Diagenesis of the Khuff Formation (Permian–Triassic), northern United Arab Emirates

Abstract

This diagenetic study (including fieldwork, petrographic, fluid inclusion, and stable isotope investigations) deals with the outcrop of Upper Permian-Lower Triassic carbonate rocks, which are equivalent to the Khuff Formation. The studied succession, which outcrops in the Ras Al Khaimah region, northern United Arab Emirates, comprises three formations, including the Bih, the Hagil, and the Ghail formations. The study focuses on unraveling the conditions and fluid compositions encountered during diagenesis of the succession. Emphasize is also made on linking diagenesis to major stratigraphic surfaces and to highlight reservoir property evolution and heterogeneity of the studied rocks. The evolution of fluids and related diagenetic products can be summarized as follows: (1) formation of near-surface to shallow burial, fine-crystalline dolomite (dolomite matrix) through pervasive dolomitization of carbonate sediments by modified marine pore waters; (2) formation of coarsecrystalline dolomite cement by highly evolved marine pore waters (13-23 wt.% NaCl eq.) at elevated temperatures (120-208°C), and (3) calcite cementation by highly saline fluid (20-23 wt.% NaCl eq.) at high temperature (170-212°C). A final calcite cement generation has been formed by the percolation of meteoric fluids during uplift. Fracture-and vug-filling diagenetic minerals are mainly restricted to the mid-Bih breccia marker level, suggesting preferential focused fluid flow through specific stratigraphic surfaces as well as along tectonic-related structures. Reservoir properties have been evolved as result of the interplay of the original sedimentary texture and the diagenetic evolution. Porosity is higher in the Bih Formation, which is dominated by dolomitized packstones and grainstones, than in the Hagil and Ghail formations, consisting mainly of dolomitized mudstones and wackestones. Image analyses were used to quantify the visual porosity in thin sections. The highest porosity values were measured in the Bih Formation, which is characterized by significant amounts of vug-and fracture-filling cements. This feature is attributed to the increase of porosity owing to substantial dissolution of abundant intergranular and vug-filling cements. In contrast, the Hagil and Ghail formations, which consist of finer-grained rock than the Bih Formation, were less cemented, and thus, the porosity enhancement by cement dissolution was insignificant.

Origin of Drusy Dolomite Cement in Permo-Triassic Dolostones, Northern United Arab Emirates

“…Fluid-inclusion microthermometry of drusy mosaic dolomite and the subsequently precipitated calcite cement indicate formation from hot brines during compressional tectonic events in conjunction with the obduction of Oman ophiolites [16,36]. However, it is not immediately clear whether the drusy mosaic dolomite has been precipitated as cement [13] or formed by dolomitization of drusy mosaic calcite by the hot basinal brines [15,37].…”

“…The relatively systematic T h zoning from pore walls inwards (i.e., increase in crystal size) supports the formation of drusy dolomite by precipitation from pore fluids than by dolomitization of precursor drusy calcite cement. Apparently, the chemistry of the dolomitizing fluids has changed with time and reflected on the variation of Fe-rich and Fe-poor zoning of CL pattern of the drusy dolomite cement (cf., [2,36]).…”

Quantification of diagenesis impact on the reservoir properties of the Jurassic Arab D and C members (Offshore, U.A.E.)

“…Fontana et al . ): (i) cross‐cutting relationships suggesting that dolomitization occurred before burial pressure‐dissolution (bed‐parallel stylolites) and (ii) O, C and Sr isotopic values of dolostones and associated cements within the range of isotopic signatures typical of carbonates precipitating from evaporated seawater (Veizer et al . ).…”

“…Demonstrating that evaporative sabkha and shallow burial seepage reflux models prevail for a certain dolostone body is often based on, at least, two considerations (e.g. Fontana et al 2010): (i) cross-cutting relationships suggesting that dolomitization occurred before burial pressure-dissolution (bed-parallel stylolites) and (ii) O, C and Sr isotopic values of dolostones and associated cements within the range of isotopic signatures typical of carbonates precipitating from evaporated seawater (Veizer et al 1999).…”

“…To date, classical diagenesis studies make use of a wide range of descriptive methods and analytical techniques converging into conceptual models that explain specific, relatively time-framed, diagenetic processes to deduce their impacts on reservoir rocks (e.g. Nader et al 2004;Gasparrini et al 2006;Fontana et al 2010;Ronchi et al 2011;Swennen et al 2012). Currently used techniques combine petrographic (conventional, cathodoluminescence, fluorescence and scanning electron microscopy with energy dispersive spectrometer -SEM/EDS), geochemical (major/ trace elements, stable oxygen and carbon isotopes, strontium isotopes) and fluid inclusion analyses (microthermometry, Raman spectrometry, crush-leach analysis), providing independent arguments to support or discard any of the proposed models.…”