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Prospectivity Of Reservoir Potential On The Middle Miocene Ngrayong Formation In Grobogan Area: Implications For Petroleum System


Ngrayong Fm is one of rock formation in Grobogan regency Central Java which is deposited above Bulu Formation as older formation in North Rembang Anticlinorium Zone (Pringgoprawiro, 1983). Stratigraphically, Ngrayong Fm is a part of East Java Basin as one of best reservoir as arenite quartz sandstone which deposited during middle Miocene. In order to define Ngrayong Fm reservoir potency, field geological mapping could be one of best method continued by laboratory analyses as thin section description. East Java Basin is one of the prolific oil basin with complete petroleum system elements in Indonesia. In this research, regionally geological mapping consist of 5 (five) formation which is older to younger formation consist of Tawun Fm (clastic limestone-source rock), Ngrayong (sandstone-reservoir rock), Bulu, Wonocolo (shale-seal rock) and Mundu. Based on micropaleontology analysis and reconstruction of a geological cross section, this formation thickness approximately 183 meters with age prediction was in Middle Miocene (N9-N13) based on planktonic foraminifera presence (Bolli & Saunders classification, 1985). This unit deposited on shallow shelf environement or upper neritic (bathimetric zone), based on the presence of Bentonic foraminifera such as Paratrochamina, Haplopharagmmoides pussilus and Articulata carinata (Tipsword, 1956). On the other hand, thin section gave information of porosity and mineral occurences in Ngrayong Fm. Average porosity value of Ngrayong Sandstone is 15.08% (good) and average permeability value is 203,8 mD (very good). It is caused by quartz dominated in Ngrayong Fm without highly cemented (clean sandstone) during diagenesis processes. Based on the analysis of porosity and permeability sandstone samples taken on this shallow marine facies then it can be inferred that the formation of Ngrayong is a good reservoir for this area.