REVISIT GEOLOGY AND GEOCHEMISTRY OF BUTON ASPHALT DEPOSITS, SE SULAWESI: IMPLICATIONS FOR PETROLEUM EXPLORATION OF BUTON AREA

摘要

Asphalt deposits of Buton Island, SE Sulawesi, have been known since 1920’s and it is the only natural asphalt deposits mined in Indonesia. The deposits contain 15 to 35 % asphalt/bitumen making reserves of 100 to 132 million tones of asphalt. The deposits occurred mainly as impregnated materials within carbonates of Pliocene Sampolakosa and/or sandstones of Miocene Tondo formations. The traps are uplifted and intensively thrusted anticlines formed by collision of Buton-Tukang Besi micro- continent with Muna block. All geochemical data indicate that the asphalt deposits are biodegraded crude oils derived from marine, type II kerogen. Good to excellent correlations based on biomarkers were obtained between asphalt deposits and the Triassic calcareous shales and bituminous limestones of Winto Formation. Severe deformation due to collision eroded cap rocks of the traps, causing meteoric water flushing and biodegradation of oils, leading to asphalt deposits. Occurrences of large asphalt deposits and numerous oil seeps show that petroleum system of Buton area is working. However, exploration efforts in this area are so far disappointing. The main risks include complicated structures and absence of cap rocks. Poor quality seismic data hinder detailed imaging of structures. The paper presents some ideas of exploring Buton area based on revisited geology and geochemistry of the asphalt deposits and recent exploration results.