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The Orpheus Graben, offshore Nova Scotia : palynology, organic geochemistry, maturation and time : temperature history Lyngberg, Erik


Palynological biostratigraphy and maturation data are combined to reconstruct the burial and thermal history of the Orpheus Graben, offshore Nova Scotia. Palynological analysis indicated that, 1) the age of sediments in the Orpheus Graben range from Upper Triassic to Turonian (Upper Cretaceous) with unconformities occurring in two wells, 2) recycled palynomorphs are ubiquitous in the wells examined, but do not occur in large numbers, 3) the Jurassic section is dominated by Classopollis, indicating an arid environment. The Cretaceous section is dominated by Cyathidites, Taxodiaceapollinites, Gleicheniidites and bisaccate pollen indicating wet tropical or sub-tropical conditions. Maturation zones were determined primarily by spore colour and correlation to vitrinite reflectance and pyrolysis data. Thermally mature rocks occur as shallow as 500 m in one well. The Dawson Canyon and Logan Canyon Formations contain the riches oil prone source rock. Enhanced levels of maturity were found to occur over salt structures. Burial history diagrams are shown for all six wells drilled in the Orpheus Graben as well as burial diagrams for a basin cross section through time. Two models of Tertiary subsidence are considered, 1) with minimal Tertiary deposition, 2) with up to 2000 m of Tertiary deposition. The temperature history of each well drilled in the Orpheus Graben can be inferred by combining the Time-Temperature Index and maturation data. Two temperature distributions are considered, 1) a simple temperature history with geothermal gradients twice the present and up to 2000m of Tertiary deposition and, 2) a complex Temperature history with geothermal gradients up to six times the present with minimal Tertiary deposition. The complex temperature history yields Time-Temperature Index values that are closer to the observed maturity. The future exploration potential is discussed, with enhanced exploration objectives suggested for the region further eastward along the axis of the Orpheus Graben.

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