TEM study of Mg distribution in micrite crystals from the Mishrif reservoir Formation (Middle East, Cenomanian to Early Turonian)

Abstract

Microporous limestones composed of micritecrystals constitute sizeable hydrocarbon reservoirs throughout the world and especially in the Middle East. However, the crystallization history of micrites is poorly understood. Scanning electronic microscopy (SEM) with X-ray energy dispersive spectroscopy (EDS) studies give morphological and bulk composition information about micrites, but no information exists on the distribution of minor elements inside micrite grains. This study proposes Mg maps obtained with X-ray EDS combined with scanning transmission electron microscopy (STEM) of micrite crystals from the Mishrif reservoir Formation (Middle East, Cenomanian to Early Turonian). Three types of Mg distribution were observed through micrite crystals from five different samples: (1) homogenous Mg concentration, (2) small Mg-enriched areas close to the center of the crystal, and (3) geometric Mg impoverishments near crystal edges and parallel to present crystallographic faces. The homogenous Mg distribution is the most frequent and is found both in microporous and in tight micrites. The second type of distribution showing small Mg-enriched areas inside micrite crystals relatively close to their center comes from a microporous sample located below an emersive surface. These enriched areas may correspond to crystal seeds. The third type of distribution was observed in micrite crystals from another microporous sample situated just below an emersive surface. The Mg-poor zones probably represent overgrowths that precipitated in contact with less Mg-rich meteoric fluids

The complex diagenetic history of discontinuities in shallow‐marine carbonate rocks: New insights from high‐resolution ion microprobe investigation of δ18O and δ13C of early cements

“…It is thought that micrite crystals, which compose marine lime‐mud, precipitated with an aragonitic or high‐magnesium calcite mineralogy throughout the Phanerozoic (Folk, ; Bates & Brand, ). However, recent studies suggest that the shallow‐marine micrite of lime‐mud initially crystallized into low‐magnesium calcite during the Cretaceous – as attested to by the similar morphology between lacustrine Miocene micrites and marine Cenomanian to early Turonian micrites (Volery et al ., , ,b, ) – and the Jurassic (Aalenian; Turpin et al ., ). Coimbra et al .…”

“…It is thought that micrite crystals, which compose marine lime-mud, precipitated with an aragonitic or high-magnesium calcite mineralogy throughout the Phanerozoic (Folk, 1974;Bates & Brand, 1990). However, recent studies suggest that the shallow-marine micrite of lime-mud initially crystallized into low-magnesium calcite during the Cretaceousas attested to by the similar morphology between lacustrine Miocene micrites and marine Cenomanian to early Turonian micrites (Volery et al, 2009(Volery et al, , 2010a(Volery et al, ,b, 2011 and the Jurassic (Aalenian; Turpin et al, 2014). Coimbra et al (2009) show that Jurassic marine micrite can display high stable oxygen isotope ratios, but they interpret this as the consequence of early stabilization of micrite in lowmagnesium calcite in seawater.…”

“…Moreover, carbon and oxygen stable isotope geochemistry may also allow for the determination of the initial mineralogy of early cements. While some authors claim that marine micrite precipitated in aragonite and high‐magnesium calcite throughout the Phanerozoic (Folk, ; Bates & Brand, ), recent studies suggest that low‐magnesium micrite can form during calcite sea periods (Volery et al ., , ,b, ; Turpin et al ., ).…”

Classifying chalk microtextures: Sedimentary versus diagenetic origin (Cenomanian–Santonian, Paris Basin, France)

“…Many authors have described micrite, based on samples from different locations, differing in age, deposit environment and type of diagenesis (e.g. Friedman, ; Le Roux, ; Masson, ; Loreau, , ; Wright et al ., ; Lambert et al ., ; Fournier & Borgomano, ; Vincent et al ., ; Volery et al ., ; Loucks et al ., ; Neveux et al ., ; Ehrenberg & Walderhaug, ; Tavakoli & Jamalian, ). Loreau (, ) proposed a system to classify micrite structure.…”

Mineralogy and geochemistry of Mishrif Formation from selected oilfield, south east of Iraq

“…It is descripted in the zubair area as a very heterogeneous detrital limestone contain some time algal, Rudist and coral reef limestone capped by limonitic fresh water limestone, and this first description was by [6], but in type section area Mishrif Formation compose of grey-white, massive, algal limestone with gastropods and shell fragment in the upper part, and of brown, detrital, porous, partly very shelly and foraminiferal limestones, with banks of Rudists in the lower part [7]. During the Cretaceous period in the Middle East, the majority of stratigraphic stages units contains one or more shallow-marine microporous carbonate reservoirs [5,8]. The Mishrif Formation which belong to Late Cenomanian age [9] is one of the most important oil reservoir in the Mesopotamian Basin ( Figure 2), southern Iraq [10,11,5,12], containing f 0% f m I q's s v s [13].…”