Low-frequency reflection seismic and direct hydrocarbon indication

Abstract

Low-frequency reflection seismic has been used as an indicator to detect hydrocarbons in petroliferous reservoirs. A number of spectral decomposition methods have been developed to derive seismic from time to frequency domain. The application of low-frequency seismic computed by the Gabor-Morlet transform (GMT) and instantaneous spectral analysis (ISA) methods revealed one of lowfrequency shadow effects, amplitude attenuation, in the southern North Sea. According to computed low-frequency (< 20 Hz) instantaneous spectral profiles through discovered wells, strong responses are observed on hydrocarbon-saturated intervals. By contrast, on higher frequency (> 20 Hz) instantaneous spectral profiles, frequency amplitude is vanished from the petroliferous intervals, however, strong responses are presented either above or below these pay zones. The loss mechanism of high-frequency energy is considered to be contributed by P-wave attenuation of stratigraphic heterogeneity. This type of low-frequency shadow effect can illustrate the probable existence of an oil reservoir and can be applied to detect the existence of hydrocarbon directly. Based on the distribution of attenuated frequency amplitude, a total of 140 km2 of prospects have been identified from both Cretaceous-Paleogene chalk reservoirs and sandstones beneath the basal Cretaceous unconformity (BCU). The identified chalk prospects are mainly distributed along the rim of anticlines on the Lindesnes Ridge, while prospective clastics are stratigraphically entrapped on the slope of the Grensen Nose. In addition to using low-frequency reflection seismic to detect hydrocarbon directly, frequency components can be applied to calibrate the interpretations of horizons and faults as well.