Hilbig oil field, Bastrop County, Texas

The Hilbig oil field produces from a porous serpentine plug. The discovery well was completed, February 15, 1933, and the development of the field was completed with 18 wells in August, 1933. Evidence of surface structural features has not yet been found in the outcropping Wilcox formation; neither can evidence of subsurface structure be found in the underlying Midway, Navarro, or Taylor formations. The serpentine is an altered, or metamorphic, igneous intrusion. The original igneous rock intruded the top of the Austin chalk, folding the chalk into a dome around the flanks of the intrusion. The Austin chalk shows evidence of contact metamorphism near the serpentine. Part of the lava was probably extruded on the floor of the Taylor sea, for Taylor sediments were deposited over and around the structure and are unaltered where they are in contact with the serpentine. The serpentine body is in the shape of a laccolith, with an areal extent between 700 and 800 acres. Salt water is found in the base of the serpentine. This water is probably native water of the Taylor sea, which was trapped within the serpentine. A uniform oil-water level is not present. The porosity of the serpentine varies from 3 per cent to 22 per cent. The oil is 35.5 degrees A.P.I. in gravity, light green in color, and has a paraffine base.