Hydrocarbon‐induced Diagenetic Dolomite and Pyrite Formation Associated With the Hormoz Island Salt Dome, Offshore Iran

Abstract

Hormoz Island, a salt diapir in the eastern Persian Gulf, is dominated by the Infracambrian Hormoz Complex comprising both evaporites (Hormoz Salt) and carbonates, siliciclastics and volcanic rocks. Minerals include black, white and grey dolomites, pyrite, gypsum, anhydrite, apatite and iron oxides. Formation of some of the dolomite crystals is interpreted to be linked to the oxidation of hydrocarbons.The δ 13 C values of black dolomite crystals range from -0.8 to -2.07‰ VPDB, indicating that little if any of their carbon is derived from hydrocarbon oxidation but that sea water has provided carbon and Mg for dolomite precipitation. The δ 18 O values for these dolomites range from -9.2 to -15.3‰ VPDB, reflecting a temperature effect on isotopic fractionation. By contrast, δ 13 C values for white to grey dolomites range from -17.81 to -35.68‰ VPDB, indicating that the carbon may be derived from the oxidation of hydrocarbons. Based on the δ 18 O dolomite and temperatures obtained from fluid inclusion studies (215ºC), the calculated δ 18 O water in equilibrium with these dolomites (+2 < δ 18 O fluid < +12‰ ) indicates the involvement of brines evolved via the interaction of seawater with the Hormoz Salt and associated sedimentary rocks. Some of the dolomite may have precipitated from post-Cambrian seawaters at lower temperatures (ca.100 ºC). Thus, the dolomites may have begun to form during deep burial but have also formed during salt diapirism at more shallow depths.Pyrite and native sulphur are interpreted to have formed in reducing conditions where the source of sulphur was H 2 S produced by the thermochemical reduction of sulphate in the Hormoz Salt evaporites. Heavy δ 34 S values for the anhydrites (ranging from 28.7 to 30.8‰ ) and for sulphides (ranging from 17.2 to 23.4‰ ) preclude a major contribution of sulphur from volcanic sources or from Early Cambrian shales.Pyrites, apatites and dolomites formed at depth within the diapir. It is envisaged that hydrocarbons leaked along the flanks of the Hormoz Island salt dome, resulting in reducing conditions which promoted the formation of diagenetic minerals.

Shallow burial dolomitization of an Eocene carbonate platform, southeast Zagros Basin, Iran

“…Where undeformed, the salt is overlain by more than four km of sedimentary rocks (Edgell, 1996;Jahani et al, 2009). The emergent diapirs provide an opportunity to study the diagenetic history of the affected units in association with the Hormuz Salt (Ghazban and Al-Aasm, 2010). The Hormuz Salt started to mobilize as early as Jurassic to Early Cretaceous based on geological evidence, but most diapirs did not reach the surface until the folding of the Zagros Mountains during the Paleogene (Ala, 1974;Jahani et al, 2009).…”

Multi‐phase Hormuz Salt Diapirism in the Southern Zagros, Sw Iran

“…In the absence of a complete stratigraphic sequence of the Hormuz complex at outcrop in the Zagros, the stratigraphy has been reconstructed on the basis of indirect observations from salt diapirs (e.g. Player, 1969;Edgell, 1996;Ghazban and Al-Aasm, 2010). The complex is considered to consist of two sequences of salt (1-2.5 km thick) separated by a few hundred metres of carbonates and red beds (Kent, 1970;Ala, 1974).…”

Diagenetic evolution of lower Jurassic platform carbonates flanking the Tazoult salt wall (Central High Atlas, Morocco)

“…Major controls on diagenesis in such settings include localised uplift, fracturing, brecciation and focused fluid flow along the diapirs flanks, among others. The understanding of the interaction between these elements is essential in the analysis of reservoir quality linked to diagenesis (Beavington‐Penney et al, ; Ghazban & Al‐Aasm, ; McManus & Hanor, ; Schoenherr et al, ). The referenced studies however are mostly handicapped by the lack of continuous outcrops or well data availability.…”