Abstract: Late Cretaceous (Austin Group) Volcanic Deposits as a Hydrocarbon Trap

Peter J. Hutchinson

A Late 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 these features is normally attributed to the shoal-water carbonate facies developed on top of the volcano.

The heat energy of the volcano may have thermally matured the calcareous sediments of the Austin Chalk contiguous with the volcano. 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 the mobile hydrocarbons, and early emplacement 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 is maintained through early emplacement of oil and the overlying volcanic clay prevents vertical migration. Marcelina Creek, 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 over seven million bbl of oil from over 40 wells from fractured and porous rock beneath the volcano.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994