AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Bio-Pipeline Networks in Low Permeability Gas Charged Bioturbated Reservoirs
(1) Earth & Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada.
Permeability enhancement in bioturbated media has been recognized in five interrelated scenarios. Of these categories, discretely packaged and weakly defined heterogeneities, and cryptic bioturbation represent primary fabrics generated by burrowing infauna.
Burrow-associated enhanced permeability is afforded little attention in the geological literature due to a matter of scale. The permeability of geological media is a bulk character. Thus, the three-dimensional arrangement of sediment heterogeneity must be understood if flow behavior is to be predicted. Unfortunately, the bulk characteristics of sediment dominated by small-scale heterogeneities are difficult to both assess and numerically model. Overlooking the potential impact of these structures can lead to inaccurate assessments of the flow characteristics of a sedimentary rock, and misidentification or non-recognition of permeability streaks in a hydrocarbon reservoir. This is especially important in gas-prone reservoirs where slight variations in permeability can have dramatic effects on storativity, reserve calculations and deliverability. This is why considerable reserves can be generated from what was considered to be a poor-quality bioturbated facies. A developing play-type concerns low permeability gas-prone non-associated reservoirs. These fields produce from shallow zones in thin bedded units that can be laterally extensive and continuous and in most cases are interbedded with the source rock. The gas is biogenic in origin and it may not have to migrate into the reservoir but will accumulate in any internal zones that show permeability and porosity enhancement such as burrow systems.
Just as important is the recognition that hydrocarbon production from bioturbated rock is generally more complex than producing from laminated media. This is because flow paths through burrow-related flow conduits are comparatively tortuous. Further complicating geological and reservoir models this tortuous, heterogeneous media presents a notable complication for reservoir development. Burrows may provide flow conduits that interact extensively with the surrounding matrix; their tortuous nature implies that dead ends and cut-offs may be 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.