The Sirikit oil field lies in the Phitsanulok basin, one of a series of Tertiary rift-related structures in central and northern Thailand. The tectonic history of the area is complex: the original extensional half graben was deformed during deposition of the upper reservoir sequence by left-lateral strike-slip faulting.

Careful interpretation of a three-dimensional seismic data set has resulted in the reliable mapping of seismic markers. Detailed correlation of well logs, based on field-wide, acoustically soft, lacustrine claystone markers, confirmed a lacustrine-deltaic reservoir geologic model derived from core analyses. The Sirikit deltas are of constructive lobate type with sheetlike mouth-bar (bay-mouth bar) reservoir sandstone geometries.

Interpretation of seismic horizon seiscrops (amplitude maps) of mapped seismic horizons has refined the understanding of the complex local reservoir structure. Subtle amplitude anomalies, interpreted as small displacement faults, are visible on horizon seiscrop maps. These faults may reduce the drainage area inferred from log correlations. The resolution of small dislocations and flexures by seiscrop analysis may apply to well targeting.

Production decline rates are variable in the Sirikit wells, showing no simple relationship to the perforated reservoir thickness. The geologic model explains this behavior in terms of effective drainage area: wells perforated in thin sheet sandstones commonly have lower decline rates than those completed in thicker, better reservoir quality, but isolated, channel sandstones.