The Kendeng Basin, the major depocenter of the East Java Basin, has been filled with deep-sea clastic volcanic deposits since the Middle Tertiary, which developed into volcanic deposits in the Quaternary. With thick Quaternary volcanic deposits covering almost the entire basin, outcrops are only found in the north, forming a fold-thrust belt structure. The oldest known stratigraphic unit is the early Miocene Pelang Formation, which was deposited in the lower to the upper bathyal zone. Rocks older than the Pelang Formation have not been identified in this basin either from outcrops or drill-hole data. However, the geochemical analysis of oil seepage proves to be different because the oil source rock in Kendeng Basin was interpreted to be of older lithology than the Pelang Formation, indicating a potentially older stratigraphic unit in the Kendeng Basin that has not been revealed to date. Mud volcanoes transported rock material from the Kendeng Basin to the surface, uncovering the stratigraphy that has been an enigma. The material in question includes Nummulites limestones, conglomerates, and quartz sandstones. Paleontological analysis results on rock fragments indicate that they belong to the Middle Eocene age, so they are older than the Pelang Formation. Mud volcano also carried younger limestones to the surface identified as Miocene Age (equivalent with the Pelang Formation), which were deposited in a middle neritic environment, so they provided information that shallow areas possibly formed a horst-graben structure during the Miocene in the Kendeng Basin. The appearance of Eocene and Miocene rock fragments can be used to complement the stratigraphy and it also provides a potentially new concept of source-reservoir rock in the Kendeng Basin.
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