東インドネシア・イリアンジャヤのBintuni堆積盆の石油地質

―巨大ガス田の探鉱・評価の実例―

抄録

Bintuni Basin is a fore-deep basin located in Irian Jaya, Indonesia. A third party has certified a 14.4 TCF of proven gas for the Wiriagar, Berau and Muturi PSCs in Irian Jaya. This paper describes gas-bearing reservoirs including Jurassic and Paleocene. The Palaeocene sandstones had not been recognized as a reservoir before the Wiriagar Deep #1 was drilled. In addition to the description of these reservoirs, the reasons why giant gas fields remained undiscovered to date are discussed. Top Jurassic horizon was picked up and mapped based on an assumption of a constant thickness between base of Cretaceous and top of Palaeocene. In the swampy onshore Wiriagar PSC, a so-called "deep reflector" was picked as intra-Eocene. There was no tie line throughout the transitional area between onshore Wiriagar and offshore Berau PSCs. Palaeocene was found to be thick beyond operator's prognoses because turbidite lobes are developed in and around the location of Wiriagar Deep #1. As a result, a spill-point between Wiriagar Deep structure and nearby structures was deepened, and P structure was interpreted to be connected with Wiriagar Deep structure. On the other hand, high velocity due to the presence of thick carbonates in the western flank of the Vorwata structure was encountered. The saddle of the depth map between Vorwata and Wiriagar structures was deeper than that of the time map. Depth conversion of "pre-carbonates" is as difficult as that of "pre-salt". Although the Vorwata and Wiriagar fields have proven to be giants, these giant fields have had lain undiscovered due to the above-mentioned underestimations.