Geophysical Corner

Multicomponent Seismic Proves Its Value

Contributors: Bob Hardage

A fundamental thesis of elastic wavefield seismic stratigraphy (or multicomponent seismic stratigraphy) is that S-wave seismic data have equal value to P-wave data for geological interpretation.

Seismic stratigraphy analyses, then, should be based on interpreting P and S data in combination (the full elastic wavefield) rather than restricting interpretation to only single-component P-wave data (traditional seismic stratigraphy).

An example illustrating differences between P-wave and S-wave definitions of reflecting interfaces and the rock physics principles that cause this behavior are discussed in this month’s column.

The particular S-wave mode used in this example is the converted-shear (P-SV) mode.

Marked contrasts between compressional-wave (P-P) and P-SV seismic sequences and seismic facies occur across numerous stratigraphic intervals. The example chosen for this discussion is from west Texas (figure 1).

The arrows on the P-P and P-SV images of this figure identify a significant difference between P-P and P-SV reflectivities for a targeted reservoir interval – the Wolfcamp formation.

Well log data across the Wolfcamp interval local to this seismic profile are displayed on figure 2.

P-P and P-SV reflectivity behaviors are analyzed across the Wolfcamp interface, shown at a depth of approximately 10,300 feet, to demonstrate the geological reason for the difference in P-P and P-SV reflection amplitude strengths exhibited on figure 1.

Compressional-wave and shear-wave velocities and formation bulk density values were averaged across 300-foot intervals immediately above and below this internal Wolfcamp interface, and these average rock properties were used to calculate the reflectivity curves shown as figure 3.

These curves confirm that for this particular interface, P-SV reflectivity is greater than P-P reflectivity when both reflectivity curves are evaluated over a large range of incidence angles.

For example, P-P reflectivity exceeds 0.04 only for incidence angles between 0 and 15 degrees, but P-SV reflectivity has a magnitude greater than 0.04 for incidence angles between 15 degrees and 45 degrees – an angle range that is twice as large as that of the high-amplitude P-P response.

Because the multicomponent seismic data across this study area were acquired with a full range of incidence angles, the difference in P-P and P-SV amplitude behavior shown on figure 1 has a valid rock-physics basis.

P-P amplitudes should be weaker than P-SV amplitudes, and the data exhibit that behavior.

The principle documented by this example is that an elastic wavefield seismic stratigraphy interpretation based on both P-P and P-SV data can provide a different – and often a more valid – geological model of seismic sequence boundaries and seismic facies than can a single-mode seismic stratigraphy interpretation based on P-P data only.

Future applications of seismic stratigraphy probably will rely more and more on full-elastic wavefield seismic data than on only single-component seismic data.

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Geophysical Corner

Geophysical Corner - Satinder Chopra
Satinder Chopra, award-winning chief geophysicist (reservoir), at Arcis Seismic Solutions, Calgary, Canada, and a past AAPG-SEG Joint Distinguished Lecturer began serving as the editor of the Geophysical Corner column in 2012.

Geophysical Corner

The Geophysical Corner is a regular column in the EXPLORER that features geophysical case studies, techniques and application to the petroleum industry.


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The Geophysical Corner is a regular column in the EXPLORER, edited by Satinder Chopra, chief geophysicist (reservoir), at Arcis Seismic Solutions, Calgary, Canada.

