OLIGO-MIOCENE CARBONATES OF JAVA, INDONESIA: TECTONIC-VOLCANIC SETTING AND PETROLEUM IMPLICATIONS

摘要

On Java, carbonates of Oligo-Miocene age are widely distributed. This time period is also noted for its volcanism which is commonly referred to as the period of the "Old-Andesite" volcanism and is distributed across southern Java. This interesting contemporaneity has been evaluated to understand the inter-relationships between the tectonic-volcanic setting of the carbonates and their petroleum implications. Two trends of Oligo-Miocene carbonates can be recognized: (1) A Northern Trend, including the Cepu-Surabaya-Madura, North Central Java, and Ciputat-Jatibarang areas, comprising the carbonates of the Kujung, Tuban, Baturaja and Middle Cibulakan formations and(2)A Southern Trend, including the Gunung Kidul - Banyumas - Jampang - Bayah -Sukabumi - Rajamandala areas. The Northern Trend carbonates developed in a back-arc setting, 75-150 kms away from a contemporaneous volcanic arc located in present-day southern Java. The Southern Trend developed within an intra-arc setting where contemporaneous volcanism took place. No reefal carbonates are found to be developed contemporaneously with the volcanism in the Gunung Kidul-Banyumas-Jampang areas of the Southern Trend. The Oligo-Miocene reefs which were growing on ridges in the Bayah-Sukabumi-Padalarang areas were not deposited contemporaneously with the volcanism. A window of volcanic quiescence occurred across Java from 18 - 12 Ma (uppermost Early Miocene -Middle Miocene). During the same period, the sea significantly transgressed many areas in SE Asia. This condition caused deposition of abundant reefal carbonates along the Southern Trend and included the following formations: Wonosari/Punung Formation in the Gunung Kidul area, the Jonggrangan Formation in the Kulon Progo area, the Karangbolong/Kalipucang Formation in the Banyumas region, and the Bojonglopang Formation in the Jampang area. The Northern Trend carbonates are very prolific petroleum reservoirs. Volcanic impurities are absent to very minor and are diagenetically insignificant. The geological setting of the carbonates is very supportive for the existence of a viable petroleum system. The Southern Trend carbonates in part show good-excellent porosities such as the Wonosari Formation carbonates, while others show severe neomorphism such as the Rajamandala Formation carbonates. There is no hydrocarbon discovery along the Southern Trend but the area has been inadequately explored. Nevertheless, the presence of a viable petroleum system is considered of higher risk than that of the Northern Trend.