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Geology/Natural H2S in oils or from recent SRB activity

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Question
Can you advice how to prove/disprove the H2S we see in these heavy oils are from ancient SRB activity or pyritic hydrolysis, or from recent (7years) SRBs in waterflooding exercises?

Answer
Dear Dev,

Thanks for sending me your question. Please accept my sincere apology. I hate to offer the excuse for not responding you earlier due to my engagements but I have to admit that I should have responded your earlier.

The temperature and pressure are key determinant for SRB activity in oilfield reservoirs.

There is a wide range of arguments at what temperature SRB may remain active and alive. Firstly, it was considered  that Sulfate reducing bacteria (SRB) thrive in shallow depth and relatively lower temperatures (less than 45 C) but experience has taught us that depth and temperature could be extreme and this has supported by  some laboratory experiments that certain type of bacteria may survive up to 105 C. So we may assume that SRB activity cannot be overruled in reservoir with temperature up to 105 C or so.  Broadly speaking there are 3 kind of SRB; hyperthermophilic SRB  are those bacteria that may survive in temperature range up to 105 C whereas others are mesophilic (20-40 C) or thermophilic (40-80 C) SRB temperature ranges.  
Genus  Desulfovibrio, is usually mesophilic bacteria which lives in low temperatures, halophilic bacteria love to live in saline conditions. Desulfotomaculum species may live in relatively higher temperature and sometime referred as thermophiles (preferring relatively high temperatures.

Geologically speaking, bacteria may live in all sort of rocks ageís regardless young or ancient in the rock, as old as billions years old, and possess the ability to stay dormant for thousands of years and can be reactivated in favorable conditions. The hydrogen sulfide(H2S) is the product of these processes;  H2S only hoards in nature in the conditions devoid of oxygen or anaerobic settings; H2S has tendency to be oxidized in the presence of air to elemental sulphur.

Letís see what happen in oilfield reservoirs. The water chemistry of produced water from an oil reservoir may vary over the field life.

It is not unusually that fields that have been initially considered sweet but produce more
H2S in later stage with substantial increase of concentrations totaling up to many thousands of parts per Million by volume in the gas.  Whatís the temperature of your reservoir water flooding is taking place. If reservoir temperatures are extreme or more than 100 C itís less likely that SRB causing H2S.

However, if injecting cold seawater  with SRB with high pressure in the reservoir SRB survival to higher pressure and temperature condition will be difficult but at the same time the surrounding of the reservoir temperature will be lowered due to water injection and bacteria may survive into new temperature and pressure conditions. Also, you have to consider the Ph of injected sea water and formation waters within the reservoir which will be a sort of mix Ph range between 6 to 9 where microbial activity may be easy.

You may also collect the sample of the produced water for laboratory test to identify the level of SRB and determine the genus of bacteria to reach the logical reasoning of the production of sour crude.


Hope, this answer helps.

Regards  

Geology

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Syed Tariq Hasany

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