MOLECULAR GEOCHEMISTRY ASSISTS EXPLORATION

Gary H. Isaksen
Exxon Production Research Co.
Houston

Molecular geochemistry has found its place within petroleum exploration as an important tool for the assessment of thermal maturity, organic facies, and the degree of biodegradation in addition to its obvious importance for correlation work.

This paper is concerned with the assessment of organic facies from measurements of molecular properties of organic rich sediments. Sediments allow molecular assessments of the extractable organic matter or pyrolyzate of the kerogen in addition to bulk pyrolysis, optical, and lithological assessments.

The study of sediments facilitates the interpretation of oil, despite their mobility in the subsurface, mixing from different source facies, limited control analyses (optical, lithological, etc.), and a possibly questionable source-unit origin.

The research approach in this field is discussed together with the effects of variations in the salinity, oxygen level, and organic matter input of the depositional environment.

INTRODUCTION

In oil and gas exploration, molecular geochemistry has proven helpful in assessing key exploration parameters, such as thermal maturity, organic facies, biodegradation, and correlation assessments.

Within the category of molecular geochemistry this paper focuses on the use of biomarkers, in particular, for the assessment of organic facies.

Biomarkers are chemical compounds whose basic molecular skeleton can be linked to a known natural product. They exist in the geological environment because their skeleton undergoes only minor changes as a result of thermal digenesis.1

Biomarkers aid in oil and gas exploration for the assessment of the degree of thermal maturity of an oil or an organic rich rock, the organic matter type and depositional environment from which an oil was generated, the degree of biodegradation, and oil/oil, oil/source and source/source correlation work (Fig. 1).

METHODS APPROACH

When employing biomarkers for the assessment of organic facies it is important to isolate the variables so that one's attention can be focused on molecular distributions as a result of the organic input and not the effects of thermal maturity or other secondary effects.

Oils should not be used for organic facies research unless there is only one viable source rock in the basin and this source rock does not have a significant variation in organic facies.

Mixing of oil from several source units and organic facies obviously renders any research into the origin of the oil in terms of the specific organic facies rather difficult. Therefore, such research should be carried out on sediment samples collected with good geological control (paleodepositional environment) and knowledge of their "end-member" organic facies, i.e., marine hypersaline, freshwater lacustrine, paralic, etc.

The samples should not have experienced high degrees of thermal maturity-vitrinite reflectance should be less than 1.0-so as to minimize the study of molecular alterations that take place as a result of the adjustments to high degrees of thermal stress.

The sediment samples should also be free of allochthonous oil-staining and weathering effects (water washing and biodegradation). These prerequisites for sample selection for this type of research are outlined (Fig. 2).

In oil and gas exploration, however, it is more common to deal with mixed source facies as opposed to typical "end-member" facies. As an example, the molecular signatures from a normal marine siliciclastic source rock accumulated on a constructional shelf margin will be slightly different in the proximal settings (higher plant enriched) as compared with the distal settings (algal enriched).

The interpretational guidelines within a mixed facies are based on the knowledge gained from the study of the molecular distributions within 11 end-member" environments.

The selected sediment samples should undergo control analyses such as bulk pyrolysis (TOC, Rock Eval) and optical assessment (visual kerogen, lithological thin sections). This control step high-grades the confidence level of the samples selected.

Visual kerogen descriptions allow positive identification of some of the macerals. These macerals may represent the precursor material from which certain biomarker compounds are generated.

The next steps involve detailed geochemical characterizations by a variety of analytical techniques such as gas chromatography, gas chromatography/mass spectrometry, pyrolysis-gas chromatography, inorganic characterization, etc.

When employing biomarkers for the assessment of organic facies it is of interest not only to know the molecular distribution that typifies each "end-member" depositional environment but also why there are observed differences.

Such understanding is gained by making the link to biochemistry.

This enables us to better study and understand the precursor-product relations for molecular distributions and utilize biomarker information more efficiently in exploration situations.

This has helped Exxon and its affiliates improve basin-scale mapping of the sedimentary facies with the greatest oil potential and understand in which geographical direction the oil-proneness of the source rock is likely to improve.

