The field of interest is one of ExxonMobil''s development assets in offshore West Africa discovered in the late 90’s. A main appraisal well was recently drilled to confirm the existence of reserves updip of existing well control within the Lower Miocene deepwater reservoir. The oil reservoir is a steeply dipping (20-45 degrees) deepwater channel complex located on a flank trap of a piercement salt diapir. The discovery well was drilled in late 90’s by the “Well A” exploration well and possesses a class II DHI response. Poor seismic data quality prevented subsequent confident predictions of reservoir thickness and reservoir quality updip of the “Well A” well. The application of Spectral Decomposition allowed us to discriminate the most significant frequency content of the reservoir and thus optimize the seismic image quality for stratigraphic and DHI interpretation. Reservoir imaging was high-graded by applying a bandpass filter to preserve the most significant frequencies of the reservoir reflections. The resultant dataset revealed the expected DHI response for a class II anomaly, showed clear reservoir amplitude conformance to structure, improved the stratigraphic details for higher confidence interpretation and ultimately supported the presence of thick high-quality reservoir section updip from the “Well A” well. The “Well B” appraisal was drilled two years later to a target picked on the basis of the spectral decomposition analysis. The well penetrated over 154 meters measured depth (113 meters TVD) of net oil sands, confirming the presence of thick, high-quality oil sands updip of the “Well A” well.


The field presented in this paper is part of an ExxonMobil operated development area in offshore West Africa. The field was originally discovered by the exploration well, “Well A”, which penetrated two reservoir intervals, one of them is the Lower Miocene main objective. The reservoir is interpreted to be a southeast-northwest trending confined deepwater channel complex averaging 2-4 km wide and 60–220 m thick. The channel system is situated on a high-side salt/faultdependent closure juxtaposed against a large counter regional fault (Figure 1)


The “Well A” discovery well encountered 45 m of net oil pay in original hole and a thick blocky wet sand downdip by its sidetrack. “Well B” was proposed as a development well two years later to help prove the presence of thicker reservoir updip, evaluate its quality in the central portion of the hydrocarbon accumulation and better define the resource size. “Well B” was positioned to penetrate the oil bearing reservoir as high on structure as possible with our current drilling constraints due to pore pressure and fracture gradients. However, amplitude extraction from seismic data could not support the presence of thicker or better quality sands updip from “Well A”’s location and the drilling of the well was put on hold.

Rock Physics and Modeling

Rock properties for the reservoir were extracted from the exploration “Well A” well and its sidetrack. Ranges of reservoir properties were modeled to examine the expected signature for increasing reservoir quality filled with oil.

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