Differential entrapment of charged oil – New insights on McMurray Formation oil trapping mechanisms

Abstract

The Lower Cretaceous McMurray Formation is the primary host of the Athabasca oil sands deposit, one of the largest petroleum deposits in the world. Regional studies show that within the McMurray Formation, bitumen-saturated reservoir sands are encountered within the western, central and northeastern sections of the northeastern Athabasca deposit, while the southeastern part of the deposit has never been charged by petroleum, and that, the lateral contact between petroleum- and water-saturated reservoir sands is in some instances characterized by rapid changes in bitumen saturation, even between closely spaced wells. In the literature, a number of petroleum entrapment schemes have been proposed to explain how the bitumen accumulated through different trapping mechanisms, however controversy remains.This paper investigates the concept of inter-compartmental petroleum “fill and spill” charge and entrapment in a setting where compartments are clearly defined by mud-filled channel deposits. Classical molecular maturity parameters based on gas chromatography – mass spectrometric analysis of hydrocarbon compounds that are extracted from the bitumen show notable changes in composition between the geologically defined reservoir compartments. Relatively higher maturity sourced oil resides in the western compartment while maturity decreases to the eastern most compartment suggesting that the eastern compartment was filled by petroleum of lower maturity which has been displaced via fill and spill as petroleum migration from an increasing maturity source enters compartments to the west. Oil saturation and hydrocarbon geochemistry results also suggest the oil charge was very limited locally and individual compartments located towards the eastern edge of the Athabasca may not have seen the multiple charges evident in the oil sands to the west.The concept of inter-compartmental fill-and-spill provides new insights regarding the often overlooked intra-formational geological controls on reservoir charge in the McMurray Formation. The concept explains that apparently sharp lateral oil–water contacts are in fact due to mud-filled channel deposits which at least locally serve as lateral seals. This concept should be applicable to other meandering fluvial belt reservoirs worldwide and suggests a necessity to revise existing stratigraphic trap schemes by including the point bar stratigraphic play type of trap as one of the trapping mechanisms. Additionally, geochemical gradients can be used as a tool for defining the presence and extent of individual compartments as well as the level of fluid communication as an aid to well placements.