Rock Porosity Based on Trace Fossils for Interpretation of Reservoir in the Rembang Zone Choiriah, S.U 1 , Achmad Subandrio 1 , Yody Rizkianto 1 , Intan Paramitahaty 1 , Nanda Ajeng Nurwantari 1 , M.Ardiyan Syah Darmawa 1 , Hendry Wirandoko 1 1 Department of Geology Engineering, UPN “Veteran” Yogyakarta, Indonesia 1 E-mail address: umiyatunch@upnyk.ac.id Abstract The research area is located in Selorejo Formation, on the Kali Gadu Section, Sambong District, Blora Regency, Central Java Province, and is physiographically located in the Rembang Zone, North East Java Basin. In the Rembang Zone there are thick sedimentary rocks of Cenozoic age, containing various types of fossils of Eocene to Pleistocene-aged. The Selorejo Formation is dominated by limestone which is a gas and oil reservoir rock in the North East Java Basin. The Selorejo Formation is deposited in a shallow marine environment, so many foraminifera fossils and abundant fossil traces are found. This study aims to determine the variations of fossil traces are found in each layer of limestone of the Selorejo Formation and the level of rock porosity in the petroleum system. This research was conducted by collecting data in the field using a field mapping method measuring, measuring stratigraphic measurements with a radius of 40 meters and thickness of 25 meters and profiling outcrops, rock sampling of 10 trace of fossil analysis and rock porosity analysis using porimeter and etching. In fossil trace analysis, the method used is the method of spot sampling and gridding of rock layers containing trace fossils, with a grid area of 20x20cm2, followed by calculating the total density of fossil traces contained in the sample grid. Twenty (20) fossil trace samples were taken. The results of the analysis of fossil traces found include Planolites, Helminthopsis, Thalassinoides, Conichnus, Chondrites, Macaronichnus, Bergauria, Ophiomorpha, Skolithos, Terebellina Asterosoma and Palaeophycus. The results of rock porosity analysis show that rock layers containing trace fossils have a smaller porosity than rocks that do not contain trace fossils. The more trace fossils in the rock layer indicates that the porosity is worst Keywords: trace fossils, porosity, Selorejo Formation I. INTRODUCTION The research area is located in the Selorejo Formation, Gadu Village, Sambong District, Blora Regency, Central Java Province. This formation is physiographically located in the Rembang Zone, North East Java Basin. In this Rembang Zone, there are sedimentary rocks that are thickly carbonated and aged from Eocene to Pleistocene. The Selorejo Formation itself is one of the formations dominated by limestone which is a gas and oil reservoir rock in the North East Java Basin. The Selorejo Formation also contains abundant foraminifera and trace fossils. Research on trace fossils in the Selorejo Formation still needs to be carried out in more detail and comprehensively, considering that the abundant presence of trace fossils on the limestones will affect the level of rock porosity in the petroleum system. This study aims to determine the existence of a diversity of fossil traces and how the porosity value in rocks containing trace fossils and in rocks that do not contain trace fossils in the same rock layer. II. LITERATURE REVIEW The research area based on the physiographic map of East Java according to Prasetyadi (2007) (Figure 2.1), is included in the Rembang Anticlinorium or Rembang Zone, which is an undulating plain area with hills interspersed with alluvial plains. These hills extend to the West - East, from Purwodadi City to Blora, Jatirogo, Tuban to Madura Island. The average width of this zone is 50 km with the highest peaks 515 m (Gading) and 491 (Tunganngan). The morphology in this area is divided into 3 units, namely the lowland morphological unit, undulating hills morphological unit and steep hills morphological unit, with the