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Diagenesis of early Miocene sandstones in the Pattani basin resulted in rapid cementation and degradation of reservoir quality with increasing depth of burial. These subquartzose sandstones provide an example of accelerated burial diagenesis in an area of unusually high geothermal gradient. Burial compaction, progressive cementation by quartz overgrowths, and development of authigenic kaolinite and illite have substantially reduced porosity and impaired permeability at depth. Abundance of quartz overgrowths increases with depth, indicating continuous or episodic silica cementation. Kaolinite occurs as a pore-filling cement between depths of 4,500 and 10,000 ft (1,375-3,050 m). Illite is common as pore-linings and also bridges pores in deeper zones (8,000-10,000 ft or 2,45 -3,050 m). Minor cements include calcite, dolomite, siderite, pyrite, mixed-layer illite-smectite, and chlorite. Feldspars display textures that indicate progressive dissolution with increasing burial depth. Large intergranular pores are present in permeable sandstones between 3,000 and 7,500 ft (925-2,275 m). In low-permeability sandstones from deeper zones (7,500-10,000 ft or 2,275-3,050 m), porosity is largely restricted to voids within detrital feldspar grains. Many of these secondary pores are partly filled by authigenic kaolinite and illite, and their pore apertures are usually smaller (1-15 µm diameter) than intergranular pore apertures (10-75 µm diameter). Good reservoir properties in the Pattani basin are generally restricted to sandstones above 7,500 ft (2,275 m) that contain large intergranular pores. Abundant secondary porosity below 7,500 ft (2,275 m) is generally associated with poor reservoir properties; however, favorable reservoir properties may occur locally where large feldspars have been leached from coarse-grained sands.
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