ABSTRACT: Reservoir Delineation of the Sibayak Geothermal Field, Indonesia Derived from Resistivity and Magnetotelluric Data

Daud, Yunus¹, Keisuke Ushijima², Sayogi Sudarman³, and Bambang Soegijono⁴
(1) Exploration Geophysics Laboratory, Department of Earth Resources System Engineering, Faculty of Engineering, Kyushu University, Fukuoka, Japan 
(2) Exploration Geophysics Laboratory, Department of Earth Resources System Engineering, Kyushu University, Fukuoka, Japan
(3) Geothermal Division, Pertamina, Jakarta, Indonesia
(4) Department of Physics, Faculty of Mathematics and Natural Sciences, University of Indonesia, Depok, Indonesia

Sibayak Geothermal Field is located in the North Sumatera Province, Indonesia. Reconnaissance survey had been started since 1989 by Geothermal Division of Pertamina continued by detail geophysical exploration in 1990 including electrical resistivity. The exploration results suggested that the Sibayak area is a high potential geothermal field. To delineate the resistivity distribution to the economic depth, magnetotelluric (MT) measurements were carried out in 1992. This paper presents the result of a multidimensional interpretation of the electrical resistivity and MT data that was performed in an attempt to delineate the complex conductivity structures of the geothermal area and to gain information of the geothermal reservoir. Key resistivity soundings were interpreted using a 2.5D finite-difference algorithm modified from Dey_fs method. The MT data were interpreted using a 2D finite element code. A conductivity model was developed, integrating available geological and drill-hole information. The combined interpretation of the resistivity and MT data for several sections shows that a thick conductive layer is commonly found in the study area underlain by a resistive layer. The former is associated with the strongly altered rock (cap rock) and the latter is associated with the moderately altered reservoir layer. It is proven from the study that the combined resistivity and MT data interpretation can effectively delineate the reservoir structure of the Sibayak Geothermal Field.

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