Thermal Effects of Basaltic Sill Emplacement in Source Rocks on Maturation and Hydrocarbon Generation

Abstract

In sedimentary basins sills have a tendency to become emplaced in litholgies of lower mechanical competence, like shales, which can possess source rock qualities.

In this study has the thermal effects of thick sills on sedimentary rock units at Svalbard, Norway, been investigated. This study should be directly relevant for sills in large scale sedimentary basins. If sills were emplaced in contact with the source rocks at great burial depth, a "source rock on a hot plate" type of situation could arise with, a temperatureheat history quite unlike that of small dikes in shallow, waterrich sediments. An additional effect could be related to a regional increase in heat flow if the sills are in a sense "symptoms" of massive ingenious activity as known from other regions.

It is well proven that thin sills and dikes can locally mature source rocks, increase biomarker ratio in extractable compounds and at the same time cause systematic increases in kerogen maturity parameters. Few organic geochemistry results on large scale sills has been published. It is therefore not clear if massive sills simply could be viewed as a upscaling phenomena of the smaller dike and sill systems.

Samples were collected from eight di_erent profiles with sills. The Triassic Botneheia black shale source rock formation, with dominantly marine kerogen, were collected from the hills of Botneheia (Central Spitsbergen), Teistberget (eastern Central Spitsbergen), Kre_tberget and Høgrinden (Barentsøya Island). Samples were also collected from the Jurassic Janusfjellet black shale (with dominantly terrestrial kerogen) from Domen (eastern Central Spitsbergen) and the Wilhelmøya Island. Samples from identical black shale units in nearby sections not thermally a_ected by the sills were also collected and used for comparison.

Vitrinite reflectance, kerogen description, Rock Eval, TC and TOC were determined on all 170 samples, whereas EOM, Iatroscan TLCFID, GCFID and isotopic studies of C and O (in carbonates, kerogen and extracts) were conducted on 48 selected samples.

Results from this study would not only be relevant for sill intruded sedimentary basins but would possibly also be relevant for deep water exploration e.g. the Atlantic volcanic margin were sediments have experienced increased heat flows due to magmatic underplating.