Petrographic and Graphic Analysis of Shelf-Edge Porosity in Lower Cretaceous Sligo Formation, South Texas

Brenda L. Kirkland

During Aptian to Albian time, over 2,000 ft (610 m) of reef, back reef, and lagoonal facies were deposited along the Sligo shelf margin in south Texas. Ninety thin sections of whole core from five wells (Hunt 1 Reuthinger, Mobil 1 McElroy, Cities Service 2A Nueces Minerals, Gulf 1 Friedrichs, and General Crude 1 McDowell) were described and point counted to determine the nature and amount of visible porosity (average 0.7%; maximum 20.9%). Porosity (open pore space) is entirely secondary, usually intergranular, and generally associated with dolomite rhombs or saddle crystal dolomite.

The most abundant cement type in these shelf-edge samples is blocky equant calcite spar. It occludes primary porosity, as well as porosity associated with early dissolution and fracturing. Early and late generations of spar are evident from petrographic relations.

Thermally altered bitumen is abundant in the Sligo Formation. This bitumen is present along stylolite seams, micritic grain rims, and between microspar crystals. Trapped organic material was converted to oil and then to solid organic residue (residual bitumen) with maturation. If visible porosity and porosity filled by altered bitumen are considered together, porosity values average 2.3% with a maximum of 20.9%.

Along the shelf edge, porosity in the Sligo Formation is greatest in sediment deposited in high-energy platform shoals. Porosity filled by altered bitumen is greatest in low-energy lagoonal sediment. Secondary porosity, formed in association with dolomitization or fracturing, is the key factor controlling reservoir quality in the Sligo Formation of south Texas.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.