NEW INSIGHT INTO LOWER CRETACEOUS OIL SYSTEMS OF THE SOUTH FLORIDA BASIN

The USGS is preparing for an assessment of petroleum resources in the South Florida Basin. The basin was last assessed in 2001, and identified two Lower Cretaceous total petroleum systems (TPS): Sunniland-Dollar Bay TPS and Pre-Punta Gorda TPS. Current work suggests the presence of three total petroleum systems based on source-rock character analysis of ~130 samples of micritic limestone and finely crystalline dolostone that were collected from the Dollar Bay, Sunniland, and pre-Punta Gorda Anhydrite formations. Samples from the Dollar Bay, which has had no commercial oil production to date, contain average (avg) total organic carbon (TOC) content of 2.0 weight (wt) %, avg thermal distillate (S1) of 1.2 grams (g) hydrocarbon (HC)/g rock, and avg pyrolysis yield (S2) of 12.9 g HC/g rock. Avg solid bitumen reflectance (R_o) of 0.4%, avg T_max of 431°C, and avg production index (PI) of 0.1 indicate the unit is immature, but because sulfur-rich Type IIs kerogens are present, the unit may produce early hydrocarbons at low maturity. High TOC, S1, and S2 are likely due to the presence of heavy oils and asphaltenes, as seen in petrographic analysis and identified in extract fractionation. Samples from the commercially producing Sunniland contain avg TOC of 1.2 wt %, S1 of 2.0 g HC/g, and S2 of 10.0 g HC/g. Avg solid bitumen R_o of 0.33% and avg T_max of 428°C show thermal maturity to be similar to the Dollar Bay. Petrographic analysis indicates a Type II/IIs kerogen in Dollar Bay and Sunniland source rocks with abundant unstructured marine algae and minor Type III terrestrial component. Analytical data indicate the Dollar Bay is self-sourced as opposed to charged, as previously hypothesized. Moreover, the presence of kerogen-rich source facies within the Dollar Bay and the presence of thick anhydrites that separate the Dollar Bay and Sunniland suggest the Dollar Bay can be separated from the Sunniland-Dollar Bay TPS.

Avg TOC (0.47 wt %) in pre-Punta Gorda samples are lower than overlying sampled units, and avg T_max of 433°C and one measurement of solid bitumen R_o (0.46%) show thermal maturity is slightly higher. Although this study attempted to identify potential source rocks in pre-Punta Gorda rocks and better assess thermal maturity, locating organic-rich samples for analyses is difficult, and further evaluation is recommended.