ABSTRACT

The Davis Hill dome is a relatively large salt stock located in the northeastern part of the Houston salt dome province. The minimum depth is about 800 ft to the cap rock and about 1,200 ft to salt. The source layer (Jurassic Louann Salt) lies at a depth of at least 30,000 ft. Seismic and well log interpretation suggests that diapir growth took place from Late Cretaceous to Plio-Pleistocene. Passive growth (downbuilding) of the dome ceased during the Early Oligocene (Vicksburg) and was followed by active growth (upbuilding) of the dome. Present-day topographic relief at Davis Hill is 175 ft above the surrounding coastal plain. A radial pattern of oxbow lakes above the dome suggests that the dome still is rising, but the topographic maximum does not correspond to the apex of the dome.

Studies of cores indicate that the cap rock is about 440 ft thick and comprises, from bottom to top, anhydrite, gypsum and calcite zones. The anhydrite zone represents the less soluble components of the salt that accumulated as the halite was dissolved by pore fluids. The calcite zone formed by bacterial alteration of sulfate accompanying hydrocarbon destruction. The calcite zone in these cores contains significant amounts of sulfide minerals and oil. The sulfiderich calcite zone is approximately 40 ft thick and is dominated by locally massive iron sulfides. These sulfide concentrations resulted from the interaction of deep heated metal-bearing formational brines with bacterially derived reduced sulfur in the ambient cool ground waters.