地質・探鉱部門シンポジウム 「熟成堆積盆における石油探鉱開発―新技術の適用―」
ノルウェー領北海Astero構造
―AI-Vp/Vsによる砂岩の抽出―
2008 年 73 巻 1 号 p. 47-56
詳細
2008 年 73 巻 1 号 p. 47-56
Hydrocarbon was discovered in the Astero structure located in the northern part of Norwegian North Sea. The reservoir, Oxfordian turbidite sandstone, is also reservoir of a nearby field and commonly indicates Class-IV AVO anomaly. The sandstones can be separated based on the type of fluids on the cross-plot of well-based AI vs. Vp/Vs. Since the structure is compartmentalized with different contacts, in-house seismic inversion was planned and carried out for investigating the hydrocarbons in place.
Seismic data used in this study was acquired in 1992. Offset records (Near-, Middle- and Far-offset) re-processed in 2005 were used in this study. Hampson Russell's software was used in the inversion and the model base inversion was adopted.
A low AI zone is generally expected to suggest possible hydrocarbon bearing reservoir. AI results in this study, however, do not have enough vertical resolution to separate sandstone with different fluids. Most of the low AI zones extend further deeper depth than the OWC confirmed by the wells. On the other hand, generally low Vp/Vs ratio is observed in the sandstone interval and does not show difference in different fluids.
A 3D cube multiplying AI and Vp/Vs was made to predict spatial variation of the reservoir quality. Based on the cross-plot of AI and Vp/Vs of the well data, hydrocarbon-bearing sandstone has lower AI and Lower Vp/Vs and could be emphasized in this product. As a result, sandstones are fairly well separated by this product from over- and under-lying shale. Sandstones with different fluids, however, are not distinguished. This is partly because that the thickness of the hydrocarbon bearing zone is as thin as a half dominant wavelength. There is a well which penetrated the equivalent Oxfordian sandstone 5.5km away from the discovery well. In this well, the Oxfordian sandstone becomes thin and is cemented by calcite. The product of AI and Vp/Vs seems to indicate changes in the reservoir quality as well as thickness between these two wells and shows potential being utilized in the lithostratigraphic interpretation and geo-model construction, etc..
