Partners of the Kashagan field were faced with the option of acquiring and processing a new seismic survey or using more advanced modern techniques to reprocess the existing data, which was an OBC dataset acquired in 2001/2002. The decision was made to proceed with reprocessing as acquiring new data would have added another two years to the project timeline. The objective of the reprocessing was to deliver a step change in seismic data quality to conduct further field development activities and plan production wells.

• First Break October 2022 - 3D demultiple techniques dramatically improve the imaging of a giant Kashagan reservoir in the ultra-shallow North Caspian Sea

Shallow Water and Multiple Challenges

The Kashagan oil field is located in the Southern Pre-Caspian Basin (4-4.5 km depth), comprising a Paleozoic carbonate platform. The reservoir is overlain by mobilized, diapiric Triassic evaporites and a mixture of clastic and carbonate layers (Jurassic to Cretaceous). Large faults have significant displacement from the evaporites to the near surface. The basin is draped by recent thin sedimentation and is capped by approximately 3-10 m of water.

The geophysical challenges included significant shallow-water multiples, inter-bed multiples, and strong lateral and vertical velocity contrasts. In addition, the available field-wide 3D survey (OBC data acquired in 2001-2002) was relatively sparse by modern seismic standards and lacked long offsets and true full azimuthal coverage.

Legacy Versus SWIM 

The figure below shows a comparison of the legacy PSTM volume with the SWIM reflectivity cube. SWIM has reconstructed near-angle information needed to image both the water bottom and the Cretaceous carbonate layer. The highly reflective near-surface geology is the source of many short-period multiples which have historically been poorly represented, although they affect the entire seismic section in the form of high-order reverberations and peg-leg multiples.

The example also illustrates spatial variability in this reflectivity series which emphasizes the need for accurate information in order to minimize the risks associated with adaptive subtraction for multiple suppression.

As shown in the figure above, the SWIM cube allows for the use of a shallow reflectivity cube for demultiple. This would have been extremely difficult if not impossible with the legacy near surface image. 

Modern Solutions for Complex Problems 

Modern reprocessing techniques such as SWIM, 3D SRME, 2D IME and wavefield extrapolation demultiple helped to provide significant improvements in signal-to-noise and image quality, largely due to advanced multiple attenuation. In addition to the improvements in the PSTM processing, modern techniques such as FWI and least squares Kirchhoff and reverse time migrations provided even greater enhancements to the image in the PSDM processing flow. These new seismic volumes have led to improved seismic interpretations providing higher confidence structural maps as well as better seismic attribute maps. They have also enabled the employment of more advanced techniques such as diffraction imaging and seismic inversion, all of which help to define the matrix porosity distribution away from well control and the non-matrix porosity distribution of fractures and karst features.

Undoubtedly, acquisition of new modern seismic data would have been preferable, but given the time and budget constraints, the reprocessing of the existing seismic data proved to be the right decision and provided the required step change in seismic image quality to meet the business objectives.

Acknowledgments 

This project required work from many teams, imaging by PGSK, support from PGS in Oslo, supervision by ExxonMobil in Houston, and the interpretation by NCOC in Atyrau. It was through the collaboration and work of all of these teams that this complex project was possible.