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Geoelectrical characterization of fluid in hydrocarbon reservoirs: case study on the deep saline aquifer of Hontomín

Abstract

To know the physical properties of fluid contained on the reservoir rocks is a key point on the assessment for its exploration or its use as a gas storage. Electrical conductivity is a transport property of the medium, which is sensitive to the rock constituents (its minerals) but also to the characteristics of the pore space; it depends on fluid salinity, temperature, porosity, pore connectivity, saturation and pressure, and it is more sensitive to oil and gas saturation compared to other properties. In this context Archie's law describes reasonably well the bulk electrical conductivity of the rock as a function of these parameters.

This contribution presents a summary of the geoelectrical characterization of the deep saline aquifer of Hontomín from Previous HitmagnetotelluricNext Hit data. The Hontomín site is located at the southern sector of the Basque-Cantabrian Basin (Western Pyrenees, Spain). Exploration activities have provided geological and geophysical information of the area since 60s. It is a smooth Jurassic domed anticline structure cored by Upper Triassic evaporites. The stratigraphic sequence shows several reservoir and seal units at different ages. The primary reservoir (saline aquifer) and seal units are Jurassic in age.

The Previous HitmagnetotelluricNext Hit (Previous HitMTNext Hit) data were acquired on a grid of 109 closely-spaced broadband Previous HitMTTop soundings covering an area of 3×5 km2. The 3D inversion of the data in the frequency range of 1000 – 0.1 Hz provided the geoelectrical structure of the site, which was correlated with the different lithologies and formations. The model shows the more conductive behavior of the primary reservoir-seal system as it contains the saline aquifer, and the more resistive behavior of the secondary reservoir-seal system. The electrical resistivity of the primary reservoir indicates porosity values between 9% and 17% according to Archie's law. The model also images an EW fault in the south associated to the Ubierna Fault.