Depositional environment and oil generation in Ordovician source rocks from southwestern Ontario, Canada: Organic geochemical and petrological approach

The Ordovician Trenton Group (Sherman Fall and Cobourg formations) and the Lindsay (Collingwood Member) and Blue Mountain formations of southwestern O ntario were examined using Rock-Eval pyrolysis, gas chromatography, gas chr omatography-mass spectrometry,and incident-light microscopy to evaluate the ir paleodepositional environments, thermal maturities, and source rock pote ntial. All units contain sufficient amount of oil-prone (type II), predomin antly marine organic matter to be considered as petroleum source rocks; Uns tructured bituminite with varying proportions of unicellular alginite are t he dominant dispersed organic matter macerals. The bituminite typically occ urs in massive to laminated, granular or patchy populations that commonly s how microtextural relationships. Persistent inclusions of Leiosphaeridia te lalganite demonstrate that planktonic algal debris was a primary organic su bstrate for blooming microbes. Disseminated coccoidal Gloeocapsomorpha pris ca is found in minor amounts, usually in association with common to abundan t acritarchs, Zooclasts (chitinozoa, graptolites, scolecodonts)and solid bi tumen also are present as maceral inclusions within the bituminite network. The biomarker distributions for all of the studied units are those expected for marine organic matter deposited in a elastic-dominated environment. Th e extracts are characterized by a smooth n-alkane profile, with low abundan ce of C20+ members, typical for marine derived organic matter. Pristane/phy tane ratios range from 0.37 to 1.72, indicating dysoxic conditions during d eposition. Smooth C-31-C-35 homohopane profiles, T-s/T-m ratios (typically above 1.0), and a higher concentration of diasteranes relative to regular s teranes all appear to indicate the clay-bearing character of these rocks. T he predominance of C-30 hopane over C-29 regular sterane is interpreted to reflect a primary microbial input and extensive reworking of the organic ma tter. Optical (reflectance, fluorescence) and geochemical <T-max, biomarker data) thermal maturity parameters indicate that the Trenton and Blue Mountain st rata are within the zone of prolific oil generation throughout the whole ar ea of study. The Collingwood shales are mature with respect to petroleum ge neration in the eastern part (Toronto area) and only marginally mature in t he northern part (Georgian Bay area) of the study area. In general, the bio marker composition of the extracts from all examined units is compatible wi th that of the oils found in the Trenton reservoirs of southwestern Ontario ; however, geochemical and geological evidence suggests that the organic-ri ch shaly laminae within the Trenton Group are the principal source of these oils. Accumulation of organic carbon in the Ordovician sediments of southern Onta rio is suggested to derive from low-energy, normal-marine environments grad ing from shallow-shelf into deep-shelf and open-basinal settings. The nutri ent availability and, consequently, higher bioproductivity, more intense co nsumption of oxygen, and progressing anoxia, controlled by a low-latitude l ocation, diminished water circulation, stratification of the water column, and a depressed pycnocline resulted in high preservation rates. The amorpho us nature of kerogen reflects significant microbial interaction at the wate r/ sediment interface and within the sediments where reducing conditions mu st have periodically predominated.