Isotope diagenesis and palaeofluid movement : Middle Jurassic Brent sandstones, North Sea
The Middle North Sea, have simplified to a Jurassic deltaic Brent Group sandstones, northern complicated diagenetic sequence which may be kaolinite Fe,Ca carbonates - High porosities and Ness and Tarbert quartz overgrowths - Fe,Mg carbonates - illite. existed in the Etive, throughout diagenesis. By permeabilities have Formation sandstones contrast, the highly micaceous and r,elatively finer grained Rannach Formation sandstones have good porosities but very poor permeabilities due to burial compaction and carbonate cementation. Oxygen and hydrogen isotope studies indicate that early diagenesis occurred in a dominantly meteoric pore water (6180 = -7%0). The early diagenetic cements of siderite, vermicular kaolinite and calcite started to precipitate at 14, 26 and 32DC respectively. With the onset of burial, below 1.2 km, the Brent sequence eventually became sealed off from the meteoric 'head' by Lower Cretaceous sediments. Blocky kaolinite precipitated and by the end Cretaceous quartz overgrowth formation commenced. Fluid inclusions in the overgrowths indicate formation from a warm, dominantly low salinity water (1 - 5 wt.% eq. NaCl). Homogenisation temperatures range from 73to 131 DC. Illite precipitation is cogenetic with the latest stages of quartz overgrowth precipitation (K/Ar illite date, 58Ma.) and has reduced porosity and permeability markedly in different locations prior to oil migration. The depth of burial at which this last cementation event occurred is 2.3 km. Fluid inclusion microthermometry indicates that quartz overgrowth and latest ankerite precipitation occurred in a geothermal gradient of 70DC/km. After this heat excursion the reservoirs have cooled back to present day temperatures of 85 - 11SDC. Water values computed from the mineral cements indicate precipitation from a porefluid which has gradually evolved isotopically to its present day composition (5180 = -7 to +2%.SMOW) in an isotopic system that has become closed during burial. However the quartz and ankerite suggest one unusual episode of open system hot fluid input.