FRODESEN, ERIC, Atlantic Richfield Indonesia, Inc., Plano, TX

ABSTRACT: Offshore Northwest Java (ONWJ) 1000 Wells Later: Application to Exploration in Non-Marine Rift Basins

Cumulative data from 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 sub-basins within the block. These sub-basins formed when riffing commenced in late Eocene or early Oligocene time. The earliest sediments in these half-grabens were predominantly lacustrine and continental in nature. These non-marine sediments were followed by a fluvial-deltaic sequence and 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 c mplicated hydrocarbon mixture in most fields. Faulting provides an important mechanism of migration to carrier beds and traps. Most fields can be correlated to seven trap styles. These trap styles and their success histories have been applied in the exploration program, which has resulted in the discovery of over 1.2 billion barrels of oil in the block to date. Fifty-six percent of these reserves were found in the first five years and are represented by the four major fields in the block, (E, B, F&K). These fields are in the 50-200 MMBO size. Fields discovered in the last fifteen years are estimated to be under 40 MMBO with an average size of 11 MMBO. A total of 291 wildcat and 198 delineation wells have been drilled for an average of 13.2 wildcats and 9 delineation wells per year. W ldcat success ratios are 41% geologic with an economic realization closer to 28%. Delineation wells enjoy a 66% success rate while development success is an amazing 98%. An average of 2.5 MMBO per well has been discovered in recent years. ONWJ is currently producing 135,000 barrels of oil per day. This is increasing due to exploration successes and an aggressive development strategy. More recently gasexploration has been accelerated as ONWJ will deliver gas to PERTAMINA in late 1993 for power generation in Jakafta. Exploration for dry gas is taking place in shallow carbonate buildups, while wet gas Is being pursued in the deeper basinal traps. The source, migration, and trap information in ONWJ can be applied to other non-marine rift basins in the world.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.