Abstract

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).

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