Arthur S. Trevena¹, J. Reed Glasmann², M. Charles Manske³, David O. Martens⁴

(1) Unocal Corp, Sugar Land, TX
(2) Willamette Geological Service, Philomath, OR
(3) OMNI Laboratories, Houston, TX
(4) Unocal Thailand, Bangkok, Thailand

ABSTRACT: Sandstone Diagenesis and Reservoir Quality at High Temperatures, Pattani Trough, Thailand

The Pattani Trough in the Gulf of Thailand is a trans-tensional basin that opened during the middle Tertiary and filled with non-marine and shallow marine clastics during the Neogene. Thermal gradients have been extremely high (40 - 66oC/km), based on kaolinite/dickite and illite/smectite geothermometers. These high temperatures have resulted in rapid diagenesis at moderate burial depths (1500-3000 m). Nevertheless, the Miocene post-rift strata contain numerous, highly productive, gas/condensate reservoirs and some oil reservoirs.

Major secondary minerals in sub-quartzose (Q72F04L24), post-rift sandstones include calcite (55-70oC), kaolinite (90-140oC), dickite (>130-140oC), illite/smectite, illite (>120oC), quartz (mostly > 90oC), and ferroan carbonates, based on petrographic, X-ray diffraction, and stable isotope analyses of more than 1500 core samples from an area of nearly 10,000 km2. In general, abundance of mineral cements in these sandstones increases markedly with increasing temperature and burial depth, as porosity and permeability decline. However, in post-rift deposits from below 2100 m, very coarse- to coarse-grained sandstones (0.5 -2 mm) contain less cement and have higher porosities and permeabilities than lower medium- to fine-grained sandstones (0.125-0.350 mm). We infer that at temperatures of 140-190oC, diagenetic reactions proceeded more rapidly in the finer grained sandstones than in the coarser grained sandstones. This probably occurred because finer sandstones have greater surface area/unit volume than coarser sandstones, which increased cementation rates.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado