Reservoir Heterogeneity in Marginal Marine Settings of the Athabasca Oil Sands: Apparent Continuity of Mud Laminae Between Closely Spaced Cores

Abstract

In the Athabasca Oil Sands Deposit, substantial bitumen resources lie along a secondary trend in the northwest. This trend is a major tributary to the main Athabasca trunk valley system of Lower Cretaceous time and is known informally as the Ells paleovalley. A comprehensive study of core near Brion Energy's MacKay assets indicates that mud beds and mud laminae deposited in marginal marine embayments within the Ells paleovalley probably have significant lateral extent. The study area includes two vertical wells spaced 15 metres apart located on Southern Pacific's McKay SAGD project. One is an older exploration well and the other was drilled as an observation well for SAGD operations. Both wells, 1AA/05-18-91-12W4 and 100/05-18-91-12W4, are cored and available in the public domain. These two cores contain multiple instances where patterns of mud bedsets and mud laminae within the prime reservoir sands can be clearly correlated from well-to-well at equivalent stratigraphic depths, indicating that individual mud laminae are continuous over 15 metres between the two cores. Correlative mud laminae appear to trap lean zones of identical thickness in both cores, presumably where structural closure is present. This observation provides further evidence for the continuity and seal potential of the mud laminae. The mud beds and laminae are interpreted to have been dynamically deposited from fluidized flow. Evidence of fluidized flow is provided by the presence of millimetre-scale current ripples in the muds. Correlative muds are observed at depths below, between, and above the trajectory of a nearby well pair at the Southern Pacific McKay SAGD project. The full lateral extent of the correlative muds may be several tens to hundreds of metres. In areas with SAGD potential where these mud beds may be abundant, they could act as tortuous steam baffles and may hamper gravity driven drainage. Although it is rare and extraordinary to have such closely-spaced cores, mud bed continuity may be inferred nonetheless in single cores from at least two phenomena: the presence of lean zones that are apparently trapped below mud laminae; and the presence of millimetre-scale current ripples in mud. A useful strategy in areas of risk may be to obtain core from closely spaced wells to determine the lateral extent of mud beds. This need not entail additional expense if scheduled observation wells can be cored and strategically placed in close proximity to older, cored, exploration wells.

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