Fit-for-Purpose Depth Imaging of Sub-Thrust and Strike-Slip Fault Regime Onshore Myanmar
The onshore structures of Central Myanmar basin are a result of regional tectonics which created steeply dipping thrusts and strike-slip faults in the area under study. In order to map potential structural prospects, a 3D seismic data was acquired in 2015 using split-spread orthogonal shooting geometry. The survey area is covered largely by paddy fields, small towns and a reserved forest which indirectly limited the options for seismic source. Accessibility was one of the main issues apart from cultural noise during the seismic acquisition phase. Inaccessible reserved forest area resulted in missing near offsets and complex near surface conditions resulted in shot gathers with poor S/N ratio. This dataset was previously processed in 2016 using 3D Anisotropic Pre-stack Time Migration (APSTM). The APSTM processing was pushed to the limits, but it could not image the complex structures accurately due to presence of strong lateral velocity variations observed during APSTM velocity analysis phase. To overcome the lateral velocity variations in the sub-thrust region and position the fault planes accurately, Anisotropic Pre-stack Depth Migration (APSDM) was deemed necessary. Keeping these limitations in mind, a technical decision was made to reprocess the seismic data through APSDM within 3 months to meet project planning deadlines. Using the experience from the APSTM processing, the dataset was reprocessed again from field tapes with an objective to enhance the S/N ratio. A 5D interpolation algorithm was used to interpolate traces in low fold areas, especially in the region underneath the reserved forest. The 5D interpolation uses five independent parameters (X, Y, Time, Offset and Azimuth) to preserve the amplitude integrity of the interpolated traces while filling in large spatial gaps. This generated a uniform high fold common-offset-vector (COV) input to migration and resulted in reduction of migration noise during Kirchhoff APSDM. Diving Wave Tomography using inversion of first break picks was the first model building step and helped to resolve near surface lateral velocity variations. Following this, reflection tomography, using non-parametric residual move-out picks, was used to update a TTI anisotropic velocity field and model the strong structural velocity variations seen in this region. The reprocessed pre-migration gathers, after 5D interpolation, were migrated using this velocity model through Kirchhoff and Beam APSDM algorithms. A robust pre-processing sequence, accurate velocity model which honours the geological structures and the migration algorithms used have resulted in an APSDM volume which shows significant improvement when compared to the 2016 APSTM volume. Both Kirchhoff and Beam APSDM volumes were able to image the sub-thrust zone and confirm the presence of strike-slip faults in areas which were challenging to interpret using the APSTM volume. Beam APSDM produced cleaner images with less migration noise and higher S/N ratio compared to Kirchhoff APSDM which helped in confident structural interpretation. In conclusion, to further explore oil and gas opportunities in complex geological settings like the ones present in Central Myanmar basins, a 3D seismic data is required. This dataset needs to be optimally processed and imaged using a structurally correct velocity model and TTI-APSDM. Thus, a fit-for-purpose depth imaging solution can handle complexities caused by thrusting and strike-slip faulting onshore Myanmar, and result in a high quality seismic image within a very short turnaround time.
AAPG Datapages/Search and Discovery Article #90336 ©2018 AAPG Asia Pacific Region, The 4th AAPG/EAGE/MGS Myanmar Oil and Gas Conference, Myanmar: A Global Oil and Gas Hotspot: Unleashing the Petroleum Systems Potential, Yangon, Myanmar, November 13-15, 2018