Geochemistry as a Powerful Tool in Oil Exploration: A Case Study from the Karstified Reservoirs of the Middle East: SW Iran

Abstract

Abstract The Cretaceous-aged reservoirs of Iran and their counterparts on the Arabian platform are among the most important carbonate reservoirs developed under the warm equable climate. Intense tectonic activities combined with the eustatic sea-level changes caused sporadic exposures of these carbonate successions in SW Iran and some other areas of the Middle East. Under the domination of warm and humid climate and long-term exposures, several erosional disconformities formed throughout these successions. To investigate these palaeoexposure surfaces, petrographic and geochemical studies were carried out in four subsurface sections of giant and supergiant oilfields located in northern and southern parts of the Dezful Embayment. Some of the most important diagenetic features (such as extensive meteoric dissolution/karstification, palaeosol development, and collapsed-dissolution brecciation) were used as evidence for recognition of main palaeoexposure surfaces and associated karsts. Isotopic composition (δ13C and δ18O) and trace element concentrations of these carbonate rocks were used to elaborate the effects of these surfaces on diagenetic evolutions of palaeo-karstic Bangestan reservoirs. Both carbon and oxygen isotope ratios show a spectacular decline just below these disconformities in comparison with the expected values for the simultaneous marine carbonates, indicating that the Bangestan carbonates underwent extensive meteoric diagenesis. Elemental concentrations (Mn, Fe, and Sr) also point to moderate to intense meteoric diagenetic alterations associated with the palaeoexposure surfaces. Introduction Palaeoexposures have had an irrefutable role in the formation of carbonate reservoir rocks under warm equable conditions (Weidlich, 2010; Mazzullo, 2004). These reservoir rocks ?namely palaeo-karstic carbonates? host an important portion (20 to 30%) of the world's hydrocarbon (Mazzullo and Chilingarian, 1992; Mazzullo, 2004). Owing to the influences of palaeoexposure surfaces on the reservoir quality of carbonates, they must be traced through rock record by considerable changes in facies characteristics, diagenetic features, paleontological evidence, and of the most importance, geochemical tracers (Immenhauser et al., 2000; Weissert et al., 2008). Abiogenic and most of biogenic carbonate sediments precipitate geochemically in equilibrium with their contemporaneous sea waters (Veizer et al., 1999). Carbon and oxygen isotope compositions change through carbonate alterations. Ancient and modern exposure surfaces indicate a clumped variation in oxygen and carbon isotope compositions. Weight of eo-diagenetic alteration on the isotopic composition of carbonates is mainly controlled by several factors including soil-gas CO2, primary mineralogy and geochemical composition, meteoric waters characteristics, exposure duration, and system openness (Hudson, 1977; Allan and Matthews, 1982).