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The Chalk Group (Cretaceous/Tertiary) in the Machar Field (British North Sea) contains both fracture-filling and microcrystalline calcite cements. Modeling of fluid-rock interaction using data on light stable isotopes obtained by whole rock analyses and laser ablution analyses of calcite cements reveal that the fracture and matrix diagenetic systems were largely decoupled. The calcium and carbonate of the fracture-filling calcite cements were derived largely from the adjacent chalk matrix. The fracture diagenetic system had a high water-rock ratio, which maintained a relatively stable water 18O ratio during calcite dissolution and precipitation. The chalk matrix on the contrary, had a low molar water-rock ratio during r crystallization, which resulted in increases in the pore-water 18O value during recrystallization at elevated temperatures. This evolution of the pore-water 18O value is manifested by highly variable cement 18O values. The present-day formation waters of the Machar Field have 87Sr/86Sr ratios significantly higher than the whole rock and fracture-filling cement calcite values, evidence that the chemical composition of the formation waters is not representative of that of the pore waters during chalk recrystallization. Little diagenesis is therefore now occurring in the Machar Field. The diagenetic systems of the chalk matrix and fractures both had a high degree of openness with respect to carbon, because of the introduction of organically derived bicarbonate rather than advection of water through the chalk. The hulk of calcite cementation in fractures and the recrystallization and cementation of the chalk matrix occurred at temperatures in the 80-100°C range, at or just belo the present-day reservoir temperature of 97°C.

The Miocene Sihapas Formation sands of the Central Sumatra Basin (Indonesia) are being cemented by quartz, clay, and carbonate minerals, today. Precipitation of these minerals in the pore spaces has reduced the porosity of the sandstones from 30% to 20%. Four independent sets of data are interpreted to show that cementation began only 2 million years ago and that it is active today: A major reverse fault split a once continuous Sihapas sandstone about two million years ago. In the hanging wall of the fault the Sihapas sand is uncemented. In the footwall the same sandstone is cemented. Compaction-induced porosity loss in these sandstones is the same as that expected for the present burial depth of the sandstones. The temperature at which quartz cement precipitated is the same as the present-day formation temperature. Water trapped within fluid inclusions in the quartz cement is of similar low salinity to the formation water, and the oxygen isotopic composition of the quartz cement and modern formation water are approximately in equilibrium. These data for the Sihapas sandstone add to a growing body of evidence which indicates that cementation of sandstones can occur over geologically short periods of a few millions of years to a few tens of millions of years (Lee et al. 1985; Glassmann et al. 1989; Robinson and Gluyas 1992a).