Investigating borehole-density, sonic, and neutron logs for mapping regional porosity variation in the Silurian Lockport Group and Salina Group A-1 Carbonate Unit, Ontario

Abstract

The Oil, Gas and Salt Resources Library (OGSRL) is a repository for data from wells licenced under the Oil, Gas and Salt Resources Act for Ontario. It has approximately 50,000 porosity and permeability drill core analyses on bedrock cores. It also has in analogue format, geophysical logs (e.g., gamma ray, gamma-gamma density, neutron, sonic) from approximately 20,000 wells. A significant challenge for geotechnical and hydrogeological studies of the region is the accessibility of digital data on porosity and permeability. Recent work completed on approximately 12,000 core analyses for the Silurian Lockport Group and Salina Group A-1 Carbonate Unit are geographically concentrated within productive oil and gas pools. An opportunity therefore exists to expand the bedrock porosity characterization for southern Ontario by using geophysical logs collected in open-hole bedrock wells that are more geographically dispersed. As part of this study, hard copy files of analog geophysical logs are converted to digital data (LAS format), followed by quality assessment and quality control (QAQC) to obtain meaningful results. From the digitized geophysical data, density, neutron, and sonic logs are selected to mathematically derive porosity values that are then compared with the corresponding measured core porosity values for the same depth interval to determine the reliability of the respective log types. In this study, a strong positive correlation (R²=0.589) is observed between porosity computed from a density log (density log porosity) and the corresponding core porosity. Conversely, sonic log porosity and neutron porosity show weak (R2 = 0.1738) and very weak (R2 = 0.0574) positive correlation with the corresponding core porosity data. This finding can be attributed to different factors (e.g., the condition of the borehole walls and fluids, the type and limitations of the technology at different points in time, knowledge of formation variability for calculations), and as such requires more investigation. The density log measures the bulk density of the formation (solid and fluid phases), and as such the derived porosity values indicate total porosity i.e., interparticle (primary) pore spaces, and vugs and fractures (secondary) pore spaces. The sonic log measures the interval transit time of a compressional soundwave travelling through the formation. High quality first arrival waveforms usually correspond to a route in the borehole wall free of fractures and vugs, which ultimately result in the derived porosity reflecting only primary porosity. As molds, vugs and fractures contribute significantly to the total porosity of the Lockport Group and Salina A-1 Carbonate strata, sonic porosity may not reflect true bulk formation porosity. The neutron porosity log measures the hydrogen index in a formation as a proxy for porosity, however, the current limitations of neutron logging tool fail to account for formation-related complexities including: the gas effect, the chloride effect and the shale effect that can lead to over- or underestimation of formation porosity. As a result, the density log appears to be the most reliable geophysical log in the OGSRL archives for total porosity estimation in the Lockport Group and Salina A-1 Carbonate Unit. Nonetheless, sonic porosity can be combined with density porosity to determine secondary porosity, whereas a combination of density and neutron porosity logs can be used to identify gas-bearing zones.