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

Abstract

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.

Lacustrine microporous micrites of the Madrid Basin (Late Miocene, Spain) as analogues for shallow-marine carbonates of the Mishrif reservoir Formation (Cenomanian to Early Turonian, Middle East)

“…This similarity of microfabrics confirms the essential role played by the mineralogical composition of the precursor mud in the development of such microporous limestones (Volery et al 2009). During the Cenomanian to Early Turonian, the seawater was favorable to the precipitation of low-Mg calcite (Dickson 2002(Dickson , 2004Hardie 1996;Lowenstein et al 2001;Sandberg 1983;Siemann 2003).…”

“…The last question was already debated in a previous paper (Volery et al 2009): the study of stratigraphic occurrence of shallow-marine microporous carbonate formations in the Middle East reveals that such carbonates developed during periods of calcite seas. It clearly indicates that mud mainly composed of low-Mg calcite crystals is required to produce these microporous limestones.…”

“…During the Cretaceous in the Middle East, the majority of stratigraphic stages contains one or more shallow-marine microporous carbonate reservoirs (Alsharhan and Nairn 2003;Volery et al 2009). These reservoir formations are characterized by a microporous micritic matrix composed of sub-rhombic low-Mg calcite crystals with diameters generally below 8 lm.…”

Morphometry of micrite particles in cretaceous microporous limestones of the Middle East: Influence on reservoir properties

“…Various diagenetic models have been suggested for the mineralogical and textural stabilization of microporous matrixes in ancient neritic limestones. While some of them argue for stabilization in sea-waters (Moshier, 1989b;Holail and Lohmann, 1994;Al-Aasm and Azmy, 1996;Richard et al, 2007;Coimbra et al, 2009) most others argue for an influence of meteoric waters in very shallow to shallow burial conditions (Budd, 1989;Perkins, 1989;Jones and Kahle, 1995;Volery et al, 2009Volery et al, , 2010b. Determining such controlling sedimentologic and diagenetic factors is a key point to understand and predict the distribution of the C, F and D petrographical classes.…”

“…Because their comparatively low porosity and permeability make water/oil saturation/recovery difficult to predict and achieve (Kieke and Hartmann, 1974;Petricola et al, 2002;Ahr et al, 2005;Mallon and Swarbrick, 2008), microporous limestones have until now received less attention than coarser macroporous limestones. Accordingly, factors controlling the spatial distribution of fluids remain poorly understood (Lambert et al, 2006;Richard et al, 2007;Maliva et al, 2009;da Silva et al, 2009;Volery et al, 2009Volery et al, , 2010aVolery et al, , 2010b.…”