Combination and stratigraphic traps may contribute with significant gas reserves in Bongkot Field, Gulf of Thailand. However, such stratigraphic play cannot be easily defined as conventional seismic interpretation provides mainly structural information. To identify stratigraphic prospects in this area, a seismic pre-stack inversion and reservoir characterization study was carried out. The input dataset consisted of 365 km2 of 3D seismic, six wells, and interpreted time horizons. Following seismic pre-conditioning and rock physics analysis, wavelet extraction and well-ties were performed for each individual well, considering every input angle stack. Constrained by input time horizons, low frequency models were built based on well log and seismic stacking velocity. Inversion parameters were tested; subsequently, final inversion results were subjected to Bayesian classification to obtain a litho-facies volume. In addition, multi-linear regression was used to derive elastic-petrophysical relationships, to generate petrophysical property volumes. The final results included inverted elastic properties, classified litho-facies, computed effective porosity and Vshale volumes. By analyzing these results, several channel and deltaic/sand lobe features could be observed throughout the study area. Connected sand-filled channels with high porosity were mainly observed in the shallow section, as sand distribution appeared sparser and more isolated with increasing depth. Also, the predicted reservoirs in the deeper section were mostly filled with gas, while shallower sand bodies were mostly filled with brine. This observation implied that the high net-to-gross reservoir distribution in the shallow section can be a key factor that hinders effective trapping of hydrocarbons at this level. Since reservoir distribution plays a key role in hydrocarbon trapping mechanism, upside stratigraphic potential was identified from isolated gas-filled channels, mapped from seismic inversion products, to implement a more successful field development strategy in a mature field.
In seismic exploration, checkshot survey is an important method to obtain accurate time-depth profile for depth-to-time or time-to-depth conversion. However, the standard geometry correction for a rig-source survey in a high-inclination well will not provide a reliable time-depth result due to ray-bending effect. The objective of this study was to enhance the accuracy of the time-depth velocity by utilizing an advanced technique called model-based correction or pseudo walk-above simulation. To obtain model-based vertical times, a flat-layered velocity model was built by using the checkshot velocities as initial guess. The model was then inverted to match the observed checkshot travel times, which were the actual measurement of travel path from source to receivers. The model was iterated to minimize the residual between the observed and modeled travel times in a least-square sense. A pseudo walk-above checkshot simulation was run on the inverted model by positioning the sources exactly on top of the receivers to get the vertical times, which were used for the final time-depth relationship and further image processing. The residual times of less than 1 millisecond (ms) were observed between the actual measured transit times and modeled travel times from a fixed source to the receivers in the inverted model. This demonstrated the inverted model was realiable to use for obtaining more accurate vertical time-depth through pseudo walk-above checkshot simulation. This optimal inverted model was considered as the best estimation of the true earth model in this case. A comparison of modeled vertical times estimated through the pseudo walk-above simulation and calculated vertical times using standard geometry correction were done. The difference between the two scenarios was 6 ms one-way time (OWT) demonstrated the reduction of 6 ms uncertainty of using the advanced model-based correction versus the standard geometry correction. In short, the advanced technique delivered more reliable time-depth velocity information to reduce depth uncertainties for drilling operation. The walk-above or vertical incidence checkshot survey, which required boat and navigation system, was unable to acquire in a highly deviated well due to very bad weather conditions. The rig-source survey was carried out instead of employing the walk-above survey in this project. The standard geometry correction using simple trigonometry was not able to provide the correct vertical times. The advanced model-based correction was the optimal solution to improve the accuracy of checkshot time-depth velocity data.
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