Source rock and petroleum system of the Paleogene-Neogene forearc basin in the Kakegawa-Sagara oil field, Shizuoka Prefecture, central Japan

Abstract

Oil fields occur in the Sagara district of Shizuoka Prefecture, despite the disadvantageous geological conditions of a fore-arc setting. We investigated the petroleum system of this fore-arc basin in southern Shizuoka Prefecture based on hydrocarbon compositions of oils, source rock potential, organic matter type, and burial history of the Paleogene to Neogene sediments. Argillaceous rock samples from the IVIITI-Omaezakioki well and from outcrops in the Kakegawa and Sagara districts were analyzed for geochemical properties by CHNS elemental analyzer, Rock-Eval pyrolysis, and GC/I4'IS. Various maturity parameters in the Sagara oils indicate differing maturity levels, ranging up to the condensate zone. The range in maturity parameters is wider in the northwest flank of the Megami anticlinorium than in the southwest flank. The upper layers of the Lower Miocene Towata Formation and the lower layers of the Lower Miocene Matsuba Formation have relatively high potential for hydrocarbon generation (up to TOC-0.9 wt'., S_, -2.8mg/g and HI - 350mg/gC). The highest-potential layer of the N'Iatsuba Formation was probably deposited in a highly productive marine environment during deposition of siliceous clastic sediments. No equivalent high¬potential layers were identified in the hIITI-Omaezakioki well, suggesting that the organic-rich layer formed only in the northern part of the basin. In the Eocene-Oligocene N'Iikura Group, layers relatively rich in TOC (up to 0.7 w•t°'a in over-matured black shale) were observed in the Kakegawa area and also in the N'IITI-Omaezakioki well. According to epimerization of sterane and hopane isomers, thermal gradient was higher in the northern part of the basin than in the south. In both areas the Kurami Group only reached medium-light oil zone, whereas the Mikura Group reached the condensate zone. The thermal structure of the basin thus changed significantly between the Paleogene and Neogene. On the basis of basin modeling simulation of the MITI-Sagara well, medium-light oils (Ro-0.7-1.0°0) were generated in the Nlikura Group at ca. 29 N'Ia and 15 Ma, and condensates (Ro- L2°-'o ) at ca. 2 Ma. The Kurami Group generated medium-light oils (Ro-0.7-1.0°0 ) at ca. 15 Ma and 2 Ma. Therefore, the ca. 2 Ma condensates from the hlikura Group migrated and were contaminated by oils from the Kurami Group and bitumens from the upper reservoir layers (e.g. Sagara Group) .