Publications

The Tunu Shallow Zone (TSZ) corresponds to Pliocene fluvio-deltaic series in the depth of 600-1500 meters subsea. The gas sands of TSZ are scattered all over the field and were once considered as hazard during Tunu Main Zone (TMZ) drilling. Despite dense well coverage in the TMZ, the TSZ was not adequately covered by wells. This is where seismic becomes a paramount tool to map the gas sands distribution. Three seismic surveys (3D95, CT3D, and NWT3D) were acquired and (re)processed for the TSZ interval. Subsequently, a reliable seismic- interpretation techniques, with the purpose to identify and characterize the gas sands based on angle sub-stacks, gathers, AVO, inversion, and well correlation analyses, was formulated. All drilled reservoirs are thinner than seismic resolution, in consequences seismic-derived isopach does not represent the true reservoir thickness. For this reason, the method incorporates the use of seismic amplitude values for net-pay (thus resource) estimation. The biggest reservoirs which are easiest to access have been drilled and produced earlier. Today’s challenge is identifying the next drilling targets in the most efficient manner. A 1500 km2 full field geobody inventory is then built by imitating this seismic interpretation method in a more automatic process. Moreover, this inventory integrates not only seismic targets, but also well architecture and surface facility constraints to optimally evaluate the developable resources. Comprehensive use of the reliable-method is the key in finding robust shallow reservoirs to support the TSZ development. Since its application in 2012, more than 150 purely seismic-driven wells of 3 development phases have been drilled, with 82% success ratio in finding targeted reservoirs. TSZ currently contributes 40% of the Tunu’s total production of 700 MMscfd. This success story has triggered extensive studies in similar shallow zones in other Mahakam fields.