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The geometries of clay smears produced in a series of direct shear experiments on composite blocks containing a clay-rich seal layer sandwiched between sandstone reservoir layers have been analyzed in detail. The geometries of the evolving shear zones and volume clay distributions are related back to the monitored hydraulic response, the deformation conditions, and the clay content and strength of the seal rock. The laboratory experiments were conducted under 4 to 24 MPa (580–3481 psi) fault normal effective stress, equivalent to burial depths spanning from less than approximately 0.8 to 4.2 km (0.5 to 2.6 mi) in a sedimentary basin. The sheared blocks were imaged using medical-type x-ray computed tomography (CT) imaging validated with optical photography of sawn blocks. The interpretation of CT scans was used to construct digital geomodels of clay smears and surrounding volumes from which quantitative information was obtained. The distribution patterns and thickness variations of the clay smears were found to vary considerably according to the level of stress applied during shear and to the brittleness of the seal layer. The stiffest seal layers with the lowest clay percentage formed the most segmented clay smears. Segmentation does not necessarily indicate that the fault seal was breached because wear products may maintain the seal between the individual smear segments as they form. In experiments with the seal layer formed of softer clays, a more uniform smear thickness is observed, but the average thickness of the clay smear tends to be lower than in stiffer clays. Fault drag and tapering of the seal layer are limited to a region close to the fault cutoffs. Therefore, the comparative decrease of sealing potential away from the cutoff zones differs from predictions of clay smear potential type models. Instead of showing a power-law decrease away from the cutoffs toward the midpoint of the shear zone, the clay smear thickness is either uniform, segmented, or undulating, reflecting the accumulated effects of kinematic processes other than drag. Increased normal stress improved fault sealing in the experiments mainly by increasing fault zone thickness, which led to more clay involvement in the fault zone per unit of source layer thickness. The average clay fraction of the fault zone conforms to the prediction of the shale gouge ratio (SGR) model because clay volume is essentially preserved during the deformation process. However, the hydraulic seal performance does not correlate to the clay fraction or SGR but does increase as the net clay volume in the fault zone increases. We introduce a scaled form of SGR called SSGR to account for increased clay involvement in the fault zone caused by higher stress and variable obliquity of the seal layer to the fault zone. The scaled SGR gives an improved correlation to seal performance in our samples compared to the other algorithms.
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Considerable effort has been devoted to the development of simulation algorithms for facies modeling, whereas a discussion of how to combine those techniques has not existed. The integration of multiple geologic data into a three-dimensional model, which requires the combination of simulation techniques, is yet a current challenge for reservoir modeling. This article presents a thought process that guides the acquisition and modeling of geologic data at various scales. Our work is based on outcrop data collected from a Jurassic carbonate ramp located in the High Atlas mountain range of Morocco. The study window is 1 km (0.6 mi) wide and 100 m (328.1 ft) thick. We describe and model the spatial and hierarchical arrangement of carbonate bodies spanning from largest to smallest: (1) stacking pattern of high-frequency depositional sequences, (2) facies association, and (3) lithofacies. Five sequence boundaries were modeled using differential global position system mapping and light detection and ranging data. The surface-based model shows a low-angle profile with modest paleotopographic relief at the inner-to-middle ramp transition. Facies associations were populated using truncated Gaussian simulation to preserve ordered trends between the inner, middle, and outer ramps. At the lithofacies scale, field observations and statistical analysis show a mosaiclike distribution that was simulated using a fully stochastic approach with sequential indicator simulation.

This study observes that the use of one single simulation technique is unlikely to correctly model the natural patterns and variability of carbonate rocks. The selection and implementation of different techniques customized for each level of the stratigraphic hierarchy will provide the essential computing flexibility to model carbonate settings. This study demonstrates that a scale-dependent modeling approach should be a common procedure when building subsurface and outcrop models.

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Offshore Angola has to date delivered recoverable reserves in excess of 20 billion barrels of oil equivalent. This has been encountered in two distinct play systems: the Upper Cretaceous Pinda carbonates sourced by Lower Creatceous lacustrine mudstones and Tertiary deepwater slope turbidite sands sourced by underlying Upper Cretaceous marine mudstones. An extension of the Girassol play into Block 18 to the south will be used to describe how high quality 3D seismic data coupled with a detailed analysis of rock properties led to an unprecedented 6 successes out of 6 wells in the block, including the giant Plutonio discovery. Industry is turning once more to the carbonate play potential - this time in deepwater. It would seem that the Angola offshore success story is set to continue for some time to come.

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