Interpretation and mathematical modeling of temporal changes of temperature observed in borehole Yaxcopoil-1 within the Chicxulub impact structure, Mexico

SummaryGeothermal research of the Chicxulub impact structure on the Yucatan Peninsula, Mexico, included repeated temperature logs following 0.3–0.8, 15, 24, 34 and 50 months after shut-in of drilling operations at the 1.5 km deep Yaxcopoil-1 borehole. A gradual distortion of the linear temperature profile by a cold wave propagating downward from 145 m to 317 m was detected within the observational period of 49 months (March 2002–April 2006). The amplitude of the cold wave was increasing with depth and time in the range of 0.8–1.6 °C. As an explanation of this unusual phenomenon, the hypothesis of downward migration of a large volume of drilling mud, reported lost during drilling within the overlying and cooler highly porous and permeable karstic rocks, has been proposed.The thermal effects of the migrating fluid have been evaluated by solving numerically the heat conduction–convection equation in appropriate geothermal models. The best coincidence between the observed data and the simulations was yielded by the model of the drilling mud migrating as a large body. Parameters of this model are constrained by the measured temperature logs relatively tightly: (i) the vertical extent of the downward migrating fluid body is about 5–10 m, possibly increasing within the observational period of 49 months by a factor of 2; (ii) the horizontal extent of the body must be at least 15–20 m, i.e. by order(s) of magnitude larger than the diameter of the borehole; (iii) the average speed of the migration is about 5 metres per month and (iv) the fluid must migrate through a highly porous rock (80% porosity or more). This high porosity, which is necessary for the model to fit the observed data, and the observed relatively stable velocity of the migration indicate the existence of a well-developed system of interconnected cavities down to more than 300 m about 150 m more than the deepest cave system known in Yucatan yet.