Prestack amplitude analysis methodology and application to seismic bright spots in the Po Valley, Italy

Geophysics ◽  
1990 ◽  
Vol 55 (2) ◽  
pp. 157-166 ◽  
Author(s):  
Alfredo Mazzotti
Keyword(s):  
Amplitude Analysis ◽  
Borehole Data ◽  
Seismic Section ◽  
Po Valley ◽  
Gravel Layer ◽  
Amplitude Versus Offset ◽  
Bright Spots ◽  
Head Waves ◽  
Amplitude Anomaly ◽  
Zero Offset

The amplitude‐versus‐offset (AVO) characteristics of three separate bright spots on the same seismic section are analyzed. One of the bright spots results from a water‐bearing gravel layer, and the others correspond to gas‐saturated sandy beds. The amplitude analysis includes reflections from the entire range of incidence angles available from the survey; for the shallower amplitude anomaly, these angles reached values up to 66°. Extension of the analysis to longer offsets is aimed at detecting possible critical‐angle phenomena in order to reduce the uncertainty when the zero‐offset reflection’s polarity is unknown. The reflection from the gravel layer has this property. Its amplitude exhibits an initial decrease followed by a sudden rise in the AVO trend due to critical reflection and head waves. The gas‐related anomalies have a much different AVO characteristic, one in which the amplitude increases with offset distance. Two seismic events located above the bright spots were also investigated to further verify the validity of the seismic amplitude processing. The AVO trends of the three bright spots and of the two reference levels were compared with analogous trends of synthetic seismograms that were computed from models derived from borehole data.

Case study: The importance of gas leakage in interpreting amplitude‐versus‐offset (AVO) analysis

Geophysics ◽  
1991 ◽  
Vol 56 (11) ◽  
pp. 1886-1895 ◽  
Author(s):  
M. K. Sengupta ◽  
C. A. Rendleman
Keyword(s):  
Seismic Energy ◽  
Gas Content ◽  
Bright Spot ◽  
Seismic Section ◽  
Gas Leakage ◽  
Hydrocarbon Reservoir ◽  
Reflection Amplitude ◽  
Amplitude Versus Offset ◽  
Amplitude Anomaly ◽  
Amplitude Versus

The amplitude‐versus‐offset (AVO) method has been shown to indicate the presence of gas sands if the reflection amplitude from the seal/reservoir‐sand interface, measured in a common midpoint (CMP) gather, increases rapidly with increasing shot‐to‐geophone distance (or offset). However, in a few instances, it has been observed that the seismic reflection amplitude does not increase with offset and may even decrease if there is widespread gas leakage above the hydrocarbon reservoir causing partial gas saturation in the overburden sediments. Gas‐charged sediments are known to attenuate seismic energy. Depending on the size and shape of this gas leakage zone, there may be higher attenuation of seismic amplitudes with increasing offset. We present one such case that involves a prominent “bright‐spot” amplitude anomaly corresponding to a 56‐ft‐thick (17 m‐thick) gas sand in the Gulf of Mexico slope. The reflection amplitude for the sand top was found to decrease with increasing offset. There is also evidence of gas leakage into the sediments above the reservoir. Color amplitude displays of the seismic section show a low‐amplitude diffused zone above the bright‐spot amplitude anomaly, which suggests gas leakage. Further evidence of gas leakage can be inferred from the significant gas content (including heavier hydrocarbons) observed in the mud log. Gas leakage is also confirmed by gather modeling in which the effects of leakage‐caused attenuation are accounted for in matching the variation of seismic amplitude with offset.

Exploration application of seismic amplitude analysis in the Krishna-Godavari Basin, east coast of India

2014 ◽  
Vol 2 (4) ◽  
pp. SP5-SP20 ◽  
Author(s):  
Ram Janma Singh
Keyword(s):  
Amplitude Analysis ◽  
Seismic Amplitude ◽  
High Impedance ◽  
Data Phase ◽  
Amplitude Versus Offset ◽  
Bright Spots ◽  
Strong Amplitude ◽  
Very High ◽  
Amplitude Versus ◽  
Godavari Basin

Seismic amplitude anomalies are attractive exploration targets in the Krishna-Godavari Basin offshore India. These bright spots mostly have very high amplitudes, so confident interpretations have been possible. We distinguished between hydrocarbon-bearing sands, water-bearing sands, and high-impedance nonreservoir bodies. Also, we mapped channel architecture and accurately predicted reservoir thickness. Strong amplitude anomalies, prospective seismic character based on an understanding of data phase and polarity, flat spots, and amplitude versus offset have all provided valuable evidence.

