The interpretation of strong seismic reflections as direct indications of gas accumulation (often called ?direct detection of hydrocarbons?) has changed the probability of a successful gas strike and significantly affected bidding for offshore leases. The physical principles underlying such interpretation are discussed. Oil and gas in a rock1s pore space affect the density, seismic velocity, reflectivity and absorption. However, many different factors give rise to amplitude anomalies and data must be processed so that irrelevant amplitude variations do not obscure those which are significant. Amplitude anoma1ies can be created by factors which do not relate to the subsurface, subsurface situations other than hydrocarbon reservoirs can give rise to amplitude anomalies, and gas accumulations do not necessarily give rise to amplitude anomalies. Other evidences of hydrocarbon reservoirs include shadow zones, time sags, velocity anomalies, altered multiple pattern, and detection of sea-floor seeps.
The big geophysical event of 1972-73 was the first extensive use of the amplitude of a seismic event as an indicator of in-situ gas. Gas reservoirs in a classic section have exceptionally low acoustic impedance; consequently, the interfaces at their top and base may be exceptionally strong reflectors. This has optimistically come to be called, "direct detection of hydrocarbons, II a misuse of words, because it is matter in the gaseous state whose effects are being detected rather than hydrocarbons. Also, while the correspondence between gas accumulations and amplitude anomalies may be good, gas may not give rise to an amplitude anomaly and an amplitude anomaly may exist for reasons other than gas accumulation. Nevertheless, amplitude anomalies doubtless affected recent bids in lease sales in the Gulf of Mexico and elsewhere in a very significant way.
The detection of hydrocarbons dissolved in sea water has been used for several years to find hydrocarbon seeps in the sea floor. High-frequency seismic methods have located gas bubbles leaking from sea-floor seeps. Recent seismic amplitude measurements have indicated shallow gas pockets which suggest the path where gas has 1eaked from deep reservoirs along fault zones. Changes in sea-floor reflectivity are seen which are attributed to gas leaking from below. It thus appears that 1eakage from reservoirs is more common than previously supposed.
In this paper I shall review the physical principles involved in interpreting seismic data with respect to hydrocarbon accumulations (and lithology). Inasmuch as seismic amplitude is the predominant indicator, I shall first enumerate factors which affect the amplitude seen on seismic records and then elaborate on velocity, reflectivity, interference, absorption and other important aspects.
Many factors (Figure 1) affect amplitude or our ability to recognize amplitude variations (Sheriff, 1973). Most of these have no exploration meaning but we must recognize them lest they confuse the interpretation of variations which are significant. Among the factors we may enumerate the following:
Source strength and coupling.
Hydrophone sensitivity and coupling.
Directivity of source or receiver arrays or trace mixing.
Instrument gain, filtering and balance.
Spherical divergence and raypath curvature.