Petroleum Geochemistry for Source Rock Evaluation D.K. Asiedu

Petroleum source rocks Every oil or gas play originates from source rocks The viability of each play depends on its source rock Without this source of petroleum, all other components and processes needed to exploit a play become irrelevant D.K. Asiedu

Petroleum source rocks Source rock is as any fine-grained, organic-rich rock that is capable of generating petroleum Its petroleum-generating potential is directly related to its volume, organic richness and thermal maturity Organic richness refers to the amount and type of organic matter contained within the rock Thermal maturity refers to a source rock’s exposure to heat over time D.K. Asiedu

Petroleum generating potential of source rocks When assessing source-rock potential, four questions must be answered : First, does the rock have sufficient organic matter? Second, is the organic matter capable of generating petroleum and, if so, is it oil prone or gas prone? Third, is the organic matter thermally mature? Fourth, have generated hydrocarbons been expelled from the rock? D.K. Asiedu

The question of whether or not the rock has sufficient organic matter to be considered a source rock can be answered on the basis of total organic carbon (TOC) measurements. Rocks that have insufficient TOC content can be rule out as possible source rocks. The TOC content needed for petroleum generation is thought to be greater in siliciclastic shales than in carbonate source rocks. D.K. Asiedu

Mechanism of generating oil and gas It is the thermal transformation of organic matter that causes a source rock to generate petroleum Following deposition of organic-rich sediments, microbial processes convert some of the organic matter into biogenic methane gas Greater depths of burial are accompanied by increases in heat D.K. Asiedu

Mechanism of generating oil and gas This heat causes the organic matter to gradually transform into an insoluble organic matter known as kerogen Further heating converts the kerogen, yielding bitumen and petroleum Increasing maturity causes the petroleum compounds to undergo structural changes – typically starting with oil, then wet gas and ending at dry gas D.K. Asiedu

Maturation of kerogen During the phase of catagenesis, kerogen matures and gives off oil and gas Establishing the level of maturation of kerogen in the source rocks of an area subject to petroleum exploration is vital When kerogen is immature, no petroleum has been generated With increasing maturity, first oil and then gas are expelled When kerogen is overmature, neither oil nor gas remains D.K. Asiedu

Correlation between hydrocarbon generation and temperature D.K. Asiedu

Fundamentals of source rocks Although many organic-rich source rocks are argillaceous, carbonates (typically marls) can also make excellent source rocks Generally, the quality of the source rocks share a common characteristics: They form in anoxic or highly reducing environments Are generally laminated Have moderate to high TOC Contain organic matter with atomic hydrogen/carbon ratios exceeding 1.2 D.K. Asiedu

General environment of deposition Hydrocarbons generated Kerogen types Kerogen type Source material General environment of deposition Hydrocarbons generated H/C O/C I Mainly algae Lacustrine setting Oil, Gas ≥ 1.5 < 0.1 II Mainly plankton, some contribution from algae Marine setting 1.0 to 1.4 0.09 to 1.5 III Mainly higher plants Terrestrial setting Dry gas < 1.0 0.2 to 0.3 IV Reworked , oxidized material Varied settings None Poor in H, high in C D.K. Asiedu

The second question asks what type of organic matter is present within the rock. The type of organic matter, if present in sufficient quantity, will determine if a source rock will produce principally oil or principally gas upon maturation. Algal, herbaceous (plantonic), and much amorphous kerogen (kerogen types I and II) will generate oil and associated gas upon maturation Woody kerogen (kerogen type III) and some amorphous kerogen will generate gas and possibly a minor amount of oil or condensate upon maturation. Inertinites are type IV kerogens that have extremely low hydrogen contents and are incapable of generating significant amounts of hydrocarbons. It is possible to differentiate kerogen types I, II, and III using Rock-Eval pyrolysis D.K. Asiedu

D.K. Asiedu

Paleothermometers Paleothermometers measures the maximum temperatures to which the source rock was ever subjected. This is important because they let us know whether or not the source rocks have matured sufficiently to generate petroleum or whether they are supermature and barren Broadly, two major groups of techniques are used for measuring the maximum paleotemperature to which a rock has been subjected: D.K. Asiedu

Chemical paleothermometers: Carbon ratio Electron spin resonance (ESR) Pyrolysis Gas chromatography Biological paleothermometers Pollen and spore coloration (visual kerogen analysis) Vitrinite reflectance D.K. Asiedu

