Warta Geologi, Vol. 46, No. 3, December 2020, pp. 230–234


Warta Geologi, Vol. 46, No. 3, December 2020, pp. 230–234

High resistivity reservoirs (causes and effects): Sahara field, Murzuq Basin, Libya

Abubaker Alansari1,*, Ahmed Salim1, Abdul Hadi Bin Abd Rahman1, Nuri Fello2, Hammad Janjujah3
1 Department of Geoscience, Universiti Teknologi PETRONAS, Malaysia
2 National Oil Corporation, Tripoli, Libya
3 American University Beirut, Lebanon
* Corresponding author email address: alan.abu@outlook.com

Abstract: High and low resistivity values is an alarming phenomenon that is usually associated with a very complicated reservoir history and worth looking into. Ordovician sandstone reservoirs are the primary oil producers in the Murzuq basin oil fields that is characterized with an average porosity of 14%, permeability range 410-10,760 md and clean quartz aranite composition. More than fifty wells were drilled in Sahara oil field, but only four of them were announced to have high resistivity values more than 100k ohm-m and ten others to be considered as low resistivity wells (below 50 ohm-m). Therefore, average deep resistivity was mapped in both water and oil legs using all available data set, and the top reservoir was employed as a trend map. They showed distinctive trends for low resistivity readings in oil-leg and confirmed the extreme deep resistivity nature for the wells (W7, W8, W9, and W10). Height above oil water contact and capillary pressure was also calculated for all the wells and revealed a high pressure (400 psi) at the location of the high resistivity wells. As a result, of higher capillary pressure in thicker reservoir area oil might have been able to displace water through geological time by benefitting of more considerable height above oil-water contact, higher connate pressure, and buoyancy forces support, which resulted in occupying all the larger pores and pushed the water into minor scattered pores leading to gradual alteration of reservoir wettability from water to oil-wet. Hence, the brine fluids will no longer be connected to each other inside the pore system. Therefore, they will lose their contribution to resistivity readings, and the resistivity tool will encounter a more resistant medium, which in turn will lead to underestimation of water saturation.

Keywords: High resistivity reservoirs, Upper Ordovician reservoirs, Murzuq basin, capillary pressure

DOI : https://doi.org/10.7186/wg463202011