Alan G. Nunns

Structural restoration of seismic and geologic sections in extensional regimes

Seismic sections from extensional basins can be structurally transformed to produce restored seismic sections showing subsurface geology of the past. Restoration is achieved using a simple and effective algorithm, which is suited for use in an interactive seismic interpretation environment. Seismic data are resampled along paleovertical trajectories, which are the deformed equivalents of what were originally closely spaced vertical lines. Paleoverticals are generated within the present-day section as follows: (1) origin points are equally spaced along a reference horizon that corresponds to an original depositional surface, (2) paleovertical segments are vertical within fault blocks, (3) paleoverticals are offset across faults by the reference horizon slip, and (4) restor d depth is equated to distance down a paleovertical. These rules can be modified to conform with such styles of extensional deformation as vertical simple shear, inclined shear, or rigid rotation, and to account for compaction. Structures with complex fault patterns readily are restored. Several examples from the Gulf of Mexico and offshore west Africa show that seismic restoration is useful for (1) unravelling structural and stratigraphic history, (2) revealing tectonically masked features, and (3) checking interpretations across faults for seismic correlation and structural balance. Geologic sections can be restored in similar fashion.

A temperature probe survey on the Louisiana shelf; effects of bottom-water temperature variations

A survey of temperature gradients in the uppermost sediments of the Louisiana continental shelf was done in 1983 by Gulf Oil Corporation, using a 5-m-long temperature probe. The survey covered 450 sq mi, with an average grid spacing of less than 1 mi. The purpose was to detect heat flow anomalies due to subsurface fluid flow. However, thermal perturbations due to seasonal variation of bottom-water temperature mask the temperature gradients due to geological heat flow. Bottom-water temperature variations similar to those recorded near the survey area in 1963-1965 explain most of the observed spatial and temporal changes in shallow temperature-depth profiles. The data set from this detailed survey has been donated to a publicly accessible scientific database, to spur resear h into heat flow measurement on continental shelves.

Correcting amplitude, time, and phase mis-ties in seismic data

Seismic lines from different vintages frequently mis‐tie where they intersect. The mis‐ties may stem from different amplitudes, a shift in time, or different wavelet character. We can correct these mis‐ties to a great extent by applying a scale factor, a static time shift, and a phase rotation to one of the lines. We describe algorithms to compute the amplitude, time, and phase differences at each intersection among a series of 2-D seismic lines. We can then use an iterative least‐squares technique to derive optimal mis‐tie corrections for each line. We include a necessary modification of the least‐squares technique for the nonlinear phase data. The various algorithms are stable, fast, and accurate. We have used them in conjunction with an interactive workstation seismic interpretation program for five years. The scalar mis‐tie corrections greatly enhance the consistency of the seismic data. We show the results of the mis‐tie package applied to a 20‐line survey consisting of five vintages of seismic data....

Combining multiple quantitative structural analysis techniques to create robust structural interpretations

AbstractCarefully selected 2D transects contain an abundance of structural information that can constrain 3D analyses of petroleum systems. Realizing the full value of the information in a 2D transect requires combining multiple, independent structural analysis techniques using fully interactive tools. Our approach uses quantitative structural geologic software that instantaneously displays structural computations and analyses, eliminating time-intensive manual measurements and calculations. By quickly testing multiple hypotheses, we converged on an optimal solution that is consistent with available data. We have combined area-depth-strain (ADS) analysis, structural restoration, and forward modeling of a structural interpretation of a fault-propagation fold in the Niger Delta. These methods confirmed the original interpretation and furthermore quantified displacement, strain, detachment depth, and kinematic history. ADS analysis validated the interpreted detachment depth and revealed significant layer-par...

Direct estimation of fault trajectory from structural relief

ABSTRACT Confidently defining the trajectory of faults that control structural traps is a recurring challenge for seismic interpreters. In regions with fault-related folds, seismic and well data often constrain the upper fold geometry, but the location and displacement of the controlling fault are unknown. We present a generalized area–depth strain (ADS) analysis method that uses the observed depth variation in deformed horizon areas to directly estimate underlying fault depth, dip, displacement, and layer-parallel strain from a structural interpretation. Previously established ADS methods are only applicable to structures controlled by faults that sole into layer-parallel detachments. The new technique, referred to as the fault-trajectory method, generalizes ADS analysis to contractional and extensional structures controlled by fault ramps that cut across layers and displace the regional. For structures where area is conserved during deformation and shear is minimal, laterally shifting the analysis limits across the structure defines changes in fault orientation. We validate the method by applying it to numerical forward models, analog clay models, and seismically imaged structures from the San Joaquin basin in California, the Sierras Pampeanas in Argentina, and the North Sea. The fault-trajectory method is shown to be robust, because it exactly reproduces the prescribed fault trajectories and displacements used to construct the numerical and analog models. In the natural examples, the ADS-estimated fault trajectories are consistent with independent fault-location constraints such as earthquake focal mechanisms, seismic imaging, and forward modeling.

Meren Field, Nigeria: A 4D seismic case study

In 1996 Chevron acquired a repeat 3D seismic survey over the Meren Field, offshore Nigeria, with multiple objectives: to increase structural and stratigraphic image quality, to explore for deeper targets, and to try time-lapse seismic reservoir monitoring. By comparing the 1996 survey with a Chevron legacy 3D survey dating from 1987, this paper focuses on the 4D seismic objective to look for evidence of reservoir fluid changes during the 9-year interval. To evaluate this potential, the 4D migrated data sets were first “cross-equalized” with Chevron proprietary software to normalize the effects of different source waveform, static shifts, amplitude balance, etc. between the two surveys. The cross-equalized difference cube between the two surveys shows good repeatability (minimal difference) above and below the reservoir. Significant 4D seismic difference events appear within the reservoir zone, with striking geologic and fluid-flow features such as good lateral continuity, truncation against faults, and close proximity to known producing wells and sand intervals.