--> Abstract: Natural Gas Production and Anomalous Geothermal Gradients of the Deep Tuscaloosa Formation, by Lauri Burke; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Natural Gas Production and Anomalous Geothermal Gradients of the Deep Tuscaloosa Formation

Lauri Burke1

(1) U.S. Geological Survey, Denver, CO.

Why does the deep Tuscaloosa Formation contain so many prolific natural gas fields? This study examines temperature, pressure, and production to bring insight into mechanisms of hydrocarbon accumulation in the siliclastic-rich, shelf-margin, deltaic deposit of the downdip Tuscaloosa Formation of onshore Louisiana.

Average field temperatures were used to calculate geothermal gradients for all currently producing natural gas fields in the onshore Gulf of Mexico Basin. The onshore Gulf Coast exhibits minimum, average, and maximum temperature gradients of 1.20, 1.58, 2.18 °F/100 ft, respectively. Gulf Coast temperature gradients exceed the geothermal gradients found in the majority of hydrocarbon-bearing basins worldwide.

Judge Digby field in Louisiana exhibits some of the highest cumulative production of natural gas from the deep Tuscaloosa. In this field, temperatures of approximately 400 °F are typically encountered over the production horizons. However, at this depth for all natural gas reservoirs in the Gulf Coast region, the minimum, average, and maximum temperatures are 330, 430, and 550 °F, respectively. Therefore, the temperatures encountered in this field are anomalously lower than the average temperature trends at similar depths regionally. In Judge Digby field, the highest natural gas production comes from a 6,000 ft thick reservoir zone exhibiting a temperature gradient of 1.54 °F/100 ft. This temperature gradient is also lower than the average temperature gradient in the onshore Gulf of Mexico Basin.

Based on the analysis of petrophysical data and production information for Judge Digby field, these relatively depressed geothermal gradients may be due to high sedimentation rates and high preservation rates of the Tuscaloosa. This may have delayed the thermal equilibrium of the sediment package with respect to the surrounding formations. Burial history reconstruction and thermal maturation analysis indicate that the Tuscaloosa in Judge Digby field is currently within the natural gas generation window. This result has implications for self-sourcing mechanisms of the organic rich, marine shales interbedded within the formation.

These results are applicable to other hydrocarbon-bearing basins worldwide. Specifically, targeting deeper formations with high sediment rates, high preservation rates, and relatively depressed geothermal gradients at depth may decrease exploration risks for undiscovered hydrocarbon accumulations in frontier regions.