Seamless processing new and legacy data for high-quality imaging in shallow water, offshore Myanmar
The project field is on the western edge of the Tanintharyi shelf, offshore Myanmar. The field area has discovered gas-condensate accumulation. The field structure was identified in 1992 and a discovery well was drilled based on seismic data indicating a flat spot in Lower Miocene deltaic clastic beds, interpreted as tidal-flat and estuarine sandstones, which onlap onto basement towards the east. These gas-bearing Lower Miocene sandstones exhibited a marked seismic amplitude response that terminated at the mapped flat spot and represents the gas-water contact. In this field, a few 2D lines were acquired between 1974 and 1994 and, in 1994, the first 3D seismic survey comprised of 328 km2 was acquired. The 3D legacy data are generally good; however, the data have limited bandwidth and low resolution, which hindered properly imaging the existing producing reservoir. In 2016, two adjacent 3D surface seismic surveys were acquired and processed through both prestack time migration (PSTM) and prestack depth migration (PSDM). The objectives of these processing workflows were to improve the seismic interpretability, to allow mapping fracture intensity in the basement, and to solve imaging issues to permit further analysis and facilitate future drilling plans. Due to a surface obstruction in this field area, the 2016 3D seismic survey was limited in its ability to acquire new data below the existing production platform. To ensure continuous imaging of this area, the 1994 3D seismic survey was processed together with and merged into the 2016 3D seismic survey. The processing was done from original field data tapes, which allowed us to apply the latest processing technologies. Challenges were encountered in survey merge and velocity model building, due to non-broadband, shorter offset acquisition of legacy data and broadband, longer offset acquisition of recent dataset. However, in pre-processing survey matching derived seamless amplitude, phase and frequency matching, to provide a homogeneous volume and enable the derivation of a continuous, geologically plausible velocity model. Other challenges in this project were overburden gas anomalies, a Lower Miocene reservoir that exhibits pressure depletion trends, and unclear basement imaging. To address these challenges, a velocity model building strategy was designed as part of a PSDM workflow. There were three main model building flows in this project; these were near-surface modelling, high-resolution 3D common image point (CIP) tomography, and basement high-fidelity beam velocity scanning. The near-surface modelling used diving wave tomography to resolve the gas anomalies. Six repetitions of CIP tomography were performed, iteratively reducing the model updates’ scale length and increasing the model updating depth from the layers near the water bottom to the basement. In basement scanning, different velocity percentages and azimuths were scanned and picked to enhance fault and fracture detection below basement. Final PSTM and PSDM images showed a significant improvement compared to legacy images. Both Kirchhoff depth migration and beam migration produced high resolution, cleaner, sharper imaging, with enhanced fault positioning, and simpler and more geologically plausible structure than did PSTM.
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