BURIAL, MATURATION AND MIGRATION OF CRUDE OILS FOUND IN CARBONIFEROUS SANDSTONE RESERVOIRS, BLACK WARRIOR BASIN, NORTHWESTERN ALABAMA

Hydrocarbons in the Black Warrior basin in northwestern Alabama achieved maximum burial and maturation as sediment shed from the Allegheny-Ouachita-Marathon Orogenic Belt poured into the basin during Late Permian time. The crude oils reside in Carboniferous rocks; however, the source rocks of these hydrocarbons are unknown. Our results show that the viable potential source rocks are the Chattanooga Shale with an The Chattanooga Shale has an average Tmax value of 484°C and calculated vitrinite reflectance values of 0.9-2.2%. The Floyd/Neal Shale has an average Tmax value 455°C and calculated vitrinite reflectance values of 0.92-1.71%. Utilizing these data as well as existing burial history plots, these potential source rocks achieved to expel crude oil in Mid-Late Permian time just prior to maximum burial by sediments flooding into the basin from the nearby Alleghanian Orogeny. Because the rocks remained in the oil window, the rocks maintain crude oil production potential today. The pseudo van Krevelen diagrams and Jones (1987) method indicate that the Chattanooga and Floyd/Neal Shales range anywhere from non-generative to oil prone due to the differences in maturation and organic material, which is probably determined by sediment sources and pathways in the Black Warrior basin during the Late Paleozoic orogenic events.

Gas Chromatography-Mass Spectrometry results show that the C27-29 sterane levels and pristane/phytane ratios are similar in the Carboniferous crude oils and the Floyd/Neal Shale samples. The Floyd/Neal Shale samples have a pristane/phytane average of 1.23, while the Chattanooga Shale samples have an average of 0.34. The Carboniferous crude oils have an average of approximately 1.5. The Floyd/Neal Shale is likely the main source of the Carboniferous crude oils extracted from the North Blowhorn Creek and Chicken Swamp Branch fields because the Floyd/Neal Shale is laterally continuous, is approximately 100-300 feet thick throughout the Black Warrior basin, and maintains adequate TOC levels (~1-4%). In the past, the Floyd/Neal shale was thought to be super mature and kerogen type III. However, this study shows that it is wet gas and in fact might be condensate or oil. Thus, the Floyd/Neal shale may be an unconventional oil source in the Black Warrior basin.