| Summary: | The occurrence of perhydrous (oil-prone) coal deposits within the Firkanten Formation of the Central Tertiary Basin (CTB) in Spitsbergen, is well documented and the oil present is reportedly sourced from the coals (Mokogwu, 2011; Marshall et al., 2015a). This study uses a total of 146 coal samples covering areas of the eastern coalfield (Bassen, Breinosa and Lunckefjellet) and the western coalfield (Colesdalen) of the CTB to investigate the maturity, oil source rock potential, and retorting potential of these perhydrous coals. In addition, the controls on the oil potential of the coals are considered to provide measures that could be used to determine the optimum resource areas in the basin. Samples were provided by Store Norske Spitsbergen Kulkompani AS, and include drill cores, mine sections and outcrop sections from the Svea, Longyear, Svarteper and Askeladden seams (eastern coalfield), and the Sputnik and Verkny seams (western coalfield). The vitrinite reflectance (VR) of the investigated coals are suppressed by bitumen impregnation and hydrogen enrichment of vitrinites; this is indicated by a general decrease in VR towards the top of the Longyear seam, which correlates with increasing Soxhlet yields towards the top of the seam, and a strong negative correlation of VR vs HI (Hydrogen Index) (R square between 0.73 - 0.78 in all areas), which is not maturity induced. Other evidences of VR suppression include a relatively wide range of VR values (between 0.50 - 0.79% Ro) within these seams, which are notably ≤2.1 m thick in all areas investigated, and wide ranges of VR distribution with bi-modal histograms observed in most samples (due to maceral effects rather than mixing of coal seams). Additionally, aliphatic biomarker and aromatic maturity parameters do not decrease towards the top of the Longyear seam (contrary to VR which decreases towards seam top), and indicates that the coals are generally in the maturity range of around 0.70% Ro or higher. Tmax appear suppressed, and the re-arrangements of methylphenanthrene isomers with increasing maturity also appear delayed/suppressed when there is aliphatic enrichment. True (i.e. unsuppressed) VR was estimated using the Lo (1993) method which gives thermal maturities of around 0.68, 0.78, 0.80 and 0.88% Ro in the Bassen, Lunckefjellet, Breinosa and Colesdalen areas respectively. True VR values indicate peak temperatures of around 104 °C in Bassen, 116 °C in Lunckefjellet, 118 °C in Breinosa and 125 °C in Colesdalen. Coalification gradients in the Adventdalen area equate to around 0.37% Ro/km, with an estimated geothermal gradient of approximately 55 °C/km. Peak burial depths in the Adventdalen area range from 1.9 km (up-dip), to 2.2 km (down-dip), indicating an overall overburden erosion estimate of between 0.9 – 1.2 km. In the Lunckefjellet area, peak burial depth is around 2.1 km, which implies a missing overburden of 1.1 km. In Colesdalen, peak burial depths are considerably higher at around 2.3 km, with a missing overburden estimate of 1.4 km. The implications of these results on burial and subsequent uplift and erosion are discussed. The oil potential of the studied coals appears to be mainly due to perhydrous detrovitrinites, although other vitrinites including collotelinite, in addition to some liptinites, may have significantly contributed. Rock-Eval analysis indicates that the coals are enriched in Type II and a mixture of Types II/III kerogens with high HI (150 - 410 mg HC/g TOC), variable TOC contents (44.5 – 89.8 %), and high S2 contents (109 – 368 mg/g), all of which indicate excellent oil potential. The mean S1 contents are 6.8, 11.8, 15.0 and 14.5 mg/g for the Bassen, Lunckefjellet, Breinosa and Colesdalen coals respectively, which reflect the maturity trend across these four localities (i.e. increasing maturity from Bassen, through Lunckefjellet, to Breinosa and Colesdalen). The Bassen coals are at the onset of oil generation, while the Lunckefjellet coals are at peak oil generation/onset of oil expulsion. The Breinosa and Colesdalen coals however, are already expelling oil (meaning they are in the effective oil window), although all samples (i.e. from all four localities) have low production index (PI < 0.10), which suggest that significant expulsion have not occurred. Results indicate that the Lunckefjellet coals will give the best indication of the maturity at which oil expulsion occurs in the CTB. The oil-proneness of the coals resulted from marine influence upon the peatlands, and the consequent marine sulphur enrichment (between 0.4 – 17.7 % S in the coals). The Askeladden, Svarteper and Verkny coals generally contain more sulphur than the Longyear and Sputnik coals, and this trend is consistent with that of oil potential. Between sample localities, oil potential generally increases in the direction towards the inferred palaeocoastline. The varying sulphur contents within seams and between localities are assessed, and the implications of this variation on oil potential are examined and discussed. In addition to the influence of sulphur on oil potential, there are other marine as well as non-marine related factors on oil potential which have been examined and discussed to help in delineating the optimum resource areas in the basin. Results indicate that greatest oil potential is mainly due to the combination of the following factors: a) Thermal maturity (true VR of around 0.78% Ro) b) Relative sea level rise leading to S contents in excess of 0.5 % c) Stable hydrology (i.e. relatively large/stable groundwater catchment) d) Fe/S ratio significantly <0.87 e) Optimum pH levels (alkalinity, which favours high microbial degredation) f) Relative distance to inferred palaeocoastline and local topography g) Ash content ≤30 % Retorting of the CTB coals showed highest bulk yields at Lunckefjellet (160 mg/g on dry whole coal – dwc basis), with sections within seams yielding up to 240 mg/g dwc. At Breinosa and Colesdalen, bulk yields of 140 and 100 mg/g dwc respectively were measured. Lowest bulk yield was measured at Bassen (80 mg/g dwc). Residual semi-coke ranges between 60 – 75 % of starting material in all areas. Retorting yields are notably limited by coal swelling/blocking of the reactor vessel; consequently, further work involving other methods such as the Grey-King assay are required to fully measure the retorting potential in these coals. With a maximum coal resource of 3,300 Mt, of which 600 Mt is recoverable in the CTB (Orheim, 1982), maximum hydrocarbon resource via retorting would range between 3,188 – 5,394 Mbbl in place, with 580 – 981 Mbbl recoverable by mining.
|
|---|
