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M. Sc. Thesis

Mineralogical and Geochemical Traits of the Egret Member Oil Source Rock (Kimmeridgian), Jeanne d'Arc Basin, Offshore Newfoundland, Canada

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The Jeanne d'Arc Basin is a Mesozoic failed-rift basin situated within the Grand Banks, 100 km off the east coast of Newfoundland, Canada. Several large oil and gas fields have been discovered in the area, the giant Hibernia field being the most notable. The Kimmeridgian-aged Egret Member is the primary generator of hydrocarbons within the basin and has a log response indicative of significant internal variability. Due to the absence of cores or outcrop (Egret Member), the specific nature of this litho-chemical heterogeneity is largely unknown, therefore, it has been difficult to resolve the intervals deposition, accumulation and preservation history. As part of a broader, hydrocarbon habitat study, a comprehensive organic, inorganic, chemical and physical characterization of the interval has been prepared.

Of the 22 wells that penetrate the Egret Member within the basin, 5 wells have been examined in detail, namely: Rankin M-36, Port au Port J-97, Voyager J-18, Fortune G-57, and Trave E-87. X-ray diffraction (XRD), Environmental Scanning Electron Microscope (ESEM), and whole-rock (Inductively Coupled Plasma) chemical analyses have been performed on Egret Member well cutting samples. In addition, a detailed lithological and Rock-Eval/TOC characterization has been Conducted on handpicked cutting samples from Trave E-87 and Fortune G-57.

Whole-rock XRD and ESEM analyses indicate that the Egret Member is composed primarily of quartz, calcite, feldspar, dolomite, and pyrite. Illite, mixed-layer illite/smectite (with typically < 10% smectite), and subordinate amounts of kaolinite, chlorite, and mixed-layer chlorite/vermiculite are the dominant clay minerals contained in the < 2 mm fraction. No free-swelling smectites have been identified within the interval. Quartz, calcite, and feldspar are interpreted as depositional in origin, with dolomite and pyrite being controlled by diagenetic processes. The clay mineral assemblage is also interpreted as detrital in origin, however sufficient levels of thermal maturation have been reached to account for some diagenetic modifications. Comparisons of the clay mineral assemblages and maximum burial temperatures of the interval suggest that the Egret Member is diagenetically moderate-mature in Port au Port J-97 and Rankin M-36, and mature in Voyager J-18, Trave E-87, and Fortune G-57.

Carbon-iron-sulphur relationships in Egret Member shales suggest that the Egret has experienced reducing conditions episodically in the past, and these conditions have facilitated the diagenetic modification of the source interval through anaerobic sulphate reduction. Carbon-iron relationships for the interval suggest that iron was not an limiting factor during pyrite formation, and that iron was readily available for incorporation into alternative authigenic ferroan species upon burial. Carbon-sulphur relationships suggest that during the deposition of Egret sediments, variable bottom-water oxygen levels persisted throughout the Jeanne d'Arc Basin.

Four lithological components have been identified within the Egret Member source interval. In order of abundance, they are: 1) a dark-brown laminated shale; 2) a grey to grey-brown shale; 3) a light brown marlstone/limestone; and 4) a fine-grained sandstone and siltstone. Rock-Eval/TOC properties of each lithology suggest that each rock-type contributes different levels of organic richness to the interval's overall character and may serve to control expulsion efficiencies within the source system. The dark-brown shale has the highest potential for hydrocarbon generation amongst the identified lithologies, owing to its organic richness (between 3-10 wt% TOC), oil-prone kerogen type (predominantly type II), and thermal maturity (Tmax values ranging from 426-439 oC). The grey to grey-brown shales also have a minor potential for hydrocarbon generation.

A silled basin, stratified water column model is favoured to explain observed lithological and organic geochemical variabilites in the Egret Member. Deposition was regulated by fluctuating bottom water oxygen levels in response to orbitally-controlled sea level and climatic changes during the Kimmeridgian time period. In this model, bottom water anoxia correlates with periods of high sea level, warm climatic conditions, and sluggish oceanic convection, resulting in the deposition and enhanced preservation of organic-rich shale containing marine-derived organic matter. During periods of low sea level, deposition is characterized by rapid sedimentation, cooler climatic conditions, and increased oceanic convection, resulting in the accumulation of organic-lean shale and carbonate associated with terrestrial-derived organic matter.

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Supervisor: Mark Williamson