Heterogeneity of free and occluded bitumen in a natural maturity sequence from Oligocene Lake Enspel

Abstract

Sedimentation in Oligocene Lake Enspel was rapidly terminated by a basaltic lava flow. This introduced a preservational barrier while imparting a ‘natural flash pyrolysis’, during which the organic matter in underlying stratigraphic units was subjected to rapid thermal maturation resulting in hydrocarbon generation. Samples from these strata exhibit a steep maturity gradient (0.25–1.07% optical vitrinite reflectance, or RO) over uniform organofacies. This offers the opportunity to investigate bitumen generation during rapid thermal maturation mechanistically, in particular the nature of Bitumen 2—occluded bitumen, which is only recoverable after the digestion of the mineral matrix and was frequently dismissed as an artifact of incomplete extraction. Elaborate sequential extraction of the contact metamorphic sequence of oil shales at Enspel revealed systematic changes in bitumen composition. These trend progressively towards those of occluded bitumen, which exhibits a systematically elevated thermal maturity, a higher degree of catalytic biomarker-rearrangement and the conspicuous absence of molecular signatures from vascular plants that are present in the free bitumen. One plausible explanation involves a contribution of allochthonous clay-adsorbed organic matter to Bitumen 2. This could represent a mixture of older reworked bitumen and an early-diagenetic snapshot of clay adsorbed organic matter. Alternatively, a close association of early-generated bitumen with clay minerals may have led to enhanced isomerization and catalytically influenced ‘uniformization’ of alkane signatures. Deviations from the expected relationships between various thermal maturity parameters suggest variable dependence on the time-pressure-temperature pathway (i.e. metamorphic facies). The maturation of organic matter likely behaves differently under a contact metamorphic regime or during rapid subsidence and exhumation, as compared to slow maturation during regional subsidence. Our data also suggest that geologically brief shallow intrusive or extrusive magmatism might not be as destructive to the sedimentary hydrocarbon inventory as hitherto thought. This study draws attention to the small-scale compositional heterogeneity of bitumen that can be studied using sequential extraction methods. More importantly, it suggests that occluded bitumen could potentially harbor information on organic matter that pre-dates in situ primary productivity and may be derived from allochthonous biomass and detrital input.