Layer | Fill | Outline |
---|
Map layers
Theme | Visible | Selectable | Appearance | Zoom Range (now: 0) |
---|
Fill | Stroke |
---|---|
Collaborating Authors
Results
The Impact of Pre-stack Data Phase On the AVO Interpretation Workflow
Roden, Rocky (seismic Micro-Technology) | Jones, Gary (seismic Micro-Technology) | Castagna, John (University of Houston)
Summary: In the conventional approach of interpreting stacked seismic data, it is quite common to rotate the phase of the data to match synthetic correlations and/or to get isolated reflections in the data as close as possible to zero phase. Understanding the phase of the data is extremely important in the interpretation workflow and has significant implications for the final interpretation and drilling of wells. This functionality is available on most interpretation systems today. What is not routine in the normal interpretation process is to phase rotate gathers to determine how this affects the final stacked volume, the analysis of the pre-stack data, and ultimately the final interpretation of the prospect. Depending on the phase of gathers, the generation of various AVO attributes for crossplotting can produce varied results, which may adversely obscure or highlight significant trends in the data. From a 3D data set in the shallow waters offshore Louisiana, an AVO interpretation of pre-stack gathers and AVO crossplots was found to be quite beneficial in understanding the phase of the data and its impact on defining acoustic reservoir thickness. Background The location of the 3D dataset employed for this study is located in the Gulf of Mexico offshore Louisiana. There have been several wells drilled in this area with a significant amount of production. We focus this discussion at the Raptor 5 well where no sonic and density logs were collected or available to correlate with the seismic to help definitively determine the phase of the seismic data. The NMO corrected PSTM (pre-stack time migrated) gathers were studied for correlation with hydrocarbon production in the field. The zone of interest in the Raptor 5 well contains 160 feet of gross pay. Figure 1 is a north-south line through the pay zone in the Raptor 5 well. The strong amplitude event at approximately 2.6 seconds correlates with this pay zone. A tuning chart generated in the zone of interest indicates vertical resolution or the tuning thickness to be 22-24 ms (TWT), which is roughly 110-120 feet thick based on an estimation from nearby velocity information. This tuning chart was generated from a 2.2-3.0 window on the line in Figure 1. Gather Analysis The non-muted gather located at the CDP of the pay in the Raptor 5 well is displayed in Figure 2. On the left in Figure 2 is the gather in wiggle trace format while on the right the gather is displayed in high-resolution color raster format with offset angle guides. The maximum number of offset traces in this dataset is 29, which leads to a shotpoint to receiver offset of 19,800 feet. The CDP to receiver or halfoffset is 9,900 feet and is used to compute angles for AVO interpretation. At the 2.6 second pay zone there are 25 trace offsets, which is a half offset of 8,580 feet. What is evident on this gather is that there is not much useful data beyond an incident angle of approximately 30 of offset except for the strong event correlated with the pay.
In the conventional approach of interpreting stacked seismic data, it is quite common to rotate the phase of the data to match synthetic correlations and/or to get isolated reflections in the data as close as possible to zero phase. Understanding the phase of the data is extremely important in the interpretation workflow and has significant implications for the final interpretation and drilling of wells. This functionality is available on most interpretation systems today. What is not routine in the normal interpretation process is to phase rotate gathers to determine how this affects the final stacked volume, the analysis of the prestack data, and ultimately the final interpretation of the prospect.