ABSTRACT: Structural Diagenesis--Key to the Development of Microintercrystalline Porosity in a Lower Cretaceous Limestone, Safah Field, Oman

Dennis R. Prezbindowski, William C. Benmore, Duenchien C. Mou

Extensive development of microintercrystalline porosity in the Lower Cretaceous Shuaiba Limestones of Safah field (Oman) is dominantly the result of brittle deformation and late-stage dissolution. The Safah reservoir is contained in a gently domed, stratigraphic/diagenetic trap of large areal extent. Eighteen of the 90 wells drilled to date in this field have been cored. Microintercrystalline porosity is the most important reservoir porosity type; it is characterized by 1- to 15-µ-sized pores developed in the carbonate mud matrix of the wackestone and packstone lithofacies of shallowing-upward carbonate sequences. Reservoir porosity development occurred in an interfingering assemblage of shallow-water rudist, coral/stromatoporoid wackestone, packstone, grainstone, an boundstone lithofacies.

Microfractures provided pathways for the subsurface fluid movement responsible for porosity development. Clay content of the limestone increases downward with a corresponding increase in stylolite intensity as well as a decrease in fracture density and associated microporosity development. The late-stage timing and subsurface origin of the porosity are demonstrated by (1) dissolution textures developed on micro-spar calcite crystals; (2) porosity development postdating equant spar calcite cementation, stylolitization, and microfracturing; (3) the lack of significant correlation of microporosity development with specific carbonate lithofacies or position within the depositional cycles; and (4) the absence of geochemical and textural data indicative of subaerial exposure of the carbonat sequence.

The interplay of brittle limestone deformation (i.e., stylolitization and microfracturing), enhancement of hydrologic conductivity of the limestone, clay content, and late-stage dissolution are all critical to the development of reservoir-quality microintercrystalline porosity in Safah field.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990