UNDERSTANDING THE PETROLEUM SYSTEM OF NORTH SERAYU BASIN-AN INTEGRATED APPROACH FROM GEOLOGY, GEOPHYSICS, AND GEOCHEMISTRY

摘要

The North Serayu Basin is one of two active petroleum systems in Central Java. Wells drilled in this area failed to find economic reserves though there are numerous hydrocarbon seepages in the area. One hydrocarbon seepage occurs in the Watukumpul Area, Central Java. This paper integrates regional geology, detailed geologic data, biostratigraphy, paleostress analysis, potential reservoir characteristics, organic geochemistry and gravity. This paper studies the Watukumpul Area as a possible petroleum system model within the North Serayu Basin. Geologically, the study area has three formations which are the — Rambatan Formation, Halang Formation, and Basalt Intrusion. Paleostress analyses were performed on 1606 shear fractures in study area, resulting in three interpretations of Sh-max and the tectonic evolution. The tectonic evolution began with a Pliocene tectonic phase associated with the north-south compressional Java Wrench System. Further tectonics were associated with more localized compression from the Mount Slamet volcanism. Each element of a viable petroleum system is present in this area. The source rock candidate is derived from shale of Middle Miocene Rambatan Formation. TOC and vitrinite analysis shows a mature to post-mature organic contentup to 0.99%. Source rock could have attained maturity via the appropriate depth of burial and intrusion heat effects. HI analyses show kerogen dominantly type III; meaning the source has terrestrial origins; however, biostratigraphy and sedimentological analyses indicate marine depositional environment. Intraformational sandstones of Halang and Rambatan Formations may serve as potential reservoirs, with the sand-shale ratio up to 100% with visible fractures, dissolution, and intraparticle porosity. The seal is from intraformational mudstone of Halang and Rambatan Formations-with possible Quartenary volcanics are potential seals. Structural traps are postulated in the noted anticlines or thrust faults. Generated hydrocarbons could enter the traps of the toe-thrust anticlines formed in this area (Satyana and Armandita, 2004) Petroleum evidence presented in this study will be useful for future exploration in North Serayu Basin.