Abstract: Interactions Between Hydrocarbons, Water, and Sandstones: Implications for Reservoir Quality and Secondary Migration Timing

Robert J. Pottorf, Lori L. Summa

To constrain the timing of hydrocarbon migration and cementation, we have analyzed authigenic cements in cores from the North Sea and Gulf of Mexico using an integrated approach that includes: (a) analysis of hydrocarbon and aqueous fluid inclusions in quartz, feldspar, and carbonate, (b) K-Ar dating of illite, (c) paleomagnetic analysis, (d) burial-history modeling, and (e) PVT calculations for hydrocarbons and water. We conclude that migration of hydrocarbons into sandstone reservoirs commonly results in chemical interactions that record the timing of hydrocarbon arrival. The types of interactions and their effects on reservoir quality vary depending on the mineral assemblage:

(1) High hydrocarbon saturations slow the reaction kinetics for fibrous illite formation. Therefore the volume of authigenic illite is commonly greater in water legs than in hydrocarbon legs and K-Ar ages of illite do not directly date hydrocarbon arrival.

(2) Significant quartz cementation occurs in the presence of hydrocarbons. Thus large-scale fluid flow may not be responsible for precipitation of quartz cement and the silica is presumably derived from local sources.

(3) Hydrocarbons reduce detrital iron-oxide coatings on quartz to alter the magnetic signal of the rocks. These redox reactions result in precipitation of Fe-enriched chlorites in hydrocarbon legs.

(4) Carbonate-cemented zones may occur immediately below the hydrocarbon/water contact. The net effect is that reservoir porosities may be 2-3% higher in hydrocarbon legs than in water legs. These differences are not readily detected using standard sampling and petrographic point-count analysis, but are more easily resolved with porosity logs. In addition, a <1% difference in fibrous illite between hydrocarbon and water legs can have a dramatic effect on permeability. Consequently, hydrocarbons are important for maintaining reservoir permeability in sandstones where fibrous illite occurs. Finally, there is some indication that gas is more effective than oil at inhibiting cementation.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994