SANDSTONE DIAGENESIS IN THE PATTANI BASIN (GULF OF THAILAND) - HISTORY OF WATER ROCK INTERACTION AND COMPARISON WITH THE GULF OF MEXICO

In the Pattani Basin, a failed-rift basin, extensive water-rock interaction has occurred between subquartzose alluvial sandstones of Miocene age and their pore fluids. Diagenetic rates and pathways have been strongly influenced by high geothermal gradients, high CO2 fugacities, and low pore water salinities. Depositional pore water was fresh to brackish, depending on the depositional environment of the sediments. Chloride concentrations in modern formation water are believed primarily to reflect the proportions of river and sea water in the depositional environment. However, the concentration of other important solutes and the isotopic composition of the formation waters can not be explained by roportional mixing of these two end-member waters. Dissolution of detrital plagioclase (An = 3) and K- feldspar are reactions of major significance that are reflected chemically in the Na/Cl and K/Cl ratios of the formation water. Despite the high temperature of the sandstones (120-200°C), diagenetic albite does not occur. Geochemical calculations indicate the formation water is undersaturated with respect to both orthoclase and albite. This style of feldspar diagenesis differs significantly from that of sandstones of similar composition in other basins, and has probably influenced other aspects of silicate diagenesis. Important authigenic minerals are: 1. locally abundant calcite cement (δ 13C = -12.8, δ18O = -17.3 PDB), an early diagenetic phase that formed at about 60°C; 2. pore-filling kaolinite (δ18O = 9.9, δ D = -83.5 SMOW) that was closely associated with feldspar dissolution and formed over a range of temperatures; and 3. fibrous pore-lining and pore-bridging illite (δ18O = 9.8, δ D = - 86.7 SMOW, the last significant cement, formed at temperatures of 120 to 150°C. Potassium/argon dates on illite indicate that sandstone diagenesis took place during a period of rapid sedimentation in the first two-thirds of the burial history. Comparison of Pattani Basin diagenesis with diagenesis of sandstones of similar age in other sedimentary basins demonstrates that chemical diagenesis, relative to mechanical compaction, has been especially rapid in the Pattani Basin. This reflects the effect of high temperatures on reaction rates. The net effect is a high average rate of porosity loss with burial (11% km). © 1990.