Miocene sandstones from gas fields in the Gulf of Thailand's Pattani basin provide an example of rapid decline in porosity and permeability with increasing burial depth. This decline results from rapid burial diagenesis that is related to very high geothermal gradients in the basin. Porosity loss with depth in these subquartzose sandstones results from mechanical compaction and from progressive cementation by quartz overgrowths, kaolinite, and illite. Quartz overgrowths increase with depth, indicating continuous or episodic silica cementation. Kaolinite occurs as a pore-filling cement and is abundant between the depths of 1,980 and 3,050 m (6,500 and 10,000 ft). Minor cements include calcite, ankerite, mixed-layer illite-smectite, siderite, pyrite, chlorite, and barite. >

The best porosity and permeability in Pattani basin reservoirs are generally associated with large intergranular pores in sandstones between 915 and 1,980 m (3,000 and 6,500 ft). At greater depths most interparticle pores have been occluded, and porosity is mainly secondary in origin. Skeletonized feldspars indicate progressive dissolution with increasing burial depth. In low-permeability sandstones from deeper zones (2,285-3,050 m or 7,500-10,000 ft), porosity is mainly restricted to dissolution voids within detrital feldspars. These secondary pores are usually partly filled by authigenic kaolinite and illite, and their pore apertures are generally smaller (1-15 µm) than intergranular pore apertures (10-75 µm). However, favorable reservoir properties may occur locally at de th where large feldspars have been leached from coarse-grained sandstones.