Essentially all companies involved in oil and gas exploration and development must account for the various geological risk factors associated with their specific prospects. Since seismic data (calibrated with well control if available) is one of the primary interpretation tools to determine these risk factors, the presence of seismic amplitudes that are potentially associated with oil or gas pays is extremely important. However, interpreters evaluating prospects have had to inherently know how seismic amplitudes impact the geological chance factors and ultimately the probability of geological success (Pg). From 2001 to 2004, a DHI Risk Analysis Consortium of oil companies has been working to systematically quantify how seismic amplitude anomalies impact the pre-drill estimates of Pg. This goal was accomplished by addressing specific seismic amplitude characteristics in different geological environments, and by quantifying the quality of the data from which these interpretations are made.
The conventional approach to determine the Pg, which represents the chance of finding hydrocarbons in a reservoir capable of sustained flow, is to evaluate the geologic components required for a hydrocarbon reservoir to exist. The number of chance factors and specific definitions may vary slightly from company to company, but a commonly applied system of five factors is source, migration, reservoir, closure, and containment. However, the presence of a seismic amplitude anomaly may have a significant influence on the risking of the geologic chance factors. For example, the presence of a potential hydrocarbon generated seismic amplitude anomaly may imply a working petroleum system, which would be strong evidence supporting the presence of source. A clearly defined seismic amplitude area also suggests that migration and trapping of hydrocarbons has occurred, that trap seals may be sufficient, and (depending on the geologic setting) reservoir rock may be present. The above logic is of course “circular” in that the presence of an amplitude anomaly does not necessarily indicate it came from a hydrocarbon-charged reservoir. A better approach is to assess the confidence level that a seismic amplitude anomaly is truly generated from the presence of hydrocarbons by asking the following questions:
· How many HCI or DHI characteristics does the anomaly display?
· Should an amplitude anomaly of this type even be present in this geologic setting?
· In this geologic setting, should there always be an amplitude anomaly associated with trapped hydrocarbons?
· Is the seismic data being used to evaluate the amplitude feature of sufficient quality to make reliable interpretations and valid conclusions?
The methodology developed by the DHI Consortium addresses all these issues with an interpretation workflow concept (Figure 1) that has proven to be systematic, consistent, and produced very realistic results based on a calibration data set of almost 100 drilled wells.
In order to determine which seismic characteristics are pertinent for specific geologic trends, it is necessary to classify hydrocarbon generated amplitude anomalies based on their geological setting. The Consortium chose the approach described by Rutherford and Williams (1989) based on three Classes of AVO responses from the top of gas sands.