ABSTRACT: Offshore Northwest Java (ONWJ) 1000 Wells Later

FRODESEN, ERIC W., Atlantic Richfield Indonesia Inc., Jakarta, Indonesia

All of the cumulative 1000 wells drilled in ARCO Indonesia's ONWJ Block give new insight into risk assessment and reserve size predictability. To date, oil and gas have been discovered in three half-graben subbasins within the block. The subbasins resulted when rifting commenced in late Eocene/early lacustrine and continental, followed by a fluvial-deltaic sequence and eventually shallow water carbonates as marine transgression reached its peak. Oil source rocks are a mixture of sapropel-rich lacustrine shales and canneloid coals. By contrast, gas is thought to originate primarily from vitrinite-rich coals and lower quality carbonaceous shales. Multiple sources provide a complicated hydrocarbon mixture in most fields. Generally most discoveries have been strongly tied to prevailing st uctural styles with faulting providing an important mechanism of migration to carrier beds and traps.

Over 1.2 billion bbl of oil have been discovered in the block to date. Over half of these reserves were found in the first five years and are the 50-500 MMBO size. Fields discovered in the last 15 years are estimated to be under 40 MMBO with an average size of 11 MMBO. The recent discovery of a 200-300 MMBO field in the Asri basin to the north of ONWJ have encouraged ARII to look for similar plays in our basins.

A total of 291 wildcat and 199 delineation wells have been drilled for an average of 12 wildcats and 8.3 delineation wells per year. Wildcat success ratios are 42% geologic with an economic realization closer to 25%. Delineation wells enjoy a 65% success rate while development success is an amazing 98%. An average of 1.8 MMBO per well has been discovered in recent years. ONWJ is currently producing 135,000 bbl of oil per day and is on the increase due to exploration success and an aggressive development strategy.

More recently, gas exploration has been accelerated as ARII will deliver gas to Pertamina in January 1994 for power generation in Jakarta. Exploration for dry gas is taking place in shallow carbonate buildups, while wet gas is being pursued in the deeper basinal traps.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)