Definition, Modes of Occurrence and Pitfalls in Understanding the Term ‘Bitumen’ in Conventional and Unconventional Petroleum Systems
Definition: With a viscosity >10,000 cP at reservoir temperature, bitumen is effectively ‘solid petroleum’ on a production timescale. Because its density, including any dissolved gas, may reach or exceed that of water, it has little or no buoyancy. Heavy oil has lower viscosity, is less dense than water and is buoyant. The term ‘tar’ is used synonymously with both: Athabasca ‘Tar Sands’ in Canada are mined for their bitumen; the Orinoco ‘Tar Belt’ wells in Venezuela produce heavy oil. Modes of Occurrence: One form of bitumen originates as a petroleum liquid phase that accumulates under its own buoyancy and is subsequently altered in the accumulation, either through biodegradation or de-asphalting by addition of light saturate compounds to a liquid phase. The second form is transitional with OM/kerogen: it is OM whose structure has been mechanically weakened by bond breakage during maturation. Lacking the ability to migrate under its own buoyancy, it can only be re-distributed under the overburden load within the source bed - and sometimes well beyond in bitumen dykes. This is a non-Newtonian fluid whose characteristics change from solid to mobile in response to high pressure and temperature. Looking at a microscopic or macroscopic (outcrop) section, any OM that is seen in a load-bearing position is a non-Newtonian fluid. Further, there is no alteration mechanism to turn an original petroleum fluid into bitumen in the tiny pores of fine-grained rocks. Bitumen extrusion is not petroleum phase migration! Inside the source bed, this bituminized OM is squeezed into load-protected positions where open pores can develop within it – this state of mechanical evolution usually coincides with the late oil expulsion window and the gas expulsion window – hence the OM-hosted porosity of gas ‘shales’. During this redistribution, the mechanically weakest parts of the OM will be among the remaining reactive kerogen moieties, rather than in the aromatizing inert kerogen, leading to a preferential mobilization and ‘mechanical sifting’ that favors the development of porosity in the extruded bitumen – as commonly observed in SEM images. Pitfalls: By definition, bituminized kerogen is measured in the Py2 (S2) yield and thus its decomposition is measured and modeled as part of kerogen decomposition. There is no need to model an intermediate ‘bitumen phase’. Geochemists cause unnecessary confusion when they use the term as a synonym for Extractable Organic Matter or EOM.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017