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It is difficult to understand that exploration for oil and gas based on the distribution of pyrite in sedimentary basins should have a rational geochemical basis.


530x386 Figur1The slow micro-seepage of hydrocarbons gives rise to mineralogical changes high above a reservoir, according to ORG Geophysical. Illustration: ORG Geophysical

In an article in geoforskning.no Ronny Setså attempts to explain how oil can be found by detecting the presence of pyrite by geophysical methods (IP: Induced Polarization). This method is sold by ORG Geophysical.

The basic principle claimed for this method, is that it can detect sulfide mineral anomalies which have been caused by hydrocarbon leakage from reservoirs below.

It is true that pyrite forms assisted by bacteria and that this occurs normally at or below the red/ox boundary.

Here sulfate (SO42-) from seawater is reduced by sulfate reducing bacteria to sulfides (H2S) which react with iron in the sediments to form monosulphides (FeS) and then pyrite (FeS2).

The anoxic conditions are due to surplus of organic matter when all dissolved oxygen in the pore water has been used in aerobic degradation.

The sulfate reduction can be expressed as:

2CH2O + 2H++SO42- = H2S + 2CO2 + 2H2O

H2S is a weak acid which will dissolve iron oxides or other minerals containing iron so that monosulfide (FeS) or pyrite (FeS2) can precipitate.

4 H2S +2Fe2O3 + CH2O = 4FeS + CO2 + 5H2O
4 H2S +2Fe2O3 + 4H+ = 2FeS2 + 2Fe++ + 6H2O

Sulfate is rather stable in the absence of bacteria at temperatures exceeding about 80 - 100 °C, and abiotic reduction is not very effective and sulfate minerals like anhydrite exist at high temperatures in evaporates.

To precipitate significant volumes of pyrite however there must be a source of both sulfur and iron.

In most marine sedimentary basins nearly all the sulfur (from seawater) trapped in pore water is precipitated out as pyrite and other sulfides. This takes place below the red/ox boundary, which is normally found at a few cm or up to one meter below the sea floor.

At greater depth sulfate exists mainly in evaporates (gypsum and anhydrite). Low sedimentation rate of clastic material and a high content of organic matter will favour precipitation of pyrite because there is more time for sulfate reduction and uptake of sulfate from the seawater.

It is therefore possible that a certain concentration of pyrite may coincide with structural highs and thereby with traps. It is however uncertain to what extent this is the case.

It is difficult to see how the presence of migrated oil and gas can precipitate significant amounts of pyrite when there is no source of sulfur in the pore water.

In many sedimentary basins like the North Sea basin there is an abundance of shallow biogenic gas which could also serve as a reducing agent for sulfate reducing bacteria. In pock marks where this gas escapes there are not unusually high concentrations of pyrite.

It is true that oil may contain some sulfur, but it is not clear that this will be transported from the oil phase to the water phase so that it can be precipitated as pyrite. There are many sulfur-rich heavy oils in the world where the sulfur has not been precipitated as pyrite.

In most sedimentary basins, like the North Sea basin, the oil generated contain little sulfur (<1.0%). When oil or gas reservoirs leak through cap rocks into an overlying sequence it will tend to migrate laterally along permeable layers and shales will be barriers for such migration except at the top of new traps where there is overpressure.

There is little information about the stratigraphic level the detected pyrite accumulations occur in.

In most cases the upper Jurassic source rocks reached maturation in Tertiary or Pleistocene time and pyrite should have formed on the sea floor in these sediments. It is not likely that migration of oil can supply significant amount of sulfur to form pyrite.

It is difficult to understand that exploration for oil and gas based on the distribution of pyrite in sedimentary basins should have a rational geochemical basis.

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