Diagenesis and reservoir characteristics of theLithocodium-Bacinellafacies in a Lower Cretaceous reservoir, eastern Persian Gulf Basin

摘要

In this paper, the importance ofLithocodium-Bacinellafacies (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 (phi < 15% andK < 1 md). These intervals are composed of an alternation of open to restricted marine facies. The facies are characterized by extensive bioturbation, high clay content (argillaceous limestones), displaying marine (micritization) and shallow to deep burial diagenetic processes (pyrite mineralization and solution seams). The reservoir intervals preferentially occur within the highstand system tracts (HST's), which are commonly composed of vertically stackedLithocodium-Bacinellafacies. This facies with aggradational patterns is capped by subaerial exposure surfaces at the top of the sequences and is associated with leaching and cementation. Meteoric dissolution during subaerial exposure resulted in the formation of vuggy and moldic pores as the main pore types in these system tracts, which survived in significant amounts until present-day. Therefore, the LB facies usually have greater than 15% porosity and 1 md permeability. Diagenetic studies in these units show that the marine diagenesis was interrupted by subaerial exposure and meteoric diagenesis before burial and development of stylolites. Our findings show that despite the uncertain taxonomy and paleoenvironmental setting ofLithocodium aggregatumandBacinella irregularis,the LB facies has great reservoir importance.