The development of tectonics and depositional basins in southern Java is closely related to the collision between the Southeast Asian continental plate and the Indian-Australian plate since the Late Cretaceous-Early Tertiary, which has produced diverse geological resources. One of these resources is oil and natural gas which have accumulated in a sedimentary basin, one of which is the Banyumas basin which is an intermontane basin in a rifting valley (Geological Agency, 2009).

The source rocks considered to have the most potential in the southern part of Central Java are the Nanggulan Formation (Mid-Late Eocene), Kalipucang, Pemali, and Upper Halang Formations which are Early-Middle Miocene in age. The carbon shale of the lower Nanggulan Formation has a TOC value of 0.28-3.68 percent, T-Max 402 - 520 deg Celcius, and HI 1 - 347, as well as kerogen types II-III with immature - early mature maturity levels and tend to produce gas. In bituminous shale, carbonaceous claystone, and lignite, from the lower Pre-Halang and Kalipucang Formations have TOC values of 0.7 - 15 percent, T-Max 410-440 deg Celcius, and HI 38 - 24 (Lemigas-Pertamina, 2001), showing the level of maturity from early mature to mature (Muchsin et al., 2002 in Armandita et al., 2009). The calcareous claystone of the lower Pemali Formation contains type II-III kerogen with a TOC value of 0.5-1 percent, T-Max 411-443 deg Celcius, and HI 46 - 167. In general, the source rock of carbonaceous shale is Middle Miocene, has an immature - early mature maturity level (T. Max 414 - 438 deg Celcius and HI 102 - 200) (Santoso, et al., 2007). The rocks that are thought to act as reservoirs are intercalated clastic sediments of medium-coarse grain size from the Eocene-old Nanggulan and Karanggulung Formations, but may be too deep to be reached by drilling wells. The targeted reservoir is likely to be beneath an Oligocene volcanic deposit which can generally act as an economic bedrock. In the Paleogene tuff layer, it has high porosity (up to 40 percent), as found in the upper Oligocene interval in the Alveoliva-1 well. Volcanic sandstone layers from various Neogene formations and several limestone horizons can also have good reservoir potential. On the onshore seismic section no carbonate reefs were identified, but on the offshore seismic section identified carbonate reefs from the Wonosari Formation which has a porosity of up to 20 percent. Reservoir potential can also be found in reef and chalky limestone from the Early-Middle Miocene Kalipucang Formation with porosity reaching 13.5 - 18.5 percent which comes from the presence of vuggy, interparticle, and mouldic (Santoso et al., 2007). The fine-sized sedimentary rocks from the Penanjung and Pemali Formations can act as cap rocks for the reservoir of the Gabon Formation, besides that the limestone shales and marl at the top of the Halang Formation can also serve as cap rock for the Kalipucang limestone reservoir and sandstones at the bottom of the Halang Formation. Shale and claystone inserts or interbedded in the pre-Halang, Halang, and Pemali Formations, can act as intraformational cap rock. In the Kulon Progo area, besides as a source rock, the shale of the Nanggulan Formation can also be an intraformational cap rock for the formation's sandstone reservoir (Pertamina/BEICIP, 1985; Santoso et al., 2007). The most common hydrocarbon traps are drape anticlines that are influenced by volcanic height, as well as structures associated with fault faults (Pertamina-BEICIP, 1985), but they can also be closed anticlines such as those formed in the Cipari-Gunung Wetan area and continue to south (Mulhadiono, 1973). The most potential offshore traps are carbonate reefs such as those found in the Alveolina-1 well, besides that the presence of Late Miocene - Pliocene sediment draping on the reefs as well as the height of the Paleogene bedrock and volcanic will increase the potential of these traps. Trap conditions are still difficult to interpreted, because the quality of the existing seismic data is not good. Other potential traps include the Plio-Pleistocene anticline trending southwest-northeast, as well as stratigraphic traps associated with the growth of Miocene reef limestone and turbidite reservoirs (Santoso, et al., 2007). The formation of traps related to diapirism is likely to occur in the Banyumas area, besides that the Middle Miocene volcaniclastic lowstand play from the Rambatan Formation makes it possible to form traps. The phenomenon of onlap or wedges in the fault plane from the Paleogene height which limits the Neogene basin can also form traps (Pertamina-BEICIP, 1985) which may be related to the results of erosion at the top of the fault block, fault blocks that are clipped, drape over the fault block, and pinch out which is inverted by block faulting (Armandita, et al., 2009 and Santoso, et al. 2007). Hydrocarbon charging formed can occur vertically from Paleogene and Neogene source rocks deposited in rifts in the Majalengka-Banyumas lowland, while laterally it can originate from adjacent depressions to the east and west of the Majalengka-Banyumas high structure (Armandita et al, 2009). Oil seeps were found in Parungkamal Village, Cilacap Regency. Seepage is found in rock fractures in breccia sandstones on the banks of the Losari River so that the seepage is mixed with river water flow, with the characteristic of dark colored oil with low viscosity. Based on joint analysis, it can be seen that the geological structure formed is Normal Left Slip (Rickard, 1972). Another oil seep is found in Cipari Village, Sidareja District, found in an outcrop alternating sandstones and claystones with calcarenite limestone inserts on the banks of the Cidandur River. The characteristics of oil seepage are dark brown in color with low viscosity. In addition, oil seeps were also found in Penusupan Village, Cilacap Regency, in fine sandstone outcrops on the banks of the river, so that the oil found was mixed with water. The characteristics of oil seepage are dark brown in color with a high viscosity level.