T he times and technology comparable to the increasing need for petroleum . Challenges faced by the oil companies is to increase exploration , exploitation and development of oil fields , in order to generate maximum production along with the increasing demand for oil and gas as an energy source Geological and geophysical studies are always done in order to improve productivity in the oil fields in Indonesia . One of the studies is often done by evaluating the formation of which is a way to get information on rock formations that were analyzed . Such information may include information lithological and petrophysical properties in a formation . Petrophysical properties obtained after petrophysical analysis to determine the physical parameters of rock , such as shale content , porosity , permeability and water saturation in a formation . Reservoir property that has been obtained is then mapped their distribution patterns , so we get an idea of the lateral spread of these properties , to volumetric hydrocarbons can then be calculated for each reservoir . Location of the study lies in the Field " A " which is part of the Tertiary Sedimentary Basin of the North East Java that is focused on the reservoir layer " X " , Tuban Formation . Field " A " located on the North East Java Cekugan . This basin is influenced by a complex structure with several times the deformation . Depositional basin in the early Tertiary controlled by extensional force horst graben formation in pre tertiary . Then in the mid- Miocene formation of further structural causes interaction of three plates , namely : Australian Plate , Pacific Plate and the Sunda - Eurasian Plate . In this research, petrophysical analysis in the form of an abundance of clay ( Vsh ) , the value of water saturation ( Sw ) , and Porosity . Logs used are logs Gamma Ray ( GR ) , True Resistivity ( Rt ) , Density ( RHOB ) , and Neutron ( NPHI ) . In this research also total gas readings to see the content of the gas contained in the reservoir . Then proceed dengna map-making structure depth ( Depth Structure) , Map Layer Thickness ( isopach ) , and map hydrocarbon Net ( Net Pay ) of the reading log . Reservoar " X " is a layer that has good hydrocarbon . This layer is a carbonate rock is limestone where the limestone has good porosity . In the field " A " calculation of petrophysical and total gas ( lab analysis ) , where the reservoir ' X ' is known that the hydrocarbons contained is a gas . The results of petrophysical calculations are performed is the value of water saturation ( Sw ) from 0.12 to 0.34 , or 12 % -34 % ( gas ) with a porosity of 0.08 to 0.39 , or 8 % - 39 % ( poor to excellent ) . The spread of hydrocarbon zones in the study area is West - East later identified research areas are large reserves of 65.0 BSCF.