Recent developments in the recording and processing of seismic data have permitted the enhancement of certain diagnostic characteristics which have been found to be direct indicators of the presence of hydrocarbons within the sedimentary section. Anomalous changes in the amplitude, frequency, and phase of the seismic wave and variations in the rock velocities and apparent structural attitude of the beds can all be used for recognizing the presence of oil and gas. Under certain conditions it is possible to identify the actual fluid contact within a formation.
The proper interpretation of each of these characteristics can be facilitated through the use of easily generated seismic models which can be compared with the observed data. The geophysicist can vary the model parameters of interval velocity, bulk density, formation thickness and structural configuration which have been obtained from well bore data or which have been hypothesized in areas of unknown subsurface conditions.
The method is found to be applicable to seismic data recorded both on land and in the marine environment; however, the best results have been associated with offshore data. There are some geologic conditions existing which give similar hydrocarbon indicator characteristics but which in reality are not associated with oil or gas. Caution must be exercised in the interpretation and conclusions reached.
Hydrocarbon indicators derived from seismic data are-Just that - indicators. It may be argued that unless we actually can see, touch, or smell petroleum deposits on our seismic cross sections then we do not, in reality, have a direct detection method. Certainly seismic wave propagation theory tells us that we should expect to see various indications of hydrocarbons provided that the proper geologic conditions exist. If one or more of the indicators are observed on our data and, more importantly, if we can correlate these with actual subsurface information where petroleum accumulation is known to exist, then the probability of successful prediction within the immediate area is significantly increased.
The most commonly used seismic hydrocarbon indicators at present are:
In discussing each of these in detail we will relate to various actual data examples. Figure 1 shows a portion of a seismic transverse recorded in the Gulf of Mexico over a known producing gas field. This section is the result of what are considered to be standard processing techniques. Figure 2 is derived from the same seismic data but has had different computer processing procedures applied.
In this discussion we will not detail the how of processing methods but rather we will be concerned with the what of observed indicators. Suffice to say that the processing variations between Figures 1 and 2 involve changes in amplitude recovery, normalization, deconvolution, and filtering.
Let us first examine the geologic situation and reservoir parameters of this field. The top of a sand layer occurs at a depth of approximately 3,800 feet or at a two way seismic time of 1.3 seconds.