Modern Seismic Imaging To Address Complex Velocity And Its Role In Manora Field Development, Gulf Of Thailand
The Manora Field, which was discovered in 2009, sits on the western fault scarp of the Kra Basin. The Kra Basin is located in the northern part of the present day Gulf of Thailand and is separated from the Pattani Basin by the Ko Kra Ridge, approximately 200km south of Bangkok. The Manora Field structure is defined by two large north-south running faults that separate the field into two adjacent structures, frequently referred to as the Central Fault Block (CFB) and the Eastern Fault Block (EFB). The Manora-1 discovery well was drilled into the CFB based on 3D Pre-Stack Time Migration (PSTM) seismic data. Three subsequent appraisal wells drilled downdip of Manora-1 in the CFB as well as in the EFB, demonstrated sufficient oil reserves to warrant a development project. However, inconsistent velocities exhibited between the wells and mediocre seismic data quality posed a challenge for the forthcoming field development. At first impression, the Kra Basin appears like other extensional Tertiary rifted lacustrine basin commonly found in South East Asia. The synrift section predominantly exhibits lacustrine deposits where the Manora main pay found in a low net to gross turbiditic sand succession. A secondary pay is found in the overlying high net to gross postrift fluvial deposit. Even though a normal faulted half graben seems structurally uncomplicated, large-scale vertical fault displacement coupled with fault footwall uplift, steep formation dips and angular unconformities in the Kra Basin introduce a complex three-dimensional velocity pattern along the Manora boundary fault. Such a velocity pattern is problematic to resolve with conventional time processing, and therefore a Pre-Stack Depth Migration (PSDM) project was initiated in 2011. The seismic data was PSDM reprocessed with the main objective being to produce a high quality seismic dataset with an accompanying reliable velocity field. Four Manora exploration/appraisal wells, an older exploration well and seven key horizons were supplemented to build the velocity model. Anisotropy was incorporated as part of the velocity model building mainly in the thick late synrift shale directly overlying the main pay section. The 2011 PSDM results in considerably better quality seismic than the PSTM with clearer faults and less noise. This result formed the basis for the Manora initial field development. Given the improvement in the PSDM data, it was disappointing that the development wells still exhibited significant depth and fault positioning errors, such that volume estimation and well placement remained a key concern in the field. After struggling with these issues for a number of years, in 2016 it was decided to reprocess the seismic data again utilizing the latest advances in technology as well as the additional data acquired. 25 wells, seven horizons and several major faults were used as inputs to the velocity model and combined with the most advanced appropriate modern algorithms. Significant focus was placed on optimizing the velocity model (in particular the anisotropy) with wells and seismic data scanning. The PSDM reprocessing was completed in 2017 and recent drilling has shown that the new data provides significantly improved fault positioning and reduced depth uncertainty. The broadband nature of the data helps stratigraphic correlation, even though minor fault interpretation in the CFB remains difficult. Using examples from Manora Field, this paper demonstrates the modern processing flow and how it yields enhanced seismic data quality, providing an invaluable contribution to field development.
AAPG Datapages/Search and Discovery Article #90331©2018 AAPG Asia Pacific Region GTW, Back to the Future – The Past and Future of Oil and Gas Production in the Asia Pacific Region, Bangkok, Thailand, September 26-27, 2018