--> ABSTRACT: Giddings Edwards (Cretaceous) Field, South Texas: Carbonate Channel or Elongate Buildup?, by A. J. Lomando and S. J. Mazzullo; #91022 (1989)

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Giddings Edwards (Cretaceous) Field, South Texas: Carbonate Channel or Elongate Buildup?

A. J. Lomando, S. J. Mazzullo

Giddings Edwards field, located in Fayette County, Texas, is situated on the broad Cretaceous (Albian) shallow shelf, approximately 20 mi north of the main Edwards shelf-margin reef trend. The Giddings field produces gas from an elongate stratigraphic trap approximately 9.5 mi long and 1.8 mi wide, encased in argillaceous lime mudstones and shales; the field is oriented normal to the contiguous Edwards reef trend. Available cores and cuttings samples from the central portion of the field indicate that the field reservoir is composed of biopackstones and grainstones interpreted to have been deposited in a high-energy shelf environment. The facies system is characterized by stacked reservoirs having a maximum gross pay thickness of over 100 ft, containing primary interparti le and secondary biomoldic porosity, both of which have been modified slightly by chemical compaction and partial occlusion by sparry calcite and saddle dolomite cements.

Despite reasonable subsurface sample and mechanical log control within and surrounding the field, its depositional origin remains equivocal. Such uncertainty has important bearing on predictive models for the exploration for additional Edwards shelfal hydrocarbon reservoirs. The elongate, biconvex geometry of the productive carbonate sands, their northward thinning, and apparent updip bifurcation suggest deposition in a shallow-shelf channel system. By contrast, an alternative correlation and interpretation based on geometry and facies is that of an elongate in-situ carbonate buildup. A number of modern analogs of elongate buildups normal to major reef systems are available from which to compare and model the depositional system of Giddings Edwards field. The evaluation of this field erves as an example of using a multiple working hypothesis to develop an accurate exploration model.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.