Low Saturation Gas Reservoirs Discrimination – An Integrated Approach

Low saturation gas reservoirs are commonly found in the deep to ultra-deep water
environment of the Gulf of Mexico. Most low saturation gas reservoirs result from a breached seal or insufficient charge in a petroleum system. The breached seals could take place either from past or on-going leakages. A breach in a top seal and/or a fault seal can easily result in a low saturation gas reservoir. It has been a major technical challenge to identify low saturation gas reservoirs before penetrating them. Since deepwater exploration drilling has been extremely expensive, assessing risk for those amplitude targets can have significant economic impact.

AVO analysis, amplitude vs. background (A/B), amplitude conformable with depth structure, and low frequency shadow are the technologies commonly applied to validate gas reservoirs in Tertiary Basins. We analyzed seismic amplitudes, AVO responses, instantaneous frequencies, and effective stress over 10 discoveries and 12 dry holes in an area near the intersection of Mississippi Canyon, Atwater Valley, Desoto Canyon, and Lloyd Ridge of Eastern Gulf of Mexico. The study area falls in the vicinity of the Independent Hub producing fields. The water depths range from 7,000 to 9,000 ft. Based on the analysis results, the team formulates an integrated workflow, by applying differential weights to the index of A/B, AVO, low frequency shadows, and effective stress, to mitigate the uncertainty of risk for all Miocene amplitude plays. The workflow was applied to rank 13 prospects. The integrated methodology has been proven as an effective way to reduce the uncertainty on predicting the occurrence of low saturation gas reservoirs.