FUTURE EXPLORATION PLAY CONCEPT IN WESTERN KENDENG FOLD THRUST BELT: BASED ON COMPREHENSIVE STRATIGRAPHIC AND GEOCHEMICAL ANALYSES OF OUTCROPPED MIOCENE KEREK AND PELANG FORMATION AND OIL SEEPS

摘要

A comprehensive stratigraphic and geochemicalrnanalysis of Cenozoic sequence outcrops and oil seepsrnwere conducted in western Kendeng Zone fold thrustrnbelt and foreland area in order to determine thernpotential for future exploration play concept.rnAnalysis consisted of stratigraphic analysis ofrnoutcropped Miocene Kerek and Pelang Formationsrnwhich comprise of detail-measured section,rnbiostratigraphy, thin section, and depositionalrnenvironment determination. In addition,rngeochemical analysis of oil seeps and rock fromrnMiocene Kerek and Pelang Formation were alsornconducted in order to establish oil-source rockrncorrelation, which include total organic carbonrn(TOC), Rock-Eval pyrolysis, and vitriniternreflectance analyses for rock samples and biomarkerrnanalysis for oil seeps.rnApproximately 1000m Miocene Kerek and PelangrnFormation were deposited in the study area betweenrnN2-N20 (Early Miocene-Pliocene). Detail measuredrnsections and biostratigraphic analysis revealed thatrnMiocene Kerek and Pelang Formation wererndeposited in deep marine environment (lower fanrnsub-environment), which dominated by thickrnmudstone (approximately 20m-50m) and thin coarsernto fine-grained sandstone (approximately 3m-7m).rnHowever, either proximal or medial fan subenvironmentrnwith thick sequence of good qualityrnsandstone sealed by mudstone was not present in thernMiocene Kerek and Pelang Formation. Furthermore,rnthin section analysis from the sandstone intervals ofrnMiocene Kerek Formation show low porosityrn(<10%) and low permeability. These findings showrnthat the potential of Miocene Kerek Formation to berna good reservoir is rather low.rnGeochemical analysis of Miocene Kerek and PelangrnFormation shows that both formations have lowrnpotential to serve as source rock. Several samplesrntaken from both formations show low organicrncontent (TOC: 0.27-0.75 wt%) and low maturity (Ro:rn0.21-0.31% and Tmax: 424-427). Biomarkerrnanalysis revealed that the oil seeps found in the arearnappear to originate from terrestrial source rock thatrnwas deposited under oxic-dysoxic condition asrnshown by pristine/phytane, pristaneC17,rnphytaneC18 and the other biomarker ratios.rnFrom this research, it can be concluded that MiocenernKerek and Pelang Formation have a potential to servernas a great seal rock, instead of potential source orrnreservoir rocks as previously suggested by manyrnresearchers. Furthermore, terrestrial biomarkerrnsignature of the oil seeps revealed that the activernsource rock is most likely syn-rift terrestrial depositsrnthat still buried at the subsurface. These twornimportant facts, integrated with the regionalrnknowledge, lead to the idea of the future petroleumrnplay concept of Kendeng Basin. Faulted anticline andrninverted rift structure with Paleogene syn-riftrnterrestrial source rock and Miocene Kerek/Pelangrnseal as well as possible post rift shallow marinernsandstone reservoir can be played in Kendeng Zonernfor future hydrocarbon exploration play concept.