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Sedimentology, geochemistry and gas shale potential of the early Jurassic Nordegg member, northeastern British Columbia Ross, Daniel John Kerridge

Abstract

The Early Jurassic Nordegg Member in north-eastern British Columbia (NEBC) is composed of 25-30 m of organic-rich marine strata. The unit was deposited in a restricted basinal environment, west of a high standing carbonate platform. The Nordegg Member consists of four lithological facies: (A) a basal conglomeratic lag deposit; (B) a lower phosphatic marlstone that was deposited during highly productive conditions; (C) an overlying marlstone which represents an anoxic phase of sedimentation; and (D) an upper phosphatic mudstone reflecting a productive water column. Geochemical analysis reveals depositional conditions where high productivity is associated with enrichments of P and Fe, K, Ti, V, Cr and Zn and total organic carbon values (TOC) between 0-8 wt%, likely due to upwelling currents introduced nutrient-rich water increasing algal productivity during relative sea-level rise. The marlstone contains higher TOC (6-20 wt'%) and reduced concentrations of P, Fe, K, Ti V, Cr and Zn. The elevated TOC's and lack of productivity-proxying elements (e.g. P) infers organic matter incorporation into the sediment was primarily controlled by redox conditions when basin conditions were persistently anoxic. The TOC concentrations are a reflection of the depositional environment and have a strong influence on potential gas capacity. The TOC-rich samples have improved adsorption capacities compared to their organic-lean counterparts due to the highly microporous nature of organic matter to which the gas molecules physically adsorb to. Nordegg adsorbed gas capacities range from 0.05 cc/g to over 2 cc/g in organic-rich zones. The relationship between TOC and adsorption is complicated by other geologic factors including moisture. Moisture competes for adsorption sites with methane and blocks pores and pore-throats, reducing the transmissibility of the methane to the microporosity of the organic matter. Twenty to eighty percent of total gas storage is free gas (gas occupying open pores), ranging from 0.1 - 1.3 cc/g. Nordegg total gas-in-place ranges from 1 - 24 BCF/section. The greatest potential for gas shale production is to the south-west of the study area (93-P-5). TOC concentrations (up to 20 wt%), thickness, maturity and fracture-potential improve the gas shale potential in this region making it a prime gas exploration target.

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