SS: Exploration Prospect Interpretation and Risking Using Modern Geophysical Technology - DHI Threshold Effect in Prospect Risking
Authors: Roden Rocky , Forrest Michael C. , Holeywell Roger C. 
Published: July 2009

Seismic amplitude anomalies that are DHI's, Direct Hydrocarbon Indicators, have played a major role in oil and gas exploration since the early 70's. These features are commonly found in offshore Tertiary age sandstones, but are also present in older rock onshore areas. The technology and methods to identify and risk seismic amplitude anomalies have advanced considerably over the years, especially with the use of AVO (Amplitude vs. Offset) seismic technology. A good risk analysis work process is required to rank DHI prospects so oil company management can make business decisions to upgrade their exploration portfolio.

During the past eight years, 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. Almost 150 prospects have been reviewed and documented in a database where the geology risk factors, seismic and rock physics data quality and amplitude anomaly characteristics (as many as 37) 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 DHI Index (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. Published results by ExxonMobil in a similar DHI database study suggests these same threshold effects (although different quantification system). 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.

Introduction

The impact the presence of oil and especially gas has on the seismic amplitude response in many geological settings can have a dramatic impact in risking prospects. Seismic amplitude anomalies that display direct hydrocarbon indicators, DHIs, have played a major role in oil and gas exploration since the early 1970's (Forrest 2000, Hilterman 2001). Over the last 35 years there have been tremendous advances in seismic acquisition, processing, and interpretation, as well as, improvements in the methodology to identify and risk seismic amplitude anomalies. Even with the use of advanced seismic technology and well-equipped interpretation workstations, the interpretation of hydrocarbon induced seismic amplitude characteristics is not always easy or straight forward. An oil industry consortium and at least one major oil company have published information on how DHI's have impacted prospect risking based on hundreds of drilled wells. These studies have shown that when there is a high confidence in the presence of DHI's in any given prospect, there can be a significant uplift in the probability of geological success (Pg) greater than conventional thinking would suggest.