Stylolites characterization of lower Cretaceous Upper Thamama reservoir in Asab Field, U.A.E.

Abstract

Chemical compaction is one of the most significant diagenetic processes that could considerably affect the reservoir quality and prevent the vertical movement of fluids due to cements and accumulated insoluble residues along stylolites. The bed-parallel stylolitization is a widespread feature in the lower Cretaceous Kharaib Formation which was deposited in a shallow-water marine ramp setting. Several studies in different fields in the Middle East have been conducted to understand the distribution of stylolites and their impacts on the reservoir quality. This study aims to: (1) clarify the types of the stylolites in Asab Field; (2) classify the cements along and in the vicinity of the stylolites and elucidate their origin; (3) constrain the diagenetic processes associated with the pressure-dissolution and mineral precipitation. To achieve these goals, samples have been selected from 19 wells, and studied by optical microscopy, SEM, Carbon-Oxygen and Strontium isotopic analysis, fluid inclusion microthermometry analysis and rock-eval pyrolysis. This study reveals that the main types of filling materials along and in the vicinity of stylolites include calcite, dolomite (rhombic dolomite and saddle dolomite), pyrite, kaolinite, celestine, anhydrite and a mixture of organic matter and clay. The low δ18O values and high 87Sr/86Sr ratios indicate that the fluids are of a hydrothermal and radioactive origin. Hot basinal brines were suggested to have flowed along stylolites supported by the high Th and high salinity of fluid inclusions within different cements along stylolites. The presence of kaolinite in the vicinity of stylolites indicates that there used to have a flux of organic acids and the pyrite leads to an assumption that fluids containing S2- ions were derived from the process of TSR in deep basin. The occurrence of anhydrite points out that these hot basinal brines have gone through anhydriterich layers (e.g. Arab and Hith formation). The rock-eval pyrolysis indicates that most of the organic matters along and in the vicinity of stylolites are at immature to early-mature stage and derived from the thermal maturation process of in-situ kerogens during stylolitization. Based on all of the analysis above, a three-stage fluids evolution model is proposed. Clarifying the formation and evolution of stylolites and the fluid flow along these stylolites is of great help in a better understanding of the related overall diagenetic evolution. The results of this study should be relevant to oil production in the giant U.A.E. fields in lower Cretaceous reservoirs as stylolite-rich intervals can form important barriers to vertical flow and basis for lateral correlation. Similar studies have been carried out in other UAE oil fields and this Asab work could be used for reference and comparison.

Date of Award	2015
Original language	American English
Supervisor	Andrea Ceriani (Supervisor)