Abstract

The Middle Miocene Upper Cibulakan Formation reservoirs approximately 75% of all the hydrocarbons discovered to date in ARCO Indonesia's Offshore Northwest Java PSC. One of the largest accumulations is in the B-Field where nearly 150 MMBO have so far been produced from the B-28/29 "Main" interval. Conventional cores from the B-28 interval of the field reveal an interbedded succession of sandstones, silty sandstones, shales and thin limestones. Depending upon sandstone percentage, degree of bioturbation, bioclast type and organic content these lithologies are subdivided into 10 depositional facies. These facies reflect deposition in a series of deltaic to nearshore sub-environments, most notably mouth-bar, channel and channel fringe settings. Reservoir quality in the sandstones is primarily controlled by grain size and sorting and enhanced by dissolution of former carbonate constituents. Most favourable reservoir quality reflected by high porosity and permeability and reduced capillarity occurs in the coarser grained, better sorted sandstones with the largest numbers of interconnected moldic pores. Sandstone distribution over the B-Field appears to have been strongly influenced by syn-depositional structuring. The thickest pay intervals at the B-28 level occur preferentially on the western and eastern flanks of the field and thin dramatically over the crest of the structure. This sandstone distribution is reflected in the production characteristics of the field with the highest oil rates being achieved from those platform wells located in elongate, channel sandstone bodies lying off the main crestal area of the structure.