Cement stratigraphy: image probes of cathodoluminescent facies

Cement stratigraphy of carbonates aims to establish the chronology of processes involved in the rock diagenesis. Regional cement stratigraphy allows correlations and understanding of the petrological heterogeneities in reservoirs and aquifers, but is a long and rigorous approach. This article exposes a methodology of image analysis that facilitates the spatial correlation of diagenetic events in carbonate rocks. Based on the statistical comparison of signals extracted from the red spectrum emission of cathodoluminescence digital images, it gives via crosscorrelation a measure of similarity (values scaled from minimum −1 to maximum 1) between two cathodoluminescence facies. Cementation events and diagenetic chronologies can thus be quickly correlated without the support of a full chronology, the model normally established on cement morphologies, petrological analyses and cathodoluminescence zonation sequences. A case study from two Upper Kimmeridgian Mount Salève outcrops (France) illustrates this methodology. Their diagenetic sequences recorded in cathodoluminescent cements are presented and being compared. The final statistical similarity between the two outcrops reaches an index of R=0.78. This result is sustained by petrological and geochemical analyses such as alizarine-ferricyanure stained thin sections, X microfluorescence mapping of elements, and microthermometry of fluid inclusion

Hydrodynamic behaviour of Nummulites: implications for depositional models

Large benthic foraminifers are considered to be good indicators of shallow marine carbonate environments in fossil series. Over the last 50 years, the palaeoenvironment of Tertiary Nummulites accumulations has been a matter of debate, particularly because of difficulties in interpreting these deposits, and in this way, the absence of analogues in present-day seas does not help. The aim of this paper is to insight the different ways Nummulites tests and clasts may accumulate according to their hydrodynamic behaviour. Based on experimental measurements and on SEM observations, it appears that the high primary skeletal porosity of Nummulites made them easily transportable. The calculated threshold shear velocities confirm that large-sized Nummulites can be moved by weak wave-driven currents. This peculiar hydrodynamic behaviour of Nummulites could explain the diversity of depositional models. Depending on local hydrodynamic conditions, autochthonous Nummulites deposits can be preserved as in situ winnowed bioaccumulations or be accumulated offshore, onshore or alongshore, away from the original biotop

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)

Shallow-marine microporous limestones account for many carbonate reservoirs. Their formation, however, remains poorly understood. Due to the lack of recent appropriate marine analogues, this study uses a lacustrine counterpart to examine the diagenetic processes controlling the development of intercrystalline microporosity. Late Miocene lacustrine microporous micrites of the Madrid Basin (Spain) have a similar matrix microfabric as Cenomanian to Early Turonian shallow-marine carbonates of the Mishrif reservoir Formation (Middle East). The primary mineralogy of the precursor mud partly explains this resemblance: low-Mg calcites were the main carbonate precipitates in the Cretaceous seawater and in Late Miocene freshwater lakes of the Madrid Basin. Based on hardness and petrophysical properties, two main facies were identified in the lacustrine limestones: a tight facies and a microporous facies. The tight facies evidences strong compaction, whereas the microporous facies does not. The petrotexture, the sedimentological content, and the mineralogical and chemical compositions are identical in both facies. The only difference lies in the presence of calcite overgrowths: they are pervasive in microporous limestones, but almost absent in tight carbonates. Early diagenetic transformations of the sediment inside a fluctuating meteoric phreatic lens are the best explanation for calcite overgrowths precipitation. Inside the lens, the dissolution of the smallest crystals in favor of overgrowths on the largest ones rigidifies the sediment and prevents compaction, while partly preserving the primary microporous network. Two factors appear essential in the genesis of microporous micrites: a precursor mud mostly composed of low-Mg calcite crystals and an early diagenesis rigidifying the microcrystalline framework prior to buria

Facies characteristics and diversity in carbonate eolianites

Carbonate eolian dunes can form huge sand bodies along the coasts but are seldom described in the pre-Quaternary record. The study of more than 600 thin-sections collected in present-day, Holocene and Pleistocene dunes from Sardinia, Crete, Cyprus, Tunisia, Morocco, Australia and Baja California confirms that these deposits can be easily misinterpreted as shallow marine at core or thin-section scale. The classical eolian criteria (fine-grained and well-sorted sands) are exceptional in carbonate dunes because the diversity of shapes and densities of carbonate particles lowers the critical shear velocity of the sediment thus blurring the sedimentary structures. Wind carbonate deposits are mainly heterogeneous in size and often coarse-grained. The paucity of eolianites in the pre-Quaternary record could be due to misinterpretation of these deposits. The recognition should be based on converging sedimentological and stratigraphic elements at core scale, and diagenetic (vadose diagenesis, pedogenetic imprints) and petrographical (grain verticalization, scarcity of micritic envelopes, broken and/or reworked foraminifera) clues in thin-section. Bioclastic or oolitic grainstones showing evidence of vadose diagenesis or pedogenetic imprints, should always be suspected of having an eolian origi

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

Microporous limestones composed of micrite crystals 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 fluid

L'analyse minéralogique quantitative des roches sédimentaires: nouvelle méthode de dosage basée sur la composition chimique des échantillons

La composition minéralogique quantitative de n'importe quel type de roche sédimentaire peut être déterminée sans ambiguïté à partir d'une analyse chimique partielle de la roche, à condition que la nature et la composition chimique des minéraux qui la constituent soient connues et que le nombre d'éléments ou d'oxydes analysés soit au moins égal au nombre de minéraux présents dans la roche. Ces conditions impliquent souvent une étude minéralogique qualitative préliminaire par diffractométrie X. La méthode de dosage proposée, qui repose sur la résolution d'un système d'équations linéaires, est rapide et générale. La principale restriction réside dans l'impossibilité de doser sélectivement des composants isochimiques. La précision des dosages minéralogiques dépend de la précision des analyses chimiques de la roche et de la précision avec laquelle la composition chimique des minéraux a été estimée