Publications

Accurately predicting reservoir temperature variability across the West Natuna Basin Block B PSC has improved petrophysical water saturation calculations, particularly in lower quality pay zones. Prior to this study the last publication to include geothermal gradient data for the West Natuna Basin was based on only nine exploration wells across the approximately 31,000 square kilometer area. Calculated geothermal gradients from the prior study were shown between 1.9o F/100 ft to 2.72o F/100 ft. (Aadland et. al, 1981). Since the publication of the 1981 study, hundreds of new wells have been drilled and several fields are currently under development in Block B. In this assessment, reservoir pressure and temperature data acquired with the MDT (Modular Formation Dynamics Tester) downhole tool across eleven fields in the block have been used to generate more accurate geothermal gradients. A newly constructed temperature profile incorporating these data shows a relatively coherent profile across the fields. At least two linear geothermal gradients are observed, the first from sea bottom until around 3000 ft. TVDSS and the second below 3000 ft TVDSS. The geothermal gradient for depths more than 3000 ft. TVDSS is calculated to be 2.38o F/100 ft. In this case study, the fluvial Miocene Lower Arang reservoir in the Belut-1 well (~ 3600 ft. TVDSS) was analyzed to compare predicted water saturation differences due to variability of geothermal gradient from the 1981 publication. Due to zonal temperature differences of about 28o F between the two studies, when applying the water saturation equation for a shaly sand reservoir, a difference in water saturation (Sw) was shown to be between 2%-10%, significantly impacting ranges of gas in-place for development scenario planning.