Hydrocarbon fingerprinting and palaeoenvironmental reconstruction of Cambrian-Ordovician sedimentary rocks, western Newfoundland, Canada

Abstract

For this study, we have investigated marine carbonates and shales deposited as rhythmites from the lower Shallow Bay Formation and upper Green Point Formation of the Cow Head Group spanning the Cambrian-Ordovician boundary in western Newfoundland. Eighteen carbonate and shale samples were collected and processed for their hydrocarbon biomarkers. The resulting solvent extracts were analyzed using comprehensive two-dimensional gas chromatography-mass spectrometry (GC×GC-MS). Several petroleum biomarkers such as n-alkanes, acyclic isoprenoids, steranes, hopanes as well as various aromatic hydrocarbons were quantified. These compounds are used as parameters to reconstruct the paleodepositional conditions of the depositional environment, and the thermal maturation of the extractable hydrocarbons. Pristane/Phytane (Pr/Ph) ratios of the rock samples range from (~0.16 to ~3.1), indicating that the organic matter was deposited under anoxic to dysoxic and possibly oxic conditions. A comparison of the Ph/n-C₁₈ and Pr/n-C₁₇ suggests the organic matter is derived from mixed Type II/Type III kerogen. The rocks also contains C₂₇, C₂₈, C₂₉ αββand ααα-steranes 24(S+R) and αβ- and αβ-diasteranes in which the C₂₉ stigmastanes dominate. Gammacerane, formed from the biological precursor, tetrahymanol found in ciliated detritivores that commonly thrive in marine stratified water columns was also detected. Gammacerane Index (GI) values are low, ranging from ~0.74 to ~3.5, indicating there is minimal stratification in the water column during sediment deposition. Maturation of source rocks was assessed by monitoring the ratio of trisnorneohopane (Ts) and trisnorhopane (Tm) as well as C₃₁ αβ-hopane 22S/C₃₁ αβ-hopane (22S+22R). Ts/(Ts+Tm) and C₃₁ ⍺𝛽 S/(S+R) values are 0 to ~0.68 and ~0.45 to ~0.54, respectively. These low values indicate the organic matter in most of the samples is early to moderately mature. Samples with the highest range of Ts/(Ts+Tm) from 0.57 to 0.62 likely experienced the main phase of oil generation. The δ¹³C profile across Shallow Bay Formation and Green Point Formation shows alternation of positive and negative δ¹³C excursions (SPICE event, post-SPICE event, and HERB or TOCE event). Changes in organic matter ratios are observed by using steranes/hopanes and Pr/Ph ratios. Distinct negative shifts of values for the two parameters correspond to negative δ¹³C_carb excursions during SPICE, post-SPICE, and HERB events. At the Cambrian-Ordovician boundary, an inverse relationship between these two parameters (high steranes/hopanes percentage accompanied by low Pr/Ph ratios) exists that may be linked to oxygen drawdown from increased productivity in the water column that affected redox conditions at depth.