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Abstract

Geologic Problem Solving with Microfossils: A Volume in Honor of Garry D. Jones

SEPM Special Publication No. 93, Copyright © 2009

SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-137-7, p. 111–126.

Foraminiferal and calcareous algal biofacies of the Upper Permian to Lower Triassic carbonates provide important guides to the depositional paleoenvironment of these important gas-reservoir carbonates of the Khuff Formation in Saudi Arabia. Thin-section analysis has revealed smaller foraminifera that have been variously considered as useful for subdividing the Permian section, despite the absence of the conventional fusulinid species. The Permian–Triassic boundary lies within the Formation, and is defined by locally significant biostratigraphic evidence.

Because all species are extinct, paleoenvironmental interpretation is based on the vertical stacking arrangement of various biofacies based on the principle of anticipated paleobathymetric tiering, assisted by the application of morphogroup characteristics. This approach is linked to biological expectations of carbonate secreting organisms associated with successive transgression-linked retrogradational facies movement and regression-linked progradational facies movement, in conjunction with various carbonate textures. Foraminiferal and calcareous algal biocomponents have been used to determine subtle paleoenvironmental variations, both vertically and horizontally, in the subsurface. Paleoenvironmental factors are here suggested to include those that are based on the paleogeography, in terms of seawater temperature, solar incidence, prevailing wind direction, and proximity to land. Eustatic and autocyclic paleobathymetric variations would be expected to cause higher-frequency paleoecological changes in salinity, turbidity, wave energy, and carbonate productivity rates. The following biofacies have been established: Spirorbis–ostracod–gastropod; gastropod–brachiopod–ostracod; ostracod–Mizzia-Gymnocodium; Agathammina–Hemigordius; Agathammina–Hemigordius–Globivalvulina; Nankinella–Staffella; bryozoa; and Pachyphloia–Protonodosaria. The reservoirs here considered display subtle paleoenvironmental variations of an extensive, generally shallow marine, carbonate platform that ranged from intertidal to depths within intrashelf basins that probably rarely exceeded storm wave base. Paleoenvironments ranged from intertidal, shallow to deep subtidal, and shallow shoals. In addition, certain localities display sufficiently diverse events that there is the potential for refined interwell correlation as well as their use for biosteering development wells.

Morphogroups have been used to assist in paleoenvironmental interpretation.

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