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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. 127–152.

Depositional and potential reservoir layers in the Jurassic carbonates of Saudi Arabia cannot necessarily be recognized by core description or wireline log interpretation alone. Recent studies based on thin-section micropaleontology of closely spaced core samples have led to elucidation of such layers in heterogeneous reservoir carbonates, based on the vertical tiering of various paleoenvironmentally sensitive biocomponents. When integrated with core descriptions, such biofacies and their lateral variations provide a powerful tool for optimal reservoir development. The biofacies guide sequence-based frameworks towards three-dimensional lithofacies and reservoir facies distribution models, with the added power of facies prediction away from drilled locations.

The 13 carbonate reservoirs in the Jurassic Shaqra Group of Saudi Arabia are developed in seven formations, of which the Arab Formation hosts the world’s largest onshore reservoir. The lithostratigraphic succession is composed of the Lower Jurassic Marrat Formation, the Middle Jurassic Dhruma and Tuwaiq Mountain formations, and the Upper Jurassic Hanifa, Jubaila, and Arab formations, which terminate with a succession of interbedded carbonates and evaporites of which the final thick evaporite is termed the Hith Formation. The Marrat Formation, of Toarcian age, hosts the Marrat Reservoir. The Dhruma Formation, of Bajocian–Bathonian age, hosts the Faridah, Sharar, and Lower Fadhili reservoirs. The Tuwaiq Mountain Formation hosts the Upper Fadhili and Hadriya reservoirs, and is of Callovian age. The Oxfordian Hanifa Formation hosts the Hanifa reservoir, which is overlain by the Jubaila and Arab formations, of Kimmeridgian age, and together host the Arab Reservoir. The Hith Formation is of probable Tithonian age and hosts the Rimthan and Manifa reservoirs. An ascending order of tiered deep- to shallow-marine foraminiferal assemblages has been determined for each formation and applied to distinguish both long-term and high-frequency paleobathymetric variations.

Micropaleontology has regained its importance in such industrial applications, and has significantly outgrown its traditional, but equally important, chronostratigraphic use. Because age-significant foraminiferal species are rare in the Shaqra Group, micropaleontological biofacies and their paleoenvironmental aspects are of especial importance. In addition to the intrareservoir applications, recent studies are focusing on the application of biofacies for regional studies and delimitation of potential reservoir targets in frontier areas. Using this technique for the Hanifa Formation, a regional lithofacies trend map has been deduced, which is currently assisting seismic program planning for exploration activities.

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