The aromatization and isomerization of hydro carbons and the thermal and subsidence history of the Nova Scotia margin

Abstract

The temperature and burial histories of the sediments deposited on the continental margin off Nova Scotia have been modelled. A good match between predicted and observed present-day bottom-hole temperatures is only achieved when the thermal conductivities of sediments are derived from laboratory measurements of representative samples; values commonly quoted in the literature predict too low a geothermal gradient. The temperature history is used to predict the extents of geochemical reactions involving steroid hydrocarbons together with previously derived reaction constants. The reaction extents were measured on 33 samples. In general, agreement between observations and predictions is good. The reactions in some samples, however, appear to have progressed anomalously far for their depths. This is thought to be the result of impregnation of these sediments with oil, which has migrated from depth. The good general agreement between model predictions and observations encourages extrapolation of the model to areas of no sample control, and the reconstruction of the temperature and burial history of the margin. Tentative reconstruction of oil generation from the principal oil source rock of the region may also be attempted. The top of the oil-generation zone currently lies 4 km below the shallow regions of the continental shelf. Model predictions suggest that the Verrill Canyon formation and its equivalents are mature but that younger sediments are not. Maturation of deeper regions of the Verrill Canyon started in the Early Cretaceous, but significant volumes of sediment only entered the oil-generation zone during the Tertiary.