Abstract

The study area is located in the middle of the northeast Java Basin. The tectonic evolution can be traced from the Late Cretaceous to Recent. During the Paleogene a tensional tectonic regime was active and was followed by a Neogene compressional regime which resulted in folding and faulting in the study area. During the Middle to Late Miocene six sea level falls can be interpreted from analysis of outcrop data, seismic sections and well log data. In the Middle Miocene 4 sea level falls (15.5, 13.8, 12.5 and 10.5 ma) can be interpreted, and a further two sea level falls can be interpreted in the Late Miocene (6.3 and 5.5 ma).

In response to Middle – Late Miocene sea level falls in the northeast Java Basin, some of the sediments were eroded and clastic material was transported towards the south and deposited as submarine fans, as evidenced by the presence of mounded geometries observed on seismic sections in the study area. Sequence boundaries dated as 15.5, 13.8, and 12.5 ma. can be clearly identified in outcrops along the Braholo River to Ngampel Village, to the north of Blora town and can be readily interpreted from well log data from Banyubang-1, Nglobo P, and the EMI from KWP in Kawengan Field.

The sequence boundary identified at 10.5 ma, can be clearly observed in the Kawak River – Pucakwangi Pati area and from the EMI log in wells from the Kawengan Oil Field. The sequence boundaries dated as 6.3 and 5.5 ma., can be interpreted in the study area by the presence of cross-bedding in sandy limestones of the Ledok Formation. The concept of a submarine fan model is a new idea for hydrocarbon traps in the study area.

The recognition of a hydrocarbon system in the study area is supported by the presence of Eocene shales of the Ngimbang Formation as a potentially mature source rock; hydrocarbon migration occurred via the Ngraho reverse fault zone. Pliocene marls of the Mundu and Kalibeng formations are potential cap-rocks in the study area.