Pyrolysis Pyrolysis is the heating of kerogen or source rock Pyrolysis is carried with a flame ionization detector Analysis can be carried out on whole rock samples using the Rock-Eval pyroanalyzer Sample is heated and the expelled hydrocarbon gases recorded with a hydrogen flame ionization detector At low temperatures (200 – 300ºC) any free hydrocarbons in the sample are measured (S1). At increasing temperatures hydrocarbons are expelled from the kerogen itself; the maximum amount of hydrocarbon is measured (S2) D.K. Asiedu

The temperature at which the S2 peak occurs is termed Tmax With further heating to some 390ºC, carbon dioxide is expelled to generate a third peak (S3) The temperature at which the S2 peak occurs is termed Tmax The three readings can be used to determine the maturation level of the source rock Where migration has not occurred, the ratio S1/(S1 + S2) shows the amount of petroleum generated compared with the amount capable of being generated. The ratio is referred to as the production index (PI) S2/TOC (Hydrogen Index) relates directly to the potential of rock to generate oil rather than gas. The higher the HI, the higher the potential to generate oil S2/S3 ratio is an indicator of hydrogen richness in the kerogen D.K. Asiedu

Rock-Eval pyroanalyzer D.K. Asiedu

Cross plots D.K. Asiedu

Cross plots D.K. Asiedu

Cross plots D.K. Asiedu

Visual kerogen analysis The level of thermal maturity can be evaluated using visual kerogen analyses. Kerogen has many colours and shades, which are dependent on both maturation and composition Spores and pollen begins life essentially colourless As they are gradually heated they changes to yellow, orange, brown (light to dark), then to black Based on calibrated colour charts, the sample is assigned a numerical value (Thermal Alteration Index or TAI), which ranges from 1.0 (immature) to 5.0 (metamorphosed). This is a standard method of determining thermal maturity that can be used to confirm measurements made with other techniques D.K. Asiedu

Vitrinite reflectance The shininess (or reflectance) of vitrain, a coal maceral, increases with maturity and can be measured optically Vitrain occurs widely throughout sedimentary rocks Kerogen, which includes vitrain, is separated from sample by solution in HCl. The residue is mounted on slide and then polished A reflecting-light microscope is then used to measure the degree of reflectivity, termed Ro An empirical relationship has been noted between vitrinite reflectance and hydrocarbon generation Crude oil generation occurs for Ro values between 0.6 and 1.5; Gas generation takes place between 1.5 and 3.0. Above 3.0 the rocks are graphitic and devoid of hydrocarbons D.K. Asiedu

Vitrinite reflectance equipment QDI CoalPro™ Vitrinite Reflectance Measurement System D.K. Asiedu

Correlation between hydrocarbon generation, temperature and some paleothermometers D.K. Asiedu

D.K. Asiedu

Transmitted light optical techniques Microscopic study is very useful for kerogen identifiction and characterization This method of kerogen characterization may complement data obtained by chemical analysis or pyrolysis Some type III kerogens may be confused with other types of kerogens and result in misleading characterization of kerogen types when using pyrolysis Oxidation of kerogen may also alter its Rock-Eval character. Furthermore, pyrolysis cannot discern the different varieties of kerogens present in samples with mixed kerogen assemblages. For these reasons, Rock- Eval pyrolysis should be used only as reinforcement for petrographically determined kerogen identification D.K. Asiedu

photomicrographs of kerogens (in transmitted light) structured amorphous woody fragments, (c) unstructured amorphous, (d) spore and pollen, (e and f) dinoflagellates D.K. Asiedu

Vitrinite – woody tissues Inertinite – burned organic tissue Alginite – algae, plankton Exinite – higher plant protective tissues, spores, pollen, resins D.K. Asiedu

The fourth and final question concerns the expulsion of generated hydrocarbons from the source rock. This question is more difficult to answer than the other three questions. For the most part, studies of thermal maturity of a source rock are empirically correlated with the presence of oil in associated reservoirs. It is generally assumed that once a sufficient volume of hydrocarbons have been generated in a source rock, they will be expelled and migrate into reservoirs. Source rocks that are thinly interbedded with reservoirs will expel hydrocarbons at lower levels of thermal maturity than thick source rocks that contain few or no interbedded reservoirs. D.K. Asiedu