How a mud-dominated ramp changed to a carbonate–clastic oil reservoir: Sea-level fluctuations in cretaceous of the central Persian Gulf

Abstract

Mud-dominated facies distribution of Barremian–Turonian succession in the northeastern Arabian Plate in the central Persian Gulf represents a shallow ramp platform. On the basis of petrographical studies, core analysis data and gamma ray logs, this succession has been studied for facies analysis, defining depositional environment, sequence stratigraphy and reservoir properties. A total of 6 microfacies and 3 petrofacies types are recognized in this succession. Lagoonal environment was dominated during the deposition of Dariyan, Kazhdumi and Sarvak formations. The lagoon rapidly changed to a sand shoal by falling sea-level and increasing energy level of the depositional environment. Sequence stratigraphy of Barremian–Turonian succession shows that mud-dominated facies have been deposited in TSTs and grain dominated facies in HSTs of the sequences. Although grain-dominated facies like packstone to grainstone and algal floatstone were spread during the HSTs of sequences, the changes in the texture and size of the grains had a negligible effect on reservoir quality. During this period (Barremian–Turonian), two major sea-level falls caused considerable environmental changes such as platform emergence, incised valley fills and clastic sediments influx into the carbonate platform. Meteoric diagenesis during the falling sea-level had a major effect on the reservoir properties of the reservoirs, under the sequence boundaries. Grains, matrix and micrite dissolution and subsequent selective blocky and equant calcite cementation, strongly modified the primary reservoir properties. Local dolomitization also enhance the porosity and permeability of the samples. While large pores have been filled by cements, microporosities remained unfilled until the late stages of diagenesis and burial. © 2020 Elsevier Ltd