Source-rock evaluation of outcrop samples from Vanuatu (Malakula, Espiritu Santo, Maewo, and Pentecost)

Abstract

Thirty-nine outcrop samples of mudstones, siltstones, and limestones were collected for source-rock evaluation of the sedimentary column in Vanuatu. Twenty-nine samples showing the least evidence of oxidation were selected for total organic analysis from the Miocene Matanui Group and Port Sandwich Formation in Malakula, and from the Miocene Ilava, Peteao, and Pelapa Formations in Espiritu Santo. An additional 13 samples from the upper Miocene to Pleistocene sequence in the islands of Maewo and Pentecost were taken from the sample collection stored at the Department of Geology, Mines, and Rural WAter Supplies in Port Vila and analyzed for total organic carbon (T.O.C.). Of the 42 samples analyzed for total organic carbon content, four yielded values above the generally accepted minimum for source rock: three from the Peteao Formation (T.O.C. ranging from 0.52% to 1.44%), and one from the Pelapa Formation (T.O.C. = 0.54%). Rock-Eval pyrolysis on the rocks and reflected light microscopy on the separated organic matter indicate that the organic matter is poor in hydrogen and is dominantly type III kerogen; therefore, it is of a chemical type that does not yield liquid hydrocarbons but may havev limited potential for generating gas. The Rock-Eval pyrolysis and the vitrinite reflectance analysis indicate that the organic matter in these samples is thermally immature. The samples collected for the present study represent only a portion of the sedimentary column in the various sedimentary basins of Vanuatu. The characterize only the outer margins of the sedimentary basins and do not necessarily reflect the source-rock potential of the deeper (offshore) parts of the basins.