Speciation of trace elements in the Bowland Shale

Abstract

Samples of Bowland Shale and equivalents in Ireland, representing a range of thermal maturities from oil window to gas window, contain trace elements distributed through multiple residences. The trace element distribution reflects the depositional environment, and influences subsequent element release. Measurements of Se, Cu, As, U and Mo indicate variable retention in loosely-bound and strongly-bound forms. The trace elements are especially associated with diagenetic pyrite. Shales from Ireland, at higher thermal maturity, have relatively high proportions of loosely-bound Se, Cu and As, which may reflect expulsion from the pyrite during recrystallization. The shale samples consistently contain high contents of organic matter, reflecting deposition in an anoxic environment. The Se, Mo and U in particular may be bound to organic matter, which may have adverse environmental impacts upon weathering of the shale.