A Mass Balance Charge Model Explaining the Volume and Distribution of Asphaltene at the Crest of a Deep-Water Reservoir: Gulf of Mexico

Bruce Wagner and Leon Dzou
BP America, Houston, TX

King Field located at Mississippi Canyon Block 129 is a structurally trapped Miocene oil accumulation on the north flank of a salt associated anticline. During delineation drilling, a “tar mat” was penetrated by the structurally highest well in the field. In this zone the reservoir pore spaces were completely occluded with a “tar-like” solid. Subsequent geochemical evaluation of those solids indicates they were primarily composed of asphaltenes. Location of a tar-mat at the crest of a structure is not typical for petroleum accumulations.

Geochemical evaluation of solvent rock extracts, combined with compositional and biomarker data from associated oils and solution gases in this reservoir discriminates among the common causes for deasphalting. The proposed charge mechanism producing this asphaltene distribution is vertical entry of petroleum near the structural apex, which focuses a flux of petroleum at the crest. The reservoir fluid approaches bubble point pressure at the structural crest, causing precipitation of asphaltenes. Continued charge at the crest displaces the asphaltene-reduced oil downdip, where pressure and temperature both increase and the oil is less saturated.

Pore occlusion requires more asphaltenes than is contained in the oil in the pore space. Additional asphaltenes must be brought to the pore, cycling out the spent oil. A mass balance approach indicates three to sixteen times the original oil volume was exchanged in the asphaltenes-occluded section of the trap. Appraisal drilling determined the occluded reservoir was ~10% of the total reservoir volume (10x cycling), within the range defined geochemically.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005