Many oil companies routinely evaluate prospects for their drilling portfolio and seismic amplitude anomalies play an important role in this process. When these anomalies occur at a potential reservoir level, they are often called DHIs or direct hydrocarbon indicators, which are changes in reflection response that may be related to oil and/or gas accumulations. Examples of DHIs include bright spots, flat spots, dim spots, character/phase change at a projected oil or gas/water contact, and an amplitude variation with offset. Many uncertainties should be considered and analyzed in the process of assigning a probability of success and resource estimate range before including a seismic amplitude anomaly prospect in an oil company's prospect portfolio.

In the process of quantifying resources/reserves, geoscientists attempt to employ all the available pertinent information to produce the most accurate results. The presence of direct hydrocarbon indicators (DHI) on seismic data can have a significant impact on the reserve/resource calculations not only for volumes, but also uncertainty levels. In 2001 a consortium of oil companies was organized in an attempt to understand seismic amplitude anomalies interpreted as DHIs and their impact on prospect risking and resource calculations (Roden et al., 2005; Forrest et al., 2010). The geologic setting, seismic and rock physics data quality, DHI characteristics, and calibration of drilling results are all incorporated into a database in a consistent and systematic process. From this process, the evaluation of 217 prospects and associated well results has enabled an in-depth understanding of the relevant key aspects of seismic amplitude anomalies and how they relate to drilling results.

Essentially all companies exploring for oil and gas should perform a risk analysis to understand the uncertainties in their interpretations and to properly value order prospects in a company’s drilling portfolio. For conventional exploration in clastic environments, primarily sands encased in shales, a key component of the risk analysis process is evaluating direct hydrocarbon indicators, which can have a significant impact on the final risk value. We investigate the role AVO plays in the risk assessment process as a portion of a comprehensive and systematic DHI evaluation. Documentation of the geologic context and quantification of data quality and DHI characteristics, including AVO characteristics, is necessary to properly assess a prospect’s risk. A DHI consortium database of over 230 drilled prospects provides statistics to determine the importance of data quality elements, primarily in class 2 and 3 geologic settings. The most important AVO interpretation characteristics are also identified based on statistical results and correlated with well success rates. A significant conclusion is the relevance of AVO in risk analysis when it is the dominant component in the DHI portion of the risk. Critical in the risk assessment process is understanding the role AVO and DHI analysis play when prospects approach class 1 geologic settings. The impact that hydrocarbons have on the seismic response is significantly diminished in this setting versus the other AVO classes. All of these observations confirm the necessity of properly evaluating a prospect’s geologic setting and implementing a consistent and systematic risk analysis process including appropriate DHI and AVO components.

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Essentially all companies involved in oil and gas exploration and development must account for the various geologic risk factors associated with their specific prospects. Since seismic data (calibrated with well control if available) are a primary interpretation tool 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 geologic chance factors and ultimately the probability of drilling 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 predrill estimates of Pg. This goal was accomplished by addressing specific seismic amplitude characteristics in different geologic environments, and by quantifying the quality of the data from which these interpretations are made.