Petrographic and isotopic constraints on the diagenesis and reservoir properties of the Brent Group Sandstones, Alwyn South, northern U.K. North Sea
In this study the paragenetic sequence and isotope geochemistry of diagenetic cements are used to reconstruct the pore fluid history of the Middle Jurassic Brent reservoir sandstones in the Alwyn South Field of the U.K. North Sea. This reconstruction is used to understand the controls on the distribution of diagenetic cements and variations in reservoir quality within the relatively limited area of three adjacent reservoir blocks at successively higher structural levels. The authigenic assemblance of quartz, kaolinite and illite has resulted in severe deterioration of normally good reservoir properties. Siderite, pyrite, ankerite and albite are also present as minor cements. Early precipitation of vermiform and late blocky kaolinite was succeeded by a relatively intense illitization. Quartz cementation took place in as many as five phases through much of the diagenetic history. Siderite and pyrite were precipitated during shallow burial in marine or brackish depositional pore fluids. Vermicular kaolinite and later blocky kaolinite were precipitated from meteoric fluids which invaded the reservoir during late Jurassic emergence and were flushed to around 1km depth during continued burial. Illite formation took place between 60 and 38 Ma b.p. from compositionally homogeneous waters with marine isotope compositions (δ18 O 0-2% SMOW) and salinity (3-4 wt% NaCl) at temperatures of 106-123°C. This indicates that the early meteoric fluids were replaced by saline pore-water probably derived from compacting argillaceous rocks at depth. The fluids precipitating illite were the precursors of hydrocarbon emplacement which began between the Middle Eocene and Lower Oligocene. Fluid migration into the reservoir was probably along the deep seated Alwyn South-Ninian Fault system. K-Ar dating of illites shows that migration between reservoir panels proceeded from the deepest reservoirs up-structure. Wells adjacent to fluid inlet are extensively silicified. Within panels the pattern of fluid movement is facies dependent. In the lithologically homogeneous deep reservoirs fluid circulation was relatively simple and illitization proceeded from the base of the well upwards at rates of approximately 5.5 mMa-1. The duration of illite formation at any one level was between 2 to 5 Ma. In lithologically heterogeneous reservoirs fluid migration occurred initially along high permeability channels within the reservoir and consequently illite ages do not vary systematically with depth. Overall variation in reservoir quality between reservoirs depends on proximity to the source of diagenetic fluids. Variation in the timing of diagenesis and reservoir quality within panels is the result of facies control or fluid migration pathways.