Impact of diagenesis on the spatial and temporal distribution of reservoir quality in the Jurassic Arab D and C members, offshore Abu Dhabi oilfield, United Arab Emirates

This study is based on petrographic examination (optical, scanning electron microscope, cathodo-luminescence, backscattered electron imaging, and fluorescence) of 1, 350 thin sections as well as isotopic compositions of carbonates (172 carbon and oxygen and 118 strontium isotopes), microprobe analyses, and fluid inclusion microthermometry of cored Jurassic Arab D and C members from 16 wells in a field from offshore Abu Dhabi, United Arab Emirates. The formation was deposited in a ramp with barrier islands and distal slope setting. Petrographic, stable isotopic and fluid-inclusion analyses have unraveled the impact of diagenesis on reservoir quality of Arab D and C within the framework of depositional facies, sequence stratigraphy, and burial history. Diagenetic processes include cementation by grain rim cement and syntaxial calcite overgrowths, formation of moldic porosity by dissolution of allochems, dolomitization and dolomite cementation, cementation by gypsum and anhydrite, and stylolitization. Partial eogenetic calcite and dolomite cementation has prevented porosity loss in grainstones during burial diagenesis. Dolomitization and sulphate cementation of peritidal mud are suggested to have occurred in an evaporative sabkha setting, whereas dolomitization of subtidal packstones and grainstones was driven by seepage reflux of lagoon brines formed during major falls in relative sea level. Recrystallization of dolomite occurred by hot saline waters (Th 85-100\ub0C; and salinity 14-18 wt% NaCl). Anhydrite and gypsum cements (Th 95-105\ub0C; fluid salinity 16-20 wt% NaCl), were subjected to extensive dissolution, presumably caused by thermal sulfate reduction followed by a major phase of oil emplacement. The last cement recorded was a second phase of anhydrite and gypsum (Th 95-120\ub0C; 16-22 wt% NaCl), which fills fractures associated with faults

Ferroan dolomitization by seawater interaction with mafic igneous dikes and carbonate host rock at the Latemar Platform, Dolomites, Italy: numerical modeling of spatial, temporal, and temperature data

Numerous publications address the petrogenesis of the partially dolomitized Latemar carbonate platform, Italy. A common factor is interpretation of geochemical data in terms of heating via regional igneous activity that provided kinetically favorable conditions for replacement dolomitization. New field, petrographic, XRD, and geochemical data demonstrate a spatial, temporal, and geochemical link between replacement dolomite and local mafic igneous dikes that pervasively intrude the platform. Dikes are dominated by strongly altered plagioclase and clinopyroxene. Significantly, where ferroan dolomite is present, it borders dikes. We hypothesize that seawater interacted with mafic minerals, causing Fe enrichment in the fluid that subsequently participated in dolomitization. This hypothesis was tested numerically through thermodynamic (MELTS, Arxim-GEM) and reactive flow (Arxim-LMA) simulations. Results confirm that seawater becomes Fe-enriched during interaction with clinopyroxene (diopside-hedenbergite) and plagioclase (anorthite-albite-orthoclase) solid solutions. Reaction of modified seawater with limestone causes ferroan and nonferroan replacement dolomitization. Dolomite quantities are strongly influenced by temperature. At 40 to 80 degrees C, ferroan dolomite proportions decrease with increasing temperature, indicating that Latemar dolomitization likely occurred at lower temperatures. This relationship between igneous dikes and dolomitization may have general significance due to the widespread association of carbonates with rifting-related igneous environments

Empirical Calibration for Dolomite Stoichiometry Calculation: Application on Triassic Muschelkalk- Lettenkohle Carbonates (French Jura) Calibration empirique pour le calcul de la stoechiométrie de la dolomite : application aux carbonates triasiques du Muschelkalk-Lettenkohle (Jura français)

