大水深におけるジュラ系沿岸〜浅海成砂岩ガス貯留層―インドネシア・アバディガス田の例―

Abstract

This study discusses a burial history and diagenesis of the Jurassic sandstone reservoir that was deposited in a coastal to shallow-water environment and buried deeply in the recent deep water area. The study example is the sandstone reservoir of the Plover Formation in the Abadi gas-field in the West Arafura Sea, Indonesia. The results of this study are summarized as follows :(1) The Plover Formation is of the Middle Jurassic (partly the lower-most Upper Jurassic), based mainly on Dinoflagellata chrono-biostratigraphy. The Plover Formation is subdivided into the upper unit and the lower one by the Bathonian muddy layer (MFS). The sandstone of the upper unit is the main reservoir in the Abadi gas-field.(2) The burial history of the Plover Formation has two remarkable, rapid deepening events in the Late Cretaceous (deposition of a thick, muddy deltaic succession) and the Pleistocene (deepening of the Timor Trough).(3) The diagenetic processes are represented by (a) shallow burial stage : siderite- or kaolinite-precipitation, (b) shallow-to-intermediate burial stage : replacement of detrital feldspars by kaolinite, (c) intermediate burial stage : quartz-overgrowths or Fe-calcite, (d) compaction associated with increase of overburden and cementation, and (e) hydrocarbon-charge to the gas-field and its related formation-fluid movement that controlled the diagenetic processes. The texture of the sandstone and the process of its compaction are affected by the precipitation and dissolution of these cements and by the condition of grain contact. The rapid subsidence is a probable cause of formation of microstylolites in the sandstone.(4) Three types of pores are observed in the sandstone reservoir ; such as intergranular pores, micro-pores, and dissolution-pores. Diversity of sedimentary and diagenetic facies controls the difference in relative abundance of the three types of pores in the sandstone. It is also a controlling factor for the diversity of diagenetic facies whether the sandstone is hydrocarbon-bearing or not.