The Role of Bioturbation in Low Permeability Gas-Charged Reservoirs

George Pemberton¹, Murray K. Gingras¹, and James A. MacEachern^2
1Earth & Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada
²Earth Sciences, Simon Fraser, Burnaby, BC, Canada

Recent research shows that ichnology has significant applications in production geology. Permeability enhancement in bioturbated media has been recognized in five interrelated scenarios: 1) surface-constrained textural heterogeneities; 2) non-constrained textural heterogeneities; 3) weakly defined textural heterogeneities; 4) diagenetic textural heterogeneities; and 5) cryptic bioturbation. Of the aforementioned categories, discretely packaged and weakly defined heterogeneities, and cryptic bioturbation represent primary fabrics generated by burrowing infauna. Burrow-associated enhanced permeability is accorded little consideration in the geological literature. One reason, perhaps, is the matter of scale. Permeability of geological media is a bulk character, and the 3-D arrangement of sediment heterogeneity must be understood if flow behavior is to be predicted. Unfortunately, bulk characteristics of sediment dominated by small-scale heterogeneities are difficult to assess and model. Overlooking the potential impact of these heterogeneities can lead to inaccurate assessment of the flow characteristics of a reservoir, and misidentification of permeability streaks. This is especially important in gas-prone reservoirs, where slight variations in permeability can affect storativity and resource deliverability. Equally important is the recognition that hydrocarbon production from bioturbated rock is generally more complex than from laminated media. This is because flow paths through burrow-related flow conduits are comparatively tortuous. Burrows may provide flow conduits that interact extensively with the surrounding matrix; their tortuous nature implies that dead ends and cut-offs are common. An understanding of how burrow-associated heterogeneities control fluid flow within sedimentary units is necessary, if production from bioturbated reservoirs is to be optimized.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery