, by their geometry, can be extremely elusive targets, even within the realm of conventional development drilling. It is likely that the best targets in the Peco Field have probably not been encountered at present due to the limitations of 160 acre (64 ha) spacing. Furthermore, several northeast-trending channel fills were encountered both north and south of the major producing unit at Peco. These other trends have yet to be actively explored in the subsurface to the east. Thus, the play can be chased both within and beyond the Foothills. To facilitate efficient exploration, a rigorously designed geophysical program geared to recognizing Brazeau channel fills within a highly structured area should be a necessary initial component of any exploration strategy.

For more information contact:

Leslie Sears
Petrel Robertson Consulting Ltd.
500, 736- 8th Avenue S.W.
Calgary, Alberta
T2P 1H4

Phone: (403) 218-1618
Fax: (403) 262-9135
lsears@petrelrob.com
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