ABSTRACT

Reef limestone as much as 377 feet thick occurs in the Heterostegina zone of the Anahuac formation (Oligocene) in part of northwestern Brazoria County about 40 miles south-southwest of Houston, around Damon Mound, Nash, and West Columbia piercement salt domes. This locally-developed limestone is completely surrounded by normal Heterostegina limy shale or shale with one or more very thin limestone beds.

At Nash dome the upper part of the limestone is porous, consisiting 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 terminate at the end of Anahuac time but continued 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 varying from approximately 2,000 feet at Damon to 4,350 feet at Nash.