PGS Access Westralia: Imaging Results

The 2D long offset reprocessing performed for the PGS Access Westralia Study provides the first regionally consistent 2D PSDM framework across the Northwest Shelf, and has achieved the objective of recovering images of the deep basin scale reflectors.

Reprocessing from field tape has enabled the use of the latest broadband deghosting and dephasing techniques which were optimised to maximize the data bandwidth. Dual-sensor wavefield separation was used for deghosting the GeoStreamer data, and the conventional single-component data used a wave equation receiver deghosting technique. The incorporation of source deghosting based on emergence angle and dephasing using a carefully designed de-bubble operator has provided sharp stable data for the subsequent processing.

Figure 1: GeoStreamer data deghosting and dephasing; Legacy PSTM (left) and Access Westralia PSDM (right).  Note the significantly enhanced geological texture and resolution on the right.Figure 1: GeoStreamer data deghosting and dephasing; Legacy PSTM (left) and Access Westralia PSDM (right). Note the significantly enhanced geological texture and resolution on the right.Figure 2: Conventional data deghosting and dephasing; Legacy PSTM (top) and Access Westralia PSDM (bottom)Figure 2: Conventional data deghosting and dephasing; Legacy PSTM (top) and Access Westralia PSDM (bottom)The improvements in imaging achieved for the PGS Access Westralia Study were accomplished by enhanced pre-processing, accurate velocity model building and the application of pre-stack depth migration.  The demultiple component of the pre-processing forms an integral part of this equation by removing multiple energy that would otherwise interfere with the velocity model building process. By using adaptive subtraction of both a wave-equation multiple models and convolutive SRME multiple models, a superior degree of demultiple was achieved across the region. Figure 3: Superior demultiple; Legacy PSTM (left) and Access Westralia PSDM (right)Figure 3: Superior demultiple; Legacy PSTM (left) and Access Westralia PSDM (right)The velocity model building was performed in both a localised and a regional sense.  Several tomographic velocity model updates were performed on each line to update the shallow section and provide the localized model updates needed to improve the imaging of the shallow to mid depths.  In some areas, this provided significant improvement over time imaging (PSTM).

Figure 4: Velocity model building improvements; Legacy PSTM (left) and Access Westralia Beam PSDM (right)Figure 4: Velocity model building improvements; Legacy PSTM (left) and Access Westralia Beam PSDM (right)

PGS Beam migration (Beam PSDM) was used throughout the velocity model building workflow for high-density PSDM gather and stack output.  Because both the PGS Beam and Kirchhoff PSDM algorithms use the same ray tracing methodology, both migrations image data to the same spatial location but the particular strengths of each individual imaging algorithm define the final image.

Figure 5: VMB improvements and imaging algorithms; Legacy PSTM (left), Kirchhoff PSDM (middle), and PGS Beam PSDM (right)Figure 5: VMB improvements and imaging algorithms; Legacy PSTM (left), Kirchhoff PSDM (middle), and PGS Beam PSDM (right)

In order to ensure a regionally consistent and geologically accurate velocity model throughout the mid to deep section, a velocity flood model was derived from regional interpretations and existing seismic and well velocities.  This was incorporated into the model and a final pass of deep tomographic velocity model updating was performed.  Combined with the uplift from the reprocessing, the PSDM imaging was then able to image the deep data more coherently and uncover previously unseen structures.

Figure 6: Imaging improvements in the deep; Legacy PSTM (top), Kirchhoff PSDM (bottom)Figure 6: Imaging improvements in the deep; Legacy PSTM (top), Kirchhoff PSDM (bottom)

PGS Beam migration further enhances coherency and reduces background noise to provide clearer deep structure under most conditions.

Figure 7: Imaging improvements in the deep; Legacy PSTM (top), Kirchhoff PSDM (middle), and PGS Beam PSDM (bottom)Figure 7: Imaging improvements in the deep; Legacy PSTM (top), Kirchhoff PSDM (middle), and PGS Beam PSDM (bottom)

All data from the PGS Access Westralia Study has been both line-tied and depth-tied to a regional set of wells to ensure a regionally consistent 2D PSDM framework.  The imaging improvements and accurate positioning of the data are providing confidence in forming new ideas about the architecture and tectonic evolution of the region, and are leading to speculation of new petroleum plays throughout the North West Shelf.

Figure 8: Imaging improvements in the deep; Legacy PSTM (left), Kirchhoff PSDM (right)Figure 8: Imaging improvements in the deep; Legacy PSTM (left), Kirchhoff PSDM (right)