Is that Extra Wiggle Real? Validating Added Resolution from Spectral Blueing Using 3-Way Seismic Well Ties & Reflection Coefficient Modelling; A Case Study from the Mishrif Reservoir in the Rumaila Oil Field
Authors: Olatoke Olabode Joseph , Soulas Sebastien , Cooper Charles , Tough Kenneth , Sharma Raman 
Published: November 2018

We present a case study that demonstrates the use of our robust Seismic-Well Tie (SWT) process and seismic attributes to validate the added resolution from Seismic Spectral Blueing (SSB) on the carbonate Mishrif reservoir in the Rumaila oil field. Our SWT process included Vertical Seismic Profile (VSP) corridor stack traces and Reflection Coefficient Modelling (RCM). Seismic attributes generated following the interpretation of the SSB data, revealed geological features that weren’t previously visible on the full- stack seismic. All of these provide validation that the extra wiggle from the SSB is real in this case study.

SSB outputs bandlimited reflectivity traces derived from shaping the amplitude spectra of the input seismic to that of the well log-based reflectivity series. SSB adds seismic bandwidth to the full-stack data that is expressed as an extra trough within the Mishrif reservoir in certain parts of the field. Three-way SWTs, achieved by including a VSP corridor stack trace to a more conventional tie between well log synthetic and seismic trace, is typically seen as a thorough approach. It can help to reinforce confidence in seismic events observed in all three data types and to highlight events or intervals where well logs or seismic may contain significant anomalous data. Three-way SWTs tying full-stack synthetic, full-stack seismic and 8-12-30-45 Hz VSP corridor stack traces, as well as SSB synthetic, SSB and 8-12-50-75 Hz VSP corridor stack traces are of good-quality, with a comparable extra trough also identified on the broader bandwidth VSP corridor stack trace.

Reflection Coefficient Modelling (RCM), a part of the SWT process, is a way of deconstructing a synthetic seismic trace by looking at the intermediate step in wavelet convolution to isolate the contributions of individual Reflectivity Coefficient (RC) contrasts to the resulting seismic event, often referred to as a ‘wiggle’. RCM suggests that the extra trough observed on the SSB data is associated with the development of a rudist-dominated grainstone shoal body. VSP data was used to generate both conventional primary reflectivity response, as well as multiple corridor stacks based on key interbed multiples to understand their generation and kinematics. Different wavefields were generated to allow the discrimination between surface and interbed multiples. This provides support for amplitude fidelity for multiple events and helped identify the adverse effect of multiples on a different reservoir interval trough.

Due to the large well stock, with over 700 wells with porosity logs penetrating the Mishrif reservoir, this case study is peculiar in the sense that the previous Geomodel had no direct seismic attributes used in property distribution. Therefore, seismic attributes generated were compared to the Geomodel properties, such as porosity to see if geological features were identifiable on seismic. A grainstone shoal body on a Geomodel average porosity map, also clearly delineated on the SSB sections and attributes, was only subtlely expressed and not properly identifiable on the full-stack data. One of several sinuous features, interpreted as grainstone-dominated tidal channels, targeted using seismic attributes was recently drilled and encountered good reservoir quality channel facies.

This case study shows how a SWT process (three-way tie, RCM), seismic attributes and results from a recently drilled well provide validation of the authenticity of the added SSB resolution.