This study concerns an approach for dolomite quantification and stoichiometry calculation by using X-ray diffractometry coupled with cell and Rietveld refinements and equipped with a newly substantial database of dolomite composition. A greater accuracy and precision are obtained for quantifying dolomite as well as other mineral phases and calculating dolomite stoichiometry compared to the classical “Lumsden line” and previous methods. The applicability of this approach is verified on dolomite reference material (Eugui) and on Triassic (Upper Muschelkalk-Lettenkohle) carbonates from the French Jura. The approach shown here is applicable to bulk dolostones as well as to specific dolomite cements and was combined with petrographical and isotopic analyses. Upper Muschelkalk dolomites were formed during burial dolomitization under fluids characterized by increased temperature and variable isotopic composition through burial. This is clear from their Ca content in dolomites which gradually approaches an ideal stoichiometry (from 53.16% to 51.19%) through increasing dolomitization. Lettenkohle dolostones consist of near-ideal stoichiometric (51.06%Ca) and well-ordered dolomites associated with anhydrite relicts. They originated through both sabkha and burial dolomitization. This contribution gives an improved method for the characterization of different dolomite types and their distinct traits in sedimentary rocks, which allows a better evaluation of their reservoir potential. <br> Cette étude propose une approche pour la quantification de la dolomite et le calcul de sa stoechiométrie grâce à l’utilisation de la diffraction des rayons X couplée aux affinements de maille et de Rietveld et complétée par de nombreuses données issues de la littérature. Elle permet d’obtenir une meilleure justesse et précision pour la quantification de la dolomite (et des autres phases minérales) ainsi que pour le calcul de sa stoechiométrie par rapport à l’équation de Lumsden et de méthodes antérieures. L’approche proposée est vérifiée grâce à l’analyse d’un échantillon référence de dolomite (Eugui) et appliquée à des roches carbonatées du Trias (Muschelkalk supérieur-Lettenkohle) du Jura français. Elle est combinée à une étude pétrographique et isotopique et peut être appliquée tant aux roches qu’aux ciments dolomitiques. Les dolomies du Muschelkalk supérieur se sont formées au cours d’une dolomitisation d’enfouissement associée à des fluides dont la température augmente et ayant une composition isotopique variable au cours de l’enfouissement. Outre la pétrographie, ceci est également mis en évidence par le pourcentage de Ca calculé dans les dolomites, qui atteint progressivement une stoechiométrie idéale (de 53,16 % à 51,19 %) parallèlement au développement de la dolomitisation. Les dolomites du Lettenkohle montrent une stoechiométrie proche d’une stoechiométrie idéale (51,06 % Ca), sont ordonnées et associées à de l’anhydrite. Leur formation est liée à deux étapes de dolomitisation : de type sabkha et d’enfouissement. Cette étude permet ainsi une meilleure caractérisation des différents types de dolomites dans les roches sédimentaires et par conséquent une meilleure détermination de leur potentiel en tant que réservoir

Growth of layer-bound normal faults under a regional anisotropic stress field

International audienc

Impact of Diagenetic Alterations on the Petrophysical and Multiphase Flow Properties of Carbonate Rocks Using a Reactive Pore Network Modeling Approach

Sedimentary reservoir rocks generally have complex and heterogeneous pore networks that are related to the original depositional rock texture and subsequent diagenetic alterations. Such alterations are in part controlled by the original mineralogy and sedimentological facies, the compaction history, the involved fluids (and rock/fluid interactions), the flow history and the related physico-chemical conditions. During the diagenetic evolution (paragenesis), cycles of alternating dissolution (porosity enhancement) and precipitation (porosity destruction) caused by changes in chemical and thermodynamic conditions may lead to heterogeneous rock structure at both local and reservoir scale. In the absence of cored plugs to measure the petrophysical properties (i.e. porosity, permeability and formation factor) and multiphase flow properties (i.e. capillary pressure, relative permeability and resistivity index), a numerical tool that calculates these properties from pore structure data by predicting its evolution during the diagenetic cycle is of great interest for the petroleum industry and reservoir characterization studies. A Pore Network Model (PNM) provides opportunities to study transport phenomena in fundamental ways because detailed information is available at the pore scale. It has been used over the last decades to understand basic phenomena such as capillarity, multiphase flow or coupled phenomena. In particular, this modeling approach is appropriate to study the rock/fluid interactions since the mass exchange at surfaces can be modeled explicitly. It can provide quantitative information both on the effective transport property modifications due to the reactions and on the structure evolution resulting from dissolution/precipitation mechanisms. In the present paper, this approach is used to study the effect of the diagenetic cycle on the petrophysical properties of carbonate rocks. It involves three discrete steps. The first step consists of replacing the original complex pore structure of real porous media by a conceptual network. The second step consists of resolving the governing equations of the precipitation and dissolution phenomena (i.e. reactive convection diffusion equation) in the conceptual 3D pore network and deducing the local reactive fluxes and the motion of the fluid-solid interface. The third step consists of updating the new pore structure and calculating the new petrophysical properties of the modified porous media. Those steps are repeated in order to mimic a given diagenetic scenario. Finally, the multiphase flow properties of the current porous media are calculated. The impact of one diagenetic cycle of dissolution and precipitation on the pore networks’ heterogeneity and consequently on the petrophysical properties (i.e. porosity and permeability) and multiphase flow properties (i.e. relative permeability and capillary pressure) have been investigated. The permeability and porosity evolution during a given diagenetic cycle are calculated and analyzed as a function of the relevant dimensionless numbers (Peclet and Damköhler numbers) that characterize the flow and reaction regime. The correlation between these numbers and the dissolved/precipitated layer thickness distribution is investigated. This work contributes to improve the understanding of the impact of dissolution and precipitation on permeability and porosity modification. Using the PNM approach, multiphase flow properties and permeability-porosity relationship have been determined for different reactive flow regimes. These relationships are relevant input data to improve the quality of reservoir simulation predictions

