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.