Southern North Sea Inversion

SNS-map-350x245The successful appraisal drilling in the Breagh Field (42/12) had demonstrated the commercial potential of the early Carboniferous clastic reservoirs within the Scremerston Coal Formation (SCF) in non-inverted Base Zechstein structural highs. Previous exploration in the SCF play has been hindered by the lack of regional, high-resolution seismic data over the play fairway. During the second half of 2013, Spectrum acquired and processed 4,017km of new 2D Multi-Client data over open acreage to facilitate exploration of the Scremerston Formation (SF) and Fell Sandstone Formation (FSF) play fairways, north-east of the Breagh-Crosgan fields.

Time Processing
The data was collected with an 8km cable, nominal 160 fold, 10 second record length, and the lines were around 2km apart resulting in a very dense grid. The shallowest start time for the water bottom was about 100ms, therefore the dataset was heavily contaminated with multiples and noise, the removal of these was a major concern throughout the early part of the processing. Close collaboration between interpreters and processors from an early stage identified the Top Chalk as a generator of strong multiples which masked the deep section.

The processing sequence derived following extensive testing and parameterization produced excellent final PreSTM results. In particular, the Carboniferous section is well imaged, which has allowed Spectrum’s geologists to identify new plays. Key factors in obtaining these results were;

• Multi-domain iterations of Tau-P noise attenuation and deconvolution
• Targeted Multiple suppression (delayed start SRMA)
• Effective imaging using pre-stack time migration
• High density velocity analysis post PSTM (6.25m)

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Data before Tau-P processing and after all Tau-P processing (Auto-correlations between 0 – 2000ms and 1500 – 4000ms)

SRMA uses the seismic data to predict the multiple model and iteratively subtract it. The key advantage of SRMA is that it needs no velocity information. The main multiple generator in this dataset was identified as the top Chalk horizon. Therefore, instead of the traditional method of applying SRME from the water bottom, the interpreted top Chalk horizon was the point of application for SRMA, thus targeting the multiples generated from this formation. This approach proved very effective and significantly improved the CDP gathers and semblances for velocity picking in the deeper section. The improved velocity estimation allowed the application of an aggressive Radon pre-migration which enhanced the velocity semblance displays even further, resulting in well-defined deep primary events within the gather and improved the migration velocity field definition.

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‘)Pre-SRMA data with revised velocity field (picked post SRMA) and Post Radon data

Pre-Stack Inversion
The area for the inversion trial was covered by two wells, 43/02-1 & 43/06-1, and the tied 4 seismic lines. The interpretation performed by Spectrum’s Geoscience team identified multiple independent structural closures, fault-closed structures and erosional updip terminations against Zechstein salt at SF level. Potentially very large one-way fault and three –way dip closed structures as well as independent structural traps were mapped at FSF level. Various crossplots derived from the prestack inversion results, highlighted data points corresponding to potential gas-bearing sandstones, these were successfully integrated with leads identified from seismic interpretation and depth conversion.

SNS-Well-Loc-MapLocation of the two wells and seismic lines used for pre-stack inversion

Careful attention was given to the initial clean-up and verification of these wells, involving anomalous sample edit, missing log data transforms and well to seismic tie/correlation using time-depth charts exported from the interpretation project.

SNS-densityWell 43/06-1 Line SNS2D-13-005

The final density log was a combination of the original (Density) without the anomalous high values at the start (circled) spliced with the fully calculated log (Density_gardners). P-impedance and S-impedance logs were achieved by transformation within the software and used over the internal computed logs, this is due to potential issues discovered when using the default computed logs.

Nine regional horizons from the MC interpretation were used which were augmented within the inversion software to delineate faults, along with using both wells, this information was used in the model building stage.

SNS-modelPre-Stack model – Line SNS2D-13-004

The pre-stack inversions for the individual lines were run using consistent scalars gained from initial tests, which allowed meaningful comparisons across the results. Lambda Rho and Mu Rho volumes were also generated from the P and S wave Impedance volumes. Cross-plotting of the various data volumes gave consistent results for the predicted distribution of gas bearing sands.

AVO-SNS---1000-pxMontage displaying the results from Simultaneous Inversion.