Abstract

Lapindo Brantas, Inc. (formerly Huffco Brantas, Inc.) discovered the Wunut Field in 1994 as a gas accumulation in volcaniclastic reservoirs of Pleistocene age. The field is located onshore Java approximately on the axis of the East Java Basin. The lowermost producing interval was deposited in an outer neritic, turbiditic environment whilst the upper part was deposited in a deltaic setting. Overall, shallowing-and coarsening-upward sequences can be clearly recognized.

The Wunut structure is a simple, four-way-dip closure, aligned east-west, which formed as a response to magmatic uplift which occurred 10-15 km to the south. The uplift resulted in northward-directed detachment along an intra-Pliocene surface, creating the Wunut structure during the Late Pleistocene. It was charged immediately with hydrocarbons (mainly gas) from the deeper section. The source of the gas is believed to be from hydrocarbons originally trapped in the Porong Reef, of Miocene age, located 10 km to the east and which appears to have leaked due to the collapse of the overlying sediments. This collapse took place at approximately same time as the Wunut structure was formed.

Geochemical data show that the gas is thermogenic; its gravity and wetness both increase with depth which are thought to be caused by a process of fractionation. At a shallow depth the structure is partially filled and it becomes progressively more filled with depth. At the deepest pay levels the structure is filled to the spill-point. This suggests that the sealing capacity of the caprocks becomes better with increasing depth.

The discovery of the Wunut Field reveals significant exploration potential in the East Java Basin particularly, and Indonesia in general. This success represents the first commercial, volcaniclastic-reservoired gas discovery in the Pleistocene, which is generally considered as containing poor quality reservoirs. Although relatively small in size, the Wunut Field is located within an area of well-developed infrastructure in industrial East Java thus making its development economically feasible.