Oil-source correlations have been improved to understand the precursor organisms responsible for the best oil-prone sedimentary units as opposed to a mere fingerprint matching as in the early days of this science. This approach has also enabled us to better distinguish between which molecular distributions are due to facies, maturity, or an interplay of both.

Increasing thermal maturity causes the molecular distribution of a sample (sediment or oil) to change because of isomerization, aromatization, and cracking of larger molecules into smaller. The ultimate products after extreme metagenesis are methane and graphite.2

Bacterial degradation of oils normally follows a series of events in which lighter nalkanes, branched alkanes, and cyclo-alkanes are preferentially degraded prior to the main degradation of aromatics. The order in which various molecular groups are degraded is discussed by Connan.3

With different degrees of thermal maturity and biodegradation it is important to establish the confidence level that can be assigned to organic facies assessments (Fig. 3).

The highest degree of confidence can be placed on samples having undergone the very earliest digenesis, less mature than the principal zone of oil formation for the kerogen in question, and no biodegradation to moderate biodegradation.

Just as it becomes more difficult to backtrack a sediment sample's hydrogen index when the sample is at a maturity level corresponding to % Ro 1.2, so are the biologically inherited molecular signatures erased with increasing maturity.

INTEGRATING DISCIPLINES

In addition to the care in sample selection as discussed above, assessment of organic facies is greatly improved, and assigned a higher degree of confidence, when it is carried out with integration of various geoscience disciplines (Fig. 4).

On a regional scale sedimentary basins are classified, their environments of deposition assessed, and the sequence stratigraphic framework built by tools such as seismic stratigraphy and wire line log interpretations.4 More detail is obtained from lithological studies (outcrops, conventional cores, etc.) and trace fossil assemblages. Visual kerogen, biofacies and biostratigraphic observations can stand alone in defining the paleodepositional environment and function as good points of control when investigating the molecular geochemical signature from the same sediment extracts or pyrolysis of their kerogen.

PHYSICOCHEMICAL CONTROLS

Each organic facies or depositional "end-member" environment has characteristic geochemical properties, which are controlled by physicochemical conditions (salinity, oxygen level, etc.) and the type of organisms that populated that environment.

In other words, the molecular signatures seen in a paralic environment dominated by terrigenous higher plant organic matter are very different from that of a marine carbonate environment dominated by algal material.

SALINITY

Some important physicochemical controls on organic facies together with various "geoscience tools" to assess each of the physicochemical conditions are listed in Table 1.

Variations in salinity obviously place constraints upon the type of organisms that can inhabit the environment (Fig. 5).

Unicellular algae present in many freshwater lacustrine settings5 often give rise to a high 4-methyl sterane/desmethyl sterane ratio. The 4-methyl steranes are monitored by their m/z 231 common fragment ion from gc/Ms.

Marine algae are considered the precursors of the C30 steranes6 monitored by their m/z 217 or 218 common ions. These are commonly observed with normal seawater salinities.

Hypersaline environments are hostile to all but a few classes of organisms. Cyanobacteria, other halophilic bacteria, or algae can give rise to the triterpane distribution shown by the m/z 191 mass fragmentogram and could also be the precursors of gammacerane.5 7 8

Consequently, certain biomarker signatures can be very helpful in assessment of the salinity of the paleodepositional environment.

OXYGEN LEVEL

The oxygen level during organic matter deposition can also be assessed by biomarkers.

In many of the case studies involved with assessment of oxygen level at the time of organic matter accumulation, the pristane/phytane ratio has proven valuable.2 9

The fate of the phytol side chain of the chlorophyll molecule is shown (Fig. 6).

Under anoxic conditions the alcohol functional group is reduced to form phytane with the isoprenoid chain retaining all 20 carbon atoms, whereas oxic conditions result in decarboxylation (loss of CO2) of the intermediate phytanic acid10 and consequently formation of pristane (C19 isoprenoid).

Work by Han, Philp, Brassell et al., and others11 12 13 had demonstrated other sources of the isoprenoids pristane and phytane. Consequently the limitations upon the pristane-phytane ratio as an indicator of the oxygen level is not yet fully understood, and care should be taken when using this parameter for such assessments.

