Local frequency as a direct hydrocarbon indicator

Abstract

L Lo oc ca al l f fr re eq qu ue en nc cy y a as s a a d di ir re ec ct t h hy yd dr ro oc ca ar rb bo on n i in nd di ic ca at to or r L Lo oc ca al l f fr re eq qu ue en nc cy y a as s a a d di ir re ec ct t h hy yd dr ro oc ca ar rb bo on n i in nd di ic ca at to or r S Sh he en ng gh ho on ng g T Ta ai i, , C Ch ha ar rl le es s P Pu ur ry ye ea ar r, , J Jo oh hn n P P. . C Ca as st ta ag gn na a, , U Un ni iv ve er rs si it ty y o of f H Ho ou us st to on n. . SummaryAs a seismic wave propagates, it loses energy due to spherical divergence, scattering, intrinsic absorption and reflection at interfaces where rock properties change. The amplitude and frequency responses of the reflected seismic wave are influenced by a variety of factors including: geologic structure, layer thickness, lithology, and pore fluid properties. When the seismic wave travels back to the surface, it also bring back the information related to stratigraphic features, rock property changes and hydrocarbon accumulations. Each reservoir has its own characteristic seismic frequency response because of its unique rock and fluid properties discriminating it from the surrounding environment. We utilize a spectral decomposition method to extract the characteristic frequency components from seismic data and identify low frequency anomalies. To understand the underlying physical factors of the low frequency anomaly, we build a set of wave-equation based synthetic forward modeling. The result of our analysis shows that seismic waves travel more slowly through gas zone than the background material is a main reason for seismic time series delay and low frequency anomaly in the thin layer reservoir. Our explanation has been applied in the analysis of frequency anomalies corresponding to gas-bearing sands in the Gulf of Mexico fields.

Spectral analysis of ambient ground-motion—Noise reduction techniques and a methodology for mapping horizontal inhomogeneity

“…4b, suggest that some stations exhibit elevated V/H-amplitudes around 2 Hz. It was suggested that these or similar features could carry information on reservoir-related modifications in the wave-field (Dangel et al, 2003;Lambert et al, 2009;Saenger et al, 2009;van Mastrigt and Al-Dulaijan, 2008). Four spectral seismic attributes are used in this study to quantitatively describe these features and to map their distribution in space and time.…”

“…Therefore the studies are not able to falsify the hypothesis for these reservoirs. Results from other case-studies are more positive with respect to the hypothesis but unfortunately these studies either (i) offer only limited conclusiveness because of high anthropogenic noise and/or complex geology Saenger et al, 2009;Schechinger et al, 2009), (ii) only provide an incomplete description of the applied processing methods (van Mastrigt and Al-Dulaijan, 2008) or (iii) present only isolated measurements (Dangel et al, 2003).…”

“…Wave propagation phenomena related to high attenuation and the associated increase in reflectivity are utilized in seismic exploration for direct hydrocarbon indication, for example within the framework of low-frequency shadows below gas reservoirs (Chopra and Marfurt, 2005). Mechanisms that have been suggested to play a role in this context include: frequency-dependent reflection of waves due to frequency-dependent attenuation within the reservoir (Korneev et al, 2004); peak-frequency shifts and amplitude distortion caused by overlapping reflections from top and bottom of a thin layer (tuning effect) (Kallweit and Wood, 1982;Tai et al, 2009); resonant scattering, excited by incident seismic waves, which causes an inhomogeneity to oscillate with its natural frequency and to re-emit waves into the medium (Korneev, 2008;Korneev and Johnson, 1996;Liu et al, 2000). All these phenomena are frequency-dependent and particularly observed at low seismic frequencies (e.g.…”

Low-frequency seismic analysis and direct hydrocarbon indicators

“…Tai et al, (2009) described that when a seismic wave travels through the subsurface, the properties of the seismic wave -e.g. amplitude and frequency content -are affected by, among other things, geological structures, layer thicknesses, lithology, maturation of porosity and fluid properties in the reservoir.…”

“…It is this information that we try to decipher based on our objectives. Tai et al (2009) consider the phenomenon of low-frequency energy anomalies associated with reservoir zones (Figures 1 and 2). One sees in their analysis that the connection between hydrocarbon occurrence and the character of low-frequency anomalies is not always simple.…”