ABSTRACT

Cores from Glass Bluff field indicate that reservoir compartmentalization in thin peloidal-pisoidal grainstones of the porous Nesson zone [< 20 ft (6 m) thick, with porosities of 5-14% and permeability of 0.01 md] is caused by paleocaliche crusts and paleosoil zones developed during subaerial exposure.

The Nesson in most of the analyzed cores is inter- to supratidal deposits of pale, oxidized, porous, bedded, crustose, peloidal-pisoidal packstones and grainstones; and darker, unoxidized, silty, peloidal, kamaenid algal, shallow-shelf or lagoon wackestones and packstones. One core consists solely of gray, unoxidized, peloidal, kamaenid-Ortonella packstone and grainstone thought to be a tidal channel deposit. The strata are overlain at a local unconformity by tight, normal marine deposits of the Midale beds of the Charles Formation, which serve as a seal for the reservoir.

The extremely low fossil diversity in the Nesson suggests a restricted marine environment. Stable oxygen isotopes measured in samples from micritic layers and fossils suggest a paleotemperature of formation near 86°F (30°C).

Depositional models and cross sections through Glass Bluff field suggest that the reservoir is developed mainly in a northwest-southeast trending tidal flat and in tidal channels bisecting the tidal flat. An arid climate contributed to the development of crusts which impede vertical permeability in the tidal flat lithofacies. The absence of crusts in the tidal channel facies gives it the best combination of porosity, multidirectional permeability, and production.