understanding source rocks

Understanding source rocks

Edegbai, A.J (Department of Geology, University of Benin, Nigeria;

https://www.researchgate.net/profile/Aitalokhai_Edegbai )

Source rock attributes3 features characterize source

rocks:• Organic richness• Kerogen type• Thermal maturity

Organic richness and type of kerogen is a function of depositional setting; whilst tectonic history determines maturity

1 2 3 4 5 6 7 8 9 10

Spore Colour Index0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.3 2.0 3.0

What is a source rock (SR)• ‘A source rock is a (fine grained) rock that is

capable of generating or that has generated movable quantities of hydrocarbons’ (Law, 1999)

• A source may be termed potential source rock- if it hasn’t been sufficiently cooked (immature); active source rock if it’s is currently generating (early, mid or late mature); spent if it is over mature or lacks OM to continue generating

• Effective source rock which contains organic matter and is presently generating and/or expelling hydrocarbons to form commercial accumulations.

• Relic effective source An effective source rock which has ceased generating and expelling hydrocarbons carbons due to a thermal cooling event such as uplift or erosion before exhausting its organic matter supply.

Examples of SR lithologies: mudrock, limestone and coal

Organic richness• This is the percentage or quantity of organic carbon (TOC) in a

rock, which includes both kerogen (insoluble) and bitumen (soluble) (Peters and Cassa, 1994)

• It represents the amount of carbon, oxygen and hydrogen atoms are available for hydrocarbon generation (Kennedy et al., 2012)

• As earlier said, this attribute is a function of conditions prevalent during deposition of source material

• Generally, depositional conditions that favour accumulation of OM are low energy, reducing dysoxic – anoxic conditions

• Deserts (< 0.05% OM); Abyssal Ocean Plains (< 0.1% OM); High Energy Coasts (0.2–0.5% OM); Low Energy Coasts (0.5–5% OM); Distal Floodplains and Deltas (0.5 – > 10% OM); Silled Basins, Enclosed Seas (< 2 – > 10% OM); Epeiric Seas (< 1 - > 10%); Lakes, Coastal Lagoons (< 1 - > 10%); Coastal Swamps (10 – 100%)

Rock Type TOC Value, %Average for all shales 0.8

Average for shale source rocks 2.2

Average for calcareous shale source rocks 1.8

Average for carbonate source rocks 0.7

Average for all source rocks 1.8

Average TOC Values for all source Rocks (Chin, 1991)

Generation Potential wt.% TOC, Shales wt.% TOC, Carbonates

Poor 0.0-0.5 0.0-0.2

Fair 0.5-1.0 0.2-0.5

Good 1.0-2.0 0.5-1.0

Very good 2.0-5.0 1.0-2.0

Excellent >5.0 >2.0

Guideline for Assessing Organic Richness of Source Rocks (Law, 1999)

Determination of organic richness

Tissot and Welte (1984)

TOC is not measured directly, but can be calculated via the formula below:%TOC = [0.082(S1 + S2) + S4]/10

Law (1999)

Peak Measured parameter CommentS1 (mgHC/g rock)

Free hydrocarbons present in sample before analysis

Akin to residual hydrocarbon phase

S2 (mgHC/g rock)

Volume of hydrocarbons formed during thermal Pyrolysis of the sample

Used to estimate the remaining hydrocarbon generating potential of the sample

S3 (mgCO2/g rock)

The CO2 yield during thermal breakdown of kerogen

Most prevalent in calcareous source rocks

S4 (mg carbon/g rock)

The residual carbon content of sample

Residual carbon content of sample has little or no potential to generate hydrocarbons due to lack of hydrogen and chemical structure of the molecule

Peters and Cassa (1994)

LECO Method• This method entails the use of LECO carbon analyzer to

estimate TOC. Samples are pulverized and treated to remove carbonates (of inorganic origin) before combusted in an oxygen rich environment (Law, 1999).

• The amount of CO2 liberated is equivalent to the organic richness of the source rock.

•  The main disadvantage of this method is that TOC is often overestimated. This is due to the presence of water, inorganic carbonates and compounds of sulphur which were not properly chemically treated prior to combustion (Law, 1999).

• Furthermore, the TOC estimated by this method does not take into consideration free hydrocarbons present in sample prior to combustion (S1 peak of Rock Eval Pyrolysis).

• Kerogen is a portion of the organic richness in a sedimentary rock (Peters and Cassa, 1994; Tissot and Welte, 1984; Durand, 1980).

• It is a macro-molecular complex with a polymer-like structure (organic compound) that is insoluble in non-oxidising acids, alkaline solvents or organic solvents

• which can yield hydrocarbon when subjected to increased temperature and pressure.

