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

Palaeoproterozoic magnesite: lithological and isotopic evidence for playa/sabkha environments

Magnesite forms a series of 1- to 15-m-thick beds within the approximate to2.0 Ga (Palaeoproterozoic) Tulomozerskaya Formation, NW Fennoscandian Shield, Russia. Drillcore material together with natural exposures reveal that the 680-m-thick formation is composed of a stromatolite-dolomite-'red bed' sequence formed in a complex combination of shallow-marine and non-marine, evaporitic environments. Dolomite-collapse breccia, stromatolitic and micritic dolostones and sparry allochemical dolostones are the principal rocks hosting the magnesite beds. All dolomite lithologies are marked by delta C-13 values from +7.1 parts per thousand to +11.6 parts per thousand (V-PDB) and delta O-18 ranging from 17.4 parts per thousand to 26.3 parts per thousand (V-SMOW). Magnesite occurs in different forms: finely laminated micritic; stromatolitic magnesite; and structureless micritic, crystalline and coarsely crystalline magnesite. All varieties exhibit anomalously high delta C-13 values ranging from +9.0 parts per thousand to +11.6 parts per thousand and delta O-18 values of 20.0-25.7 parts per thousand. Laminated and structureless micritic magnesite forms as a secondary phase replacing dolomite during early diagenesis, and replaced dolomite before the major phase of burial. Crystalline and coarsely crystalline magnesite replacing micritic magnesite formed late in the diagenetic/metamorphic history. Magnesite apparently precipitated from sea water-derived brine, diluted by meteoric fluids. Magnesitization was accomplished under evaporitic conditions (sabkha to playa lake environment) proposed to be similar to the Coorong or Lake Walyungup coastal playa magnesite. Magnesite and host dolostones formed in evaporative and partly restricted environments; consequently, extremely high delta C-13 values reflect a combined contribution from both global and local carbon reservoirs. A C- 13-rich global carbon reservoir (delta C-13 at around +5 parts per thousand) is related to the perturbation of the carbon cycle at 2.0 Ga, whereas the local enhancement in C-13 (up to +12 parts per thousand) is associated with evaporative and restricted environments with high bioproductivity

Saddle dolomite and calcite cements as records of fluid flow during basin evolution: Paleogene carbonates, United Arab Emirates

Field observations indicate that tectonic compression, anticline formation and concomitant uplift events of marine Paleogene carbonates in eastern United Arab Emirates, which are related to the Zagros Orogeny, have induced brecciation, karstification, and carbonate cementation in vugs and along faults and fractures. Structural analysis, stable isotopes and fluid inclusion microthermometry are used to constrain the origin and geochemical evolution of the fluids. Fluid flow was related to two tectonic deformation phases. Initially, the flux of moderately 87Sr-rich basinal NaCl-MgCl2-H2O brines along reactivated deep-seated strike-slip faults have resulted in the precipitation of saddle dolomite in fractures and vugs and in dolomitization of host Eocene limestones (δ18OV-PDB -15.8‰ to -6.2‰ homogenization temperatures of 80-115 °C and salinity of 18-25 wt.% eq. NaCl). Subsequently, compression and uplift of the anticline was associated with incursion of meteoric waters and mixing with the basinal brines, which resulted in the precipitation of blocky calcite cement (δ18OV-PDB -22‰ to -12‰ homogenization temperatures of 60-90 °C and salinity of 4.5-9 wt.% eq. NaCl). Saddle dolomite and surrounding blocky calcite have precipitated along the pre- and syn-folding E-W fracture system and its conjugate fracture sets. The stable isotopes coupled with fluid-inclusion micro-thermometry (homogenization temperatures of ≤50 °C and salinity of <1.5 wt.% eq. NaCl) of later prismatic/dogtooth and fibrous calcites, which occurred primarily along the post-folding NNE-SSW fracture system and its conjugate fracture sets, suggest cementation by descending moderately 87Sr-rich, cool meteoric waters. This carbonate cementation history explains the presence of two correlation trends between the δ18OV-PDB and δ13CV-PDB values: (i) a negative temperature-dependent oxygen isotope fractionation trend related to burial diagenesis and to the flux of basinal brines, and (ii) positive brine-meteoric mixing trend. This integrated study approach allows better understanding of changes in fluid composition and circulation pattern during evolution of foreland basins