Influence of Fluid Chemistry and High Geothermal Gradient on Sandstone Diagenesis: Pattani Basin, Gulf of Thailand
Paul D. Lundegard, A. S. Trevena
Miocene fluvial sandstones have undergone extensive diagenesis and material transfer in the presence of low salinity, high carbon dioxide fugacity pore fluids, and high geothermal gradient. Present-day pore waters are sodium-bicarbonate-chloride-acetate waters with total dissolved solids of 3,600 to 7,200 mg/L and oxygen isotopic compositions of 0.2 to -4.4 per mil (SMOW). Carbon dioxide content of produced gas increases with depth and averages approximately 12 mole %.
The importance of fluid chemistry as a control on diagenetic pathways is demonstrated by feldspar diagenesis in the Pattani basin. Low concentrations of sodium and high CO2 fugacities caused extensive feldspar dissolution rather than albitization, even though temperatures are well into the range where albitization has been documented in other basins. Absence of feldspar albitization is further documented by increased molar sodium/chloride ratios in the pore water (avg. = 4.10) relative to either seawater (0.85) or the probable depositional water (1.4 to 1.7). Dissolution of K-feldspar is complete by 160°C, while plagioclase dissolution occurs over a wider range of temperatures and is essentially complete by 180°C. Kaolinite and illite, reaction products of the fel spar dissolution, occur within dissolution pores and indicate relative immobility of aluminum in the system.
Carbon isotopic compositions of early calcite cement and dissolved carbon dioxide indicate that the prime source of carbon dioxide in the pore fluids was the maturation of organic matter. Rapid burial and a high geothermal gradient promoted rapid production of carbon dioxide from kerogen. Consequently high carbon dioxide fugacities were produced and promoted extensive feldspar dissolution.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.