ABSTRACT: Comparison of Feldspar Diagenesis in Tertiary Basins: Pattani Trough (Thailand), Upper Cook Inlet (Alaska), and Northwest Gulf of Mexico

A. S. Trevena, P. D. Lundegard

Feldspar diagenesis is significantly different in Tertiary sandstones from the Pattani basin (Thailand), upper Cook Inlet (Alaska), and the northwest Gulf of Mexico. Both plagioclase and K-feldspar have undergone partial to complete dissolution in the Pattani basin, where present-day pore waters are relatively dilute (3600-7200 mg/L). In the Pattani basin, dissolution of K-feldspar is complete by 160°C, whereas plagioclase dissolution takes place over a wider range of temperatures and is nearly complete by 180°C. Little or no albitization has occurred in Pattani basin. In Cook Inlet Tertiary sandstones, partial dissolution of plagioclase is widespread at temperatures above about 50°C, but K-feldspar is relatively little affected by diagenetic reactions. In pite of fairly calcic plagioclase compositions (average = 35% An), only small amounts of partial albitization have been noted in Cook Inlet sandstones that have been exposed to temperatures in excess of 90°C. Feldspars in Gulf Coast Tertiary sandstones have undergone both dissolution and albitization reactions in contact with pore waters that range from fresh to highly saline. K-feldspar dissolution is generally complete at temperatures exceeding 120°C, and albitization of plagioclase is widespread at temperatures in excess of 100°C in the Gulf Coast.

Comparison of feldspar reactions from these three basins shows the importance of pore-water chemistry (in addition to temperature) in controlling feldspar diagenesis. In the thermally warm Pattani basin, low sodium activities and high CO₂ fugacities have promoted feldspar dissolution but not albitization within the same temperature range in which albitization is common in other basins. In thermally cool Cook Inlet, a limited amount of albitization has occurred in deeply buried sandstones in pore waters of probable low to moderate sodium activity. Substantially higher sodium activities in Gulf Coast formation waters have promoted more extensive albitization in that basin.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990