ABSTRACT

An Upper Cretaceous submarine igneous extrusion occurs in the subsurface of southwestern Wilson County, Texas. The Coniacian-Santonian-aged (Austin Group) volcanic eruption discharged large volumes of magnetite-rich olivine nephelinite that upon quenching formed an extensive nontronitic clay layer. This clay deposit formed a trapping mechanism for hydrocarbons beneath the volcano. Production from volcanic plugs is normally attributed to the shoal-water carbonate facies developed on top of the volcano, the palagonite tuff ("serpentine"), and overlying sandstones.

The heat energy of the volcano may have thermally matured the calcareous sediments of adjacent parts of the Austin Chalk. The normally grayish-colored Austin Chalk in contact with the intrusive portion of the igneous material displays a greenish color suggesting thermal alteration. The overlying nontronite trapped mobile hydrocarbons, and this early emplacement of oil may have preserved some of the original porosity and permeability of the Austin Chalk.

Austin Chalk-aged volcanic deposits produce hydrocarbons from stratigraphic traps within the volcanic material, within the porous beachrock, and structurally within overlying sandstones. The intruded Austin Chalk also behaves as a reservoir because the original porosity and permeability are maintained by early emplacement of oil and the overlying volcanic clay acts as a seal by preventing vertical migration. Marcelina Creek field, discovered in 1980 from an "augen"-shaped seismic signature and an aerial magnetic survey, produces from the fractured chalk beneath the nontronitic clay layer. This field has produced more than 1.5 million barrels of oil from more than 60 wells in fractured and porous rock beneath the volcano.