Publications

The East Java Basin, including the North Madura Platform is a large area of low relief, that connects to the Sunda Craton in the north. It has been subsiding slowly from the Eocene onwards forming a wide shelfal depocentre for most of its Tertiary history. A tensional stress regime was active from the Middle Eocene up to the Early Miocene in the Bawean area and is considered to be the major structural control. The extension was marked by extensive rifting during the Eocene, directly followed by a second stage of basin-wide subsidence in the Early Oligocene as the rifting transitioned to post-rift subsidence. Stratigraphically, The East Java Basin comprises the Ngimbang carbonate member (CD Carbonate) deposited in Early Oligocene in response to overall transgression by seas from the south and the foundering of Sundaland continental area to the north and west. Shallow water carbonate buildups accumulated on the JS 19, Bukit Panjang, JS 44 and JS 8 ridge of the Northern Platform, forming a linear belt with excellent reservoir characteristics. Step 1 of this study was to determine productive zones in the limestone reservoir of Ngimbang Formation based on well log data, DST, and core data. Reservoir porosity, was calculated from density-neutron and sonic log. The difference between the two porosity logs is the SPI (Secondary Porosity Index) and can indicate the presence of vuggy limestone. According to Aguilera (1980), separate vug systems have m>2 and touching vug systems have m<2. Porosity values from log analysis were compared with porosity values derived from core analysis to determine the relationship between ‘m’ as tortuosity factor and rock type in limestone reservoir facies in each well to determine permeability values. Every rock type was influenced by both depositional facies and diagenetic processes. Depositionally, there are the keep-up phase and drowning phase for the Ngimbang carbonate. Wells P-1 to P-6 were dominated by dissolution by meteoric water in the vadose zone. Connected vugs and touching vugs are important parameters to determine reservoir permeability. In Wells P-2 to P-4, the m<2 indicated touching vugs. Thus, the Ngimbang limestone reservoir has good porosity (primary and secondary) and permeability as indicated by the ‘m’ value. This is a quick look method that may be used to determine touching vugs and permeability in wells with no core. The log ratios and m value are compred to logs from cored wells. Keywords: Diagenesis, Touching Vuggy, Limestone Reservoir, Ngimbang Formation, North Madura Platform.