Whitepaper: Rock Physics and Fracture Attributes from PSDM
WIT is one of the few, if not the only, vendor using true angle gathers in depth from PSDM for attribute applications such as AVA and azimuthal anisotropy (fracture) measurement.
Why is this important? See the diagram to the left. In a simple (i.e., 1D) earth, it is very simple to relate offset to incidence angle at the reflector. The same could be said for azimuth angle. However, when the earth has a complex velocity structure and dipping layers, it may be impossible to uniquely convert offset to incidence angle or surface azimuth to reflection azimuth. There are really only two ways to relate offset/surface azimuth to incidence/azimuth angle -- 3D ray tracing in an interval velocity model or PSDM angle decomposition technology like WIT Angle.
Who should be using WIT Angle?
AVA Attributes from WEM Incidence Angle Gathers
In 2011, we filed our third US Patent application, primarily covering our use of the "hyperbolic mudrock line" to relate VP to VS. AVO technology has utilized Castagna's two-parameter linear mudrock line for years. However, the hyperbolic mudrock line only uses a single parameter to relate VP to VS (see figure at left). Please see our 2011 SEG Abstract to view how a tight range of hyperbolic mudrock line parameters can accurately relate VP to VS for any rock type. The AVA "slope" and "intercept" may be corrupted by biases induced by acquisition of processing. In the abstract, we show that the hyperbolic mudrock line parameter can be related to the ratio of slope and intercept. If, after inversion of angle gathers for slope and intercept, the predicted hyperbolic mudrock line parameter is out of range for the dominant lithology, we can compute a survey-wide calibration for the slope, based on realistic rock physics constraints. At left, please see an example of how our attributes technology has uncovered a known commercial accumulation of gas in a fairly tight Wilcox-age reservoir in South Texas.
Fracture Attributes from WEM Azimuth Angle Gathers
Fracture prediction from wide-azimuth (WAZ) seismic is taking the onshore E&P world by storm. From hard rocks to unconventional shale plays, operators realize that standard P-wave data can accurately predict fracture orientation and magnitude. WIT is the only vendor producing true azimuth angle gathers in depth, with its patent pending WIT Angle technology. The advantage of working in depth, with true azimuth angle becomes apparent in complex plays like the Niobrara example at left. When the earth exhibits reflector dip and lateral velocity variation, the predicted fracture orientation and magnitude will be skewed if analyzing time-domain, surface azimuth gathers. We compute an azimuth angle gather at every image point, and fit a sine curve to the amplitude versus azimuth angle to invert for fracture orientation and magnitude, as well as fracture "quality".
After fracture analysis, we utilize fracture quality and proprietary conditioning algorithms to process the computed 3D volumes of fracture orientation and magnitude. Many clients prefer to view the fracture information in horizon slice form, as seen on the left. A structure contour map illustrates that most production on this mature field are on the structural crest. The "hot" colors on the fracture map, shown in yellow and pink, indicate highly fractured zones. The green arrows indicate fracture orientation. Notice how many of the producers in this mature field lie in fractured areas. Our PSDM work allowed the client to identify several undrilled fault blocks, which appear to be fractured.