Resurrecting Ray Tracing in the Papuan Fold Belt as a Standard Procedure
The Papuan Fold Belt (PFB) is notoriously difficult for acquiring, processing and interpreting seismic data for the purpose of de-risking prospects and leads. Thick jungle and even thicker karstified Miocene carbonates at the surface often wreak havoc on the downgoing and upcoming seismic wavefield. Recently discovered vertical forelimbs, on wells coincident with seismic profiles, have proven that recorded seismic data in these zones often is inadequately sampled and can appear unfocussed and completely ambiguous on the final imaged profiles. Application of forward modelling and ray-tracing were once part of the standard procedure to assist in interpreting and depth conversion of all PFB prospects and leads. With greater emphasis being applied to imaging optimisation and additional time constraints, ray tracing has become less routine in modern interpretation workflows. Recent application of 3D raytracing across a variety of subsurface scenario models, combined with 2D forward modelling and synthetic seismic sections have successfully aided both image processing and data interpretability – hence creating improved subsurface confidence. By developing a suite of plausible models, the synthetic responses may be easily compared with their real equivalents to investigate the range of realistic scenarios. It is well documented that the best process for imaging such complex structures is the acquisition and processing of 3D seismic surveys. Given the problems with the rugose karst topography covered in thick jungle, acquiring such surveys would require an almost insurmountable logistic solution, not to mention the astronomical cost involved. This means we are limited to acquiring expensive, poor quality 2D seismic surveys with dubious results. It is also well documented that 2D seismic suffers from multiple imaging problems, such as sideswipe and out-of-plane reflections, as well as a plethora of noise artefacts. After these problems are stacked together, 2D migration (both pre-stack and post stack) smears the events over the section and may sometimes render the data almost impossible to interpret. Ray tracing offers the opportunity of constraining the interpretation of the data by creating a controlled noiseless simulated seismic response and helps determine whether the assumptions and models coincide with the real data. As with many model-based approaches, the proposed solutions in a given geometric or seismic velocity model are non-unique. Exponential advances in computing power now allow construction and review of multiple models in real time without impacting time constraints. The following case studies demonstrate how ray tracing as a standard procedure has assisted with the planning of seismic acquisition in the PFB, as well as processing and interpreting the seismic data, and defining geological models. With the ever-rising, significant costs of drilling and seismic data acquisition in the PFB, techniques such as this play an important role in exploration and appraisal de-risking.
AAPG Datapages/Search and Discovery Article #90371 © 2020 AAPG Asia Pacific Region, The 1st AAPG/EAGE PNG Geosciences Conference, PNG’s Oil and Gas Industry: Maturing Through Exploration and Production, Port Moresby, Papua New Guinea, February 25-27, 2020