The role of organic matter decomposition during meteoric diagenesis in reservoir quality, a case from SW Iran

The neritic Cretaceous carbonates of SW Iran―mostly deposited on the middle ramp―host major Iranian hydrocarbon reserves. Detailed environmental, diagenetic, and biostratigraphic examinations revealed two exposure-related unconformities at the Cenomanian – Turonian Boundary (CTB) and mid-Turonian with approximately 1 and 3.5 Myrs gap, respectively. Karstic and other diagenetic features related to subaerial exposure led to the formation of two reservoir horizons beneath these disconformities, of which the CTB-related shows higher reservoir quality in spite of its shorter exposure. C-O isotope analyses of the whole-rock samples corroborate the existence of these paleoexposures. Greater variations in δ13Ccarb values (up to - 10.2‰) associated with the “Inverted J” pattern indicate a far severer meteoric diagenesis at the CTB despite its shorter subaerial exposure. Whereas, a positive excursion in δ13Ccarb values of 4 - 5.5‰ is evident through rock records worldwide at the CTB, known as Oceanic Anoxic Event 2 (OAE2). The very negative δ13Ccarb values are nicely explained by sub aerial diagenesis below an exposure surface under the control of 12Cenriched organic matter (OM) deposited during OAE2. The incident is justified by chain interactions of tectonically controlled paleogeography, pale climate, and the resultant diagenetic processes. OM-rich carbonate sediments of OAE2 were immediately subjected to meteoric diagenesis under a warm equable climate due to tectonic activities dictating new pale geographic states through the SW Neo-Tethys. This led to oxidation of the OM (with δ13C of about -23 to -29‰) via percolating meteoric waters during mineral stabilization. The oxidized OM resulted in aqueous CO2 produced carbonic acid in reaction with meteorically-derived pore waters. The acid accelerated dissolution-reprecipitation processes with continuous carbon isotopic exchange between the host sediments and solution. Consequently, the precipitated meteoric calcite cements inherited isotopic composition of 12C-enriched solution. The accelerated dissolution beneath the CTB formed a reservoir horizon of higher quality beneath this exposure surface in comparison with the mid-Turonian exposure.