SEISMIC STRATIGRAPHY AND ARCHITECTURE OF THE JURASSIC ABENAKI MARGIN, AT COHASSET-MIGRANT, AND POTENTIAL FOR DISTAL ORGANIC-RICH FACIES

Abstract

This study uses well data, extensive 3D seismic data and geologic analogs to test and extend stratigraphic concepts and models in a mixed clastic-carbonate depositional setting: the Middle Jurassic to Early Cretaceous of the Sable Sub-basin, offshore Nova Scotia. The study incorporates stratigraphic concepts and models developed by government, industry and academic workers during the hydrocarbon exploration, development and production phases of the Sable Sub-basin. The study focuses on basinward mapping of third-order depositional sequences identified in the Abenaki carbonate bank at Deep Panuke Field. Source rock potential in coeval basinal calcareous mudstones; changes in bank margin morphology related to underlying basement; the transition from a dominantly carbonate system at the shelf margin to a fluvio-deltaic system of the Sable Delta adjacent to basinal mudstones outboard of the carbonate bank are discussed. These depositional systems are imaged by 3D seismic data in the area around the Cohasset L-97 and Migrant N-20 well penetrations. Core studies of wells which penetrated the Abenaki carbonate bank and field studies in the Lusitanian Basin, onshore Portugal, provide calibration and analogs to shallow water carbonates in the Cohasset area. Further ancient and modern analogues are discussed from Morocco and at the termination of the Great Barrier Reef in the Gulf of Papua. Using multiple geological, petrophysical and geophysical interpretation methods, the depositional cycles of limestones and calcareous shales that were deposited in increasingly deeper water outboard of the Abenaki margin were mapped within a stratigraphic framework. This framework formed the basis for a 3D geocellular model that was then populated with lithologies from well data via a seismic inversion. This model was then interpreted in terms of environments of deposition and source rock potential. The third-order sequence stratigraphic framework employed here was extrapolated from a framework established at Deep Panuke gas field, where commercial production began in August 2013. This third-order chrono-stratigraphic framework (“Abenaki 1-7 surfaces”) incorporates multiple litho-stratigraphically defined formations: the Mohican, Mohawk, Mic Mac, Abenaki, Missisauga and Verrill Canyon formations. Thick fluvio-deltaic successions adjacent to basinal mudstones in the Migrant N-20 well are interpreted to be structurally controlled, deposited in local depocentres that formed in response to sediment loading, normal faulting and mobile salt substrate. It is proposed from the 3D geocellular model, from examining analogous basins (Lusitanian Basin in Portugal and the Moroccan Margin), and from a published carbonate source rock model, that condensed sections of the distal carbonate depositional system within Abenaki 1-4 sequences have potential to host organic-rich material. The basinal shales of these sequences are estimated to have been deposited in up to 200 m water depths and have type 2 source rock potential. A change in seismic signatures and facies occurs between Abenaki sequences 1-4 and Abenaki sequences 5-7, reflecting encroachment of the Sable Delta, and it is interpreted that the Abenaki 5-7 sequences have predominantly type 3 source potential, with some potential for a type 2 source in intervening calcareous mudstones.