Hydrocarbon Potential of Early Mesozoic Basins of Eastern United States
Steven Schlamel
The exposed Triassic-Liassic rift basins in the eastern United States are
half-grabens filled with up to 7 km of continental sediments. The location and
sense of asymmetry of the half-grabens are closely tied to the structural grain
of the Appalachian crystalline terranes on which they have formed. In many
instances, the faulted margins of the basins are older thrusts or terrane
boundaries reactivated as listric normal faults. The sediment fill of the basins
reflects their structural asymmetry. Coarse alluvial fan deposits along the main
border faults pass basinward into a complex assemblage of fluvial, paludal, and
lacustrine facies. The oldest sediment fill in the rift basins is dated
palynologically as late Ladinian to late Carnian. Perhaps reflecting the
northward ope ing of the central Atlantic, the youngest rift-fill sediments are
older in the southern basins than in the northern--Carnian in the Richmond basin
vs. Toarcian in the Hartford-Deerfield basin. Floral evidence points to a
tropical to near-tropical environment, with severe oscillations between
xerophytic (dry) and hydrophytic (wet) conditions. The degree of thermal
maturation
, as estimated from vitrinite reflectance and clay mineralogy, varies
widely from basin to basin; however, most of the basins are within the oil to
dry gas generative window. The basins with highest thermal maturities are those
having large volumes of diabase intrusives and presumed higher paleogeothermal
gradients. The peak of thermal
maturation
/migration may have occurred as early
as the Jurassic.
In general, all of the exposed basins have adequate, albeit tight, clastic
reservoirs. However, an insufficient volume of source rock, the early timing of
maturation
/migration, and the paucity of traps severely limit their hydrocarbon
potential. Nevertheless, various non-commercial discoveries of hydrocarbons have
encouraged ongoing exploration in the larger basins.
AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.