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Stratigraphy of the Austin and Associated Formations and its Influence on Hydrocarbon Resources, Pearsall Field Area, South Texas

Thomas Ewing
[email protected]

"Detailed well-log correlations in the area of Pearsall field (150 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 Base Austin unconformity. In Pearsall Field, intervals Austin 'A' (top) through 'E' (base) are defined. The 'E' interval is highly resistive and somewhat impure chalk with stringers of Eagle Ford lithology. Above lies a thick 'D' interval of low-resistivity chalk. The middle Austin 'C' interval includes two resistive, high-GR (high U) beds (the 'Two Finger Zone') marking the return of dysoxic environments. Overlying is the Austin 'B-2' interval of clean chalk. An upper 'B' ('B-1') zone of interbedded 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 northeast, 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. 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 #90167©2013 GCAGS and GCSSEPM 63rd Annual Convention, New Orleans, Louisiana, October 6-8, 2013