Abstract

Seismic hydrocarbon indicators (HCI's) have aided in identification and mapping of gas accumulations in the clastic Miocene Arang Formation in the South China Sea. Two structures drilled in Block B, Indonesia, Bawal and Tembang, provide excellent control for the comparison of known pay zones and their associated seismic expression, with seismicamplitude anomalies and waveform changes present on adjoining structures.

In Block B, Arang gas has been found in anticlines on the upthrown side of reverse faults which formed from Early Miocene to Pliocene time. Gas may be biogenic in origin or may have migrated from mature gas prone source rocks in deeper basinal areas. Gas composition typically ranges from 85 to 98 percent methane, a few percent of ethane, propane, and butane, one percent carbon dioxide, and no hydrogen sulphide or condensate.

Gas accumulations in the Arang Formation are easily identified by seismic amplitude anomalies down to approximately 5000 feet. Coals also create amplitude anomalies, however, these are not restricted to structural culminations. Additionally, numerous other waveform anomalies are present which are definitive of gas accumulations. The Tembang structure exhibits velocity related structural sag over its crest and seismic flat spots at the gas/water contact within thicker sands and polarity changes of seismic events at gas/water contacts.

Reserves can be estimated prior to drilling by modeling the seismic anomalies. One of the hydrocarbon indicators drilled below 2500 feet has been related to low gas saturations. Deeper pay zones may be masked by the top hydrocarbon indicator and are therefore harder to model. Seismic hydrocarbon indicators are a major tool in the successful exploration for gas reserves in Indonesia Block B.