Madura Island is one of the many islands which are situated in East Java. Since the seventies drilling exploration has been done to discover hydrocarbon existence in this Island. First drilling exploration was done in 1973 (drilling Arosbaya-1 well); this exploration was done by the indonesian state oil company, Pertamina. The main objectives were the Oligocene and Eocene coarse clastics which were derived from land areas to the north and west. However drilling results showed that the limestones have more potential than the clastics reservoir, limestone reservoir perforation has resulted in 42 BOPD, 1.2-0.97 MMSCPD gas, and 8.1 MMCFPD Condensate (this Formation into Kujung I, Kujung II, and Kujung III carbonate). In 2006 drilling exploration was also dine by JOB-Pertamina Medco Madura, and have also found 0.768 mmscfpd gas and 61.9 bcpd condensate (This Reservoir was found within the Tuban Carbonates). Looking at this data it appears that the limestone reservoirs in Madura Island have good potential. According to major structural features dividing within the basin, Madura Island is a part of the Rembang – Madura Kangean structural Province, a relatively east-west trending fold belt along the northern coastline. The Zone has west-northwest or east-southeast anticlinal axes, which are particularly expressed by the older strata. The present day structural configuration of the Madura Island and adjacent areas is dominated by a major E-W left lateral wrench faults system (the Sakala Fault System) running from Kangean Area in the east to Madura and Java islands in the west. Along this E-W trend of strike slip fault, huge structures of inversion, showing local strong erosion, were created. The sedimentary succession of the onshore Madura area that are used to relate the formation names to chronostratigraphy in this report. Numerous oil companies (Philips Petroleum Company, 1984; Total Indonesie, 1994) have described this Stratigraphic column in detail. The Tertiary Strata has been subdivided into eight lithographic units overlaying the Pre-Tertiary Basement. They consist of Ngimbang, Poleng, Kujung, Tuban-Tawun, Ngrayong, Wonocolo, Kawengan and Lidah Formations. In onshore Madura Island, the formation is dominantly argillaceous which contain fine grained, tuffaceous sandstone and limestone bank sequence. The lithology of Kujung Formation (Upper Oligocene – Lower Miocene) mainly consists of marl and shale with subordinate amounts of sandstone and bank of limestone. During the Kujung Formation deposition, a wide spread transgression covered western and central part of the East Java sea. This event implies a further advance of the sea to the Western and Northwestern part of the NE Java Basin. Tuban Formation (Early – Middle Miocene) in the north of Madura consists of interbedded sandstone, shale, and limestone. In the southern part, it dominantly composes of shales and silts. Based on seven wells that have been drilled in Madura island, it showed that carbonate is one of the active reservoirs in this area. Carbonate characteristics maybe divided into two. Carbonates in the north part, and Carbonates in south part. In south part Madura island is showing bioclastic carbonate with texture mudstone to wackestone and packestone, large foraminifera contain, red algae, trace stylolite, and benthic foraminifera show that limestone in this formation relatively deposition in shallow marine to Open marine (outer neritic until Upper Bathyal), the age in N4 (prupuh Formation/Lower Miocene until upper Oligocene) and have porosity from 5-10%. Meanwhile in northern carbonates have characteristics which are Bioclastic carbonate with texture from mudstone to wackestone, entire forams, and shells and bryozoans fragements, Sometimes containing siltstone, gastropods fragments, foram fragments, the limestones in northern part is showing that deposition relatively in Shallow marine (landward to inner neritic) with age at N4 or lower Miocene until upper Oligocene, and have porosity value from 10-20%. Biostratigraphy correlation was done in seven well, it shows that the southern area is deeper than northern area, with that result carbonate prospect with N4 Age or Lower Miocene-Upper Oligocene is more prospect in northern Madura island than southern area.
Author : William Andrearto, Beiruny Syam