--> Depositional Setting of Paleozoic Organic Rich Lacustrine Oil Shale Deposits of the Maritimes Basin, New Brunswick, Canada

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Depositional Setting of Paleozoic Organic Rich Lacustrine Oil Shale Deposits of the Maritimes Basin, New Brunswick, Canada

Abstract

Oil shale deposits of the Lower Carboniferous Frederick Brook Member of the Albert Formation are world class with yields of 80-160 l/ton (25-45 gallon/ton). However, the vertical and lateral distribution of the oil shale deposits are highly complex due to their deposition in a steep margined lake within a fault bounded rift basin, several deeply incising post-depositional angular unconformities, subsequent folding and thrusting, and syn-sedimentary and post-depositional migration of hydrocarbons and deep basinal fluids. The 100-400m Frederick Brook Member forms an overall upward shallowing lacustrine succession of intercalated organic rich mudstones, dolomitic mudstones and grainstones, and fine grained to conglomeratic sandstones. Organic rich mudstones occur at several stratigraphic levels, both intercalated within the basal relative deep water lake deposits but also with relative shallow water deposits within the upper part. The shallower water organic rich mudstones are the most prospective oil shale deposits due to their higher organic content and slightly higher Hydrogen Index, 8-12% versus 6-8% wt% TOC and 850-950 versus 700-850 mg HC/g HI. However, the upper shallow water oil shale deposits show significant facies variation in thickness, organic richness, and proportion of interbedded dolomite and sandstones. The sandstones are mainly textural immature, indicating short transport and were likely derived from erosion of nearby highlands. The organic matter is dominated by Type I and II, and are predominantly filament algae derived from the freshwater alga Pediastrium. This together with the general low Boron content support deposition in a freshwater lake. However, several samples have a high boron content, suggesting infiltration of saline fluids from underlying evaporite deposits. Organic petrology show organic content in immature oil shale samples is a combination of original deposited organic matter, two types of bitumen (Naphthene rich asphalt and asphaltene-rich asphalt), and light oil fluid inclusion. The presence of bitumen and oil inclusions within immature oil shale samples further indicate fluid migration, with the bitumen and oil likely sourced from nearby fault bounded mini-basins where oil shale deposits of the Frederick Brook Member is buried to within the oil window and generated hydrocarbons from these thermal mature oil shale deposits likely migrated up along faults and fractures to the shallower immature oil shale deposits.