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Abstract

Over the past decade, discoveries of super-giant, multibillion barrel presalt oil fields in Brazil’s offshore basins and related discoveries in its African conjugates highlighted the great importance of synrift/prebreakup fluvial-lacustrine successions to the success and efficiency of their petroleum systems. Improvements in seismic acquisition and processing technologies were keys in imaging the architecture of the underlying rift basins, and interpreting the basin fill and internal depositional facies later confirmed by drilling.

Middle Triassic to Early Jurassic synrift basins are exposed onshore eastern North America and extend into adjacent offshore areas, including equivalent basins in Northwest Africa. Organic-rich lacustrine successions occur in a number of the U.S. basins and although no commercial discoveries have been made, hydrocarbon shows in outcrops and wells confirm that a working petroleum system exists in virtually every basin.

The basin-fill model for these extensional basins’ sedimentary successions defines four tectonostratigraphic (TS) units. In the Fundy-Chignecto rift basin complex, TS I is an unconformity-bounded, early synrift fluvial-eolian sequence of Late Permian age. TS II is a dominantly fluvial (with some lacustrine) sequence believed representative of an underfilled, hydrologically open basin (subsidence < sedimentation). This is followed by either a closed basin or one in hydrological equilibrium (subsidence ≥ sedimentation) dominated by lacustrine (TS III), and later playa/lacustrine (and basal CAMP volcanics) successions (TS IV). The climate sensitive lacustrine facies—especially in TS III—are exquisite recorders of paleoclimate, and with paleomagnetic data refine the determination of the basins’ age and paleolatitudinal positions.

Seismic profiles in the Fundy-Chignecto (Canada) and Newark (USA) basin reveal high-amplitude, laterally continuous reflections adjacent to the respective border faults. In the Newark basin, these are calibrated against academic and industry wells revealing a correlation with large scale climatic cycles and lacustrine facies in TS III. In both basins, similar reflections are observed in the undrilled distal portion of TS II fluvial successions and are interpreted as indicating similar lacustrine successions. This architecture departs from the original TS II model (subsidence < sedimentation) by inferring high levels of tectonically driven extension resulting in the basins being closed from their inception (subsidence ≥ sedimentation) thus facilitating lake formation.

During TS II deposition (approximately Late Anisian to Early Carnian), paleomagnetic data positions these basins within the north equatorial humid belt. This is a favorable setting for the evolution of lakes; i.e., elevated precipitation coupled with tectonic extension, and most importantly, under conditions for organic matter creation and preservation. If correct, this interpretation would have a significant impact on the potential for hydrocarbons sourced from Late Triassic lacustrine successions in presalt synrift basins offshore Nova Scotia and Morocco. Importantly, a potential new oil-rich resource play may exist beneath the shallow waters of Chignecto Bay. In the deep water portion of the offshore Scotian basin, presalt synrift basins having similar lacustrine source rock potential may also exist.

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