Source and Thermal History of Oils from Lockhart Crossing, Livingston Parish, Louisiana

Clifford C. Walters, Douglas D. Dusang

In 1982, lower Tuscaloosa production at the Lockhart Crossing field was supplemented with a major discovery of significant petroleum reserves in the Wilcox. Whole-oil and biomarker source parameters definitively prove that the oils found in Wilcox reservoir rocks constitute one single oil family. Oil produced from the Tuscaloosa Formation comprise a separate oil family unrelated to the oils in the Wilcox Group. Within each formation, the oils become progressively more mature with depth. This is believed to be due to thermal cracking which has taken place after the oil charged the reservoirs.

The chemistry of the oils produced from Wilcox reservoirs shows that these oils were generated from clay-rich shales with a significant input of bacterial organic matter. Eukaryotic algae and higher plants also contributed organic matter to the source rocks. Comparisons with cuttings from nearby wells prove that Wilcox shales are the source facies for these oils. Biomarker and other maturity parameters indicate that the Wilcox oils were generated from source rocks that were within the thermal regime associated with early oil generation.

Tuscaloosa reservoir fluids are significantly more thermally mature than the petroleum in the Wilcox. The Tuscaloosa oils were generated in the thermal conditions associated with late-stage oil/gas condensate generation. Because of this advanced degree of thermal stress, correlation of the Tuscaloosa oils to their source sediments is difficult. Data from updip well cuttings indicate that the lower marine Tuscaloosa shales are viable source rocks. Equivalent downdip shales are the likely source for the Lockhart Crossing Tuscaloosa gas condensates. The study proves that future Wilcox discoveries in the area need not be associated with deeper Tuscaloosa production.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.