Structural and Reservoir Uncertainties in old mature field; A perspective from Kawengan Field (East Java Basin

Abstract

As an old mature field, Pertamina EP’s Kawengan Field (East Java Basin) had been developed with hundreds of well and several decades of production. Although Structural configuration and reservoir presence had been established and not considered as major uncertainties, those features still had uncertainties in some area to be reckon in developing the fields. In these cases, the problem came from not acquiring enough, fit for purpose data during early development program in 1930’s and 1990’s. The NW-SE Thrusted Anticline Kawengan Field is determine into 3 region; west, central, and east which each of the region separated by NE-SW tear faults. All of the region were a productive area. The west and central area of the field were well developed in terms of data, interpretation, reservoir presences, and structural configuration. But in east area, it is remain unsolved. This issue was first notice when revise the seismic interpretation (with enhanced quality of 3D Seismic processed on 2014) and well correlation using sequence stratigraphy concept (previous interpretation only focused on sand to sand correlation in Ngrayong Formation) from western to eastern area. The main structural and reservoir uncertainty came from the fact that the wells in the eastern part of the field are old (drilled between year 1930’s to 1950’s and only one wells was drilled in 90’s) with limited available logs and most importantly no VSP or checkshots. Seismic interpretation from more developed area in the west and central, which have at least one VSP or checkshot and more complete logs, becomes a key to understanding structural configuration and reservoir in eastern area. However, seismic interpretation in east area are reveal that the continuity issue of Ngrayong reservoir, whether the reservoir present in shallow (where most likely same with previous interpretation) or deeper interval. To define the explanation above, well correlation should distinguished first between Ngrayong Formation and older formation; The Prograding Tawun Formation. Unfortunately, not many wells in Kawengan Field encountered the Prograding Tawun Formation which is lies underneath average of 1300 m TVD. Among those several wells, there is one well that penetrated deeper section in east area; KWG-115 well which first drilled on 1955 and then undergo a deepening drilled section in 2001. A revised of well correlation using sequence stratigraphy concept of key wells in Kawengan area (including the deepening of KWG-115), deliver an unexpected result of Ngrayong Formation existence and already productive reservoir (which is considered as “Ngrayong” from the beginning) younger than Ngrayong Formation in east area. A revisit of the well correlation suggest that the top of Ngrayong Formation, the main productive interval, is actually significantly deeper and thus, opening the possibility of untapped productive interval in the eastern area. The other outcome is a prolific younger reservoir than Ngrayong is characterized by more carbonate material dominant than Ngrayong which has typical of loose-friable quartz sandstone as reservoir, and these younger reservoir (eq. Bulu/Wonocolo Formation) has uncover the idea that this interval could become a proven reservoir across East Java Basin, especially in Rembang Zone.

AAPG Datapages/Search and Discovery Article #90354 © 2019 AAPG Asia Pacific Technical Symposium, The Art of Hydrocarbon Prediction: Managing Uncertainties, Bogor, (Greater Jakarta), Indonesia, August 7-8, 2019