--> Origin of the Raised Rim in the Kashagan Buildup, Kazakhstan: A Hypothesis for Diagenesis Associated With Fractures and Burial Compaction

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Origin of the Raised Rim in the Kashagan Buildup, Kazakhstan: A Hypothesis for Diagenesis Associated With Fractures and Burial Compaction

Abstract

Kashagan Field is a Carboniferous isolated platform complex that features a structurally elevated margin 100-200 m higher than the platform interior. The margin contains fractured, mechanically rigid facies that were more resistant to compaction than the facies in the interior. Numerical models of compaction are consistent with formation of the elevated margin by differential compaction during burial under hydrostatic conditions, however under the state of overpressure that exists today, such compaction would not occur. Development of overpressure probably occurred during rapid early burial of the Kashagan buildup under thick Permian evaporites (mainly halite). The margin elevation formed when the reservoir pressure dropped temporarily from an overpressured state to a near-hydrostatic state. Fluid inclusions from calcite cements precipitated contemporaneously with formation of bitumen indicate depressurization during the early stages of the hydrocarbon charge ∼150 Ma (late Jurassic), when both oil and water were present in the reservoir. Th(aq) in the calcite indicate a reservoir temperature of ∼90-100°C while variable Th(oil) in two-phase inclusions indicate decreasing oil density corresponding to a pressure decrease of ∼6000 psi, culminating in formation of asphaltene-rich bitumen. In addition to structural compaction, the pressure variations fueled burial diagenesis associated with fractures and stylolite formation. Diagenesis is characterized by fracture reactivation and dissolution, modification of pre-existing karst features, and matrix dissolution/cementation in the vicinity of the elevated margin resulting from organic acids, renewed circulation of groundwater, and temperature disequilibrium between the reservoir and the active fluids.