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Organic and Inorganic Petrographic Analysis of the “Nordegg” – Gordondale Member, Fernie Formation, West Central Alberta

Abstract

The Gordondale (“Nordegg”) member of the Fernie Formation is a shale oil-prone formation that is the target of much unconventional exploration in the Western Canada Sedimentary Basin (WCSB). It is a tight (low permeability and porosity) organic-rich source rock characterized by fine-grained, fossil-rich, phosphatic, calcareous mudstones, deposited as a shelf facies in the lower Jurassic. This study highlights the detailed organic and inorganic characteristics of the Gordondale, the nature of association of organic matter and mineral matter, and the implications for the hydrocarbon potential and reservoir quality. Samples were collected from a 30m core. Scanning electron microscopy (SEM) with energy dispersive x-ray (EDX) mapping, and optical petrography using white and ultraviolet reflected light microscopy, were utilized for this study. Preliminary results indicate that the samples are rich in organic material with up to 13 wt. % TOC, and within the oil generation window with eq. VRo of 0.9%. Organic petrology indicates the presence of bitumen, exsudatinite, liptodetrinite, inertinite and zooclast. Bitumen is the dominant organic maceral in the samples. EDX maps indicate the mineralogy consists of calcite, dolomite, quartz, pyrite, apatite and clay minerals – manly illite and kaolinite. Two main forms of bitumen (matrix and solid) were identified, based on mode of occurrence, maturity and accumulation. Matrix bitumen is fine grained and invasive. It fills intergranular pore spaces within argillaceous and calcareous fractions. Reflectance and fluorescence measurements on matrix bitumen indicate BRo averaging 0.8%, and a yellow-brown fluorescence. Matrix bitumen is associated with clay mineral aggregates (mainly kaolinite) and apatite nodules. The matrix bitumen & exsudatinite in clay aggregates degrade rapidly and release light hydrocarbons (bright blue fluorescence) after exposure to ultraviolet light. Solid bitumen is solid granular bitumen of larger accumulations that have migrated from the matrix and accumulated within large isolated pore spaces, vacuoles of zooclast, and micro fractures. It has a measured BRo between 0.8% and 1.1%, and does not fluoresce. Solid bitumen is associated with diagenetic calcites that are likely the bi-product of bacteria sulphate reduction. Each type of bitumen with their mineral associations could influence the reservoir properties in the Gordondale, with respect to release of hydrocarbons at the micro to nano scale.