PALAEO-EXPOSURE SURFACES IN THE APTIAN DARIYAN FORMATION, OFFSHORE SW IRAN: GEOCHEMISTRY AND RESERVOIR IMPLICATIONS

Palaeo-exposure surfaces within and at the top of the carbonate-dominated Aptian Dariyan Formation have been poorly studied in the Iranian sector of the Persian Gulf. This paper presents an integrated sedimentological and geochemical study of the Dariyan Formation at four oil and gas fields located in the western, central and eastern parts of the Gulf. Facies stacking patterns in general indicate shallowing-upwards trends toward the exposure surfaces, which are interpreted to correspond to unconformities. The Dariyan Formation in the study area is divided into upper and lower carbonate units by a deep-water, high-gamma shale-marl interval. At fields in the western and central Gulf, significant diagenetic changes were recorded in the top of the upper carbonate unit, including meteoric dissolution and cementation, brecciation and paleosol formation. An exposure surface is also present at the top of the lower carbonate unit in all the fields in the study area, and is associated with meteoric dissolution and cementation of grain-dominated facies. Age calibration of studied intervals was carried out using microfossil assemblages including benthic and planktonic foraminifera. Negative excursions of both O-18 (-10 parts per thousand VPDB) and C-13 (-0.66 parts per thousand VPDB) were recorded in weathered intervals located below the unconformity surfaces. A sequence stratigraphic framework for the Dariyan Formation was established by integrating sedimentological, palaeontological and geochemical data. The C-13 curve for the formation in the Iranian sector of the Persian Gulf can be correlated with the reference curve for the northern Neotethys and used as a basis for regional stratigraphic correlation. Where the top-Aptian unconformity is present, it has resulted in an enhancement of the reservoir characteristics of the underlying carbonate succession. Accordingly, the best reservoir zones in the Dariyan Formation occur in the upper parts of the lower and upper carbonate units which are bounded above by significant palaeo-exposure surfaces.