Depositional and diagenetic controls on reservoir quality: Example from the upper Cretaceous Mishrif Formation of Iraq

Abstract

The Cenomanian-Turonian Mishrif Formation at the H field in southeastern Iraq displays a wide range of rock fabrics that result in highly variable reservoir quality and flow properties. Reservoir quality in this field is controlled by the complex interplay of depositional and diagenetic factors. Depositionally, seven main productive facies are recognized, which are organized into six sequences; reservoir quality occurs primarily in the grain dominated high energy shoal, shallow open marine, and rudist biostrome facies. A major change in stratigraphic style occurs midway through the reservoir, with a lower portion that is dominated by laterally continuous upward shoaling cycles (that are often shoal or tidal flat capped) and an upper part that is more heterogeneous and dominated by lagoonal and back-shoal facies. While diagenesis has overprinted and altered this initial framework, karsting - resulting in both enhanced reservoir quality due to leaching and reduced reservoir quality due to cementation, brecciation and fines infiltration - associated with sequence boundaries within and at the top of the Mishrif Formation represents the major flow-significant diagenetic effect in the reservoir. Here, diagenesis has followed the stratigraphic pattern established at the time of deposition, and reinforced its impact on reservoir quality. Mishrif reservoir quality is ultimately controlled by sea-level history, in that high energy, grain dominated textures and facies associated with sea-level highstands have the best reservoir quality, while meteoric diagenesis has produced low reservoir quality (and reservoir boundaries) at karsted exposure surfaces that formed during sea-level falls. Recognition that reservoir quality, both good and bad, results from congruent depositional and diagenetic effects ultimately driven by the sea-level history of the succession provides an important key to understanding and predicting reservoir quality in the H field and in other carbonate fields with similar depositional and diagenetic histories.