Hydrothermal dolomites in the early Albian (cretaceous) platform carbonates (NW Spain): nature and origin of dolomites and dolomitising fluids

This study documents the temporal and lateral variation in petrographic and geochemical signatures of fault-related dolomite bodies in the Ranero and El-Moro areas (Karrantza valley, Cantabrian mountains; NW Spain). These dolomite bodies are hosted in Albian carbonates, which were deposited in the Basque-Cantabrian Basin as a result of an intense rift-related subsidence with associated faulting along various orientations. Fluid circulations generated replacive and cement dolomites, paragenetically followed by various calcite cements. Petrography, mineralogical and geochemical investigations (XRD, ICP, XRF, stable and Sr isotopes) helped in distinguishing hydrothermal stages. Two major dolomite facies were observed according to their Fe-content. Early dolomites are ferroan and replace limestone more significantly than the later, non ferroan dolomites. Dolomites are generally stoichiometric (49.76 to 51.59 M% CaCO3) and exhibit a broad range of depleted δ18O values (–18.7 to –10.5‰ V-PDB), which may indicate multiphase dolomitisation and/or different degrees of recrystallisation. Decreasing δ18O values correlate with decreasing Fe content in dolomites. In the Ranero area, dolomites show less slightly depleted δ13C values (–0.15 to +2.13‰ V-PDB) relative to the host limestone δ13C signature, while these values are substantially more depleted in El-Moro area (down to –2.18‰ V-PDB). Hydrothermal calcites predating dolomitisation show less depleted δ18O values (–14.15 to –12.1‰ V-PDB) than postdolomitisation calcite (–18.1‰ V-PDB). Sr isotope data suggest that the fluids interacted with siliciclastic lithologies (sandstone, shale). The dolomite fabric is variably altered through dedolomitisation and cataclastic deformation. Dolomitisation occurred in at least two main episodes. A first episode of pervasive ferroan dolomitisation probably resulted from compactional dewatering of basinal fluids from the nearby Basque trough and hydrodynamic fluid flow along the faults/fractures in the Albian carbonate platform. Subsequently, a second episode of very hot and localised dolomitisation may be related to igneous activity and convective flow

Tectono-stratigraphic evolution of the western margin of the Levant Basin (offshore Cyprus)

International audienceStratigraphic interpretation of twenty-four 2D seismic reflection profiles coupled with available well data from the Ocean Drilling Program (ODP) offshore Cyprus enabled identification of the main unconformities and the seismic packages in the western part of the Levant Basin and their correlation with major geodynamic events. The basic concepts of seismic stratigraphy were applied to improve our understanding of the tectonostratigraphy and the sedimentary architecture of Eratosthenes isolated carbonate platform. This platform is composed of dominant retrograding and aggrading Mesozoic carbonate build-ups and its evolution partly resembles that of the Egyptian margin. Deep-water mixed carbonates and siliciclastics probably prevailed in the basin during the Early Cretaceous. During the Late Cretaceous, these platforms were drowned. Following the convergence of the two plates, both the Eratosthenes Seamount (ESM) area and the Levant Basin progressively became part of a foreland basin along the Cyprus Arc. Different units in the Eratosthenes carbonate platforms (ECPs) and the transition to the deep Levant Basin are detailed here for the first time. We present a 3D conceptual model for the evolution of the western margin of the Levant Basin. We discuss the effect of the Miocene collision between the Cyprus Arc and Eratosthenes to understand the presence of upper Miocene Mass transport complexes (MTCs) in the deep basin and the tilting of the top of the adjacent ECPs. We briefly examine the deformation mechanisms of the Messinian salt near the paleo-shelves of the ECPs