LOG-DERIVED RESERVOIR PROPERTIES AND POROSITY PRESERVATION OF UPPER JURASSIC ARAB FORMATION IN SAUDI-ARABIA

Various types of wireline log analysis were conducted for the Arab Reservoirs of Upper Jurassic age in eastern Saudi Arabia. After evaluating the values of the cementation factor m in the Archie equation and the formation water resistivity R(w), values such as total, primary and secondary porosities, lithological composition, water (or hydrocarbon) saturation and permeability were estimated using combinations of various logs. Some of the results, such as the estimated porosity and permeability, were then compared with the available core analysis results. The estimated percentage of dolomite was also plotted against the percentage porosity in an attempt to show a possible effect of dolomitization on porosity; dolomitization seems to increase the porosity in the early and intermediate stages, but tends to reduce porosity in the later or near-completion stages of dolomitization. This finding coincides with the earlier work of Power based on petrographical and petrophysical studies. The calculated average porosity of the Arab Reservoirs is up to 30%, which is exceptionally high if the effects of depth of burial between 4000 and 9000 ft and the geological age of about 160 million years (Upper Jurassic) are taken into consideration. To explain such high porosity values for the Arab Reservoirs, probable geological, physical, chemical factors, such as acidic formation fluids, reduced fluid mobility and ion-exchange capacity, tectonic forces and dolomitization are suggested.