Publications

Lateral and vertical velocity variations are among the key concerns for time to depth conversion especially in carbonate regimes. A suitable velocity model should account for issues specific to each area.The Ketapang area is well known of its geophysical and geological complexities. Targeted and proven reservoirs are believed to be a type of platform carbonate. This platform is cut by channels filled with shale throughout the whole Ketapang block, causing pull-down effects from the shallow Wonocolo horizon down to basement level. Poor seismic data quality and the unavailability of stacking velocities have added more challenges to this study.Three methods were identified to produce a reliable velocity model meant for time to depth conversion.The three methods are, average velocity model, 3D velocity model and conventional layer cake model.The first model is an application of well average velocity with main focus on the targeted reservoirs.The 3D method utilized a 3D grid as a platform to incorporate all two-way-time (TWT) surfaces, well and dip-moveout DMO velocities. A statistical reduction method was applied to populate the well (primary trend) and DMO velocities (secondary trend) into a single 3D model. Then, an anisotropy function ({well velocities/DMO velocities} X DMO velocities) was generated to integrate the anisotropy factor into the model.The third model is a conventional method based on observed velocity changes in sonic data vertically. Whilst, TWT surfaces were used to control lateral variations. Later, both well velocities and TWT surfaces were incorporated utilizing the Vo-K method to generate the model.Based on the statistical report of residual errors, the third model result has the closest fit to well control.