--> Abstract: The Geometry and Evolution of the Sarulla Graben, Sumatra, a Strike-Slip Basin Hosting Significant Geothermal Resources; #90063 (2007)

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The Geometry and Evolution of the Sarulla Graben, Sumatra, a Strike-Slip Basin Hosting Significant Geothermal Resources

 

Hickman, Robert G.1, Pat P. Dobson2, Marc Van Gerven3, Birean D. Sagala4, Richard P. Gunderson5 (1) Structural Solutions, Houston, TX (2) Lawrence Berkeley National Laboratory, Berkeley, (3) Solaria Corporation, Fremont, (4) Chevron Geothermai Indonesia, Jakarta, Indonesia (5) Consultant, Santa Rosa,

 

The Sumatra fault zone (SFZ) accommodates much of the margin-parallel motion between the Indo-Australian and Eurasian plates. In North Sumatra strands of the SFZ form a zone up to 10 km wide which overlaps the subduction-related volcanic arc. A series of linear basins are developed along these fault strands that include the Purwodadi, Sarulla and Taratung graben. The Purwodadi graben is formed by a releasing step between fault strands, while the Taratung graben is a pull-apart basin formed by two understepping dextral faults.

 

In contrast, field mapping, gravity surveys, exploration wells and limited seismic demonstrate that, the two km deep Sarulla graben does not fit standard models of strike-slip basins. The basin has a half-graben profile and is internally cut by normal-dextral faults that parallel a nearly linear part of the SFZ. The 35 km-long Sarulla graben appears to have formed as a result of extension perpendicular to the major strand of the SFZ. Within the graben the overall pattern of extension is modified by sub-basin scale releasing and restraining bends.

 

Volcanic centers within the Sarulla area include the Sibualbuali stratavolcano (~ 0.7-0.3 Ma), the Hopong caldera (~ 1.5 Ma), and the Namora-I-Langit dacitic dome field (0.8-0.1 Ma). These centers generated the majority of the ash-flow tuffs and tuffaceous sediments filling the Sarulla graben, and appear to have been localized by structural features of the SFZ. Four significant geothermal systems within the Sarulla area are linked to major faults and volcanic centers. In three of the systems, reservoir permeability is dominated by faults and fractures within the SFZ. In the fourth, permeability probably is locally influenced by faults, but highly permeable fractures are widely distributed.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California