Chapter 6

Evaluating Seal Potential: Example from the Talang Akar Formation, Offshore Northwest Java, Indonesia

by

J.G. Kaldi
ARCO Indonesia Jakarta, Indonesia

C.D. Atkinson
ARCO British Ltd. Guildford, United Kingdom

ABSTRACT

The seal potential of various lithologies in the Upper Oligocene Talang Akar Formation (TAF) is evaluated in the BZZ area of offshore northwest Java. 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 is determined by mercury injection capillary pressure (MICP) analyses. Seal geometry is derived by integrating seismic data, core, detailed well correlations, regional sedimentological/stratigraphic relationships, and comparisons to known depositional analogs. Seal integrity is evaluated qualitatively by core examination, borehole imaging, and petrographic studies. These three variables were integrated and the totals were "ranked." In the BZZ area, deltaic distributary channel sandstones and delta-front/mouth bar heterolithic sandstones comprise the main reservoirs. Possible seals include prodelta, 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 and 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 prodelta shales are poor seals due to their limited seal capacity (delta-plain) or because they are too thin (prodelta 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.