Source Rock Character of Tight Oil Systems in Lower Cretaceous Dollar Bay, Sunniland, and Pre-Punta Gorda Anhydrite Formations, South Florida Basin

Abstract

As part of a planned U.S. Geological Survey petroleum resource assessment of the South Florida Basin (SFB), samples of Lower Cretaceous micritic limestone and finely crystalline dolostone are being analyzed for source rock character. To date, ~70 rock-core and cutting samples from the Dollar Bay and Sunniland Formations, as well as rocks underlying the deeper pre-Punta Gorda Anhydrite, have been collected within the SFB and have undergone preliminary screening. Samples from the Dollar Bay (n=34) contain avg. total organic carbon (TOC) content of 1.85 wt.%, avg. thermal distillate (S1) of 1.08 g HC/g rock, and avg. pyrolysis yield (S2) of 11.36 g HC/g rock. Results suggest the Dollar Bay is immature, as indicated by avg. Tmax of 432°C and avg. PI of 0.08; however, if sulfur-rich Type IIs kerogens are present, the unit may yield early hydrocarbons at low maturity. High TOC, S1, and S2 may be due to presence of heavy oils and asphaltenes (bitumens), as indicated in petrographic analysis showing abundant low-reflectance, brownish-orange fluorescent bitumens permeating finely crystalline dolostone. At this time, the Dollar Bay is thought to be self-sourced, as demonstrated by the presence of rock fragments containing Type II marine kerogen (lamalginite). Samples from the commercially producing Sunniland (n=30) contain median TOC of 0.66 wt.%, S1 of 0.37, and S2 of 2.26. Preliminary petrographic analysis suggests a mixed kerogen with abundant Type II/IIs unstructured marine algae and minor Type III terrestrial component. A low-reflectance, brownish-orange fluorescent bitumen also contributes a Type I/II signature to the HI. Thermal maturity is only slightly higher than the overlying Dollar Bay as indicated by avg. Tmax of 432 and PI of 0.10, although Tmax values may be low due to contribution of heavy oils to the S2 peak. Pre-Punta Gorda samples (n=8) contain similar avg. TOC content (0.61 wt.%) compared to the overlying units (~1,000–2,000 ft shallower); however, high avg. S1/TOC (92) suggests the samples may be oil-stained and not source rocks. Thermal maturity is only slightly higher with avg. Tmax of 433 and avg. PI of 0.19. It is presumed that the deeper pre-Punta Gorda rocks are more thermally mature than the Sunniland; however, further evaluation is needed to make this conclusion, as well as to identify any potential pre-Punta Gorda source rocks.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015