Abstract

The Peusangan-XL structure, offshore North Sumatra, is located about 20 kilometers northwest of the giant Arun gas field on the offshore extension of the Arun/Cunda Platform. The structure is a large classic reef buildup within the Early Miocene Peutu Formation over an old basement high. This basement feature is a shallower extension of the onshore Cunda structure, just west of the Arun field.

Four wells have penetrated the reef but only one well. Peusangan-B1, produced gas, from the top 24 feet of limestone. Peusangan-B2 was drilled 2 kilometers north of Peusangan-B1 in a time low and encountered the reef updip, as prognosed, but tested water, indicating a more complex structural relationship between the two wells than the proposed simple anticline.

The structural interpretation of the reef from the available seismic data was confused by the presence of an overlying low velocity anomaly plus an apparent seismic structural feature. Both these localized shallow anomalous features were originally thought to be caused by a "Gas Chimney", whereby shallow gas charged sediments were causing severe velocity distortion. The existence of low velocity zone(s) was successfully qualified but not quantified by a long offset velocity survey recorded in the Peusangan-B1 well. However, results of the latest well, Peusangan-B2, indicate that the two anomalies are separate features requiring separate delineation.

Key seismic lines were reprocessed in order to analyze the velocity anomaly and the underlying reef structure. The large velocity break across the reef necessitated the use of a non-standard processing technique to extract additional base reef data from the far offset traces. The existence of this data, which can not be exploited by standard processing, was recognized by inspection and verified by modeling. The resulting improvement in data quality beneath the reef along with the B2 well results is supportive of complex wrench faulting within the reef.