Case story
Accounting for lateral and temporal amplitude and frequency variations in LFC-based inversion
Client: Independent Oil & Gas Operator
Location: Malaysia
Challenge
Addressing bandwidth and frequency variations caused by gas-charged sediments in the overburden, prior to reservoir-focused litho-facies class inversion.
Sharp Reflections was tasked by Operator to undertake reservoir-focused gather conditioning on an 850 sq km 3D survey in the Malay Basin, prior to litho-facies class (LFC) inversion.
The specific challenge was to account for significant spatial and temporal variations in prestack data bandwidth and frequency related to the presence of gas charged sediments in the overburden.
Solution
QC was used to identify data artefacts and conditioning workflow was applied to improve AVA compliance. Spectral balancing and a full-volume AVA preserving scalar was used to address amplitude and frequency variations.
The first step was to use full-volume and well tie QCs to identify artefacts in the data and design a gather conditioning workflow to address observed shortcomings in AVA compliance. The issues of spatial and temporal variations in amplitude and frequency were solved through the application of a full-volume AVA preserving amplitude scalar, after the application of prestack spectral balancing.
This facilitated the use of a single set of prestack wavelets for the final LFC-based PCube+ inversion.
Impact
Inversion improved reservoir bodies continuity and resolution, even beneath the shallow gas-affected areas. Giving Client confidence in the dataset for new development planning.
The resulting PCube+ inversion volumes significantly enhanced the lateral continuity of reservoir bodies in comparison to earlier Extended Elastic Impedance (EEI) studies. The resolution of thin reservoir bodies was also enhanced by the workflow, even beneath the shallow gas affected areas.
The final results provided Operator with a dataset for planning new development and infill wells with greater confidence.
Amplitude scalar derivation and application
20-26° angle stack section – use slide to see before (left) and after (right) scaling.
Comparison between legacy EEI results and scaled PCube+ inversion results
Lithology section from EEI (relative Vp/Vs ratio – legacy data)
Lithology section (shale probability) from PCube+ inversion after conditioning and scaling
Inverted lithology volumes show improved continuity and more discrete definition of sand bodies than the legacy EEI volumes. Imaging effects of shallow overburden gas are still evident to some extent in inversion results, however additional sand bodies are identified in the target interval.
Mapped lithology response at target from EEI (relative Vp/Vs ratio – legacy data)
Mapped hydrocarbon sand probability at target from PCube+ inversion after conditioning and scaling
Sand body continuity is relatively poorly defined beneath the gas cloud (green polygon) in the legacy EEI data. After conditioning, scaling and inversion, a hydrocarbon bearing sand channel is now identified with high probability running beneath the gas cloud to the west of the structural closure