Linking dolomitization to sequence stratigraphy: Insights from the Upper Jurassic Arab Formation, offshore oilfield, UAE

Abstract

This petrographic, petrophysical, and geochemical study revealed that dolomitization in the Upper Jurassic Arab Formation reservoir, United Arab Emirates, can be constrained within 2nd, 3rd, and 4th order sequences. The formation represents a 2nd order regressive sequence/cycle (highstand systems tracts, HST) that was deposited across a carbonate ramp with shoal under hot arid climatic conditions during the Kimmeridgian-Tithonian. Deposition occurred subsequent to maximum marine transgression (maximum flooding surface, MFS) and was accompanied by a progressive increase in the degree of restriction of the connectivity between the inner platform (supratidal, upper intertidal and lagoon) and the open sea. Sporadic dolomitization took place in transgressive systems tracts (TST) of the 3rd and 4th order cycles of earliest stages of the 2nd order regression, below the parasequence boundaries and within bioturbation sites. Subsequent rapid and extended 2nd order marine regression cycles were embossed by trends of progressive increase in salinity, which resulted in concomitant systematic decrease in limestone deposition and increase in intensity of seepage reflux/sabkha dolomitization by seepage reflux of hypersaline brines. Dolomitization is most prevalent in the 4th order HST sequences and was accompanied by occlusion of the moldic and intercrystalline pores in the dolostones by gypsum/anhydrite cement. Dolomitization that was accompanied by limited formation of gypsum/anhydrite in the late 2nd order HST is attributed to reflux of penesaline (salinity saturated with Ca-sulfate) /mesohaline (below Ca-sulfate saturation) brines. The reflux of the latter brines is attributed to smaller extent of relative sea-level falls, i.e., partial restriction of the inner platform. The precipitation of dolomite cement in moldic pores and as thin overgrowths around replacive dolomite is attributed to the flow hot basinal brines (i.e., hydrothermal fluids). This interpretation is supported by the presence of saddle dolomite, along with depleted δ¹⁸O values, relative enrichment in ⁸⁷Sr isotope, and high fluid-inclusions homogenization temperatures. The greater amounts of these post-dolomitization cements in the dolopackstones and dolograinstones caused stronger obliteration of their near-surface isotopic signatures compared to the dolomudstones and dolowackestones.

This study demonstrates that linking dolomitization of limestones across carbonate ramps under hot-arid climatic conditions to different orders of changes in the relative sea level and lithology of the succession indicates the involvement of various types of dolomitizing fluids and geochemical conditions, including: (i) domination of seepage reflux of hypersaline and mesohaline/penesaline brines, (ii) dolomitization below the seafloor during marine transgression, and (iii) dolomitization within bioturbation sites.