Controlling Factors and Mechanisms in the Formation of a Muddy-Normal Point Bar: A 3-D Architectural-Element Analysis of a Heterolithic Point Bar in Dinosaur Provincial Park, Alberta, Canada

Abstract

Mud-dominate point bars show deposition by tidal influence, bar tails, or counter point bars. Less understood are point bars that lack these depositional characteristics but are mud dominate. These “muddy-normal” point bars are common in hydrocarbon reservoirs. A better understanding of muddy-normal point bar formation and impact on reservoir quality will assist in establishing predictive relationships that aid in production and exploration. Late Cretaceous fluvial strata of the Dinosaur Park Formation in the Steveville badlands of Dinosaur Provincial Park, Alberta are targeted to address this issue. A complete muddy-normal point bar within these strata that has excellent 3D expression was targeted. The goal was to determine processes by which normal-muddy point bars form using LIDAR renderings and field mapping to develop a 3D model. Strikes and dips, paleocurrents, and multiple stratigraphic columns were collected to determine accretion trajectories and lithologic trends. Surfaces were mapped following rules of architectural-element analysis. This point bar was 8-10 m thick with altering layers of sand and mud–mud comprising over 50% of the bar. Flood plain deposits stratigraphically confined the point bar displaying a single complete bar. Within the point bar, grainsize had an upward fining trend for both mud and sand layers. The mud layers are silty mudstones and the sand layers are medium grained at the base and fined up to layers of lower fine sand at the top of the bar. Mud layers within this point bar are thick-bedded and have current ripples indicative of deposition by transport. This suggests that the mud layers were deposited by active accretion events and are not drapes. This point bar also consists of accretion packages with differing orientations. Accretion packages were 3-4 m thick and 10-15 m long. Sand content shifts between packages and alternating packages have contents more consistent with typical sandy point bars. Both the sand-rich and mud-rich packages contain surfaces with alternating orientations but smaller scale. Both packages do not appear to reflect location within the bar. These data suggest that the muddy deposits of the muddy-normal point bar reflect changes in bar trajectory and sudden temporary adoption of accretion orientations not conducive to sand deposition, and do not record either late stages of growth in the overall bar formation process, deviations from fully fluvial drivers, or counter point bar patterns.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016