ABUNDANCE

QUALITY

MATURITY

ORCADIAN BASIN  QUALITY (HI) v MATURITY (Tmax)

Time and temperature BASIN MODELING

 

 

 

Orkney Source Rocks II

Middle Devonian has 180m of good quality, rich, 2.3% TOC, mature  Lacustrine Laminite source rock. They constitute between 20% and 30% of the whole "Lake Orcadie" Sequence. They thus present an excellent precursor for HYDROCARBON GENERATION AND ACCUMULATION. This was postulated in 1985 by:

(Marshall, J. E. A., Brown, J. F. and Hindmarsh, S., 1985."Hydrocarbon Source Rock Potential of the Devonian Rocks of the Orcadian Basin" Scottish Journal of Geology, 21: 301-320)

Samples of all non sandy lithologies were collected over Caithness, Orkney and Shetland for the above study and some of the data is used below. The abundance, quality and  maturation is discussed.

THE ABUNDANCE

Total Organic Content (TOC) - represents the source rock richness, that is the quantity of organic carbon contained in the source rocks. It is given in weight % mass of rock.

To be considered a viable source rock more than 1% TOC of good quality is required. The average TOC of all Middle Devonian rocks measured is 1.4%.

Note that in the first graph 34% of the random sampling of the flagstones over the region fit this criteria and can be considered source rocks. Interestingly detailed cycle measurement in the Lower Stromness Flagstones  shows that 32% of the section consists of deeper permanent lake facies always considered to be the potential source rocks of the region.

Not included in the first set of data is a separate collection of permanent lake Laminites from the Sandwick Fish Bed and Upper Stromness Flagstones (occasionally multiple samples from the same unit). This shows over 77% greater than 1% TOC with an average of 2.3%. The lower TOC values come from the "Near Lake Facies".

Discounting the 20m thick Sandwick Fish Bed the average thickness of the Permanent Lake Facies is 1.5m. With 108 lake cycles including the SFB that gives 180m of potential source rock (22%) within the 800m of lacustrine sediment over Orkney.

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QUALITY

 

Hydrogen Index (HI) - The hydrogen Index represents sum of partial potential of each kinetic reaction in kerogen transformation. The higher the index the higher the quality. It is given in mg HC / g Organic Carbon. The Lake Orcadie Laminites mainly fall in the "Oil Prone" Type I & Type II categories and therefore can be considered as high quality source rocks

MATURITY  

Kerogen Parameters. Determination of the oil-generation window in an area is the objective of maturity analyses performed on possible source rocks.  Among the parameters used to measure maturity directly in rock samples are:

  • Vitrinite reflectance (Ro). Vitrinite-reflectance techniques were developed for measuring the rank of coals, in which the vitrinite maceral is usually very common. The method is based on the fact that with increasing thermal stress, the reflectance value of vitrinite increases.
  • Tmax Temperature at maximum S2 production of a RockEval pyrolysis run.

The limits of the oil generation window vary considerably depending upon the type of organic matter being transformed. Nevertheless, for most kerogen the onset of oil-generation is taken to be near 0.6% Ro. Peak generation is reached near 0.9% Ro and the end of liquid-hydrocarbon generation is about 1.35% Ro. The ultimate limit of oil stability is not known for certain, but is probably not much above 1.5% Ro.  

Although Tmax values are determined objectively, because they vary with kerogen type as well as maturity, a unified scale for comparing them with Ro values has not been adopted. The onset of maturity varies with Kerogen Type between 430 C and 440.

It is possible to model the sedimentary history of a basin and derive a theoretical picture of maturation and hydrocarbon evolution in the region.

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ORCADIAN BASIN RESULTS - QUALITY & MATURITY

(Diagrams after Brown J F & Morris M, Gearhart Geodata - 1981)

 

The modified van Krevelen Diagram for "All Data > 0.5% TOC" shows a wide range of Kerogen types from Type I high quality algal derived  (stromatolite etc) kerogen to the Type IV inert coaly kerogen. The majority of the kerogens are Type II which is derived from the non woody (cellulose) parts of land plants and will produce mainly oil on maturation.

The Tmax values show that the majority of the Orcadian Basin samples on the surface today are immature to early oil producing indicating this relatively old source rock can still be producing Hydrocarbons.

The use of the greater than 0.5% TOC filter is used for practical purposes as it was discovered that samples with less than this did not give reproducible results for HI and Tmax due mainly to interference from bitumen in the samples.

This filter eliminates the over mature samples from mainly Shetland and Wick (Hillier S.; Marshall J.E.A.1992 "Organic maturation, thermal history and hydrocarbon generation in the Orcadian Basin, Scotland" Journal of the Geological Society, Volume 149) and therefore gives a biased view of the whole Basin and is thus probably only valid for the Orkney area.

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Looking at the Orkney data, filtered by TOC > 0.5% and separated into individual formations we note that:

  • Sandwick Fish Bed is overwhelmingly Type II mainly early mature.

  • L & U Stromness and Rousay Flags cover the range from marginal Type III to Type I and like the SFB (which is in the middle of the sequence) they are early mature.

  • The Eday Flags and Marl are much more restricted being mainly on the border line of Type II & Type III.

 

This is perhaps reflecting the dominance of the two major rivers providing the majority of the sediment for the Hoy and Eday rocks. Drift wood material transported a long distance from the mountain source may have lost much of its more soluble and unstable components and the prevailing desert conditions would inhibit local growth of plants.

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