Elemental Composition and Organic Petrology of a Lower Carboniferous-Age Freshwater Oil Shale in Nova Scotia, Canada

Abstract

A 59 m-thick section of a freshwater oil shale interbedded with marlstone of Lower Carboniferous (Tournaisian) age from the Big Marsh area in Antigonish Basin, Nova Scotia, Canada, was examined using reflected light microscopy, Rock-Eval pyrolysis, X-ray diffractometry analysis, inductively coupled plasma-mass spectrometry for elemental analysis, and prompt γ for boron concentration. The oil shale was deposited in a lacustrine environment based on geology, sedimentology, variation in organic matter, and boron content (28–54 ppm). Organic petrology classified the oil shale into three broadly distinct types. Type A oil shale is a coastal facies shale containing terrestrially derived macerals, such as vitrinite and inertinite, sporinite, with some lamalginite, and amorphous bituminous matrix. Type B oil shale was deposited in a shallow-water facies and contains mostly lamalginite and some vitrinite and sporinite. Type C oil shale is a relatively deep-water facies, associated with open-water Torbanite-type oil shale and contains mostly Botryococcus colonial telalginite. The oil shale is thermally mature (Tmax is 441–443 °C). Total organic carbon (TOC) varies from 5.8 to 7.3 wt %, and the hydrogen index is between 507 and 557 mg HC/g TOC. The rate of sedimentation as determined by the Th/U ratio indicates possibility of three sedimentation periods: an irregular but mostly slow rate of sedimentation from the base of the section up to 68 m, followed by a regular and slow rate between 68 and 53 m, and a regular and fast rate between 53 m and the top of the section. The higher Th/U ratio during deposition of the shallow-water facies was due to the input of allochthonous U. The redox conditions, as reflected in the variation of Cr to Mo, U, and Ni + V, indicate that the oil shale was deposited under suboxic–dysoxic conditions. The high organic productivity by phytoplankton and bacteria is characterized by a low Cr and high V/Cr ratio and suboxic conditions. In contrast, the well-oxygenated and uniform, warm-temperature upper water level supports a dysoxic environment. Variation of Sr/Ca vs Mn/Ca ratios indicates that most samples have low values, a characteristic of colder water and high terrigenous influx. The post-Archean Australian shale (PAAS)-normalized rare earth elements (REEs) follow three trends. Type A oil shale has the highest concentration of total REEs (648 ppm) and light REEs (LREEs, 605 ppm) as compared with type C (269 and 233 ppm), which are less than half of type A. Type B oil shale has the lowest total REEs (184 ppm) and LREEs (152 ppm). The concentration of heavy REEs decreased from 43 ppm in type A oil shale to 36 ppm in type C oil shale. Comparison of PAAS-normalized REEs for the three oil shale types indicates a reduction of the negative Eu anomaly with depth, which is possibly related to sedimentary sorting as a result of accumulation of fine sediments in the deeper water zone of the lake. The concentration of most elements of environmental concern is similar to and/or lower than the world shale. How...

Controlling Factors and Formation Models of Organic Matter Accumulation for the Upper Permian Dalong Formation Black Shale in the Lower Yangtze Region, South China: Constraints from Geochemical Evidence

“…Besides, the Th/U ratio can also be used as an indicator of the sedimentary rate. 43 In general, higher values of Th/U indicate a higher sedimentary rate. As shown in Table S2, Th/U ratios for section B are in the range of 1.14−2.88 (average 2.09), being obviously higher than those corresponding values of section A (0.46−1.02, average 0.71).…”

“…Elemental geochemistry is the most extensively employed method for quantitatively reconstructing the paleoenvironment. ,− Generally speaking, organic carbon, phosphorus (P), and biogenic barium (Ba bio ) are the popular proxies for qualitatively evaluating the paleomarine productivity. − Several redox-sensitive elements (e.g., V, U, and Ni) and their ratios (e.g., U/Th, V/Cr, Ni/Co, and V/(V + Ni)) have been widely applied to interpret the paleoredox conditions over the past couple of decades. ,,,− REE distribution patterns and ratios of (La/Yb) N , Th/U, and Na/K are the important geochemical indices to qualitatively characterize the sedimentary rate. − …”

Elemental Composition of Fluvial-Lacustrine and Lacustrine Coal-Bearing Environments, British Columbia, Canada

“…Total boron (B) in seawater is much higher (4.45 mg/L) than that in freshwater (0.1 mg/L). , Therefore, boron has been used to determine the environment of deposition of organic-rich deposits such as coal and oil shale. − The organic-rich sediment has been classified based on B content as freshwater-influenced organic-rich deposits. ,,,,,− …”

Main Controlling Factors and Models of Organic Matter Accumulation in Lower Carboniferous Dawuba Formation Shale in Southern Guizhou, China

“…It also shows that the Dawuba formation is affected by terrigenous detrital input to varying degrees, and the two parameters have relatively high values in the two depth ranges of assumed gravity flow development. In addition, to more accurately reflect the terrigenous input of the study area, this paper collected the data of major elements from the Lower Carboniferous shale in Canada and compared them with the study area. − Figure shows that the sedimentation rate of the study area is almost the same as that of Canada’s Big Marsh area and the terrigenous input of shale is similar to that of Canada’s Albert area. Due to the high sedimentation rate and terrigenous input of the Albert area in the Big Marsh area of Canada, similar element characteristics suggest that the shales in the study area may also be in a high sedimentation rate and terrigenous input environment (Figures and ).…”