Warm Arctic Climates and the Potential to Generate Oil Prone Coal Source Rocks

David Large¹, Chris Marshall¹, William Meredith¹, Colin E. Snape¹, and Baruch F. Spiro^2
1Chemical and Environmental Engineering, University of Nottingham, Nottingham, United Kingdom.
²Natural History Museum, London, United Kingdom.

Using knowledge of productivity, decay and atmospheric deposition we demonstrate that latitudinal and geographic variation in low ash coal composition can be predicted and that a warm arctic is an ideal environment in which to produce hydrogen-rich/oil prone coal. This coal, a type III kerogen source rock, has specific properties; a high proportion of microbial aliphatic compounds indicative of aerobic decay; sufficient sulfur (>1 wt %) to crosslink and stabilise aliphatic compounds within the coal. It is recognised that these properties are produced in coastal settings, however a more fundamental control on the oil potential of coals are latitudinal gradients in the balance between productivity and decay. This balance determines the long term carbon accumulation rate of the precursor peat, the potential for aerobic decay and ultimately the balance between atmospherically deposited sulphur and aliphatic compounds within the coal.
An initial model is created using known latitudinal variation in peat carbon accumulation rate, depositional rates of mineral dust and sulfur and the carbon loss during coalification, derived from the van Krevelen diagram for type III kerogen. Using this model, graphs illustrating elemental concentration vs. coal rank are contoured for latitudinal change in peat carbon accumulation rate. To test the model we compare predicted Ti and S concentrations in low, mid and high latitude Paleocene coal taken from South America, North America and the Arctic region respectively. These regions were selected as their palaeogeography has remained remarkably stable since the Paleocene.

A good correlation is observed between predicted and measured Ti and S concentrations. Sulfur concentrations exceeding 1% (required for oil prone coal formation) are generated at high latitude from low sulfur deposition rates in the range 0.1 to 0.2 g(S) m-2 yr-1 whereas in tropical peat deposits these concentrations will only be reached with deposition rates > 1 g(S) m-2 yr-1, found only in coastal environments. In addition to high sulfur concentrations, high rates of decay are also required to generate lipid rich peat. Under cold climatic conditions decay rates and net primary production (NPP) are low. The effect of warming is to increase both NPP and decay rate however respiratory decay rates increase more rapidly than NPP with increasing temperature, favouring the generation of typical microbial lipids. In winter if soil temperatures remain high, decay rates will be high, but productivity will be suppressed due to low light levels. Estimates of Paleocene palaeobotanical ecosystem productivity are equivalent to those of temperate forests and this is consistent with both elevated NPP and decay. It is therefore concluded that warm arctic palaeoenvironments are ideal for the generation of oil prone, hydrogen rich coal source rocks.

 

AAPG Search and Discovery Article #90130©2011 3P Arctic, The Polar Petroleum Potential Conference & Exhibition, Halifax, Nova Scotia, Canada, 30 August-2 September, 2011.

������������������������������������������������������������������������������������������������������������������������������