Reservoir quality and lateral variability of bioturbated shallow marine successions

Tonkin, Nicola S. (Nicola Sarah) (2011) Reservoir quality and lateral variability of bioturbated shallow marine successions. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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In bioturbated reservoir facies, ichnology is integral to reservoir characterization. Ichnological analysis of a reservoir facies and subsequent classification into ichnofabrics or ichnofacies allows for characterization of reservoir properties. Bioturbation can redistribute grains and cause sorting or mixing, this physical modification of the primary sedimentary fabric causes changes to porosity, and permeability in reservoir facies. In highly bioturbated reservoir facies, bioturbation can be the first order control on petrophysical properties. -- Petroleum geologists are primarily concerned with the effect that bioturbation has on the petrophysical properties of a reservoir facies, rather than the details of ichnotaxonomic identification. The proposed bioturbation style categories do not require in-depth knowledge of ichnology. The categories can easily be applied to facies and core analysis for incorporation into reservoir models. The action of bioturbators can be considered in terms of: 1) sediment mixing; 2) sediment cleaning; 3) sediment packing; 4) pipe-work building strategies; 5) combination sediment cleaning and packing; and 6) combination pipe-work building and sediment packing. Sediment packing and sediment mixing styles commonly reduce porosity/permeability, while sediment cleaning bioturbation style enhances porosity/permeability. Pipe-work building and combination bioturbation styles are highly dependent on the lithological contrast between burrow fill, and enclosing substrate. The use of bioturbation style categories, and the classification of trace fossils into these categories may be more user-friendly for reservoir geologists than existing paleoenvironmentally-driven ichnofacies or ichnofabric analysis. -- While categorization of bioturbation style is a useful tool in reservoir characterization, lateral variations in reservoir quality and heterogeneity of ichnofacies or ichnofabric must be incorporated into geological models in order to predict fluid flow in bioturbated facies at the inter-well scale. Ichnological analysis allows insight into variations in sedimentation rate, hydrodynamic energy (erosive currents), substrate consistency, length of colonization window, and community succession (tiering and cross-cutting relationships). This ichnological dataset means that inferences regarding both physical and chemical processes acting at the time of bioturbation can be made. -- There is inherent ichnological variability within most bioturbated beds. The lateral variability, or patchiness of the ichnofabrics studied were not found to be related to proximity to the paleo-shoreline in the three wave-dominated depositional systems. Instead, the most critical factor appears to be the sediment accumulation style. Slow continuous deposition was found to produce complex and highly patchy ichnofabrics, whereas rapid, episodic, event bed deposition was found to be associated with the most uniform development of ichnofabric. -- Autocylic and allocyclic interpretations can be postulated for bioturbated key stratigraphic surfaces in siliciclastic shallow marine settings. A case study illustrating the need for careful ichnological analysis has been undertaken as part of this thesis, and focuses on Thalassinoides, and its validity as an indicator for key stratigraphic surfaces. Thalassinoides is easily identified in core and outcrop, has a pipe-work bioturbation style, and colonizes a variety of substrates including softground and firmground (Glossifungites ichnofacies) substrates. The Glossifungites ichnofacies is commonly found in association with relative sea level change, but alternative autocyclically-generated processes should be considered. With careful ichnological investigation, realistic palaeoenvironmental and sequence stratigraphic interpretations can be made by objective consideration of both autocyclic and allocyclic processes. -- This research uses bioturbated shallow marine case studies to create a first order understanding of likely reservoir quality, reservoir heterogeneity, and interpretation of candidate key sequence stratigraphic surfaces. These concepts are an integral part of reservoir to basin scale models of hydrocarbon reservoirs. With an improved, integrated, understanding of what bioturbating organisms do to sediment, ichnology can continue to grow as an important component of reservoir characterization studies, and petroleum geology in general.

Item Type: Thesis (Doctoral (PhD))
Item ID: 9679
Additional Information: Includes bibliographical references.
Department(s): Science, Faculty of > Earth Sciences
Date: 2011
Date Type: Submission
Library of Congress Subject Heading: Hydrocarbon reservoirs; Trace fossils; Bioturbation; Reservoir sedimentation

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