Osame Senoh¹, Kazuhiko Tezuka², Kunio Akihisa³, Tadaaki Shimada⁴

(1) Japan Petroleum Exploration Co., Ltd, Tokyo, Japan
(2) Japan Petroleum Exploration Co., Ltd, Chiba, Japan
(3) Oil Research, Ltd, Tokyo, Japan
(4) Japan National Oil Corporation, Tokyo, Japan

ABSTRACT: Well Log Evaluation and Geological Model in the MITI Nankai Trough Well, offshore Japan

The MITI Nankai Trough Well was drilled to evaluate subsurface characteristics of methane hydrate, that was located at the depth of 945m offshore south-east Japan and composed of six holes. The core samples showed alternations of sand and silt, which were early Pleistocene marine deposits based on the micro fossil analysis.

Wireline and LWD log data obtained from the four wells were resistivity, sonic, density-neutron, magnetic resonance (CMR), formation image (FMI) and VSP. Methane hydrate was contained in the sand beds, where obviously high resistivity, high sonic velosity, and low CMR porosity was observed. Many homogeneous hydrate-bearing thin beds, whose thickness was 200cm at maximum, were recognized between 1100m and 1240m BMSL on the FMI logs. The total thickness of the hydrate-bearing beds was over 20m. Methane hydrate-dominant beds were recognized between 1135m and 1213m on the well logs and the most of beds can be correlated between two wells with 96m distances.

The Maximum hydrate saturation was approximately 75% which was evaluated by resistivity, density and CMR logs. Abnormally low sonic velosity and low sonic amplitude was observed near at 1210m and 1240m. This fact may imply the presence of free gas bearing beds, but low resistivity indicates low gas saturation of 10% at maximum.

The reflections related to the low velosity zones, which coincide with the gas bearing zones, were observed by VSPs near at 1210m and 1240m. These reflections can be correlated to BSRs on the seismic sections.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado