Thang Ha

Quantifying the significance of coherence anomalies

AbstractSemblance and other coherence measures are routinely used in seismic processing, such as velocity spectra analysis, in seismic interpretation to estimate volumetric dip and to delineate geologic boundaries, and in poststack and prestack data conditioning such as edge-preserving structure-oriented filtering. Although interpreters readily understand the significance of outliers for such measures as seismic amplitude being described by a Gaussian (or normal) distribution, and root-mean-square amplitude by a log-normal distribution, the measurement significance of a given coherence of poststack seismic data is much more difficult to grasp. We have followed early work on the significance of events seen in semblance-based velocity spectra, and we used an F-statistic to quantify the significance of coherence measures at each voxel. The accuracy and resolution of these measures depended on the bandwidth of the data, the signal-to-noise ratio (S/N), and the size of the spatial and temporal analysis windows...

Seismic reprocessing and interpretation of a fractured-basement play: Texas Panhandle

AbstractThe Panhandle-Hugoton field, of Texas, Oklahoma, and Kansas, is a giant oil field and is the largest conventional gas field in North America. Most hydrocarbon production in this field comes from the Wichita Uplift area, where the basement is the most shallow. Although the field has been extensively produced, many local hydrocarbon accumulations have not been fully exploited. Recent drilling activity in the survey indicates that some wells produce directly from basement fractures, suggesting a new play type for the area. Because the target is shallow, the seismic data are heavily contaminated by coherent noise, such as ground roll and head waves, creating challenges for seismic processing. To improve the seismic interpretation, we carefully reprocessed the field gathers resulting in improved correlation within the sedimentary and the basement sections. Correlating well control to seismic attribute volumes indicates that a fractured basement gives rise to lower P-wave impedance and strong amplitude ...

The value of constrained conjugate-gradient least-squares migration in seismic inversion: Application to a fractured-basement play, Texas Panhandle

AbstractSeismic inversion has become almost routine in quantitative 3D seismic interpretation. To ensure the quality of the seismic inversion, the input seismic data need to have a high signal-to-noise ratio. With the current low oil price environment, seismic reprocessing is often preferred over reacquisition to improve data quality. Common filter pairs include forward and inverse f-k and Radon transforms. Forward and inverse migrations (i.e., migration and demigration) are a more recently introduced transform pair that, when used together in an iterative workflow, results in a least-squares migration algorithm. Least-squares migration compensates for surface variation in data density and, when combined with a filter applied to prestack migrated images, suppresses the operator and data aliasing. We apply a least-squares migration workflow to a fractured-basement data set from the Texas Panhandle to demonstrate the enhancement in signal-to-noise ratio, the reduction in acquisition footprint and migration ...

Calibration of attribute anomalies through prestack seismic modeling

Seismic modeling is commonly used in determining subsurface illumination of alternative seismic survey designs, in the calibration of seismic processing and imaging algorithms, and in the design of effective processing workflows. Seismic modeling also forms the mathematical kernel of impedance inversion and is routinely used to predict the amplitude-variation-with-offset response as a function of rock and fluid properties. However, the use of seismic modeling in seismic attribute studies is less common. We have evaluated four case studies in which 2D synthetic common shot gathers were computed (acoustic or elastic) and processed (including migration) to evaluate possible interpretation hypotheses. The modeling we used in our study shows that the lack of continuous coherence anomalies in a faulted Chicontepec Basin survey was due to overprinting by coherent interbed multiples. Attributes computed from the resulting processed model data revealed that subtle curvature anomalies in a Mississippi Lime survey were due to karst collapse rather than to velocity pushdown related to vertical gas migration. Impedance attributes computed from a Woodford Shale model favored the hypothesis of increased porosity correlated with the occurrence of subtle faults rather than amplitude dimming due to poor fault imaging. Finally, modeling of a fractured basement survey in the Texas Panhandle survey indicated that headwave suppression preserved the basement fracture response while increasing the signal-to-noise ratio. Seismic attribute study on seismic modeling results helped significantly in testing possible interpretation hypotheses in all of our case studies.