Abstract

In the Early Miocene, a prolonged period of gentle epeirogenic subsidence of a broad carbonate platform resulted in the deposition of a widespread, thick limestone and shale sequence (Kujung Unit I) in the region of the present day East Java Sea. The Early Miocene paleogeography of part of this region, north of Madura Island, is described with special emphasis upon the distribution, morphology and internal structure of Kujung Unit I reefs which are currently being explored for hydrocarbons by a consortium comprising Cities Service (operator), Monsanto, Ashland, Robina and Gulf.

Major Early Miocene physiographic elements present in the East Java Sea were: (a) a deep water, east-west trending open marine clastic basin in the south (the East Java-Madura Basin), (b) an extensive, east-west positive area of shallow water carbonate deposition to the north (the East Java-Madura Shelf), characterized by a wave-washed, high energy bank-edge along the southern margin, (c) a shallow basinal area (the Central Depression) of open marine, fine clastic and low energy limestone depositon, and (d) a north-east-southwest trending ridge (the JS-1 Ridge) to the north-west of the Central Depression, characterized by extensive shoal water carbonate deposition. Another basinal area, the East Bawean Trough, was present to the west of the JS-1 Ridge. Shelf margins at the southern edge of the North Madura Shelf, and along the northwestern rim of the JS-1 Ridge, were well defined by a rapid increase in water depth into adjacent basinal areas. By contrast, there was a more gradual transition into the Central Depression from the northern edge of the East Java-Madura Shelf and along the southeastern side of the JS-1 Ridge, resulting in scalloped shelf margins.

Kujung Unit I depositional trends were strongly influenced by the pre-Early Miocene NE-SW structural grain along the Asian continental margin, north of Madura Island. South of this, the main controlling structural factor was the east-west grain of the East Java-Madura Basin, a "foreland Basin" associated with both strike slip plate movement and subduction along the Indonesian island arc. A major change in sedimentation, from carbonates and shales in the northern platform area to shales and fine clastics in the south, also occurred at the boundary between these two regions.

Extensive development of bioherms took place in the subsiding Central Depression. Six types of bioherms are recognized and classified on the basis of geometry, basinal position, and chief bio- or litho- constituent. These six categories are divided into four low growth-ratio varieties comprising: (a) shelf coral reefs, (b) basinal coral reefs, (c) basinal lime mud mounds, and (d) basin-slope pinnacle reefs; and two high growth-ratio varieties comprising: (a) bank-edge foram-algal shoals and (b) bank-edge coral reefs. Geometry, lithology, nature of foundation, and character of laterally equivalent sediments are described for each. Seismic profiles are illustrated for four of the types.

The internal geometry, facies development, and porosity of the shelf coral reefs within the oil-productive zone of the Polong Field are described. Within those reefs, five major facies are distinguished, comprising coral reef core, coral debris, red algal, foram-algal, and deep water shelf facies. This reef growth began during an initial regressive phase involving progradation of facies belts, and later culminated during a transgressive phase when facies belts retreated inwards towards the reef core.