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Does Bioturbation Enhance Reservoir Quality? a Case Study from the Cretaceous Ben Nevis Formation, Jeanne D’Arc Basin, Offshore Newfoundland, Canada

Tonkin, Nicola S.1; McIlroy, Duncan 1; Meyer, Rudi 2; Turpin, Alllison 1
1 Department of Earth Sciences, Memorial University of Newfoundland, St John's, NF, Canada.
2 Department of Geoscience, University of Calgary, Calgary, AB, Canada.

The delineation well Ben Nevis L-55 located in the Hebron/Ben Nevis Field of offshore Newfoundland, targets the Ben Nevis Formation in the petroleum-rich Jeanne d’ Arc Basin. This case study focuses on the bioturbated net pay horizons, with the objective to understand the importance of animal sediment interactions in controlling the porosity & permeability of sandstone reservoir intervals. Core was logged, analyzed and sampled in order to determine the influence that bioturbation has on petrophysical properties. Laboratory analyses of samples, included petrographical analysis using standard thin sections and large thin slices; probe-permeameter measurement of permeability; and porosity is estimated using dye-impregnated thin sections.

The net-pay interval is dominated by persistent and conspicuous Ophiomorpha burrows, which upon initial hypothesis (without laboratory analyses) have a direct relationship to enhancement of porosity and permeability. Results reveal this is not the case; Ophiomorpha burrows reduce permeability relative to the host sediment. Conversely, less conspicuous unlined burrows of Thalassinoides show enhancement of permeability.

Sorting is a fundamental control on primary porosity and biogenic sediment reworking can modify grain size sorting. Results demonstrate that intra-burrow porosity increases in open burrows such as Thalassinoides, where burrow fill is commonly coarser than matrix, although the converse relationship is true when the burrow-fill is mud-rich. Intra-burrow porosity is low in Ophiomorpha burrows, where thick mud and organic rich burrow fills and linings exist. Inter-burrow (matrix) porosity is commonly enhanced in highly bioturbated fabrics, where mud grade material is removed from the matrix and incorporated into burrow linings and fills. In sparsely bioturbated facies, mud-rich inter-burrow porosity can be highly dependent on the behavior of the trace making organisms and intensity of bioturbation.

Complex relationships exist between bioturbation and petrophysical properties in the studied material. Bioturbation can enhance or reduce porosity/permeability, dependent on trace fossil morphology, composition of burrow linings/fills, burrow size and bioturbation intensity. Our data should be compared with other studies of similar reservoir intervals to establish general models for the effects of bioturbation on petroleum reservoirs.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009