Stratigraphy of the Austin and Eagle Ford Formations and Its Influence on Hydrocarbon Resources, Pearsall Field Area, South Texas
Frontera Exploration Resources
Detailed well log correlations in the area of Pearsall Field (159 MMbo) disclose offlapping carbonate sequences, unconformities, and downlap surfaces in the Austin, Eagle Ford, and Anacacho units.
Above the ubiquitous Middle Eagle Ford marker, Upper Eagle Ford impure, organic-rich limestones are truncated to the northeast beneath a basal Austin unconformity. In Pearsall Field, intervals Austin 'A' (top) through 'E' (base) are defined. The 'E' interval is highly resistive and somewhat impure limestone with stringers of Eagle Ford lithology. Above lies a thick 'D' interval of low-resistivity, somewhat impure limestone. The middle Austin 'C' interval includes two resistive, high gamma ray (high U) beds (the 'Two Finger Zone') marking the episodic return of dysoxic environments. Overlying is the Austin 'B-2' interval of clean limestone. An upper 'B' ('B-1') zone of interbed-ded limestone and clay pinches out northeastward. Marly 'A' limestone tops the Austin, above a thick set of clay-rich 'Ash Beds.'
The 'C' interval is the key to internal Austin stratigraphy. In the north-east, the 'C' lies high in the Austin, overlying a thick 'D.' Basinward, the 'C' falls in the section and downlaps into a thin 'C-D-E' condensed zone; the B zone becomes thick. The 'D' and 'C' zones represent mound morphologies, whereas the Eagle Ford and Austin 'B' show basin-centered depocenters.
Although vertical wells produced from the entire Austin, horizontal drilling has been concentrated in the pure and brittle Austin 'B.' Recent wells target the Austin 'E' immediately overlying the Eagle Ford. Austin plays use the natural fracturing of the reservoirs to produce, but induced fracturing may represent a future play for the 'C' and 'D' intervals.
AAPG Search and Discovery Article #90202 © AAPG/STGS Geoscience Technology Workshop, Eagle Ford plus Adjacent Plays and Extensions Workshop, February 24-26, 2014, San Antonio, Texas