SHALLOW-MARINE MICROPOROUS CARBONATE RESERVOIR ROCKS IN THE MIDDLE EAST: RELATIONSHIP WITH SEAWATER Mg/Ca RATIO AND EUSTATIC SEA LEVEL

C. Volery^1*, E. Davaud¹, A. Foubert² and B. Caline³

¹ University of Geneva, Earth and Environmental Sciences, Department of Geology and Paleontology, Rue des Maraîchers 13, 1205 Genève, Switzerland.

²  K.U. Leuven, Department of Earth and Environmental Sciences, Celestijnenlaan 200E, 3001 Heverlee, Belgium.

³ Total Exploration and Production, CSTJF, Avenue Laribau, 64000 Pau, France.

* Corresponding author, email: Chadia.Volery@unige.ch & chadia@infomaniak.ch

The formation of shallow-marine microporous carbonate reservoir rocks remains poorly understood in spite of their economic importance, particularly in the Middle East. In this paper, we investigate relationships between the stratigraphic occurrence of these carbonates in the Middle East and (i)  the evolution of the Mg/Ca ratio in seawater; and (ii) cyclic variations in relative sea-level.

An inventory of carbonate formations in the Middle East was compiled for three geological time intervals characterised by different seawater chemistries: the Late Carboniferous to Triassic (aragonite seas); the Cretaceous (calcite seas); and the Cenozoic (transitional from calcite to  aragonite seas). For each time interval, carbonate formations described as microporous have been listed.

During the Cretaceous calcite sea, eleven microporous carbonate formations were deposited in the Middle East. However, no microporous carbonates were formed during the Late Carboniferous to Triassic, a time of aragonite seas. During the Cenozoic, four of the five microporous carbonate formations recorded were deposited before the transition from calcite to aragonite seas. Thus, these shallow-marine microporous carbonates appear to have developed from precursor muds which were mainly composed of low-Mg calcite crystals. Moreover, during the Cretaceous and the Cenozoic, microporous carbonate formations in the Middle East were generally associated with major transgressions and highstands of relative sea level.

The relatively high stability of low-Mg calcite muds may explain why shallow-marine microporous carbonates formed during time intervals with calcite seas. In contrast to muds composed of aragonite or high-Mg calcite crystals, the original microfabric (including intercrystalline microporosity) of low-Mg calcite muds can partly survive moderate diagenesis.

Key words:  Microporosity, micrite, carbonates, reservoir rock, Middle East,  low-Mg calcite.

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