Petroleum Geochemistry Assignment 1

Name : Muhammad Hanif Haziq bin Mohammad

Matrix No. : 15323

Lecturer : Assoc. Prof. Dr. Swapan Kumar Bhattacharya

TOC COMBUSTION

Introduction

Total Organic Carbon is the amount of carbon in a rock and is usually an indicator for possibility of the rock of being a source rock for hydrocarbon generation. The source rock is described as a fine grained rock with rich organic matter that could be able to generate oil that could be migrated to the potential reservoir rock which is more porous.

Theory of Analysis

Total organic carbon in the lab is obtained by heating the rock in an oven in the lab and combusting the organic matter to carbon dioxide (CO2). The amount of CO2 released is measured by infrared spectroscopy and the value is proportional to the carbon content of the rock. However, most rocks have the presence of inorganic carbon in them from carbonate sources. Thus, the raw sample is treated with acid to remove the inorganic carbon from it and the subsequent combustion would measure the Total Organic carbon content. The total inorganic carbon content could be acquired from subtracting the total carbon content from the total organic carbon content.

Procedure

Equipment

Conditioning oven, electric, gravity convection, capable of maintaining a stable temperature of up to 200°C

Combustion furnace, electric, capable of combusting glassware at 400°C for at least 4 hours LECO CR-412 Carbon Analyser, IR detector and 36 position auto sampler rack Glazed and unglazed combustion boats Analytical balance, capable of weighing to 1 mg Calibrated weights, certified Glass measuring scoop Mortar and pestle Aluminium weighing boats Forceps Glass wool

Reagents

Phosphoric acid, 1:1 v/v Calcium carbonate, 99% purity

Anhydrone Drierite Granular copper, 20-30 mesh Oxygen, 99% purity

Procedure

1) All the glassware and ceramic ware used s combusted at 400°C for 4 hours to remove any residual carbon from them.

2) The sample is dried in oven at 40°C.3) A portion of the sample is removed, crushed and homogenized.4) An adequate amount of the sample is placed in an aluminium weighing pan and dried at 105°C

5) For ‘Total carbon’ measurement, 0.35g of the sample is taken and placed into the combustion boat and together loaded into the LECO carbon analyser.

6) For ‘Total Organic Carbon’ measurement, another 0.35g of the raw sample is taken and placed into the combustion boat. This sample is first treated with phosphoric acid until bubbling stops. The sample is dried in an oven at 40°C for 24hours and then transferred to an oven at 105°C. Once the sample is dry, it is loaded into the LECO carbon analyser.

Mechanical measurement

The following formulas are the interpretations from the combustion experiment/#.

Figure 1: Interpretation equation of the combustion method to get TOC (Bernard, Bernard, & Brooks).

Interpretation

When the TOC content is less than 0.5%, it is considered non source rock while source rocks are considered to have values in between 0.5-10%. However, this does not confirm whether the source rock could generate hydrocarbon or not which needs to be confirmed with other experiments.

Limitations

Among the limitations of the combustion technique for TOC measurements are:

TOC is taken to be as the same thing as kerogen but it must be known that kerogen is also made up of oxygen, nitrogen, sulphur and hydrogen. In average about 80% of kerogens, consists of TOC but this could be higher for more mature rocks and lower for less mature rocks.

high price of the machine The flow of the CO2 should be smooth or else the measurement of the carbon will be false. Drying of the sample and acid digestion could cause loss of some organic compounds in the

sample.

References

Bernard, B. B., Bernard, H., & Brooks, J. M. (n.d.). DETERMINATION OF TOTAL CARBON, TOTAL ORGANIC CARBON AND INORGANIC CARBON IN SEDIMENTS. TDI-Brooks International/B&B Laboratories Inc.

Crain, E. R. (n.d.). CRAIN'S PETROPHYSICAL HANDBOOK. Retrieved from http://www.spec2000.net/11-vshtoc.htm

Schumacher, B. A. (2002). METHODS FOR THE DETERMINATION OF TOTAL ORGANIC CARBON (TOC) IN SOILS AND SEDIMENTS. Environmental Sciences Division National Exposure Research Libraryr.

ROCK-EVAL

Introduction

Rock-Eval fundamentally uses the concept of pyrolysis which is the technique of heating in the absence of oxygen. The sample is heated in an environment devoid of oxygen and filled with other types of gases such as nitrogen or helium gas.

Theory of analysis

In rock-eval, the sample is placed in a vessel and is first heated at 300°C, this releases the hydrocarbon already present in the sample which is detected by the flame ionisation detector (FID) and indicated by the S1 curve. The hydrocarbon available in the sample is assumed to be gone and that left is pure kerogen. The temperature is then increased slowly from 300°C to 650°C where the assumption made is that the available kerogen will be cracked and generate hydrocarbon. The temperature at which maximum hydrocarbon is generated is called the Tmax which depends on the nature and degree of maturity of the rock sample. Besides the generated hydrocarbon, carbon dioxide is also released and measured which is detected by the S3 curve.

Figure 2: The 3 curves analyzed during rock eval (ROCK EVAL PYROLYSIS).

