The Elusive Low Frequency Shadow

Abstract

Abstract Low frequency shadows have long been accepted as direct hydrocarbon indicators, but few convincing examples have been published. A deliberate search for low frequency shadows on seismic data from multiple fields offshore Malaysia finds no clear candidates. Prospect-scale frequency shadows are difficult to detect with conventional seismic attributes because the subtle spectral changes caused by attenuation are masked by the much larger and more variable effects of reflection interference. Detectable low frequency shadows are too rare to serve as practical direct hydrocarbon indicators. Introduction Low frequency shadows have been widely accepted as direct hydrocarbon indicators since the 1970s (Sheriff, 1975; Taner et al., 1979). The prevailing view is that "low frequency shadows often have been observed beneath amplitude anomalies associated with gas reservoirs" (Chopra and Marfurt, 2007, p. 141). The idea is appealing. A low frequency shadow is a zone in seismic data characterized by anomalously low frequencies that occurs beneath a causative body. The body strongly attenuates the seismic energy, preferentially reducing the high frequencies Seismic reflections within a shadow lack high frequency content relative to surrounding reflections. Shadows have fairly clear beginnings but tend to fade quickly with depth. Because gas sands are the chief cause of anomalous attenuation, low frequency shadows serve as hydrocarbon indicators, and reduce risk in bright spot exploration. The large and obvious effects of gas clouds, gas chimneys, and shallow gas are not considered here as they are not primary exploration targets, but instead indicate gas escaping from deeper levels (Brouwer et al., 2008; Ghazali et al., 2013). In spite of a long history and popular acceptance, few examples of low frequency shadows have ever been published, and none are especially convincing hydrocarbon indicators. This contrasts markedly with the record of other hydrocarbon indicators, such as bright spots, flat spots, amplitude conformance to structure, and AVO, for which published examples are numerous and credible. This raises the question: Do low prospect-scale frequency shadows really exist? We search for prospective frequency shadows in poststack seismic data from a number of producing fields offshore Malaysia. We employ conventional seismic trace attributes, frequency decay curves, and volume spectral decomposition. We review many gas sands, bright spots, and frequency anomalies, but none clearly constitute low frequency shadows. Method We apply reflection strength, average frequency, and bandwidth attributes to poststack seismic data from offshore Malaysia to characterize patterns of spectral change and identify frequency anomalies. The frequency and bandwidth attributes represent averages weighted by the instantaneous power, and are derived in 30 ms (15 sample) windows. We supplement this analysis with a simple volume spectral decomposition that separates low and high frequency components of the seismic data to reveal local frequency anomalies.