Brekke, Howard1, Richard Evoy1
(1) Petro-Canada, Calgary, AB
ABSTRACT: Use of Dipmeter Data in the Definition of the Internal Architecture of Point Bar Deposits in the Athabasca Oilsands: Implications for the Middle McMurray Formation in the Hangingstone Area, Alberta
Multi-story point bars deposited by an aggrading and meandering estuarine system is the
common geological model for the McMurray Fm. However, new interpretation of dipmeter data
from the Hangingstone area is compatible with a pool consisting of a single, 35 metre
thick point bar deposited by a brackish river that avulsed from the main northward-flowing
McMurray Valley System to the east. This contrasts with the regional lower and middle
McMurray continental to tidal flat facies that prograded from the south and dip northward.
Surfaces interpreted from Formation Micro-Imager and dipmeter logs were identified, and facies from core were assigned to the surfaces. The dominant facies, inclined heterolithic stratification (IHS), consists of epsilon cross-stratified (ECS), interbedded sands and muds characterized by a common azimuth, and a dip profile which gradually steepens, then gently flattens. Mud beds within the IHS are lightly to moderately bioturbated, trace fossils are diminutive and have a low diversity. Two subordinate facies are mudclast breccias, which typically have no detectable bedding, and sands with steeply dipping beds.
The IHS beds are here interpreted as lateral accretion surfaces that consistently dip southward; dip azimuths radiate outward in a fan shape over an area roughly six by ten kilometers. In many of the wells, the middle McMurray consists of an ECS dip pattern, implying a single point bar. The steeply dipping sands represent current beds indicative of flow from the northeast. Locally, minor features such as current beds or breccias disrupt the continuous ECS pattern.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.