Abstract

A 4-component (4C), 3D, Ocean-Bottom-Cable (OBC) seismic survey was carried out at Attaka oil field, Kutai Basin, East Kalimantan. The objective was to improve seismic imaging beneath a gas chimney. Borehole geophysical data was acquired to determine the shear wave velocity structure in the area. This included a 3D VSP acquisition which was done simultaneously with the 3D OBC acquisition, a comprehensive walkaway VSP acquisition, and a dipole sonic log. This borehole data allows the 4C seismic to be tied to the well control, and provides needed input for 4C data processing algorithms, including common conversion point (CCP) mapping, CCP moveout corrections, and migration.

The walkaways also allowed derivation of Qp and Qs information, which indicated a very low Qp (~19) in the near surface. This low Qp is thought to be responsible for the poor quality compressional seismic data.

Findings are:

1) Extremely low Qp values associated with a known gas chimney were quantified. These are believed to be responsible for the deterioration of conventional P wave data;

2) low Vs values were recognized in the near surface. These impact depth conversion, statics computation, and moveout correction steps in 4C OBC data processing;

3) communications links between VSP and OBC recording equipment allowed a 3D VSP to be effectively acquired simultaneously with a 4C 3D OBC survey.