Pushing the Envelope: West Africa DHI'S
Alex Martinez, David McAdow, and Matthew Novak
ExxonMobil Exploration Company, Houston, TX
Tertiary clastic sedimentary rocks (primarily slope shales and turbidite sandstones) in much of deepwater West Africa have acoustic rock properties that allow seismic data to exhibit Direct Hydrocarbon Indicators (DHI's). To date, DHI technology has been successfully used to help discover billions of barrels of hydrocarbon in these basins.
Not all seismic anomalies are DHI's, and not all DHI's are of equal quality. For this reason a DHI rating and risking method has been developed to aid data analysis and determine risk of leads exhibiting seismic amplitude anomalies. The method involves comparing the observed seismic anomaly to expected seismic responses and other known DHI's for calibration. DHI attributes fall into general categories associated with the observed amplitude response, conformance to structure and fluid contact reflections, but vary by rock properties (which are typically compaction driven). Seismic data quality and overlap between expected wet and hydrocarbon reservoir responses are also key factors used in the rating and risking process. Integration and rationalization of the DHI risk with geologic risk assessment is a final, critical step to ensure plausibility and reasonableness of the interpretations.
Historically, dry holes and sub-economic hydrocarbon accumulations have been associated with anomalies exhibiting only one or two DHI criteria. These anomalies are now attributed to low-hydrocarbon saturation, anomalous shales or silts, very high porosity sands, or inadequate and/or substandard seismic data. Examples are presented to illustrate the techniques used to identify the spectrum of AVO classes and highlight the challenges in DHI prediction. Ultimately, our experience indicates that multiple DHI criteria (e.g., AVO, amplitude conformance, etc.) are associated with successful wells. Care should be taken to not technically rationalize the lack of these characteristics when fundamental rock physics suggests otherwise.