Abstract: Investigations of a Deep Drill Hole in the Ouachita Trend, West Texas

P. M. Borrego, F. Moreno, G. R. Keller, K. M. Marsaglia, N. E. Pingitore

The Exxon 1 Gatlin well was recently drilled to a depth of over 25,000 ft in Terrell County, Texas, just north of the international boundary with Mexico. This well is the deepest penetration of the interior zone of the Ouachita system to date. The well is near the apex of the interior zone gravity high, which elsewhere has been shown to approximately mark the Paleozoic continental margin. Gravity models constructed in this region, including one that was tied to this well, confirm that this well is proximal to the Paleozoic margin.

The well was spudded in Lower Cretaceous rocks, and drilled through a 23,000-ft section of low-grade metamorphic rocks. Cuttings of this metamorphic section are composed predominantly of phyllite, fine-grained schist, metaquartzite, and impure marble. Based on their mineralogy and texture, the likely precursor for these rocks was a sedimentary sequence of marls and mudstones. However, no relict sedimentary structures or fauna were observed. Phyllite and schist fragments are graphitic and locally crenulated, and large fragments exhibit multiple phases of deformation. Minor coarse carbonate and quartz may represent vein fills. Downhole textural trends include a gradual change from phyllite to schist at approximately 7500 ft, and a gradual increase in schist fragments to 19,600 ft. X-ray diffraction of bulk samples indicates a downhole decrease in total carbonate content. This metamorphic sequence appears similar to Ouachita interior zone rocks encountered in wells in central Texas and in outcrops in Oklahoma and Arkansas. However, the large thickness encountered in the well and the even larger thickness suggested by the gravity modeling is indicative of proximity to the continental margin and is likely the result of thrust faulting.

AAPG Search and Discovery Article #90980©1994 AAPG Southwest Section Meeting, Ruidoso, New Mexico, April 24-26, 1994