The Tembang gas field, located in the West Natuna Basin, was discovered in 1981 by drilling of the Tembang-1 well. Two appraisal wells were subsequently drilled. The Tembang structure is an elongate, bilobate anticline, approximately 9 kilometers long and 2.5 kilometers wide, and consists of 13 gas bearing sand reservoirs of the Lower to Middle Miocene Arang Formation. Several DST’s were carried out in some of the reservoirs and flowed at rates ranging from 6.8 to 21.7 MMCFGPD. In association with field development planning in 2000, a reservoir characterization study was conducted to better understand the critical uncertainties and identify potential field reservoir heterogeneity, in support of a value driven development plan.
Interpretation of conventional cores, well logs and biostratigraphic data shows that the reservoirs were deposited in a tide-dominated delta plain to delta front depositional setting. Reservoir fades consist of distributary channel, tidal flat, crevasse splay and delta front sands. These sands are composed predominantly of quartz, accompanied by common feldspar and minor rock fragments, with permeability and porosity ranges of 7 mD — 1720 mD and 14% – 38% respectively. Clay content ranges from 16% – 42% and consists of both detrital and authigenic clays. Good quality 3D seismic data over the field area provides excellent structural control. Seismic amplitude extractions as well as other seismic reflection attributes have been used to interpret the heterogeneities of the Tembang field reservoirs and the critical field limits. The seismic amplitude anomaly distributions agree closely with the reservoir gas water contacts derived from the RFT plots of reservoir initial pressures.
The results of the study have led to a better understanding of reservoir complexity, better estimates of GIP, and have enabled the selection of an optimized strategy for field development.
Authors : Teguh Prasetyo, Sugiharto Danudjaja & Yusti Budiningsih