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This study aimed to investigate the variations of fossil traces in the limestone layers of the Selorejo Formation, as well as the level of rock porosity in the petroleum system. Field mapping, stratigraphic measurements, profiling outcrops, rock sampling, and porosity analysis were conducted to collect data. The results showed that the Selorejo Formation contained various types of fossil traces, such as Planolites, Helminthopsis, Thalassinoides, Conichnus, Chondrites, Macaronichnus, Bergauria, Ophiomorpha, Skolithos, Terebellina Asterosoma, and Palaeophycus. Moreover, rock layers containing trace fossils had lower porosity than those without them. The more trace fossils in the rock layer, the worse the porosity. This finding could be useful in petroleum exploration, as the porosity of rock layers with fossil traces needs to be taken into account when assessing oil and gas reservoir potential. [ABSTRACT FROM AUTHOR]

Nannofossil analyses of this study was carried out of Kalibeng river section, Plandaan, Jombang, east Java. This study included of the Kendeng Zone, East Jawa Basin. This research method through 2 section of the Measurement Section, and sampling a number of 32 samples. Analysis used of smear slide and Scanning Electron Microscope analysis. The result of the analysis of there are twelve genera and forty-three species. The stratigraphy of this area consists of 3 lithostratigraphic units, namely from old to young: Marl Unit of Kalibeng Foramtion (CNM20-CNPL6/Late Miocene to Early Pliocene), Calcareous Sandstone Unit of Sonde Formation (CNPL6-CNPL8/Late Pliocene to Pleistocene), and Calcareous claystone unit of Sonde Formation (CNPL9-CNPL10/CNPL11?/Pleistocene. The nanoplankton biostratigraphic zone are arranged into 11 zones consisting of 2 Partial Zones, 10 Interval Zones. Development of sedimentation rate (RoS) of this study, there are 11 periods. The periods are (1) CNM20 Zone/Late Miocene to Early Pliocene /1.4 cm/ky), (2) CNPL1 Zone /Early Pliocene (8.44 cm/ky), (3) CNPL2 Zone/Early Pliocene (9.7 cm/ky), (4) CNPL3 Zone/Late Miocene-Early Pliocene (90.91 cm/ky), (5) CNPL4 Zone / Late Miocene-Early Pliocene (4,26 cm/ky), (6) CNPL5 Zone/Middle-Late Pliocene (8.57 cm/ky), (7) CNPL6Zone /Late Pliocene (13,26 cm/ky), (8) CNPL7 Zone/ Late Pliocene to Early Pleistocene (14.95 cm/ky), (9) CNPL8 zone /Early Pleistocene (26.42 cm/ky), (10) CNPL9 Zone/Early Pleistocene (3.11 cm/ky) (11) CNPL10 Zone/Early-Middle Pleistocene (1.34 cm/ky). In this study there is a fast sedimentation process, namely in the CNPL3 Zone which has a sedimentation rate of (90.91cm/ky), very thick thickness sedimentasion (200 m) and with a short time duration of (0.22 my). There are 2 unconformities, namely Top of the CNPL2 Zone (Late Miocene) and Top of CNPL7 Zone (Early Pleistocene) [ABSTRACT FROM AUTHOR]