Biogenically Enhanced Reservoir Properties in the Medicine Hat Gas Field, Alberta, Canada
Bioturbation is commonly regarded as a process that is detrimental to reservoir properties because burrowing organisms alter both the textures and fabrics of laminated sediments. Biogenic destruction of reservoir properties is more common in highly bioturbated, heterolithic reservoirs. At lower bioturbation intensities, interconnected burrows can potentially form fluid-flow conduits provided their fills are porous. In this study, spot-minipermeametry and micro-CT analytical techniques were employed to evaluate the effects of bioturbation on porosity and permeability distributions in interstratified muddy sandstones and sandy mudstones of the prolific Medicine Hat Gas Field, Alberta, Canada. The facies are generally moderately bioturbated (BI 3) with a suite of trace fossils including Planolites, Palaeophycus, Scolicia, Thalassinoides, Chondrites, Cylindrichnus, Teichichnus, Skolithos and rare fugichnia, corresponding to a proximal expression of the Cruziana Ichnofacies.
Spot-minipermeability results demonstrate that permeability in the bioturbated sandstones can be up to two-orders of magnitude greater than those of the muddy matrix (150 md compared with 1 md), thus corresponding to a dual-porosity fluid flow system. Graphs of bioturbation intensity versus measured permeability in bioturbated sandstones indicate that burrows are well connected horizontally and constitute flow conduits. However, micro-CT models reveal that flow conduits are generally isolated and planiform (i.e., bed restricted) in character. Rare interpenetrations between vertically oriented trace fossils (i.e., Skolithos, Cylindrichnus) interconnect these hydraulically isolated zones. Bioturbation in these facies has two main impacts: 1) decreasing anisotropy, such that Kv is increased relative to Kh; and, 2) elevated porosity and permeability, where the porosity and permeability of bioturbated sandstones are comparable to that of the better sorted laminated sandstones. Consequently, bioturbated horizons that have previously been identified as “non-reservoir rocks” should be mapped as effective reservoir and included in reserve estimations of the Medicine Hat gas Field.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California