Characterization of Discordant Surfaces within Tidally Influenced Point Bars: Implications for Fluvial System Evolution and Reservoir Development

Durkin, Paul R.; Hubbard, Stephen M.; Leckie, Dale; Boyd, Ron; Suter, John R.

Current point bar depositional models describe lateral accretion on the inner bend of meanders and erosion on the outer cut-bank, with little discussion of intra-point bar erosion and re-orientation. Analysis of outcropping point bar deposits in the Horseshoe Canyon Formation (Cretaceous, Alberta) shows the lateral accretion model to be overly simplistic. Point bar evolution here is punctuated by re-orientation events, which are associated with erosion into previously accumulated lateral accretion deposits and a slight shift in the direction of accretion (up to 30°). Point bars consist of inclined heterolithic strata (IHS), dipping at 8-12°, which are truncated by more steeply dipping (~15°) erosion surfaces. The origin of the re-adjustment surfaces is unclear, but they may be associated with major storm or flood events; paleoflow data suggests flood tide-dominated sediment transport and re-orientation may be associated with episodic high-energy fluvial discharge.

A 3 km long outcrop through a 12-16 m thick meander belt is exposed along a series of tributaries to the Red Deer River valley in central Alberta, 12 km southeast of Drumheller. The outcrop shows a transition from sand-dominated deposits to IHS dominated deposits along the main point bar studied, captured in 30 detailed sedimentary sections. Individual accretion surfaces, as well as intra-point bar erosion or re-orientation surfaces, were surveyed using a high-resolution (10 cm) differential GPS unit. Outcrop data were then imported into reservoir geomodeling software so that surfaces could be generated in 3-D. Re-orientation surfaces bound distinct lateral accretion packages that are each characterized by unique orientations of point bar migration.

Erosion surfaces attributed to point bar re-orientation have been described previously; however, their impact on the stratigraphic record is commonly overlooked. These surfaces play a key role in the continuity of siltstone layers, which may act as baffles or barriers to fluid flow in subsurface reservoirs. The quantitative data collected suggests numerous beds are discontinuous due to intra-point bar erosion, which positively impacts reservoir development and forecasting within analogous heterolithic point bar deposits of the Athabasca Oil Sands.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013