Porosity and permeability variations in the Silurian Lockport Group and A-1 carbonate unit, southwestern Ontario
Authors: Sun S , Brunton F R , Carter T R , Clarke J R , Russell H A
J , Yeung K , Cachunjua A , Jin J 
Published: October 2023

This is the first regional porosity/permeability study to incorporate petroleum industry laboratory core analyses submitted to the Ontario government and managed by Ontario's Oil Gas and Salt Resources Library. This study comprises 11,759 analyses for the Early
Silurian Lockport Group of southwestern Ontario from 150 drill cores. The Lockport Group consists of a cyclic succession of dolostones and minor limestones comprising, in ascending order: Gasport, Goat Island, Eramosa, and Guelph formations. This stacked carbonate succession was deposited on an
eastward-deepening carbonate ramp, extending from Michigan, through southwestern Ontario, to Ohio, Pennsylvania and New York. It is overlain disconformably by restricted marine carbonates, evaporites and mixed shales of the Salina Group, whereas unconformably underlain by one of four formations that
include, the Lions Head (a stratigraphic equivalent of part of the Rochester), DeCew, Rochester and Irondequoit. To ensure appropriate stratigraphic assignment of the laboratory test intervals, a quality assurance/quality control review on formational tops was carried out on the 150 cores that were
tested. This regional subsurface work resulted in the reassignment of 846 formation tops that were verified by examination of drill core, drill cuttings, and geophysical well data including gamma-ray, neutron and density logs. Core analysis datasets have been validated by summarizing laboratory
protocols and standards and reconciling data fields in the core analysis database with auxiliary data, including geophysical logs, thin sections, and core examinaion. This auxiliary data was then used to identify data outliers to update the core analysis database. The measurements of porosity and
permeability were then assigned a formation rank plotted on a subregional scale. Average porosity and permeability values have been divided into statistical populations for each formation assigned by three depositional realms. The southwestern Ontario study area has been divided into three
paleogeographic settings, based on distinctive lithofacies that correspond to different carbonate depositional regimes and regions of paleokarstification. From northwest to southeast, the lithofacies reflect an inner to outer carbonate ramp setting designated as area 1-3 from northwest to southeast.
Area 1 is the inter-pinnacle karst region and includes some of thepinnacle structures within the Lockport Group. This region has the most significant paleokarstification of the upper Lockport Group (Guelph and Goat Island formations) and overlying Salina Group A-unit. Area 2 has rare pinnacle
structures, where no porosity/permeability core analyses data are available. Area 3 is the middle to outer portion of the Lockport carbonate ramp, with local development of reef mound phases in the lower Goat Island and Gasport formations. The porosity and permeability variability corresponds with
areal distribution of paleokarstification and resulting diagenetic phases in Area 1, and lithofacies variations and temporal/spatial history of karstification in Area 3. Higher porosity and permeability generally coincide with greater thicknesses of the oil and gas reservoir within pinnacles in Area
1 and reef mound phases of Lockport Group and lower Salina Group A-1 Carbonate in Area 3. Within inter-pinnacle karst regions in Area 1, average porosity for each formation is consistently high with little variations. In Area 3, a general increase of porosity and permeability towards the southeast
corresponds with lithofacies ranging from restricted lagoonal/platform interior deposits to carbonate bank deposits with local development of reef mound phases in the Gasport and Goat Island formations. There has been significant erosion and karstification within and at the tops of these pinnacles,
resulting in higher porosity and permeability of the Guelph and upper Goat Island formations, and the overlying Salina Group A-1 unit. Paleokarstic events have enhanced various porosity types, including intercrystalline, moldic, irregular and fenestral vugs, and cavities.