Abstract

A 3D seismic survey was conducted in Ogata area, one of Japanese mature oil fields. The survey was aimed to obtain a basic data set, which is useful for studying the effects of irregular data acquisition geometry, and to provide the precise information on the subsurface structure for finding additional reserves. The survey area was previously the second largest lake in Japan and has been changed to rice fields after reclamation. Few difficulties in seismic data acquisition were expected and dense regular data acquisition geometry was planned to use in the survey. However, acquisition work encountered many hazards because a lot of unexpected constraints came from the reclaimed land. Moreover, obtained shot records showed the very complex nature of seismic wave fields. Especially, strong ringing waves masked weak reflection signals. Spatial variation of these ringing waves was investigated and the origin was modeled as critical refraction of multiple reflected waves within a very thin surface layer. Those results can provide valuable knowledge for seismic survey in areas with similar surface conditions. In seismic data processing, these ringing waves were successfully suppressed at the earlier data processing stage by FK filter in common shot and receiver domains followed by deconvolution. After this suppression, remaining data processing became more effective and gave a precise depth image. Various analyses to the obtained processing vintage such as coherency cube and attributes revealed the existence of minor faults precisely, which were not recognized previously. These could give a new interpretation of the area. As a result, the data set can be very useful for the evaluation of irregular data acquisition geometry as well as the exploration in the area.

Introduction

Recent evolution of seismic data acquisition systems and cost reduction of seismic surveys have made 3D seismic explorations practicable even in culturally congested difficult areas. In 1996, Japan National Oil Corporation (JNOC) conducted a 3D seismic survey in one of such difficult areas in Japan. The survey was successful and much useful information was obtained. However, at the same time, various problems had been clarified (Minegishi et al., 1997).

In the culturally congested difficult areas, various constraints exist and make ideal seismic data acquisition impossible, which results in insufficient or irregular sampling of complicated seismic wave fields. This causes various artifacts at data processing and introduces data acquisition footprints into final seismic images. To develop elaborated techniques to solve the problem, a data set acquired by dense uniform data acquisition geometry is required for synthesizing and analyzing various irregular geometries at an effective cost.

The acquisition area of such a data set was selected in one of old Japanese oil fields, Ogata area (Figure 1), which was previously the second largest lake and has been changed to rice fields after reclamation. Fewer surface obstacles against a 3D seismic survey, plenty of available well information and complex geological structure, characterize the area. Therefore, it was expected that shot and receiver points could be laid out densely and the obtained data set would be appropriate for the acquisition geopmetry study. This survey was also intended to rejuvenate this declined oil field.

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