Mechanical measurement

The measuremets for rock-eval analysis follows the following equations:

Hydrogen index 1: HI = 100 * S2 / TOC%

Oxygen index 2: OI = 100 * S3 / TOC%

Production index 3: PI = S1 / (S1 + S2)

Since, S2 is the measurement of the amount of hydrocarbon generated, when it is divided over the TOC multiplied by 100, it is a direct resemblance of the hydrocarbon hydrogen index. S3 measures the carbon dioxide content produced from the start of the experiment to the end. The oxygen is available from the kerogen. The production index (PI) is an indicator of the potential gradient of the rock to be a source rock.

Interpretation

When the Tmax = 400°-430°C, this represents immature organic matter;

Tmax = 435°-450°C represents mature or oil zone;

Tmax > 450°C represents the overmature zone.

When PI is in the 0.1-0.4 range, this is an indicator of the rock being a potential source rock while if the PI is less than 0.1, this indicates that it might not be a source rock.

The interpretation of the HI and OI is represented by the following diagram

Figure 3: The HI and OI could indicate the type of kerogen in the source rock (ROCK EVAL PYROLYSIS).

Limitations

-Higher temperatures are used than subsurface conditions to get considerable results at a shorter time thus this will not represent actual subsurface conditions.

-The assumption is that all free hydrocarbons containing in the source rock will release at 300° C but there have been found free hydrocarbons that are expelled at 390° C thus will expel at S2 giving a higher S2 reading than supposed to.

-S1 should also be in suspicion as the hydrocarbon contained in the rock might be migrated hydrocarbon thus would give a higher S1 value than the actual hydrocarbon generated autochthonously in the rock.

-The FID measures the C-C bonds thus S2 could be misrepresentative for hydrogen index indicator where for example, benzene rings have same c-c bond number as hexane but benzene has lower H/C ratio. The HI value for benzene in rock-eval is overestimated thus a high HI value could also indicate high aromacity (presence of carbon rings) in the compound.

References

Bhattarcharya, S. K. (n.d.). QAB 4063.

Crain, E. R. (n.d.). CRAIN'S PETROPHYSICAL HANDBOOK. Retrieved from http://www.spec2000.net/11-vshtoc.htm

McCarthy, K., Rojas, K., Niemann, M., Palmowski, D., Peters, K., & Stankiewitz, A. (2011). Basic Petroleum Geochemistry for Source Rock evaluation. Schlumberger.

ROCK EVAL PYROLYSIS. (n.d.). Retrieved July 3, 2014, from http://www-odp.tamu.edu/publications/tnotes/tn30/tn30_11.htm

Vitrinite Reflectance

Introduction and theory of analysis

The reflectance which is the reflection of 546nm light (green light) on the coals has been a rapid and accurate method of measuring the maturity of kerogen. The coal consists of organic components called ‘macerals’ such as vitrinite, inertinite, liptinite and etc. The reflectance is directly measured on the vitrinite which is believed to form through the thermal alteration of lignin and cellulose from plant cell walls. As temperature increases, vitrinite undergoes complex chemical alterations such as aromatization which increases reflectance. The sample is first crushed to small size around 20 mesh. Then, it is solidified inside a carnauba wax cube, polished on the side where reflectance is measured. The reflectance is seen by an oil immersed (to standardize the readings) objective lens microscope which contains a photometer to measure the amount of reflection. Before that, the reflectance needs to be calibrated with glass or mineral-relfectance standards such as sapphire or diamonds depending on the expected reflectance of the coals. The reflectance is measured as the percentage of light reflected in oil, Vro. Many measurements are taken from multiple samples of the same coal and the mean Vro is used.

Mechanical measurement

Figure 4: Vitrinite reflectance measured on logarithmic scale vs the sample depth (Crain).

As stated, the measurement of vitrinite in the sample is first obtained from a mean value, it is then plotted on logarithmic scale graph against the depth the sample was taken. If for example, the depth of a shale is in between two coals, the maturity indicator of the shale can be known by intersecting the line of the Vro-depth gradient.

Interpretation

- >1.5% Vro, dry gas - 1.1%<Vro<1.5%, wet gas- 0.8%<Vro<1.1%, condensate gas- 0.6%<Vro<0.8%, oil- Vro<0.6%, immature kerogen.

Limitation

The limitation of vitrinite reflectance measurements could be caused by:

Incorrect identification of vitrinite particles where the interpreter might be measuring reflectance from other particles in the microscope.

Anisotropy in the formation where in the same depth, two various mean values of vitrinite might be obtained.

Drilling mud additives in the thin section causing aberrant reflectance readings. Reworked vitrinite might occur where the vitrinite from more mature provenances are

deposited with the immature vitrinite giving incorrect interpretation of the maturity.

References

AAPG. (2014, March 13). Vitrinite Reflectance. Retrieved from AAPG WIKI: http://wiki.aapg.org/Vitrinite_reflectance

Bhattarcharya, S. K. (n.d.). QAB 4063.

McCarthy, K., Rojas, K., Niemann, M., Palmowski, D., Peters, K., & Stankiewitz, A. (2011). Basic Petroleum Geochemistry for Source Rock evaluation. Schlumberger.