Subaerial exposure and meteoric diagenesis of the Cenomanian-Turonian Upper Sarvak Formation, southwestern Iran

Abstract

The Sarvak Formation (Cenomanian -Turonian) forms one of the main reservoir rocks in many oilfields in southern Iran. Extensive lateral and vertical facies variations as well as effects caused mainly by the subaerial exposure associated with the regional Turonian unconformity have resulted in variable porosity and permeability. Dissolution affected the entire upper part of the Sarvak Formation and is the most important process related to subaerial exposure. Brecciation, development of palaeosol and formation of bauxite deposits are also limited to the upper few metres of the top of the Sarvak Formation and indicate warm and humid climatic conditions. Subaerial exposure had varying effects on the diagenesis depending on its duration, palaeotopography and the availability of meteoric water. The d 18 O and d 13 C values obtained from calcitic matrix, various generations of calcite cements and calcitic rudist shells in the Upper Sarvak overlap to a large extent, indicating their equilibration with fluids of similar isotopic composition. Negative d 18 O values (i.e. 26.6‰ to 21.7‰) suggest a significant meteoric component. More 18 O-depleted values (e.g. 212.3‰) obtained from late calcite cements indicate their precipitatation from warm fluids. Positive d 13 C values (i.e. 0.00‰ to 3.4‰) in the various carbonate phases reflect values of seawater coeval with an Oceanic Anoxic Event and later modified by meteoric waters.

Controls on depositional facies in Upper Cretaceous carbonate reservoirs in the Zagros area and the Persian Gulf, Iran

“…2). The presence of these palaeohighs has resulted in great variations in facies characteristics and diagenesis of contemporaneous carbonate formations (Taghavi et al 2006;Hajikazemi et al 2010;Mehrabi and Rahimpour-Bonab 2014). The isopachous maps depicted for the Sarvak and Ilam formations show close relationships with major structural elements (i.e., the major faults) in the study area.…”

“…c Schematic cross sections across the SW Iran illustrating tectonic setting of the study area during the Upper Cretaceous (adapted with some modifications from Ziegler 2001; Letouzey and Sherkati 2004) Buchem et al 2001;Sepehr and Cosgrove 2005;Grelaud et al 2006;Ahmadhadi et al 2007;Farzadi and Hesthmer 2007). Diagenetic history and reservoir properties of the Upper Cretaceous deposits were investigated in the context of tectonic setting and palaeoclimatic condition (Taghavi et al 2006;Hajikazemi et al 2010;Rahimpour-Bonab et al 2012a, b, 2013Mehrabi and Rahimpour-Bonab 2014;Mehrabi et al 2014Mehrabi et al , 2015.…”

Mineralogy, Geochemistry and Stable Isotope Studies of the Dopolan Bauxite Deposit, Zagros Mountain, Iran

“…Framboidal pyrite commonly forms during early diagenesis under organic-rich, anoxic conditions [7,11,47,48]. Euhedral pyrite occurs as cubic grain filling fractures and is interpreted to have formed during the last stages of diagenesis.…”

“…One sample from the calcite cement of the footwall dolomite shows δ 13 C value of´7.34‰ (Table 6). Such depleted δ 13 C values are considered to reflect subaerial exposure and influence of the karstification associated with the unconformity surface [48]. as kaolinite and boehmite are considered to be one of the main controls on the isotopic composition of bauxite deposits [28].…”

“…One sample from the calcite cement of the footwall dolomite shows δ 13 C value of −7.34‰ (Table 6). Such depleted δ 13 C values are considered to reflect subaerial exposure and influence of the karstification associated with the unconformity surface [48]. Calcite from the footwall dolomite (Khaneh-Kat Formation) plots away from the kaolinite line (δD = 7.55δ 18 O − 219); [59]; Figures 15 and 16).…”