SOURCE ROCK AND APATITE FISSION TRACK DATA PERTINENT TO PETROLEUM SYSTEM FRAMEWORK ALONG THE ATLANTIC MARGIN OF THE SOUTHEASTERN UNITED STATES

Geoscientists can inform decisions about the petroleum potential of the southern U.S. Atlantic outer continental shelf by characterizing potential source rocks. Prospective source intervals include those that are known from the eastern Gulf of Mexico, particularly the Upper Jurassic (Oxfordian and Tithonian), Lower Cretaceous (Aptian), and the Upper Cretaceous (Cenomanian-Turonian). Owing to a paucity of offshore wells, rock-based studies must necessarily focus on updip areas. However, Upper Jurassic and Lower Cretaceous strata are either broadly absent or commonly non-marine beneath onshore parts of northern Florida, Georgia, and the Carolinas. Marine Cenomanian-Turonian strata are more widely preserved across the southeastern Atlantic margin and we report here on geochemical analyses from those strata.

For each of three wells offshore of Georgia and northern Florida, we report geometric mean values of Tmax (420-430°C), total organic carbon (TOC, 0.3-0.6 wt. %), S2 (0.4-0.7 mg/g), and hydrogen index (HI, 100-140 mg/g), based on a total of 30 cuttings samples. Respectively, the data indicate that the strata are thermally immature, are lean in TOC, have limited petroleum generation potential, and contain mostly terrestrial kerogen. Upper Turonian(?) strata in one well show better source rock properties, although petroleum generation potential is still limited (S2 of ~2-3 mg/g).

To better understand the limited preservation of older strata, we present apatite fission track ages from coastal plain basement rocks along the Fall Line of five rivers located from southern Virginia to northern Georgia. Samples exhibit either a) unimodal age distributions with central ages of 155-135 Ma, or b) mixed age distributions with an apparent 155-135 Ma age mode and skewed or bimodal track length distributions. The age and length distributions seem to indicate that several km of exhumation occurred in the Late Jurassic to Early Cretaceous. Thus, the data may account for the absence of Jurassic-Lower Cretaceous strata beneath the updip Coastal Plain and anomalous high thermal maturity of nearby Triassic basins. In light of the results above, the best potential for petroleum source beds would be in higher accommodation areas that are offshore and have not been penetrated by wells.