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
ISSN
0126–5539; e-ISSN 2682-7549
