Abstract

Unconventional gas (tight gas, coalbed methane, and shale gas) has become an increasingly significant source of energy. Economic production from such low-permeability reservoirs relies upon identifying regions of the reservoir that will yield the highest gas production rates. Currently available gas recovery technologies are highly dependent on the fracturability of the reservoir. Zones of enhanced brittleness and permeability within shale-gas reservoir horizons are a prerequisite for successful shale-gas recovery. Such brittle zones are directly linked with increased quartz and/or carbonate content within the mudstone. In mudstones with high clay-mineral content, quartz may be concentrated and redistributed as a result of burrowing activities of infaunal organisms. High-quality porosity and permeability zones in shale-petroleum reservoirs may be present in the form of silty and sandy tortuous strips of selectively concentrated grains of quartz that constitute burrow halos. Grain-selective burrows therefore can improve reservoir capacity, permeability, and fracturability and thus control the storativity of the shale-petroleum reservoir. This study presents three-dimensional reconstructions of three different types of Phycosiphon-like burrows and investigates the possible fluid-flow paths caused by the ichnofabric. The volumetric approach to the bioturbation generated by phycosiphoniform burrow makers used herein shows that the volume of sediment that becomes more porous and more permeable media within such bioturbated interval can range from 13 to 26% of the total volume. The quartzose strips of sediment caused by bioturbation are highly tortuous and interconnected vertically and horizontally, thereby increasing both horizontal and vertical permeability. Additionally, the quartz frameworks created by the burrows may locally increase fracturability within otherwise nonbrittle mudstones.

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