Incomplete AVO near salt structures

Geophysics ◽  
1992 ◽  
Vol 57 (4) ◽  
pp. 543-553 ◽  
Author(s):  
Christopher P. Ross
Keyword(s):  
Seismic Profiles ◽  
Spectral Modeling ◽  
Signal Degradation ◽  
Amplitude Versus Offset ◽  
Bright Spots ◽  
Close Proximity ◽  
Pseudo Spectral ◽  
Amplitude Versus ◽  
Structure Class ◽  
Made In

Amplitude versus offset (AVO) measurements for deep hydrocarbon‐bearing sands can be compromised when made in close proximity to a shallow salt piercement structure. Anomalous responses are observed, particularly on low acoustic impedance bright spots. CMP data from key seismic profiles traversing the bright spots do not show the expected Class 3 offset responses. On these CMPs, significant decrease of far trace energy is observed. CMP data from other seismic profiles off‐structure do exhibit the Class 3 offset responses, implying that structural complications may be interfering with the offset response. A synthetic AVO gather was generated using well log data, which supports the off‐structure Class 3 responses, further reinforcing the concept of structurally‐biased AVO responses. Acoustic, pseudo‐spectral modeling of the structure substantiates the misleading AVO response. Pseudo‐spectral modeling results suggest that signal degradation observed on the far offsets is caused by wavefield refraction—a shadow zone, where the known hydrocarbon‐bearing sands are not completely illuminated. Such shadow zones obscure the correct AVO response, which may have bearing on exploration and development.

REDUCING RESOURCE UNCERTAINTY USING SEISMIC AMPLITUDE ANALYSIS ON THE SOUTHERN RANKIN TREND, NORTHWEST AUSTRALIA

1999 ◽  
Vol 39 (1) ◽  
pp. 128 ◽  
Author(s):  
D. Sibley ◽  
F. Herkenhoff ◽  
D. Criddle ◽  
M. McLerie
Keyword(s):  
Seismic Attributes ◽  
3D Seismic ◽  
Amplitude Analysis ◽  
Seismic Amplitude ◽  
Amplitude Versus Offset ◽  
Minimum Number ◽  
Gas Fields ◽  
Resource Uncertainty

Between 1973 and 1996 West Australian Petroleum Pty Limited (WAPET) discovered five major gas fields on the southern Rankin Trend including Spar, West Tryal Rocks, Gorgon, Chrysaor, and Dionysus (collectively termed the Greater Gorgon Resource). Recent discoveries at Chrysaor and Dionysus emphasise the role of subtle 3D seismic attributes in finding hydrocarbons and defining reserves with a minimum number of wells.The Gorgon, Chrysaor, and Dionysus fields were covered by 3D seismic data shot in 1991 and 1995, which led WAPET to discover Chrysaor and later Dionysus. Subsequent to the 3D acquisitions, field reservoirs have been correlated with anomalous seismic events (seismic amplitude and amplitude versus offset) that conform to depth structure. Follow-up work has shown that combining these 3D seismic attributes improves the prediction of wet sands, gas sands, and other lithologies.The resulting understanding and confidence provided by this 3D seismic has driven an aggressive exploration program and defined field reserves at a high confidence level. Results include the recent award of permit area WA-267-P to WAPET and the ongoing studies to begin development of the Greater Gorgon Resource.

Bright spots, milligals, and gammas

Geophysics ◽  
1982 ◽  
Vol 47 (12) ◽  
pp. 1693-1705
Author(s):  
Alan O. Ramo ◽  
James W. Bradley
Keyword(s):  
Seismic Data ◽  
Magnetic Data ◽  
Amplitude Analysis ◽  
Seismic Amplitude ◽  
The North ◽  
Gravity And Magnetic ◽  
Amplitude Anomaly ◽  
North Edge ◽  
Gravity And Magnetic Data ◽  
Seismic Anomaly

