Abstract

The seal potential of various lithologies in the Upper Oligocene Talang Akar Fm. (TAF) in the BZZ area of offshore NW Java is evaluated. Seal potential comprises 1) seal capacity (the calculated amount of hydrocarbon column height a lithology can support), 2) seal geometry (the structural position, thickness and areal extent of the lithology) and 3) seal integrity (rock mechanical properties such as ductility, compressibility and propensity for fracturing). Seal capacity was determined by mercury injection capillary pressure (MICP) analyses. Seal geometry was derived by integrating seismic data, core, detailed well correlations, regional sedimentological/stratigraphic relationships and comparisons to known depositional analogs. Seal integrity was evaluated qualitatively by core examination, bore-hole imaging and petrographic studies. These 3 variables were integrated and ranked semi-quantitatively. In the BZZ area, deltaic distributary channel sandstones and delta-front/mouth bar heterolithic sandstones comprise the main reservoirs. Possible seals include pro-delta, delta front and delta plain shales, channel abandonment silts, and transgressive shelf carbonates in both the upper and lower TAF. Seal potential is best in the delta front shales, which have high seal capacity, are thick, laterally continuous and very ductile. Seal potential is moderate in the thicker (upper TAF) transgressive carbonates. These rocks have high seal capacity and excellent lateral continuity, but are brittle and hence prone to fracturing. Delta plain shales and pro-delta shales are poor seals due to their limited seal capacity (delta plain) or because they are too thin (pro-delta shales). Channel abandonment siltstones have even poorer seal potential because of small lateral extent and limited seal capacity. The least favorable seal potential occurs within the thin (lower TAF) carbonates. These rocks are relatively thin as well as being prone to fractures.