Publications

This study deals with a petrographic analysis of a volcaniclastic-rich sedimentary series penetrated by Rendang-1X (R1X) deep-water exploration well drilled in the Bengkulu Basin, South-West Sumatera. Bengkulu is a 150-km-wide fore-arc basin located in between the accretionary prism, related to the Indo-Australian and Sunda plates collision, and the Great Sumatera Fault zone (GSF). This well is located ~75 km from Bengkulu city. Several electrical logs were acquired and interpreted, petrographic analysis and biostratigraphic analyses from drill cuttings were performed to characterize the stratigraphic series. Some of the intervals show a high Gamma Ray (GR) trend, associated to a Neutron-Density cross over. Petrographic analysis of the drill cuttings is complex as the whole stack of Plio-Pleistocene sediments is represented by volcaniclastic lithotypes that are related to explosive volcanic activity occurred in the nearby Sumatera volcanic arc. Two main lithological facies have been defined: tuff & pumice and accretionary lapilli. Tuff and pumice unit is a volcaniclastic facies mainly composed of sharp fragments of tuff and pumice, showing little transport and poor organization. GR log trend is shifted towards high values (> 90 API). Accretionary lapilli facies mostly consists of coarse sphaeroidal lapilli-sized aggregates of ash characterized by lower density and reduced porosity. Lithic fragments are sparsely found throughout the section penetrated, they are mainly represented by limestone, intrusive rock, shale, silty shale, marl, and sandstone fragments. The high GR electrical log response is mainly due to high potassium and thorium content due to the presence of K-rich minerals (feldspars, biotites, and serecite), commonly found in felsic volcaniclastic rocks. These volcaniclastic products are consistent with the explosive volcanic activity associated to the Sumatera stratovolcanoes. Their sedimentation into Bengkulu Basin is related to both fall-out deposition of effusive products and basin-ward deposition of pyroclastic flows as mass transport complex (MTC).