Mixed-layer illite/smectite diagenesis in the rift and post-rift sediments of the Jeanne d'Arc Basin, offshore Newfoundland, Canada

Abstract

Mixed-layer illite/smectite (I/S) clays were analyzed from 22 deep exploration wells from the Jeanne d'Arc Basin, where large commercial hydrocarbon reserves have been discovered. This basin has been subdivided into 4 areas (Southern, Trans-Basin Fault, Northern, and Outer Ridge Complex) based on general geology and characteristics of I/S-depth profiles. -- The mineralogy of fine-grain clays (<0.1μm) consists of mainly mixed-layer I/S with minor amounts of kaolinite, illite, and chlorite. Smectite and/or smectite-rich I/S clays were supplied to the Jeanne d'Arc Basin during Upper Jurassic to Tertiary time. At present smectite-rich I/S clays occur only in shallow samples irrespective of their geologic age. The proportion of illite-layers, as well as their ordering in I/S, increase with depth and temperature. The random to R1-ordered I/S transition occurs between depth intervals of 1940m to 3720m and crosses several stratigraphic boundaries. The R1 to R3-ordered I/S transition is generally observed below 4000m depth. Basin-wide I/S data indicate that smectite-rich I/S clays have been illitized in both rift and post-rift sediments of the Jeanne d'Arc Basin due to diagenesis. -- The majority of I/S clays in samples with vitrinite reflectance (R₀) values less than 0.5% are highly expandable and randomly interstratified. However, for R₀ values greater than 0.66% only illite-rich ordered I/S is observed. The random to R1-ordered I/S transition generally occurs within the upper "oil window" level of organic maturity (0.50-0.66%R₀). The relationships observed in this study should be expected in other wells of the Jeanne d'Arc Basin and may provide useful information to investigate the thermal maturity of sediments. -- The δ¹⁸O values of I/S clays range from 17.9‰ to 25.8‰ (SMOW), and calculated pore-fluid compositions vary from 1.5‰ to 11.6‰. In the Trans-Basin Fault area, high δ¹⁸O pore-fluid values are present at shallower depths and coincide with a narrow transition from random to R1-ordered I/S and reflect vertical migration of deep-basinal fluids along the fault zones. These fluids appear to be responsible for the rapid I/S-depth transition observed. In contrast pore-fluid δ¹⁸O in the Northern area increases progressively with depth, suggesting there was no significant vertical migration of deep basinal fluids in this area. -- In the regional context I/S-depth profiles reflect the influence of such factors as temperature, fluid migration, basin structure, lithology, and salt diapirism. These observations illustrate that I/S-depth profiles should be expected to vary within the same sedimentary basin due to a variety of geological factors. A single well cannot always be considered representative of a particular basin.