Indication the occurrence of geological structure in the Ngimbang-1 well, East Java, has never been reported. The Kujung and Ngimbang Formations found in the Ngimbang-1 well is always interpreted being normal. During this work, sediment samples of the Kujung and Ngimbang Formations from two wells, i.e. Ngimbang-1 and Kujung-1 were analysed for their maturity based on vitrinite reflectance technique and, by doing so, it was revealed that a geological structure should occur within the Ngimbang Formation in the Ngimbang-1 well. In Kujung-1 well, the only Kujung Formation sample analysed indicates that its maturity based on vitrinite reflectance (Ro) is 0.2%, whereas some samples obtained from the Ngimbang Formation show Ro values between 0.25 to 0.45%. These values indicate a continuation of maturation without any disturbance. In fact, both formations reveal immaturity for hydrocarbon generation. In Ngimbang-1 well which is around 22 km far from the Kujung-1 well, the vitrinite reflectance values observed in the Kujung Formation samples range from 0.37 to 0.38%, however, Ro values for the Ngimbang sediments are between 0.45 and 2.26% with a significant jump from 0.44 to 1.1%. The high Ro values may be due to a rework process, but the consistent and continue increase of the maturation (after significantly jump) in this well is most likely because of a normal maturation process rather than a rework. This is supported by SCI data that show a similar case, i.e. a sudden increase of SCI values from 4.5 to 8.0 was also observed within the Ngimbang Formation. Maturation in the Ngimbang-1 shows a span of maturity from immature to over mature, up to dry gas window. Geological data of the area reveals a normal fault occurred in between the Kujung high and the Ngimbang low. This fault is interpreted being most likely responsible for the significant increase of the maturity in the Ngimbang-1 well.
Author : Kuwat Santosa, Eddy A. Subroto