Back to main page

CONTENTS

The B-28 zone of the Middle Miocene “Main” Interval Upper Cibulakan Formation is the major contributor to the hydrocarbon cumulative production of B-Field, the oldest field in the ARII Offshore Northwest Java Contract Area. By January 1998, this zone has delivered approximately 100 MMBO and is best known for its reservoir heterogeneity. This paper discusses the most common sedimentary structures responsible for reservoir heterogeneity which plays an important role in the performance of thin bedded reservoirs. The B-28 zone is interpreted to be deposited in a tide dominated middle shoreface to offshore environment. It is subdivided into 4 major sand packages in which each sand package does not necessarily have the same lithofacies. However, they share a common characteristic which is highly bioturbated with rare evidence of physical sedimentary structures. A detailed investigation using approximately 250 meter conventional core data from 10 wells reveals that bioturbation is most extensive in thinly bedded sandy siltstone, claystone and silty sandstone lithofacies. The bioturbation is dominated by moderate diversity trace fossils of horizontal to sub-horizontal form, grading between Skolithos and Zoophycos ichnofacies such as Thalassinoides, Planolites, Chondrites, Asterosoma and Zoophycos. A low invasion coring technique conducted in one of the cored well exposed an important evidence of vast occurrence of trace fossils associated with hydrocarbon shows. Under UV light, the trace fossils are clearly observed to be fully saturated with oil. Sufficient thickness of this trace fossils assemblages ultimately defines an excellent pay zone. This thinly bedded lithofacies commonly gives low to moderate contrast between the shallow and deep resistivity reading despite having high GR reading. DST and production test have confirmed significant oil rate from this type of lithofacies, BL-1 well tested 500 BOPD. Biogenic structures are important sedimentary structures. They play a significant role in thinly bedded sandy siltstone, claystone and silty sandstone lithofacies by acting as both the hydrocarbon reservoir and conduit. Reservoir stimulation technique to increase permeability such as hydraulic fracturing may be required to optimize the production from this type of lithofacies.