Prediction of Hydrocarbon Sources in The Walio Area, Salawati Basin, West Papua, Indonesia

Abstract

Substantial seismic and well data have been gathered and used for the exploration and exploitation of the Salawati Basin since the 1970s. The interpretation of these data and implementation of findings resulted into several discoveries and producing fields associated with the Tertiary petroleum system of Klasafet-Kais, particularly in the Walio Area. The presence of pre-Tertiary petroleum systems remain speculative to date although numerous pre-Tertiary structural and stratigraphic traps can be defined, and hypothetical sourcing can be inferred to origin from shales of the Permo-Triassic Aifam Group and the Late Jurassic – Late Cretaceous Kembelangan Formation. Yet the actual hydrocarbon charging of those traps has not been proven. Surface geochemical surveys were conducted in the Walio Area with the objective to characterize the origin of hydrocarbons of the area and possibly uncover the presence of a pre-Tertiary source. Pre-Tertiary sediments are encountered in three exploration wells of the Walio Area and pre-Tertiary traps were defined by the interpretation of 3D seismic. Surface geochemical surveys were conducted in the Walio Area to map the distribution of oil and gas microseeps and try to determine their source. Whole oil chromatography and isotope & biomarker analysis were also performed on Kais reservoired oil samples to determine source rock lithology, depositional environment, and age. Hydrocarbon analysis of one-meter-deep soil samples revealed the distribution of volatile and liquid hydrocarbon microseeps in the survey area. Total Organic Carbon (TOC) and Rock-Eval Pyrolysis measurement was carried out to outcrop samples of the Klasafet Fm., Kembelangan Fm., Tipuma Fm., and Aifat Fm. The outcrop samples resulted in lean TOC, and produced very low S2 values to which the Tmax cannot be estimated. Hydrocarbon microseeps were found occurring over and off existing fields and pre-Tertiary traps. Both light oil microseeps (35° API) and heavy oil microseeps (14° API) were identified over fresh and biodegraded crude oil Kais reservoirs, respectively in the Walio Area. Microseeps with lighter hydrocarbon compositions (e.g. C5/C17 ratio) may reflect leakage from deeper pre-Tertiary sources.