The Role of The Hulusimpang Formation In Well-Scale Permeability of The Central-Southern Sumatra Geothermal Fields

Permeability is one of the most critical objects that dictates a geothermal field success story. In Sumatra hydrothermal system, most of them are believed controlled by fault or fractures, either due to tectonic or volcanic structure. The combination of crystalline and brittle lithology associated with a local extensional mechanism is common and underlain this hypothesis. In this paper, we address the evidence of the other option for hydrothermal fluid to flow: the matrix permeability through an extensive and thickest Paleogene - Neogene volcanic formation in central and southern Sumatra, the Hulusimpang formation. The evidence synthesized from the comprehensive analysis through available field data including Sungai Penuh, Bukit Daun and Ulubelu which guided by the regional geological data.

The finding of this paper is the consideration of the Hulusimpang formation sufficient contribution to the well-scale permeability and therefore could influence the well-targeting strategy. The proof is derived from the detailed analysis of borehole data integration such as cutting, core, fossils, image log and PTS spinner data. We infer most of the shallower feedzones in the southern Sumatra geothermal fields reservoir are associated with less-fractured formation zone which dominantly pyroclastic deposits such as volcanic breccia and tuff. The stratigraphy unit of these lithologies is fit to refer to the regional Hulusimpang formation, that extensively mapped from Jambi to the Lampung region, including the investigated fields.