Roger Holeywell

The impact of seismic amplitudes on prospect risk analysis

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).

The Role of AVO in Prospect Risk Assessment

AbstractEssentially 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 b...

Risking seismic amplitude anomaly prospects based on database trends

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 companys prospect portfolio.

Risking Seismic Amplitude Anomaly Prospects Calibrated to an Industry-Wide Database

Over the past decade, a consortium of oil companies in the US and Europe has developed a work process to interpret and risk seismic amplitude anomalies on exploration and development prospects. Over 175 prospects have been reviewed and documented in a database where the geology risk factors, seismic and rock physics data quality and amplitude anomaly characteristics have been analyzed to calculate the probability of geological success (Pg - flowable hydrocarbons). The drilling results for each prospect were compared to the calculated Final Pg, a function of the Initial Pg (geology) and the DHI Index which is the impact of the anomaly characteristics modified by data quality. The Final Pg and DHI Index compared to drilling result trends indicate a DHI Threshold Effect on prospects at the high end of the risk spectrum and a low Pg Threshold Effect at the low end of the spectrum. In other words, on the upper end of the spectrum there is a point at which a significant amount of the risks have been reduced to dramatically increase the Final Pg and DHI Index. On the lower end of the spectrum there is a threshold below which essentially all the wells are dry holes. Using these database trends, Pg can be calibrated to a wide range of 5% to 95% Final Pg. Therefore in DHI prospects, geoscientists should be more optimistic in analyzing the prospect risk if Final Pg and DHI Index are above certain threshold levels. Conversely, the low end of the risk profile prospects in the portfolio should probably be farmed out or not drilled with an overall goal of upgrading the exploration portfolio.