The Ujung Pangkah field is predominantly a fractured limestone reservoir, of early miocene age, that has undergone several stages of deformation, both extensional as well as compressional. The fractures observed are attributed to these differing deformation styles. The fractures are suspected to be contributing to the current production history. The datasets available (borehole image logs , mono and cross dipole sonics, VSP’s and 3D seismic) to characterize these fractures are either very local in nature or compromised by noise to differing degrees. Once we have reprocessed the 3D seismic attempting to reduce as much as possible the interfering phenomenon of statics and noise, we used different techniques (hand picking on horizons and auto-picking on continuity cubes) to generate lineament maps on and near our zone of interest. Lineaments though common in surface interpretation are less common in subsurface application. Statistics from these lineaments (dip direction and density) are generated. These are compared to our current understanding of the reservoirs structural history and more concrete local fracture/fault picks, mostly from borehole image logs, to ground truth the lineament interpretation. A qualitative comparison shows good agreement. The resulting density maps are then used to help place wells within our reservoir.
The Ujung Pangkah field is situated in the south west part of the East Java sedimentary basin, northeast of Java at the mouth of the Solo river. This region has undergone several stages of deformation which contributes to the current geometry and fracturing of our field. The initial relevant phase of deformation is related to Eocene rifting which produces NE-SW trending horst and graben features. The horst, on which our field is situated, has an abrupt southern (E-W oriented) fault bound termination that forms the southern closure to our field. The horsts, forming local highs, provided loci for some of the most intense carbonate deposition during the period between the late Eocene and early Miocene. These also determine where we find most of our reservoirs. The grabens (lows) receive most of the clastic influx from the highlands to the north and are thought to be the source of hydrocarbon for our field. The normal faulting appear to have been active until the top of the Kujung formation evidenced by missing sections. More recently, during the early Miocene, the collision of the Australian plate with the Sunda-Eurasian plate is felt in this area and a period of compression is initiated (from seismic isochrones, we determined that the influence locally to have begun after or late in the deposition of the Kujung formation and the Rancak formation). During this period of compression, which is considered to be NNE-SSW we observe that our field develops a distinct northerly tilt as well as reactivation and inversion of some of the normal faults, which were created during the rifting phase. There is one additional structural complication due to the observed left lateral strike-slip movement with underlying fault extending in an east-west direction to the north of our field.
