Reef limestone as much as 300 feet thick occurs in the Heterostegina zone in part of northwestern Brazoria County about 40 miles south-southwest of Houston, between Damon Mound, Nash, and West Columbia piercement salt domes. This locally developed limestone is completely surrounded by normal Heterostegina calcareous shale or shale with one or more very thin limestone beds.

At Nash dome the upper part of the limestone is porous, consisting chiefly of "honeycomb" corals, and the lower part is more dense. Indications are that the Heterostegina reef developed in a near-shore, shallow-water environment, and that its growth did not stop at the end of Anahuac time but continued even into the early Miocene.

Large-diameter conventional cores are recommended for evaluating the production potential of the Heterostegina limestone. Although the best porosity is found in the top of the limestone, it may develop also in other intervals. The limestone production at Nash field does not have a common oil-water contact.

Substantial oil production has been obtained from the Heterostegina limestone and more may be expected at Nash, Damon Mound, and West Columbia fields, at depths ranging from approximately 2,000 feet at Damon to 4,350 feet at Nash.

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