Effects of organic and mineral matter on reservoir quality in a Middle Triassic mudstone in the Canadian Arctic

Abstract

This study examines the hydrocarbon potential of the Middle Triassic Murray Harbour Formation (Sverdrup Basin, Arctic Canada) as a self-sourced reservoir. The Murray Harbour Formation consists of organic-rich mudstone and siltstone which has contributed oil and gas to the majority of known hydrocarbon fields in the basin. Organic matter (OM) and mineral fractions of the interval are investigated to characterize the effect that each fraction has on reservoir quality. The interval of interest is within the oil generation window, with a Tmax of 443°C and total organic carbon (TOC) value of 3.20wt.% (median, n=51). A modified method of Rock-Eval analysis, referred to as the extended slow heating (ESH) cycle, where heating occurs at a slower rate of 10°C per minute up to 650°C, was utilized to better resolve different fractions of OM. In conjunction with organic petrology, this allowed correct identification of the components of TOC and the effects each has on reservoir quality and hydrocarbon potential for the unit.Three main OM fractions are identified: i) light free hydrocarbons, ii) medium to heavy fluid-like hydrocarbon residue (FHR), and iii) kerogen and solid bitumen. The kerogen (allo- and autochthonous) and solid bitumen fraction, which is dominated by solid bitumen, occurs in two petrographically observable forms: matrix and lump solid bitumen. Matrix bitumen is primarily disseminated within the clay-rich matrix and fills smaller pores, whereas lump solid bitumen fills larger pores. TOC has a negative correlation with porosity indicating that it occupies pore space, with a detrimental effect on hydrocarbon storage and flow at the present level of thermal maturity. However, thermally overmature areas of the Murray Harbour Formation may have increased microporosity within the solid bitumen network which could provide additional adsorption sites for generated gas within the reservoir. Macro- and micro-scopic phosphate nodules (likely high-density apatite mineral) appear throughout the interval, and under fluorescence microscopy, are observed to retain light free hydrocarbons and FHR. A strong positive correlation between porosity and grain density suggests that these nodules contribute to porosity and provide capacity for hydrocarbon storage within the rock. This important observation can be applied to other phosphate-rich mudrock plays, including the coeval Doig Formation Phosphate Zone in the established Alberta Basin, to potentially identify and exploit prolific zones.