INTRA-ARC TRANS-TENSION DUPLEX OF MAJALENGKA TO BANYUMAS AREA : PROLIFIC PETROLEUM SEEPS AND OPPORTUNITIES IN WEST-CENTRAL JAVA BORDER

摘要

Intra-arc region of Java from Majalengka to Banyumas area straddles the West-Central Java border. The area has been the least-explored area in Java. This contrasts with the facts that numerous petroleum seeps and the first exploration well of Indonesia (Maja-1 well, drilled in 1871, oil discovery) are located in this area. In this study, analyses and synthesis of a series of data comprising gravity, surface geological maps, field study, seismic, and wells lead to a new look of petroleum geology of the area.Major NW-SE trending dextral fault called the Pamanukan-Cilacap Fault Zone crossed this area possibly since the early Neogene or could be slightly older. Around the Majalengka-Kuningan-Majenang areas, the fault zone formed duplex system causing pull-apart opening of trough or trans-tension duplex. The opening controlled the occurrences of pre-Late Miocene back-arc volcanism sourcing the Late Miocene-aged turbiditic Halang volcanoclastic deposits. The Majalengka-Banyumas trough was then inverted in the Mio-Pliocene forming a structural high. Pemali sediments were deposited afterward in low areas flanking the structural high.Trough and trans-tension duplex of the Majalengka-Banyumas area and their inversion had controlled petroleum system of this intra-arc region. High heatflow and geothermal gradient due to pull-apart opening and volcanism, thrust loading within inverted Halang deposits and thick Pemali burial sediments had matured the Paleogene and/or Neogene source rocks existing in this area. Petroleum was generated and migrated updip to Majalengka-Banyumas structural high and trapped in various structures. Numerous oil and gas seeps reveal that petroleum system is working in this area.Reservoir quality, trap integration and poor structure imaging due to volcanic cover may risk exploration. However, based on its active petroleum system, the area merits further exploration endeavor.