Determination of the temperature history for the U Thong oilfield area (Suphan Buri Basin, central Thailand) using a realistic surface temperature

The temperature history for the BPI-W2 well in the oil-producing Suphan Buri Basin, central Thailand, has been investigated using different surface temperatures (Ts). Two ID models using Ts values of 0°C and ∼22°C were able to fit the suppression-corrected vitrinite reflectance (VR) values and burial peak temperatures (Tpeak) in the well. The geothermal gradient averaged over 3 km is ∼54°C/km for Ts= 0°C, whereas it is ∼42°C/km for Ts∼22°C. Ts= 0°C is, however, considered to be unrealistic and the ∼54°C/km gradient is therefore too high. Similarly, a previously determined geothermal gradient of 62°C/km is considered to be an overestimate. The geothermal gradient of ∼42°C/km is plausible compared to other geothermal gradients onshore and offshore Thailand, although it is at the low end. This may be due to a too low suppression correction for the measured VR values. The obtained temperature history can be used to predict measured present-day temperatures of Ts∼22°C and 77°C in the reservoir in the U Thong oilfield. The obtained temperature history associated with the geothermal gradient of ∼42°C/km seems realistic as it predicts that the onset of oil generation at 107°C will have post-dated reservoir and trap formation in Middle to Late Miocene times.