Spatially discontinuous high‐amplitude seismic reflections were encountered in seismic data acquired in the early 1970s in northeast Louisiana and southwest Arkansas. Large acoustic impedance contrasts are known to result from gaseous hydrocarbon accumulations. However, amplitude anomalies may also result from large density and velocity contrasts which are geologically unrelated to hydrocarbon entrapment. A well drilled on the northeast Louisiana amplitude anomaly encountered 300 ft of rhyolite at a depth of 6170 ft. Subsequent gravity and total field magnetic profiles across the feature revealed the presence of 0.2 mgal and 17 gamma anomalies, respectively. The measured magnetic susceptibility of the rhyolite was 0.0035 emu and the measured density contrast was [Formula: see text]. Model studies based on the seismically determined areal extent of the anomaly and the measured thickness of rhyolite accounted for the observed gravity and magnetic anomalies. The southwest Arkansas amplitude anomaly was a sheet‐like reflection which terminated to the north and west within the survey area. Two north‐south gravity profiles exhibited a negative character over the sheet‐like reflector but did not exhibit a clear spatial correlation with the north limit of the seismic anomaly. Two north‐south magnetic profiles exhibited tenuous 4 gamma anomalies which appeared to be spatially correlated with the interpreted north edge of the seismic anomaly. A subsequent wildcat well encountered no igneous material but did penetrate 200 ft of salt at about 7500 ft. Reassessment of the gravity and magnetic data indicated that this seismic amplitude anomaly is not attributable to an intrasedimentary igneous source; it suggested a salt‐related 0.2 to 0.3 mgal minimum coextensive with the observed seismic amplitude anomaly. Present amplitude analysis technology would treat these seismic data with suspicion. However, gravity and magnetic data acquisition can provide a relatively inexpensive means for evaluation and verification of amplitude anomalies and thus should be an adjunct for land seismic exploration utilizing amplitude analysis.

Geological origins of seismic bright spot reflections in the Ordovician strata in the Halahatang area of the Tarim Basin, western China

2018 ◽  
Vol 55 (12) ◽  
pp. 1297-1311 ◽  
Author(s):  
Wei Yang ◽  
Xiaoxing Gong ◽  
Wenjie Li
Keyword(s):  
Tarim Basin ◽  
Oil And Gas ◽  
High Amplitude ◽  
Hydrothermal Activity ◽  
Hydrocarbon Exploration ◽  
Western China ◽  
Bright Spot ◽  
Gas Reservoirs ◽  
Seismic Section ◽  
Bright Spots

Anomalously high-amplitude seismic reflections are commonly observed in deeply buried Ordovician carbonate strata in the Halahatang area of the northern Tarim Basin. These bright spots have been demonstrated to be generally related to effective oil and gas reservoirs. These bright spot reflections have complex geological origins, because they are deeply buried and have been altered by multi-phase tectonic movement and karstification. Currently, there is no effective geological model for these bright spots to guide hydrocarbon exploration and development. Using core, well logs, and seismic data, the geological origins of bright spot are classified into three types, controlled by karstification, faulting, and volcanic hydrothermal activity. Bright spots differing by geological origin exhibit large differences in seismic reflection character, such as reflection amplitude, curvature, degree of distortion, and the number of vertically stacked bright spots in the seismic section. By categorizing the bright spots and the seismic character of the surrounding strata, their geological origins can after be inferred. Reservoirs formed by early karstification were later altered by epigenetic karstification. Two periods of paleodrainage further altered the early dissolution pores. In addition, faults formed by tectonic uplift also enhanced the dissolution of the flowing karst waters. Some reservoirs were subsequently altered by Permian volcanic hydrothermal fluids.

Amplitude‐versus‐offset measurement errors in a finely layered medium

Geophysics ◽  
1991 ◽  
Vol 56 (1) ◽  
pp. 41-49 ◽  
Author(s):  
Herbert W. Swan
Keyword(s):  
Measurement Errors ◽  
Residual Velocity ◽  
Event Time ◽  
Amplitude Versus Offset ◽  
Differential Tuning ◽  
Ricker Wavelet ◽  
Directivity Patterns ◽  
Zero Offset ◽  
Normal Moveout Correction ◽  
Amplitude Versus

Recently Spratt (1987) showed how amplitude‐versus‐offset analysis (AVO) can be sensitive to small residual velocity errors. However, even when the velocity is determined perfectly, serious AVO distortions remain due to normal‐moveout stretch, differential tuning as a function of offset, spherical divergence, and source and receiver directivity patterns. I have found that all of these errors can be expanded in a Taylor series about the zero‐offset event time, assuming it is much larger than the wavelet width. The first term of this series represents the residual velocity error term found by Spratt, while the second term encompasses the remaining effects mentioned. In practice, either term can be larger than the underlying amplitude variations being estimated. For example, Ricker wavelet stretch leads to a peak AVO error which is 61 percent of the peak zero‐offset reflectivity, even though the velocity field is uniform and correct. This result is independent of the wavelet frequency, and the range of incidence angles used in the analysis. Positive gradients in moveout velocity amplify this error, while narrowband filtering of the data prior to AVO analysis greatly widens its temporal extent. Aligning a particular event with static shifts instead of normal‐moveout correction can eliminate stretch, but not differential tuning error, in a finely layered target zone whose wavelets overlap.

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