A SUBSURFACE STUDY OF THE MIDDLELOWER SILURIAN THOROLD SANDSTONE FROM CONSUMERS' GAS SILVER CREEK 004 GRIMSBY POOL; NORTH-CENTRAL LAKE ERIE

Abstract

<p> Examination of subsurface cores of the Middle-Lower Silurian Thorold Sandstone from Consumers' Gas Silver Creek 004 Grimsby Pool indicate the presence of a single laminated sandstone facies deposited in a lower shoreface environment. Subsurface thin sections show greater development of quartz cement in the form of quartz overgrowths when compared to thin sections from surface outcrop leading to a porosity decrease of 20%. Petrographic and cathodoluminescence studies reveal that concavo-convex and sutured grain contacts, responsible for 80% of the grain contacts within the Thorold Sandstone, are generally between authigenic overgrowths and not detrital grains indicating that pressure solution is not the major source of silica within the Thorold Sandstone. Studies of detrital and authigenic clays utilizing a combination of thin section, scanning electron microscope and X-ray diffraction techniques show that illite (both detrital and authigenic) is the dominant clay mineral within the Thorold Sandstone in the 004 Pool, followed by approximately one half as much detrital kaolinite and minor authigenic chlorite. Shallow maximum depth of burial ( 786 to 1160m) and low diagenetic temperatures (30°c) suggest that clay minerals are unlikely to have undergone extensive diagenetic transformation. Consequently, detrital minerals represent the clay minerals present at the time of deposition while authigenic clays form by direct precipitation from pore fluids. During eodiagenesis mechanical compaction has reduced sandstone porosity from 40% to 28 to 29% while during mesodiagenesis the progressive paragenetic assemblage of authigenic quartz overgrowths, authigenic chlorite and authigenic illite have reduced porosity to its present value of 4 to 10%.</p>