Origin, distribution and paragenetic sequence of carbonate cements in the Ben Nevis Formation, White Rose Field, Jeanne d'Arc Basin, offshore Newfoundland, Canada

Normore, Leon Scott (2006) Origin, distribution and paragenetic sequence of carbonate cements in the Ben Nevis Formation, White Rose Field, Jeanne d'Arc Basin, offshore Newfoundland, Canada. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Aptian-Albian quartz arenite sandstones of the Ben Nevis Formation were deposited in a storm dominated shoreface setting during a marine transgression. Sediment grain size, and poikilotopic calcite cement are largely accountable for the reduction in porosity and permeability, resulting in mean permeabilities on the order of < 1 to 300 mD. The objective of this study is to develop a knowledge of the paragenetic sequence during early and late diagenesis, relating timing and source of cements to attempt to predict cement distribution, its effect on reservoir-scale fluid connectivity and its use as a development and exploration tool at White Rose Field and potential extensions nearby. The union of several analytical methods are required to decipher 120 million years of diagenesis. Diagenetic evolution of the authigenic cements is investigated using core description, and optical petrography, with corroboration from carbon and oxygen isotopic analyses. -- The paragenetic sequence identified in this study consisted of four phases of authigenic cement precipitation and four dissolution events, three of which acted on detrital components and the final dissolution phase acting on ferroan calcite cement. To understand the diagenetic sequence of any sedimentary rock unit, it is important to determine physical, geochemical and biological variations in depositional environment, as the compounded effect of all these will define the course of diagenesis. Transgressing a deep shelf environment over a shallow shoreface environment will decrease the rate of deposition, bioclastic content and sand/shale ratio, while increasing mud content, distance from shoreline and bioturbation. The transgressive nature of the Ben Nevis Formation compacts Ben Nevis sandstones beneath the Nautilus shales, building pore fluid pressure, transporting fluids vertically and obliquely towards the shoreline. -- While there is a significant volume of serpulid worm tubes, bivalves and gastropods, it may be thin, compacted and often discontinuous residues of micro- crystalline textured curved shell fragments that are the primary source for calcite cement. Petrographic analyses demonstrate various stages of shell dissolution, as well as multi- layered shells typically composed of aragonite. Carbon isotopes (δ¹³C values ranges from -13.9 to +8.8 [per thousand]) are consistent with an aragonite source for calcite cement with a +1.20 [per thousand] average enrichment between shells and authigenic cement, corresponding to the aragonite-calcite conversion. Unstable aragonite is dissolved in the burial process providing an excellent source of enriched δ¹³C, to then precipitate as calcite cement. -- The early stages of diagenesis enriched pore fluids with calcium carbonate providing the groundwork for the entire diagenetic history. Petrography and isotope geochemistry work collectively to confirm early precipitation of calcite cement. -- Petrographic analyses reveal minus cement packing up to 40%, nearly equivalent to original unconsolidated grain packing for fine grained sediments. Delicate ornaments on shells were also preserved within cements, indicating negligible compaction prior to cementation. Preservation of partially dissolved feldspar grains also indicates early cementation. Oxygen isotope analyses (δ¹⁸C values range from -9.2 to -1.5 [per thousand]) estabhsh an approximately 13°C increase in temperature between shell formation and cement precipitation. This equates to a burial depth of only 390 m for precipitation of calcite cement. -- Distributions of cements support vertical pore fluid flow within the Ben Nevis Formation. The storm dominated shoreface provides a much larger volume of bioclastic debris than the lower energy transition to the offshore environment immediately above but cementation intensity increases up-section. This indicates dissolution of shells in the lower Ben Nevis and transportation of fluid into the overlying offshore transition where sedimentation rates slow down, allowing precipitation of cement. -- Calcite concretions are not laterally or vertically continuous as demonstrated by the rounded boundary types and compaction of uncemented sandstones between concretions, but the enormous volume of calcite cement is occupying pore space that could otherwise be filled with oil or gas. Concentrating on identifying the east-west constraints on the distal shoreface through seismic interpretation, additional wells and basin modelling may prove beneficial for future extensions of the White Rose Field, as well as the entire Jeanne d'Arc Basin.

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