COMPARING UNCORRECTED WELL BOTTOM-HOLE TEMPERATURES WITH PUBLISHED CORRECTION METHODOLOGIES FOR THE ONSHORE U.S. GULF OF MEXICO

Understanding subsurface thermal regimes is critical for petroleum systems analysis. The most common sources for subsurface temperatures in petroleum-producing basins are the bottom-hole temperatures (BHT) recorded while drilling wells. In basins with a long history of petroleum production, such as the onshore portion of the U.S. Gulf Coast Basin, temperature measurements from wells constitute a particularly dense spatial and stratigraphic dataset from which variations in the subsurface thermal regime can be identified and analyzed. However, a primary issue with using BHT measurements is that the readings record a combination of instrument housing, drilling fluid, and formation temperatures masking the ‘true’ formation temperature. Therefore, various methods to correct BHT measurements have been developed to reflect true formation temperatures.

This study computes subsurface temperature gradients for over 11,000 wells from the onshore and State Waters of the U.S. Gulf of Mexico Basin. The data are derived primarily from a proprietary database owned by IHS to which the U.S. Geological Survey has subscribed. Using these data, thermal gradients based on uncorrected subsurface temperatures were calculated. Thermal gradients for these wells using published correction methodologies were also calculated and compared to the uncorrected BHT grid to identify trends.

First-order observations from the temperature gradient maps include (1) gradients in southwest Texas are elevated along the Wilcox, Frio, and Vicksburg fault zones, perhaps in response to circulation of hydrothermal fluids; (2) lower gradients occur along the coastal area of Texas and in southern Louisiana likely due to a younger, thicker sedimentary section; (3) there are higher gradients over the Sabine uplift; and (4) variations in gradients within salt basins are present probably due to thermal conductivity properties of salt.

Calculating and comparing thermal gradients provides valuable information to characterize regional subsurface temperature regimes, and help improve the understanding of petroleum systems. The results are important for predicting source rock maturation, petroleum expulsion histories, and petrophysical properties of reservoir rocks.