Diagenesis and reservoir characteristics of the Lithocodium–Bacinella facies in a Lower Cretaceous reservoir, eastern Persian Gulf Basin

In this paper, the importance of Lithocodium–Bacinella facies (LB facies) as reservoir rock in the Lower Cretaceous carbonates of the Persian Gulf will be addressed. The studied interval belongs to the reservoir zone of upper Fahliyan Formation, which occurs in one of the giant offshore oilfields. A detailed examination of a continuous core (58 m) from this formation allows clarifying the relationship between facies distribution, diagenetic evolution, and porosity development in function of relative sea level changes. Seven facies types are identified based on core descriptions and microfacies analysis. These facies are interpreted to have been deposited in lagoonal, shoal, and open marine settings during Valanginian–Hauterivian times. Petrographic studies and stable carbon and oxygen isotope analyses indicate that these facies have been affected by different diagenetic processes, starting with early marine diagenesis and followed by meteoric and/or burial diagenesis. Based on facies stacking patterns and evidence of subaerial exposures, four high-frequency sequences are interpreted in terms of variations in accommodation space. Combination of facies analysis, diagenetic studies and poroperm data reveals (with the exception of sequence I) that the lower transgressive and the upper highstand intervals of these sequences show different facies types, diagenetic modifications and reservoir properties. The transgressive system tracts (TST’s) correspond to tight zones (Z-1, 3, 5), with poor reservoir quality ( Φ