Abstract

The back-arc basins of the East Java Sea are a continuation of the very prolific onshore basins. Exploration in the offshore areas, which began in the late 1960's has gone through three main cycles. The first two cycles resulted in only a few discoveries, mostly in the late 1980's, such as Pagerungan and West Kangean. The current exploration cycle has revitalised this largely under-explored area through several oil and gas discoveries. These include Bukit Tua, Jenggolo and Payang within the ConocoPhillips's Ketapang block, adjacent to the open Tanjung Lapak Blocks, small discoveries on the Karapan Block and in addition other discoveries such as Oyong, Ujung Pangkah and the onshore Banyu Urip.

This paper is based on the interpretation of a 2729 km MC2D survey and 3963 sq km High Density MC3D (HDMC3D) survey acquired by PGS in 2002 and 2003 respectively. The HDMC3D survey is located over the North Madura Platform in the open North and Northeast Tanjung Lapak blocks, which are scheduled for offering in Indonesia's upcoming 4th Round. The high density nature of the 3D has enabled a detailed interpretation of the complex limestone reservoirs, whilst the regional nature of the 2D survey has allowed the identification of source rock kitchens, migration fairways, as well as the main tectonic events in the area.

Interpretation of the seismic data has revealed numerous prospects and leads along the North Madura Platform within both Kujung I and II/III carbonates. Some of these structures may have structural closures of greater than 100 km².

Structures within Kujung II/III carbonates, comprise large, low relief inversion anticlines, which represent similar play types as the Bukit Tua and Jenggolo oil and gas discoveries, as well as smaller, fault-controlled structures. The Kujung II/III carbonates represent different facies, within a stable carbonate platform area. The central part of the North Madura Platform is further characterized by the development of extensive Kujung I carbonate build-ups. Some build-ups have up to 150-250 m of structural relief. The build-ups are usually discrete, steep-walled biohermal structures, often of relatively confined nature encased in marine shales and silty carbonates of the overlying formation. This has been demonstrated by many wells in the East Java Sea area. The size of the individual build-ups range from just a few to several tens of sq kms, separated by lagoonal facies and therefore representing a potentially very complex play. The MC3D seismic data has allowed a detailed interpretation of the individual build-ups, resulting in an increased understanding of their size, architecture and geological development. It is believed that both the Kujung I and II/III carbonates have been extensively karsted, as indicated by the seismic character. This is further supported by results from some of the nearby wells. The carbonates have therefore probably been through several phases of exposure, with development of secondary porosity/permeability.

The Kujung I play represents a combined stratigraphic/structural play, depending on lateral and top seal. Even though the seismic data suggests the build-ups are encased in mostly non-permeable sequences, "thief-beds" represent a potential element of risk. The main source rock sequences are related to a deep (up to 6 km) kitchen in the southeastern part of the area, where a 3-4 km thick sequence of potentially mature source rocks are identified. These are believed to comprise a mixture of lacustrine deltaic and marginal marine sediments belonging to Ngimbang Fm, as well as lower part of Kujung Fm. As most of the traps in the area are located between 10-50 km from mature source rocks, permeable carrier beds in Ngimbang Fm or Kujung Fm are critical for this play system to work. Even though long distance migration is proven by several discoveries along the North Madura Platform, this is nevertheless perceived as representing the main element of risk in this area.