Application To The Hassi -Messaoljd Cambrian Reservoir Of A New Quantitative Interpretation Method For Shaly Sands Available to Purchase

Quantitative log interpretation in shaly sands has always been a difficult task, but the particular conditions at Hassi-Messaoud made it even more difficult. The Cambrian reservoir is overlayed with Triassic massive salt beds, sloughing shales and high pressure calcium chloride water bearing dolomite of Jurassic age. Consequently, wells are drilled with salt saturated water base muds loaded with barite and the reservoir is diamond cored with oil base muds or occasionally with fresh water base mud. Cambrian formation water is salt saturated (400 000 ppm NaCl) but the lack of a clean zone makes the SP log almost useless for PSP determination. Furthermore, the nearest shale is in the Trias, behind casing so that there is no shale base line available when logging the pay zone. In most parts of the field only the top of the sand contains oil, at irreducible water saturation whereas the bottom of the formation is water bearing, with a thick capillary transition zone, extending often over more than 50" of section. The Gamma Ray curve is also difficult to use because of the presence of radio-active minerals, mica and heavy minerals in the sand matrix and yet the shale content of the reservoir cannot be neglected in a quantitative determination of the oil reserves. The overall porosity of the reservoir varies from 5 % to 15 % (1), its shale content varies from a few percent up to more than 30 % and considerable amount of secondary silica is observed on thin sections. The R1 zone at the top of the reservoir has an average coreporosity of 7 % and an average shale content of only 7 % but porosity and shaliness are higher in the lower zones R2 and R3, respectively 10 % average core porosity and 14 % average clay content for R2 and 15 % average core porosity and 25 % average clay content for R 3.