Geological Evaluation of the Waringin Formation as the host of a Vapor-Dominated Geothermal Reservoir at the Wayang Windu Geothermal Field

The Waringin Formation is part of the Quaternary Volcanic rocks that host the upper parts of the Wayang Windu geothermal field. It is part of a medial facies of an andesitic stratovolcano with its eruptive centre in the south. Its importance is that it hosts the vapour dominated zone of the Wayang part of the field. It consists of andesitic lava flows and medium to coarse grained pyroclastics. The lava flows generally become less common with depth. The upper lava flows mark the formational break from the overlying Pangalengan Formation that consists of fine pyroclastics and epiclastics. Hydothermal alteration in the Waringin Formation that formed during earlier liquid dominated conditions that spanned the transition from argillic to propylitic alteration with depth. Smectite-illite clay and chlorite predominate in the argillic zone with the top of the propylitic zone marked by the first occurrence of epidote. This lies well above the top of the vapor dominated reservoir, which is confined to the lower part of the formation where medium to coarse grained pyroclastics predominate. This is a similar situation to that in the Gambung part of the field where the separate vapour dominated zone there has its top at the formational break between the lava rich Malabar Formation and medium to coarse grained pyroclastics of the underlying Dogdog Formation. The elucidation of similar geological controls over the top of the reservoir between the two separate vapor dominated zones is of major assistance for well design for further exploitation of the Wayang vapour dominated zone.