Terrestrial heat flow in the tertiary basin of central Sumatra

Abstract

Heat flow at 170 locations in the Central Tertiary basin of Sumatra was determined from thermal gradients obtained from the extrapolated oil well bottom hole formation temperature and the assumed temperature of 80°F at the surface. The effective thermal conductivity of the whole rock column, by which the gradient is multiplied to get the heat flow was calculated from measurements on 273 specimens of the geologic section and inspection of 92 well logs. For the whole basin the gradient averaged 3.71 i- l.O4’F/ 100 ft (67,6’C/km) the conductivity 4.83 i: 0.31 meal ‘C-l cm-l set-l, giving an average heat flow of 3.27 ?: 0.93 10s6 cal crne2 set-i which is about twice the world average. The gradient and the heat flow vary inversely with the depth of the wells most of which bottom in the pre-Tertiary basement. This may result from the basement rocks being several times more conductive than the sediments. Mocef calculations on a narrow heatflow anomaly which rises from a base level of 3.2 HFU to 8.8 HFU suggest that it can be caused by the intrusion less than 55,000 years ago of an igneous plug or Iaccolith no deeper than 3 km and 2.2 to 4.6 km wide. Using the gradients from the SEAPEX Geothermal Gradient Map and assuming a conductivity of 5 meal cm-l ‘C-l set- l, the heat flow in the North Sumatra basin, the South Sumatra Basin, Sunda Strait and West Java is 2.5 HFU, while in Java east of 1lO”E longitude it drops to 1.9 HFU. Since subduction off Sumatra dates back at least to the Cretaceous, compression of the Asian plate against the Benioff zone is preventing the opening of a back-arc basin. This does not preclude the possibility of occasional periods of crustal tension corresponding perhaps to episodes of transgression which allow magma to rise into the rocks underlying the basin.