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TitlePetrographic study of the Nanaimo Group from OW-11-01-Nanaimo Obs Well 390
LicencePlease note the adoption of the Open Government Licence - Canada supersedes any previous licences.
AuthorZhai, L; Grasby, S EORCID logo
SourceGeological Survey of Canada, Open File 7647, 2015, 74 pages, Open Access logo Open Access
PublisherNatural Resources Canada
Documentopen file
Mediaon-line; digital
File formatpdf
ProvinceBritish Columbia
NTS92B/13; 92F/01; 92F/08; 92G/04
AreaNanaimo Lowlands; Nanaimo; Parksville; Mill Bay; Deep Bay; Ladysmith
Lat/Long WENS-124.6667 -123.5000 49.5000 48.7500
Subjectsmineralogy; sedimentology; hydrogeology; petrography; petrographic analyses; diagenesis; pore size; textural analyses; textures; fabric analyses; Upper Cretaceous; groundwater; groundwater resources; groundwater regimes; bedrock aquifers; aquifers; depositional environment; depositional history; sedimentary rocks; sandstones; mudstones; Nanaimo Group; OW-11-01-Nanaimo OBS well 390; Mesozoic; Cretaceous
Illustrationsphotomicrographs; tables
ProgramGroundwater Geoscience Aquifer Assessment & support to mapping
Released2015 01 28
Thin section petrography was conducted on forty-four (44, see Table 1) samples from the OW-11-01-Nanaimo OBS well 390, to determine their mineralogical, textural, diagenetic and reservoir characteristics. Work was focused on pore systems and processes affecting the porosity evolution. The 44 Nanaimo Group samples include 43 sandstones and 1 silty mudstone. Most samples are massive; some finer-grained samples show clay and organic material-rich lamination, shale/siltstone cross lamination, burrows and bioturbated structures. All Nanaimo Group samples show low mature features in texture and composition with significant amounts of matrix clay (most samples are between 10 to 30% clay). Grain composition was characterized by abundant feldspar, chert and volcanic rock fragments. Quartz, feldspar and rock fragments are the three major components; although their proportions vary between samples. Coarser grained sandstone samples have a wide grain size range, varying from coarse silt to very coarse sand. The framework grains are angular, subangular to subrounded and poorly sorted or poorly to moderately sorted, showing a grain supported, moderately compacted fabric. Whereas finer (very fine to fine, or silt to very fine) grained sandstone samples demonstrated a grain-supported, well sorted or moderately to well sorted fabric. Grains are subangular to subrounded or rounded, with most grain size ranging from very fine to fine. Common diagenetic phases include authigenic chlorite coating, quartz overgrowth, feldspar overgrowth, dissolution, emplacement of pyrite, calcite and organic matter or bitumen. Evolution of the pore system is apparently controlled by depositional environment and later diagenesis. Various diagenetic features have affected the reservoir quality to varying degrees, both creating and destroying porosity. The original intergranular pores are blocked by matrix clay and further reduced by early chlorite lining, quartz overgrowth, feldspar overgrowth and compaction, therefore no original intergranular pores were observed in any sample. Later stage dissolution produced secondary intergranular pores and improved the reservoir quality of Nanaimo Group sandstones to some degree. The formation of pyrite, calcite cement and organic matter emplacement locally occludes secondary pore spaces, all sandstone samples show very low to low visible porosity (trace to 2-3%). The porosity types of the Nanaimo Group sandstones can be generalized as: secondary porosity (sp), microporosity (mp) and microfractures. Secondary porosity (as a result of feldspar dissolution) is the major pore type in the Nanaimo sandstones. Microporosity is abundant and is extensive within the clay matrix, leached feldspars, chert, and rock fragments (volcanic, plutonic and metomorphic rock fragments) and has little contribution to reservoir quality due to the very smaller pore size. Microfractures appeared in two directions to cut through the rocks and locally enhanced the reservoir permeability.
Summary(Plain Language Summary, not published)
This open file presents detailed descriptions of rock fabric and mineral content as well as pore space. These results are important for assessing aquifer properties in the Nanaimo area.

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