An analysis of amplitude variation with offset (AVO) observations is applied in hydrate-bearing sands, free-gas- charged sands, and hydrate-over-gas sands. The elastic model parameters (Vp, Vs, and density) are obtained from well log measurements and a rock physics model. The study suggests that presence of gas hydrate and free gas affect the AVO of shallow unconsolidated sediments containing gas hydrate and free gas. Low-concentrated gas hydrate and low-concentrated gas hydrate overlying free gas have weak AVO behaviors while highly-concentrated gas hydrate and highly-concentrated gas hydrate overlying free gas have strong AVO behaviors. Both highly-concentrated gas hydrate and highly-concentrated gas hydrate overlying free gas are Class I AVO anomalies but the intercept of AVO is stronger negative for highly-concentrated gas hydrate overlying free gas. They may occur in different locations in the AVO intercept and gradient plane.
This paper discusses two seismic data conditioning processes – Structural Filter (SF) and Gabor deconvolution. We found that these two techniques can improve AVO analysis in CMP and CRP gathers. Structural Filter greatly enhances the S/N of gathers by removing abnormal single high amplitude events, outliers and strong random noise. At the meantime, the relative amplitude is well preserved. A smoother AVO anomaly is observed in the gathers with enhanced S/N. The gather was measured for AVO fit using a 2-term Aki-Richards equation. The AVO gradient analysis shows an amplitude change discontinuity at far offsets. We speculated that this is caused by tuning effect. Gabor deconvolution is used to increase the resolution of the data and remove the Q effect. The gradient analysis showed a much more reasonable class 2 AVO anomaly with a fit of 0.85 for top of the layer after Gabor deconvolution.
The time-lapse OBN (Ocean Bottom Node) seismic data acquired by Shell in 2007 and 2010 in the Mars field, is a true wide azimuth marine dataset that provides a good opportunity to study the azimuth and offset dependence of time-lapse depth-shifts and amplitudes. The goals of such analysis are to gain a better understanding of the response of seismic waves to geomechanical changes happening in the field and to improve our ability to discriminate between pressure and saturation signals using the time-lapse AVO response. In this paper we compare the conventional time-lapse AVO analysis with an alternative method based on the combined use of time-lapse amplitudes and time-lapse depth-shifts to separate pressure and fluid-related changes on the Mars time-lapse OBN data.
Interpretable, easy-to-calculate corrective terms are added to first order (i.e., Aki-Richards type) approximate solutions of the Zoeppritz equations. The corrections quantitatively and qualitatively account for the influence of large contrasts and target anelasticity in the precritical regime. The formulation permits several observations pertinent to AVO to be made, regarding
1. The non-negligible importance of target VP to mode conversions, i.e., the angle dependence of RPS;
2.The importance of the number VP/VS = 2 to all elastic reflection coefficients;
3. Wave scattering from contrasts in QP and QS alone; and
4. The relationship between reciprocal quality factors and the frequency rate of change of anelastic reflection coefficients.
The basics of the approach are summarized and the above points are illustrated with AVO curves calculated from plausible large contrast elastic and anelastic media.
This paper applied AVO to oil and brine discrimination for cavern carbonate reservoirs in Tarim Basin, Northwest China. The intercept (P) and gradient (G) attributes are extracted from the top and base of the cavern reservoirs encountered by 16 known wells. In the P-G cross-plot domain, oil and brine saturated cavern reservoirs are well separated, with oil wells having a positive G and brine wells having a negative G. We performed AVO forward modeling for the cavern carbonate reservoir and the results show that reservoir Vp/Vs ratio is the controlling factor in the AVO signature at the top and base of the cavern carbonate reservoir. The brine saturated case with a high Vp/Vs ratio exhibits class I AVO response, i.e., amplitude decreases with incident angle, while the oil saturated case with a low Vp/Vs ratio, exhibits class III AVO signature, i.e., amplitude increases with incident angle. Based on these findings, we designed an oil-brine classifier using the P-G attributes and implemented oil-brine prediction for 21 wells before drilling. The real drilling results show that the correct rate of this oil-brine classifier is about 81%. The results are promising and show that AVO could be an effective tool for oil and brine discrimination for cavern carbonate reservoirs.
In some areas, seismic data can exhibit the effects of strong azimuthal anisotropy (AA). One of the major causes of AA can be anomalous horizontal stress regimes, which can be modeled as horizontally transverse isotropy (HTI). The Stybarrow field, located offshore NW Australia in the Carnarvon sedimentary basin, is one such area, where strong horizontal stress conditions have been present throughout the basin’s tectonic history. We find evidence for AA in repeat 3D seismic data acquired at two separate azimuths over the Stybarrow field. AA is observed in amplitude versus offset (AVO) reflection amplitude difference maps and cross plots, and is consistent with dipole shear logs and borehole breakout data in the area. We model azimuthal AVO responses using Ruger’s HTI AVO equation, using the anisotropy parameters derived from dipole shear logs, and compare the results with AVO data from the two 3D seismic surveys. Certain fault blocks (but not all) exhibit the same AAVO trend in the seismic data as those modeled from log data, consistent with a stress-induced HTI anisotropic model interpretation.