Dear  Mr. Alexandridis,

a summary of what hydrocarbon depsits from tar sand to oil shales look like is given in:

Dill et al. (2009) Diagenetic and epigenetic alteration of Cretaceous to Paleogene

organic-rich sedimentary successions in northwestern Jordan,

typical of the western margin of the Arabian Plate. GeoArabia, vol. 14, no. 2,  p. 101-140 Potential energy resources, northwest Jordan Gulf PetroLink, Bahrain.

It should be avaiable for download on the RG server.

Best regards

H.G.Dill

Dear professor Dill, Mr Vafadar,

Thank you for your answers.

Unfortunately, I haven't find in the papers you've attach, exact parts which answer to my question. Nevertheless, they do provide some reliable basis for further thinking.

Regards

Ioannis

I understand that you are investigating dead or past oil seeps, as detectable at the surface in outcrop, not necessarily exumed oil fields.

On this basis, I suggest to distinguish oil seeps in incoherent terrain and oil seeps in rocks.

Incoherent terrain or soil: the main physical characteristic is presence of some kind of hydrocarbon-related substance, like tar, asphalt, or even wax, concentrated in the soil at the spot of the dead seep. This may also indicate an active seep, possibly with low seepage rate. The main difference would be the absence of gaseous or liquid active seepage in a really dried-up paleo-seep.

Rocks: like-wise, the presence of some  kind of hydrocarbon-related substance, like tar, asphalt, or even wax, at the spot of the dead seep is the main indicator. It would be useful to distinguish seeps in porous rocks (porous and permeable sandstone, porous and permeable calcarenite or grainstone in case of carbonate rock) and naturally fractured rocks (tight clastic rocks, massive limestone or dolomite, without matrix porosity). In porous rocks, the rock in the immediate vicinity of the paleo-seep would be impregnated of heavy hydrocarbons or asphalt/tar, indicating irreducible saturation in the matrix porosity. In naturally fractured rocks, the main sign would be asphalt/tar stain on the walls of the natural fractures where hydrocarbons were seeping out. Again, a live seep can be differentiated from a dried-up seep by the presence of gaseous components and light ends, that are absent in a dead seep.

An oil spill on incoherent soil, even decades old, could be detected by the absence of bio-indicators typical of active oil seeps, because an occasional oil spill is generally not enough to induce the start of a local colony of hydrocarbon-feeding micro-organisms. An oil spill on rock would be easy to detect becouse of lack of deep impregnation in the rock matrix.

Dear Roberto,

Thank you for the detailed answer. I have already consider the most of your targeted suggestions.

The presence of an oil seep is more or less obvious, but the problematic issue for my case is the lack of "typical" observations such as a  structure or smell and in my case, the very small size (some cm's). My observations could better indicate a "dead path" to an old-dried oil seep rather than the surficial expresion of one.

Regards

Ioannis

The appearance of dried oil seeps is depends on the composition of the oil seeping (light oil or heavy, tar-;like oil, with or without associated gas, etc), the level of activity of the seep, surface conditions, etc. If the seep has been inactive for many years or decades, there may be no visible expression, However, analysis of the soil from the area of the purported seep should show elevated hydrocarbon concentrations although the hydrocarbons will be biodegraded. If the seep occurred in an area of fractured bedrock (rather than in a field or forest), look for oil staining along the fractures in the rock, and/or tar-filled fractures. Most oil seeps are accompanied by natural gas, and that gas leakage (micro seepage) may still be continuing even when no visible oil reaches the surface. Analysis of light hydrocarbons (C1-C4 or C5) may show elevated concentrations relative to nearby background areas; solvent extraction of the soil should be tested for fluorescence (2-roing and 3-ring aromatics). Other indications of active or past seepage can include increased secondary mineralization such as precipitation of pore-filling calcite, pyrite, etc.,; bleaching of redheads (if present); clay mineral changes relative to areas not exposed to the seepage.