Sandstone precipitation temperature

The type II calcite is the most common cement in the Namorado Sandstone. It was precipitated relatively early, at an estimated temperature of 25 °C, prior to oil emplacement, whereas type III calcite was precipitated after oil charging in the water zone, at an estimated precipitation temperature of 40°C. 

The relatively low 5 0 values of the Angel Field and Gidgealpa Field carbonate cements can be explained in terms of either a meteoric-water incursion and/or elevated precipitation temperatures. Low 5 0 values in minerals can also be achieved through the interaction between sea water and volcaniclastic sediments under low-temperature conditions (Morad De Ros, 1994). However, because of the late-stage origin of the carbonate cements and the fluvial nature of the Namur Sandstone, this possibility need not be considered in the present context. 

Shikazono and Shimizu (1987) concluded that Ag contents of gold precipitated from low-salinity fluids is higher than that prediction and the relationship between NAg of gold and salinity of fluid inclusions estimated from freezing temperature data. Therefore, another interpretation is that NAg of gold from shale-hosted deposits is lower than that from sandstone-hosted deposits, because shale is expected to be richer in Cl mainly due to adsorption by clay minerals included in shale than sandstone. 

This process began many millions of years ago with the development of abundant life, and it continues to this day. The sediments grow thicker and sink into the sea floor under their own weight. As additional deposits pile up, the pressure on the ones below increases several thousand times, and the temperature rises by several hundred degrees. The mud and sand harden into shale and sandstone. Carbonate precipitates and skeletal shells harden into limestone. The remains of the dead organisms are then transformed into crude oil and natural gas. Usually the underground and formation pressure is sufficient for the natural release of hydrocarbon liquids and gases to the surface of the earth. 

Modern abiogenic precipitation of chert or chert precursors is a relatively rare phenomenon, but it has been documented. Wray (1999) reported speleothems of chert in sandstone, showing that direct precipitation of quartz at low temperature is possible, even from water in equilibrium with quartz and hence having a low silica concentration. 

Table 1 summarizes the major dissolution and precipitation reactions of late diagenesis in sandstones and shales in nonvolcanogenic basins. In volcanic basins, zeohtes play a more prominent role. All of the reactions listed in the table proceed over a wide range of temperature in late diagenesis and are, in large part, synchronous. 

The notion that silica is transferred from shales to sandstones during late diagenesis (Towe, 1962) provides a mechanism to link the observed depth trends of at least three (possibly four) of the major diagenetic reactions listed in Table 1 (smectite dissolution, ilhte precipitation, quartz precipitation in sandstones, and quartz pressure solution in shales). Large-scale silica transfer is to be expected as fluids flow from regions of higher to lower temperature (Ferry and Dipple, 1991). 

Hostetler, P.B. and Garrels, R.M., Transportation and precipitation of uranium and vanadium at low temperatures with special reference to sandstone-type uranium deposits. Econ. Geol. 57 (1962) 137-167. 

Walderhaug, O. 1997. Precipitation rates for quartz cement in sandstones determined by fluid-inclusion microthermometry and temperature-history modeling. J. Sedim. Res., in press. 

There seems to be no obvious systematic difference in the amount or type of calcite cementation in sandstones that have been buried to depths of 4-5 km and temperatures of more than 150°C versus otherwise unconsolidated sandstones that have not been deeper than 1.5 km and that have not been subjected to temperatures of more than around 70°C (Plate 9 Table 1). This suggests that most calcite cementation is probably complete at shallow to moderate burial depths and at relatively low temperatures, although examples of later calcite precipitation that postdates initial quartz cementation have been documented (e.g. Saigal Bjor-lykke, 1987 Walderhaug, 1990 Taylor Soule, 1993). The oxygen isotopic compositions of calcite... 

Few fluid inclusion data have been published for calcite cement in shallow marine sandstones, and homogenization temperatures in calcite may possibly be reset (Barker Goldstein, 1990). However, the measurements reported by Saigal Bjorlykke (1987) are in the range 56-68 C and thus compatible with calcite precipitation prior to deep burial. Finally, the high intergranular volumes found in many calcite-cemented sandstone samples (Table 1) also indicate relatively early calcite precipitation. 

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