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Abstract: Distribution and Quantification of Thin Tar Mats in a North Sea Field

Bernard Carpentier, Alain-Yves Huc

Distribution of infra-meter thick tar mats has been investigated in a North sea oil field and has resulted in a high resolution description of tar mats location within 13 wells more or less evenly distributed across the field (45 km2). This paper presents a procedure to assess the 3 D distribution and the total volume of non-productive heavy oil using geostatistical approaches.

In the considered area, the tars are preferentially deposited in the southeastern part of the field in reservoir zones displaying relatively high pore volumes. The suggested mechanism of emplacement of the tar is related to the deposition of bitumen onto minerals along favored oil avenues during the process of oil feeding of the reservoir.

Multiple regression applied on each individual reservoir units provides a regional relationship between, on one hand, cumulative thickness of tar (dependent variable) and, on the other hand, total pore volume and distance to the inferred point of entry of the oil filling the field (predictor variables).

Geostatistical analysis have been performed on the reservoir facies distribution in order to build a stochastic simulation (Heresim software). The simulations are based on the interpretation of the depositional sequence using variograms and proportion curves. The results are 3D images of the reservoir with respect of the spatial structure of the lithofacies distribution. Pore volume variability, measured for each lithofacies, has been added into the simulation results. The resulting images of the reservoir porosity, used in conjunction with the multivariate correlations, led to estimate the 3D distribution of tar mats in the field, allowing a quantitative evaluation of the total tar volume.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France