ABSTRACT: Living with uncertainty in the Funan Field, Gulf of Thailand

Turner, James W. , Unocal Thailand, Bangkok, Thailand

The Funan field is located in the Pattani Trough - a rift basin filled primarily with non-marine sediments. Reservoirs are Miocene age fluvio-deltaic, sandstones, while the primary source rocks are Miocene age coals and lacustrine shales. The field is located in a north-south trending graben complex, where sedimentary strata generally dip away from the graben center, opposite to the fault dip direction. Gas is trapped through a combination of structural and stratigraphic factors, where channel complex sandstones are terminated up dip by faults.

Mapping individual reservoirs is difficult due to poor seismic resolution and lack of well control. The thickness of most sand beds is beneath the resolution of 3D seismic, and imaging thicker sand bodies is often complicated by interbedded coal. All wells penetrate multiple reservoirs, but reservoir area is limited by faulting and facies change. Hundreds of individual reservoirs are present in the field, but most have only a single well penetration.

Wells drilled in the Funan Field have relatively low reserves and steep decline rates, due to the complex reservoir geology. This requires the use of cost-saving techniques for drilling, completing, and producing wells; however, these methods create additional uncertainties in understanding how reservoirs perform.

Special techniques have been developed to deal with geologic uncertainty. Reservoir complexity makes net pay isochore mapping impractical, and a geostatistical method has been developed for estimating gas reserves. Well locations are structurally optimized in the hanging wall of faults, to increase the chance of penetrating unmapped reservoirs.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia