When Mitsui Oil Exploration Co., Ltd. (hereinafter called MOECO) evaluates hydrocarbon potential in Tenpoku Seihou Area which is located offshore north-west Hokkaido, Japan, one of our concerns is sand presence and its property. Main potential reservoirs in this area are expected to be Chikubetsu and Sankebetsu formations, which are observed in onshore well "MITI Rumoi" drilled in 1985. The challenges of formation evaluation derive from the fact that GR does not respond to shaliness clearly even though sandstone and mudstone were distinguished in cores and cuttings. This is due to complex mineral composition and volcanic influx in sandstone and mudstone. One of the approaches for volcanic sandstone reservoir evaluation is presented in this talk. Prior to log interpretation, environmental correction and depth matching were thoroughly performed. In order to understand the log response to lithology identified by mud log, principal component analysis (PCA) was carried out at first. Then, for better understanding of log response to lithology, a formation volumetric modelling with multi-minerals was performed by ELAN using wireline log, XRD and core data. The result showed that estimated porosity and permeability were comparable to core data. However, lithology was not well organized in mineralogical model due to the complex texture. Therefore, the formation volumetric model was updated using lithological concept by ELAN based on VSH from SP which showed the highest correspondence with lithology shown by PCA. For porosity estimation of the lithology model, the porosity from mineralogy model was used as initial model. The lithological volumetric model matched well with cuttings lithology, and it also matched with core porosity and permeability. This approach in MITI Rumoi will give us useful measure for volcanic reservoir evaluation in further exploration work in this area.


MOECO is planning to drill an exploration well offshore north-west Hokkaido, Japan, which is located in the area of Tenpoku Seihou 3D as shown in Figure 1. MITI Rumoi well is the best offset well to the further exploration well and it is helpful to understand the expected reservoir property for the target formations, which are Chikubetsu formation and Sankebetsu formation. These formations are commonly observed onshore outcrop in alternating beds of mudstone, siltstone, tuff and sandstone in Eocene to Miocene age.

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