Lithofacies, architecture and evolution of carbonate tidal delta system: Implications for reservoir heterogeneity (mid-cretaceous Mishrif Formation, Southeast Iraq)

Abstract

Carbonate tidal delta system used to be put much less attention on unlike the counterparts in siliciclastic systems, and its unique architecture influences the reservoir heterogeneity and corresponding development strategy significantly. This study aims to recognize and characterize the carbonate tidal delta system of Mishrif Formation in Southeast Iraq.Based on detailed observation of cores from 5 wells and integrated analysis on well-loggings and 3D seismic, five lithofacies associations were recognized: (1) subtidal mudstone to wackestone (LF-1); (2) back-margin massive wackestone to packstone (LF-2); (3) high-energy bioclastic grainstone (LF-3); (4) swamp carbonaceous mudstone interbedded with bioclastic wackestone (LF-4); and (5) incision brown mudstone to wackestone with chert nodular (LF-5).A detailed description of the recognized lithofacies and facies association of the system is given in this work, together with an interpretation of the corresponding depositional setting. The architecture of the reservoir bodies has been reconstructed in detail and placed within a high-frequency sequence-stratigraphic framework.The studied interval belongs to the highstand system tract (HST) of a single composite sequence and comprises two shallowing-upward high-frequency sequences. The internal architecture of the tidal delta system is the result of complex tidal-dominated hydrodynamics.Facies and diagenetic partitioning with each high-frequency sequences are primary controls on pore types and reservoir properties. Original lithofacies texture and subsequent meteoric dissolution resulted in regional barriers and high permeable streaks that created a complex internal architecture of reservoir bodies. The shape of reservoir bodies generally follows those of the lithofacies, but with the addition of intercalated thin flow barriers.