Velocity Variations within Tonson Fault Area, Arthit field – North Malay Basin

Abstract

Tonson fault trend is a major fault which is located in the northern part of Arthit field, North Malay Basin. Due to the large fault throw, the depth prediction is a challenge because of velocity variations within the fault blocks. Platform Alpha, which covers both upthrown and downthrown blocks of Tonson fault, results in a large error in depth prediction by using velocity modeling workflow, which uses only nearby well velocities and interpreted horizons. This study evaluated different velocity models for more accurate depth prediction and suggests appropriate workflow of velocity modelling for the area. The integration of well velocity, seismic velocity, structural control and pseudo velocity showed best statistical results for the depth prediction. This study recommends the use of pseudo velocity to improve the depth prediction in an area with high structural variation and sparsely located wells with velocity information. The velocity variation shows a conformable relationship with structure. Large vertical separation of Tonson fault causes significantly change in the velocity across upthrown and downthrown blocks. Velocities are low in the upthrown side and high in the downthrown side within the same stratigraphic interval. Moreover, the velocity trend also varies with the structural depth as high velocities were observed in deeper parts of the basin, whereas shallow parts have low velocities.