• It forms from the diagenetic modification of organic precursors (Carbohydrates, Proteins and Lipids) from organic materials like algae, miospore, etc at a depth of a few hundred metres and a temperature range of about 500C to 600C.

SOURCE QUALITY/KEROGEN TYPING

Kerogen types• Sapropelic /Type I kerogen (primarily of algal origin) with

oil generation potential• Herbaceous / Type II Kerogen (organic matter comprise of

planktonic marine organisms, cuticles and miospores of herbaceous plants (Holditch, 2011) with wet gas generation potential

• Humic / Type III Kerogen (from terrestrial plant materials) with dry gas generation potential

• Inertinite / Type IV Kerogen ( oxidized plant material) with no generation potential

SOURCE QUALITY/KEROGEN TYPING

Kerogen Type

Organic Precursors Hydrogen Product

I Algae LiquidsII Marine Algae,

Pollens, Spores, Leaf waxes, Fossil Resins

Liquids

III Terrestrial-Derived Woody Materials

Gas

IV Reworked Organic Debris, Highly

Oxidized Material

None

Table 5: Kerogen types (Waples, 1985)Modified Van Krevelen diagram (after Tissot and Welte, 1984)

Kerogen analysis Via HI

HI = S2 (mg/g)/%TOC * 100

OI = S3 (mg/g)/%TOC × 100

Kerogen analysis Via modified van krevelen diagram

Modified Van Krevelen diagram (after Tissot and

Welte, 1984)

HI = S2 (mg/g)/%TOC * 100

OI = S3 (mg/g)/%TOC × 100

Palynofacies assemblage

AUCHI (Mag.: X40)

SOBE (Mag.: X40)

IKABIGBO (Mag.: X40)

• Phytoclast and Miospore are dominant

• Type II&III kerogen (Exinite & Vitrinite) AGBANIKAKA

PALYNOFACIES ANALYSIS

Chart 1: Relative abundance of Palynofacies in Agbanikaka

• Phytoclast and Miospore are dominant

• Type II&III kerogen(Exinite and Vitrinite)

PALYNOFACIES ANALYSIS

AUCHI

Chart 2: Relative abundance of Palynofacies in Auchi

• Phytoclast predominates

• Mainly of Type III kerogen (Vitrinite)

PALYNOFACIES ANALYSIS

IKABIGBO

Chart 3: Relative abundance of Palynofacies

in Ikabigbo

• Phytoclast predominates

• Mainly of Type III kerogen (Vitrinite)

PALYNOFACIES ANALYSIS

SOBE

Chart 4: Relative abundance of

Palynofacies in Sobe

THERMAL MATURITYTOC

(wt.%)S1(mgHC/g)

S2(mgHC/g)

S3(mgHC/g)

Tmax (0C)

HI(mgHC/g)

OI(mgHC/g)

Auchi 2.58 1.64 2.97 3.68 328 115 143Ikabigbo 2.42 0.06 1.47 1 421 61 41Uzebba 8.34 0.34 10.76 0.42 440 129 5

MaturationStage of Thermal

Maturity for oilRo Tmax(%) 0C

Immature 0.2-0.6 <435MatureEarly 0.6-0.65 435-445

Peak 0.65-0.9 445-450

Late 0.9-1.35 450-470

Post mature >1.35 >470

• Maastrichtian Black Shales are Immature to early mature in the SW

Table 6:Thermal maturity levels (modified after Peters and

Cassa, 1994)

THERMAL MATURITY (SCI)R0 SCI Tmax Generalized HC Zone

0.40 4.0 420 Immature0.50 5.0 435 Immature0.60 6.0 440 Oil0.80 7.4 450 Oil1.00 8.1 460 Oil1.20 8.3 465 Oil &wet gas1.35 8.5 470 Wet gas1.50 8.7 480 Wet 2.00 9.2 500 Methane

1 2 3 4 5 6 7 8 9 10

Spore Colour Index

Table 7:Generalized correlation of different

maturity indices (Waples, 1985)

Chart 5: Spore colour index chart (modified from Pearson, 1984)

AUCHI

Plate 1:Miospores identified in Auchi Black shales

Magnification: X40

1 2 3 4 5 6 7 8 9 10

Spore Colour Index

SCI <5

Immature

SOBE

Plate 2:Miospores identified in Sobe Black shalesMagnification: X40

1 2 3 4 5 6 7 8 9 10

Spore Colour Index

SCI <5

Immature

AGBANIKAKA

Plate 3:Miospores identified in Agbanikaka Black shales

Magnification: X40

1 2 3 4 5 6 7 8 9 10

Spore Colour Index

SCI <5

Immature

IKABIGBO

Plate 4:Miospores identified in Ikabigbo Black shales

Magnification: X40

1 2 3 4 5 6 7 8 9 10

Spore Colour Index

SCI <5

Immature