Ichnotaphonomy in Dolomitization and Characterization of Mississippian Mudstone Reservoirs: Hydrocarbon Potential and Flow Dynamics in Upper Midale Beds, Weyburn Oilfield, Saskatchewan

Keswani, Arjun D.; Pemberton, S. George

Widespread abundance and quality of preservation of biogenic structures within dolomudstones of the Upper Midale Beds at Weyburn Oilfield, highlight the significance of paleobiological controls on reservoir development, and its characteristics. Accordingly, this study utilizes a paleobiological approach to reservoir modeling: roles of ichnotaphonomy in dolomitization, and retention reservoir characteristics. Petrographic analyses of dolomudstones show porosity-permeability relationships are linked to the preservation of biogenic structures. The quality of preservation defines a reservoir potential window within dolomitized muddy substrates that show a characteristic distal-Cruziana ichnofossil suite. Reservoir characteristics are linked to burrow-fabrics that resisted both mechanical and chemical compaction. Quality of preservation from relatively intact to minor deformation in biogenic structures implies that porosity-permeability relationships that influenced dolomitization of the muddy substrate has remained mostly unaffected. By withstanding adverse effects of compaction, permeable conduits within burrow-fabrics influence relatively unimpeded fluid dynamics in hydrocarbons within the reservoir strata.

Preservation of morphological aspects of burrow-fabrics, and their textural parameters define inherent porosity-permeability characteristics. Typically, reservoir significance is associated with burrow-fabrics that show quality of preservation from intact to somewhat deformed structures. Fluid dynamics within such reservoir strata varies from unimpeded to moderately impeded (cross burrow-fabric) flow, respectively. Such variations in fluid regimes are influenced by the remnants of burrow-fabrics that constitute permeable conduits; and, by local effects of compaction and occlusion of porosity. Consequently, later diagenetic breakdown of permeable conduits within paleobiologically-influenced dolomites is defined by local differences in crystal morphology, size, alignment, and abundance patterns of rhombs. Such modified fabrics in dolomites vary locally with the degree of deformation in biogenic structures within the reservoir strata. Since reservoir characteristics are inherently linked to the preservation of bioturbate textures in diagenesis, an understanding of patterns in ichnotaphonomy and fabric evolution, particularly retention of bioturbation-enhanced permeable conduits, is useful for construction of paleobiological models in development geology.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013