Facies and Flooding Surfaces Relation with Reservoir Characterization of Tuba Tight Carbonate Reservoir in Kuwait

Abstract

Abstract The Upper Cretaceous Tuba Formation in Kuwait is a heterogeneously developed unconventional carbonate reservoir, which is overlain by the regional seal of Ahmadi Shale. The purpose of this study is to identify the flooding surfaces (FS), and facies associations (FA) as they are related to the reservoir characters of Tuba Formation. Thus, the concepts of FS and FA are effectively applied to describe and characterize the depositional and stratigraphic trends, stratified flow units, and the overall reservoir-layering scheme. The Tuba stratigraphic framework consists of twelve FA packages represented by several transgressive-regressive depositional cycles. In the lower to middle part, argillaceous wackestone, packstone, cleaner skeletal grainstone dominates whereas upper FA are interlayered with argillaceous mudstone and intraclastic wackestone to skeletal packstone. Analytical methods include thin section petrography, XRD, SEM, poro-perm, MICP data to evaluate the twelve FA packages into different flow units, namely: reservoir, baffle, and barrier units. These flow units act as a rock volume that can be differentiated from the above and below based on its geological and petrophysical features that affect the fluid flow properties. The study results suggest that excellent reservoir attributes are present in FA4, FA10 and FA12 which are characterized by skeletal packstone and grainstone facies, and well-connected intergranular, moldic and fracture pores. Moderate reservoir properties with baffle quality rocks are observed in FA11, FA8, FA9, and FA3 representing intraclastic skeletal wackestone and argillaceous packstone as dominant facies with some moldic and fracture pores that have degraded the overall permeability. Poor baffle to barrier quality rocks is present in FA1, FA2, FA5, FA6, and FA7 where argillaceous wackestone, calcareous shale and mudstone type facies display isolated moldic, cemented vuggy, and very limited micro-fractures, which severely damage the reservoir quality. The concept of FA acting as stratified flow units have the potential to prevent homogenization of the reservoir fluids across the Tuba Formation and thus, may cause reservoir layering to influence the production. Multiple thin to thick (1 to >10 feet) baffle and barrier intervals are identified which are depositional and diagenetic in nature, and are present within FA units of moderate reservoir quality that have stratigraphic significance. These intervals are complex sedimentary bundles composed of various low permeability fabrics with variable sedimentary, diagenetic and stratigraphic genesis. This study demonstrates that highly stratified Tuba Reservoir has a complex fluid distribution pattern in Sabiriyah field in Kuwait. The degree of stratification usually has an influence on the production volume estimation, and thus, should be accounted for reservoir development, modeling/simulation, and planning for well testing and fluid-contact characterization. Hence, further assessment with enhanced integration of RFT/pressure, SCAL, fluid properties, and injection/production data are suggested to validate the stratified flow-unit characterization of the Tuba Formation.