Depositional and Diagenetic Controls on Reservoir Quality of the Tuba Formation, Sabriyah Field in Kuwait
Authors: Sunil Swarga , Al-Farhan Ahmad , Ferdous Hasan , Sewilan Mohammad , Al-Reja Abdullateef , Komolafe Bukola , Ali Afnan , Al-Qallaf Ali , Akther Shamima , Al-Oqab Mohammad , Ghloum Ebtisam , Abdullah Fowzia 
Published: February 2024

The Tuba Formation represents an unconventional carbonate reservoir in the Sabriya Field in Kuwait. The overall depositional environment includes low-energy ramp setting where various facies associations consisting of relatively high-energy inner/mid ramp, and low energy shallow to deep outer ramp basinal facies form approximately SE-NW trending shore parallel geobodies, which are later modified to form well-developed reservoir layers. The main objective includes understandings of depositional facies and diagenetic alterations to develop complex reservoir layers.

The study conducts detailed core description and sampling of six wells of the Tuba Formation. Samples are used in thin section petrography, SEM, and MICP analysis to establish ten major facies. Facies types are recognized based on sedimentary environments and subsequent sample analysis assist to establish a para-genetic evolution and diagenetic model. This approach contributes to proper assessment of various diagenetic controls to establish vertically developed reservoir and non-reservoir layers within the Tuba Formation.

The study results include evaluation of depositional facies complexity and the influence of post-depositional diagenesis that resulted in heterogeously distributed reservoir and non-reservoir layers of the Tuba Formation in Sabriya Field. Study findings suggest that Tuba rocks underwent multiple diagenetic phases, including the syn-genetic, early and meso-genetic, and later stage fracturing events, all of which had a significant impact on the reservoir layer distribution. The genetic processes involved compaction, cementation, dissolution, pressure-solution, neomorphism, and fracturing. In addition, original micrite and bioclast composition, their dissolution, bioturbation, precipitation, and recrystallization have generated various types of calcite and dolomite cementation, and secondary porosity generation. Preservation of primary porosity, creation of secondary porosity, and later stage fracture generation have significantly influenced the presently existing heterogeneous reservoir quality distribution. As the original depositional features and fabric greatly influenced diagenetic processes, cement composition and morphology, and fracture orientation and styles, thus, the facies dependency exist on reservoir attributes and qualities to affect the reservoir heterogeneity.

Our findings also include the correlation between pore types and pore-throat size and their distribution. The petrophysical data including core porosity-permeability are also used to characterize the pore types and other diagenetic features.

The present study exhibits that Tuba Formation has a complex facies distribution that are highly influenced by diagenetic processes resulting to greatly variable discontinuous and layered reservoir distribution. Reservoir attributes affected by depositional fabric, diagenetic alteration of carbonate cement generations, porosity, pore-throat, and permeability distribution. In future, further integration of advanced geochemical evaluation including stable and clumped isotope, image, and fracture characterization could be useful for Tuba Reservoir long-term secondary and EOR development strategy.