Unexpected Low-Resistivity Pay Zone while Drilling a Horizontal Well: A Case Study from the Oriente Basin in Ecuador

Abstract

Abstract Low-resistivity pay reservoirs have been described in many basins around the world. Technical information available explains the causes related to hydrocarbon bearing zones, represented by low resistivity values. These causes are mainly associated with mineralogy and complex pore structures. All the wells drilled and produced in the Johanna Este field in Ecuador have met the condition of pay zones with high resistivity values in relation to the well-known water formation resistivity. A step change in the understanding of the main reservoir M1 sandstone occurred after drilling the horizontal well JE34H. The first half of the lateral section was drilled with resistivity values close to water resistivity. Vertical variations in the resistivity were confirmed in the second half of the well, with no lithologic barriers between both zones observed in the logs. The oil production obtained from this well was recognized as a field record, and the first time it was associated with low resistivity values. After an exhaustive review of the available information including LWD electrical logs, the tool functioning, and cuttings description, it was possible to explain the low-resistivity pay within the M1 sandstone reservoir at the mineralogical level. The density-neutron cross-plots used for lithology identification show that the reservoir is mainly quartz with some mineral mixture. The target zone is mainly made up of medium-to-coarse grain with no visible matrix in the composition, showing no evidence of compaction or cement in density-neutron logs. Macroscopic anisotropy was also discarded as the reason for the decrease in the measured resistivity. In addition, structural compartments and preferential pathways for water invasion created by faults and/or fractures were discarded based on the azimuthal density image interpretation. The possible causes for the low resistivity pay were microporosity, presence of disseminated clay, and presence of conductive minerals. Special analysis (cores, magnetic resonance, capillary pressure, SEM) are required in order to understand the mineralogy and porosity distribution. A low-resistivity pay within the M1 has opened a door for future analysis by means of conventional or special open hole and cased hole logs in order to identify reserves that could be bypassed.