The Heterolithic Point-Bar of the l'Eyre Estuary (Arcachon Bay, France) — A Partial Analog for McMurray Formation Estuarine Reservoirs, Alberta, Canada

Abstract

The bitumen of the middle McMurray is principally hosted in tidally-influenced point-bar deposits, for which there are few known applicable modern analogs. The heterogeneities and discontinuities of these sedimentary bodies create complexity for economic bitumen extraction. In order to better understand these tidally-influenced reservoirs, an investigation of a modern point-bar located in the L'Eyre estuary (Southwest France) was undertaken. The relatively small “Tuiles” point-bar (∼100 × 150 m in size) is controlled by fluvial hydrodynamics modulated by tidal currents. Ten oriented vibracores, 4-6 m long and spaced 20- 40 m apart, were recovered in transects perpendicular and parallel to the river. From the cores, high-resolution photographs, X-ray scanning, sedimentological description, grain size analysis, porosity analysis on thin sections were carried out to identify six facies divided in two major associations. A 2-4 m thick lower sandy unit, located at the base of the point-bar and laterally continuous across the point-bar, is composed of coarse to fine sands with ebb and flood dunes. A 1-1.5m thick upper heterolithic unit overlying the sandy unit consists of alternating mud and sand layers. The principal facies are, from the bottom to top: 1) a channel lag consisting of gravels and mud clasts deposits; 2) a 2-4 m thick fining up sand unit including ebb- and flood-oriented dunes devoid of mud drapes; 3) a 1-1.5 m thick inclined heterolithic unit (IHS) with sand layers decreasing in the upward and downstream direction. Hydrodynamic processes can explain the upstream-downstream decrease of heterolithic deposits. During annual seasonal winter floods, when the fluvial discharge is high, tidal currents do not reach the point bar location and sand layers are deposited across the top of the point-bar by fluvial current. During summer, when the river level is low, a mud layer is deposited at the surface of the point bar under the influence of tidal currents. Both upstream and downstream parts of the “Tuiles” point-bar heterolithic unit are heterogeneous, but the upstream part contains less mud layers than the downstream part. So, along the yearly cycle, the tidal influence is better registered in the downstream part of the point bar. The “Tuiles” point-bar significance as an analogue is to exhibit a tidally-influenced lower sandy point-bar facies overlain by inclined heterolithic stratification, a succession that typifies the McMurray Formation.

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