Alain Rabaute¹, Claudine Durand¹, Sophie Belin¹

(1) Institut Français du Pétrole, Rueil-Malmaison, France

ABSTRACT: Pore-Lining Chlorite in Siliciclastic Reservoirs: Quantitative Estimate of Mineralogy from Logs and Consequences on Formation Evaluation

The physical properties recorded as continuous downhole measurements can be inverted to provide a continuous quantitative description of the mineralogy as a function of depth, including diagenetic clay minerals. We used commonly measured physical properties that account for variations in the mineralogical composition of the matrix, that is total gamma radiation, bulk density, photoelectric factor, K, Th and U contents. A total inversion scheme was successfully tested on two case studies in the context of sandstone reservoirs rich in grain-coating chlorite, one in the upper Silurian Acacus Formation of the Ghadames Basin, Libya, and the second in the uppermost Devonian sediments of the Berkine Basin, Algeria. Best results were obtained the more the log physical properties are discriminant for each mineral in the assemblage, i.e. presence of Fe-rich, high density chlorite, absence of solid solution, etc. Uncertainty ranged from 1 to 10% relative. Inversion results from logs were validated with core sample mineral proportions precisely estimated from a combination of elemental analysis of rocks and chemical composition of minerals. The advantages in formation evaluation are (i) a significantly more precise estimate of the total clay volume, (ii) the ability to calculate the proportions of the different clay minerals, and hence a continuous estimate of CEC and specific surface along the borehole, and (iii) an independent estimate of the porosity value. These are useful parameters when using common water saturation models (Waxman and Smits, Dual Water), and are needed for estimating permeability using new models dedicated to chlorite-bearing siliciclastic reservoirs.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado