Cenozoic Basin History of the Cepu Area Based on Calcareous Nannofossil Biostratigraphy in Eastern Java, Indonesia

Abstract

We studied in detail the nannofossil biostratigraphy of Lasem-1, Kalijati-1, Kedungtuban-1, and Ngimbang-1 wells located in Cepu Area, Eastern Java, Indonesia. Our result indicates that a total of four unconformities are traceable in the Oligocene to Pliocene sequence in this area. Unconformity 1 is situated in the Oligocene/early Miocene boundary, which is traceable in all wells, is characterized by the missing interval from NN1 to NN3 zone of the early Miocene. Unconformity 2 is correlated to the early middle Miocene, which is found in Kedungtuban-1 well. Nannofossil zone NN6 is not present in this well. Unconformity 3, which is characterized by missing the interval NN8 of the early late Miocene, is traceable to Kedungtuban-1, Dander-1, and Ngimbang-1 wells. Among the wells, the sediments from NN6 to NN8 is not present in Ngimbang-1 well. Unconformity 4, which is the biggest unconformity found in this area, is situated in the lower Pliocene base. In this area, the lower Pliocene NN14 sediment is distributed above the unconformity 4. The time gap of unconformity 4 is the biggest in Lasem-1 and Kalijati-1, located in the western area. The interval, NN6 to NN13 of middle Miocene to lowest Pliocene, is missing in Lasem-1 well, and the interval NN5 to NN13 (part) is not distributed in Kalijati-1 well. These results indicate that the LST (Low Stand Systems Tract) above the unconformities 1 and 2 are situated in the center of the studied area (Kedungtuban-1 and Dander-1 wells), and LST above unconformity 3 is found in the eastern area located in out of the sedimentary basin (Ngimbang-1 well). In contrast with LSTs of the Miocene, the location of LST above unconformity 4 moves to the western area. The thickest sandstones, which are reservoir rocks of this oil field, is correlated to the early Pliocene LST and Transgressive Systems Tract above the unconformity 4.