A triterpane compound known as "bacteriohopanetetrol" and functioning as a cell-membrane rigidifier with bacteria has also proven to be useful for the assessment of the oxygen level of certain depositional environments.

Direct reduction under anoxic conditions of the alcohol functional groups on the bateriohopanetetrol sidechain is thought to be a process that accounts for high C35/C33 homohopane values.

The resulting distribution of hopanes as monitored by their m/z 191 common fragment ion is shown (Fig. 7). Since the amounts of the higher molecular weight homohopanes will decrease with increasing maturity, it is important to take the maturity level of the sample into consideration.

ORGANIC MATTER INPUT VARIATIONS

As mentioned earlier, different types of organisms will give rise to unique molecular signatures in the geological environment.

The distribution of steranes as monitored by their common m/z 218 fragment ion illustrates this (Fig. 8). Terrigenous organic matter with high contents of higher plant organic matter typically shows a predominance of C29 steranes relative to the C28 and C27 steranes and a total sterane concentration of 500900 ppm of the extractable organic matter (EOM) at Ro < 0.8%.

The opposite is found for marine algal organic matter, typically showing a predominance of the C27 steranes. The total amount of steranes in this case is approximately 6,000-8,500 ppm of the EOM at Ro < 0.8%.

Unicellular organisms present in the harsh hypersaline (halite precipitation stage) environments often show a near equal abundance of C27 and C29 steranes and lower amounts of C28 steranes.

Freshwater green algae can give rise to a sterane distribution of near equal amounts of C27, C28, and C29 steranes or a predominance of C29 steranes. Total sterane concentrations are typically low, around 200-600 ppm of EOM for the freshwater lacustrine setting. Similar observations were made for sterols by Huang and Meinschein.14

Actually, the situation is more complex. Volkman5 demonstrated a predominance of C29 sterols in Antarctic sediments with no presence of vascular plants in the algal-dominated organic matter from the sediments in the upwelling areas along the coastal shelf of Peru.

Algal organic matter deposited within a freshwater lacustrine setting in Mesozoic rift basins of Africa show a predominance of C29 steranes.

The hopane/sterane ratio is also useful for assessment of organic facies and the depositional environment. This is shown for marine algal, mixed marine algal and terrigenous higher plant, lacustrine algal, and terrigenous higher plant organic matter (Fig. 9).

Low ratios are often found in normal marine through marine hypersaline depositional environments,15 whereas higher ratios are often found in paralic and nonmarine environments with input of triterpanes from both bacteria and higher plant organic matter.

The hopane/sterane ratio increases with increasing maturity because of the higher stability of hopanes under higher levels of thermal stress. Consequently, comparisons between samples must be done at near the same maturity levels. The samples shown in Fig. 9 are at equivalent maturity levels (0.6-0.8% Ro).

BENEFITS FOR OIL, GAS EXPLORATION

Reservoired oils, including severely biodegraded oils, contain biomarkers that link them to the type of organic matter from which they were generated.

With a detailed knowledge of the molecular characteristics of different source facies, one will be better suited to make this link in oil-source correlations.

The maturity of an oil and the organic facies that generated the oil needs to be known for play assessments when obtaining oil from relatively poorly understood basinal areas.

Geochemists are able to asses the organic facies of the source rock(s) by understanding the significant molecular properties and signatures within each depositional "end-member" environment and environments with mixed organic facies.

This knowledge base can only be built by (a) studying the source rocks or (b) studying the oil when it could have been generated from one single source facies.

Source rock prediction away from the control points (sample locations) has also been improved by the detailed study of the rocks (Fig. 2). Consequently, locating the most oil-prone sedimentary units has also been improved.

CONCLUSION

Molecular geochemistry has proven to be an essential tool in the search for oil and gas.

With a proper calibration to the rock record it allows assessment of the organic facies that generated an oil, in addition to the geological age and depositional conditions (salinity, oxygen level, etc.) of the source rock(s).

During the latter 1980s the knowledge base for organic facies assessment from molecular markers was greatly expanded, mainly due to integrated geoscience studies on source rocks and incorporation of biochemical information to better understand molecular precursor-product relations.

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