Geologic origin of aquifer with high resistivity in the carbonate reservoirs in South China: A case study from the Lower Permian Qixia Formation in West Sichuan Basin

Abstract

More and more geologic phenomena distinguished from those in a normal well log analysis appear successively along with the deepening of hydrocarbon exploration. Up till now, most reports have focused on low resistivity oil/gas layers in clastic reservoirs, but very few on high resistivity aquifers in carbonate reservoirs. In view of this, in a case study from the Lower Permian Qixia Formation reservoir in the Sichuan Basin, we studied the geologic reasons of high-resistivity aquifers in the carbonate reservoirs in respect of reservoir characteristics, reservoir filling features and formation water property. In conclusion, apart from the engineering factors, the following factors are also presented herein. (1) The tightness of carbonate reservoirs lowers the conductivity of electrons in the reservoirs, which results in high resistivity. (2) The wide distribution of bitumen in the carbonate reservoirs also lowers the conductivity of electrons in the reservoirs, thus leading to high resistivity. (3) If the aquifers contain fresh water, the amount of ions will decrease, lowering conductivity; quite similar to that of hydrocarbon layers in well log interpretation, thus the resistivity of the aquifer is very high. This understanding provides good reference for logging interpretation and exploration in the areas with tight carbonate reservoirs.