Petroleum Column Heterogeneity as a Geological Tool: Origin and Significance of Tar Mats

WILHELMS, ARND, University of Oslo, Oslo, Norway, and STEVE LARTER,* University of Newcastle, Newcastle upon Tyne, United Kingdom

Compositional heterogeneity in liquid petroleum columns can be related to variation in (1) the maturities and facies of the source rocks feeding the field, (2) the complexity of the carrier beds acting as filling conduits, (3) the controls of physical mixing, diffusion and in-reservoir adsorbtion on the homogenization of heterogeneities inherited by the field during its filling, and (4) alteration of petroleum composition post trapping. The absence of convection in most liquid petroleum columns plus the inability of diffusive mixing in petroleum columns to homogenize large scale (1 km) heterogeneity even on a geological time scale, allows the petroleum geologist/geochemist to use these heterogeneities as geological tools. We discuss a very significant petroleum column heterogeneity, n mely tar mats, and their formation through petroleum alteration and their geochemical and geological significance.

Tar mats, or viscous oil barriers, are commonly found in paraffinic oil reservoirs (as, e.g., in the North Sea Oseberg and Ula fields, and in Prudoe Bay field, Alaska). They commonly represent zones of from 1-25 m of asphaltene-rich oil (typically around 50% asphaltenes) sometimes at oil/water contacts with sharp compositional contrast and contact with the rest of the oil column. They are significant in that they represent potential flow barriers; often petroleum columns with tar mats have production and safety problems associated with deasphalting during production. Although there has been much debate on their origin, most field evidence is circumstantial. We present a new model of tar-mat formation based on physical-chemical considerations and an improved chemical structural model o asphaltenes, and we show with case histories how this information can be used to aid the geologist in geologically interpreting reservoir geochemical information relating to field-charging processes.

AAPG Search and Discovery Article #91007© 1991 AAPG International Conference, London, England, September 29-October 2, 1991 (2009)