Petroleum source-rock potential of the Cretaceous marine argillaceous rocks in the offshore Joban fore-arc Basin, northeast Japan

Abstract

Gas and condensate have been discovered in the offshore Joban fore-arc Basin. They derived from coal and coaly mudstone in the Upper Cretaceous to the Miocene sediments. Although thick Cretaceous marine argillaceous rocks deposited in the basin, especially in the east of the Abukuma Ridge, they have not been evaluated as an effective source rock. Therefore, organic geochemical characteristics and petroleum generation potential were studied for the Cretaceous argillaceous rocks in the basin. The argillaceous rock samples from the MITI Joban-oki, MITI Kesennuma-oki, and Kita-Ibaraki-oki TJ-1 wells and from outcrops of the Futaba and the Nakaminato Groups were submitted to total organic carbon, total nitrogen and total sulfur analyses, Rock-Eval pyrolysis, and bitumen extraction. Saturate hydrocarbons fractionated from the bitumens were further analyzed by GC-MS. Only a small number of samples from the Kita-Ibaraki-oki TJ-1 and the MITI Joban-oki wells show good petroleum generation potential (S2≥2.5 mgHC/gRock). These samples also have high Hydrogen Index (≥250 mgHC/gCo). Two biomarker ratios, (C27+C29+C31)/(C24∼C34) of n-alkanes and C27/(C27+C29) of steranes, indicate an evident negative correlation, suggesting source organic input. Two other ratios, pristane/phytane and C35/(C31∼C35) of homohopanes, also indicate a negative correlation, suggesting oxic/anoxic depositional environment. All these ratios correlate to petroleum generation potential, and good potential is accompanied by low ratios in (C27+C29+C31)/(C24∼C34) of (C-alkanes and pristane/phytane, and high ratios in C27/(C27+C29) of steranes and C35/(C31∼C35) of homohopanes. These results suggest that the Cretaceous argillaceous rocks in the offshore Joban Basin can have good petroleum generation potential in case of abundant marine organic matter deposited under anoxic environment. For petroleum exploration in the east of the Abukuma Ridge, it is important to predict the regional distribution of Cretaceous argillaceous source rocks that are rich in marine organic matter and deposited under anoxic environment.