Depositional and Diagenetic Sedimentological Model of Najmah-Sargelu Formation, Umm Gudair, Kuwait

Najmah-Sargelu Formation of Middle Jurassic is tight, fractured Carbonate reservoirs, spread across many fields in Kuwait. These reservoirs are often vertically and laterally heterogeneous because of depositional variability and diagenetic alteration through space and time. Understanding the distribution of hydrocarbons in relation with porosity / permeability heterogeneities is thus of major importance in effective field development and production. A proper sedimentological model is then a mandatory step in matrix characterization to make an accurate numerical reservoir model, predicting fluid flows through time in order to support development scenarios. This paper presents an overview of efforts in building a sedimentological model based on the analyses performed on cores penetrating the Najmah and Sargelu Formations of the Umm Gudair field. The data obtained from the description of cores allowed defining a sedimentary facies scheme that served a basis for building of a 3D sedimentological model. Dominant sedimentary structures and bioturbation traces have been taken into account for the facies classification. The apparent differences in texture between the core-based lithotype scheme and the thin-section based microfacies scheme is due to intense micritization of grains. This model shows a moderate-energy carbonate ramp with a relatively flat morphology, further subdivided into three major depositional environments: The mid ramp, outer ramp, and basin. The basin includes: evaporitic conditions, restricted/anoxic conditions, unrestricted/oxygenated conditions. The basinal deposits are mostly characterized by shaly mudstones that host the deposition of a calciturbidite sheet complex. The mid- and outerramp settings are mostly dominated by relatively fine sedimentary textures where the structures are obliterated by intense bioturbation.