Wednesday, November 28, 2012

How useful is vitrinite reflectance for calibrating paleotemperature ?

As basin modellers we are all taught to calibrate our models to present-day temperatures and then to paleo-temperatures using various paleo-thermometers, the most popular of which is vitrinite reflectance. We can spend a lot of time trying for find heat flow histories that match our vitrinite data. Leaving aside for the moment the fact that vitrinite is a maximum temperature recorder, how useful are the data anyway ? Unfortunately, the answer is often "not very useful at all".

Here are some published data for several wells in the Bozhong sub basin of the Bohai Basin (courtesy of  a literature search by Zhiyong: Guo et al., 2011, Fig. 1b).




Without the line to lead the eye most of us would be hard pressed to see anything but a vague increase with depth here. Imagine trying to infer anything about paleo-temperature or heat flow trends ? Not all data sets are like this of course and one could propose various reasons for the scatter observed in any particular case (cavings, suppression in hydrogen rich organic matter, thrusting etc). But if cases like this occur frequently (and in my experience they do) how do we know when we CAN trust vitrinite data ? Here are some things that make me feel more comfortable about a set of vitrinite reflectance data:

1.       When it shows a good, logical  depth trend, i.e., is not too scattered. The trend of course should be logarithmic not linear with depth, assuming a simple burial and thermal regime (i.e., no major erosions, no major transient thermal effects)

2.       When the source organic matter is humic, is not hydrogen rich and is not embedded in a matrix of H rich amorphous organic matter

3.       When the early diagenetic environment for the section covered is relatively uniform in character, e.g. a long period of deltaic conditions without major alteration in climate or periods of dessication. (moving from coals into lacustrine sediments or vice-versa as we do in many Tertiary rift grabens spells trouble for Ro-depth curves)

4.       When the time section does not span major periods of plant evolution, e.g. from the Cretaceous to the Tertiary covering the displacement of the gymnosperms by the angiosperms, beginning in the tropics (although this doesn’t seem to be a big influence on vitrinite precursors)

5.      When a single experienced analyst has provided the data, when they are the average of multiple measurements, when he or she is looking at a consistent population of macerals and has provided a description of the populations along with the standard deviations and a text statement of confidence

6.     When the trends agree broadly with the thermal history and with other indicators of maturity such as Rock Eval Tmax, spore colour, clay mineral diagenesis, extract paraffin profile etc.

7.     When it is true vitrinite being measured, i.e., we aren’t in the Silurian or older !

8.     When we have some way of controlling the influence of cavings (good mud logger reports, avoid data near casing shoes etc) and of identifying detrital (re-deposited) vitrinite (petrographer experience)

Here are some data for two wells in a area where all of the above criteria are met:


The error bars are standard deviation from multiple measurements. It’s great when it works out like this, but note that the range of values is quite narrow here – we are only just into the conventional oil window.

Once we have vitrinite reflectance data that we are confident in using, we still have to work out what it means for kerogen maturation. I’m no petrographer but as I understand it Ro is the mean or maximum specular reflectance in oil of a subjectively identified vitrinite particle. The intensity of specular reflectance is a function of surface electron mobility which in turn depends on the degree of surface aromatisation (Pi-electrons are more mobile). Ro increases with thermal stress because the degree of aromatisation increases with thermal stress. However, formation of aromatics is not a function of temperature alone: Obviously there has to be cyclic structures to begin with, e.g. the lignitic structural elements of plants, and these will vary in concentration. Also, aromatisation is an oxidation process which can and is accomplished by anaerobic bacteria at low temperature. Pentacyclic plant terpenoid alkenes are aromatised within metres of the surface in ever-wet sediments of the Amazon basin (Lohman, 1988). This early diagenetic aromatisation can be of similar magnitude to that related to thermal stress. Furthermore, it will vary according to many environmental factors. Hence, the starting point for thermally mediated aromatisation (and hence Ro) will also vary throughout a sediment column, causing scatter in depth vs Ro trends. This will be most obvious in terrestrial sequences – like the one in the first example above (and there also good examples from the same basin in Hao et al. 2007)

Happy Modeling,
AM


Guo Y.H., Zou X.H., Ling Y.X, Li, J.P, Wang F.L and Wang, J. (2011) New understandings of hydrocarbon accumulation in Penglai 19-3 oilfield, the Bohai waters. (in Chinesee), Oil and Gas Geology, 6, 327-332

Hao, F., Zou H.Y., Gong, Z.S. and Deng Y.H. (2007), 1-13. (2007). Petroleum migration and accumulation in the Bozhong sub-basin, Bohaid Bay Basin, China: Significance of preferential petroleum migration pathways (PPMP) for the for the formation of large oilfieds in lacustrine basins. Mar. Petrol. Geol, 24, 1-13.

Lohmann F. ( 1988) Aromatisations microbiennes de tritepenes vegetaux.
Ph.D. Thesis, Univ. Strasbourg.

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