Palyno Facies Subathu 2011

7/22/2019 Palyno Facies Subathu 2011

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Palynofacies characterization for hydrocarbon source rockevaluation in the Subathu Formation of Marhighat,

Sirmaur district, Himachal Pradesh

O P Thakur and N N Dogra

Department of Geology, Kurukshetra University, Kurukshetra 136 119, India.Corresponding author. e-mail: [email protected]

This paper deals with the hydrocarbon source rock evaluation of the Subathu Formation exposed atMarhighat on SarahanNarag road in Sirmaur district of Himachal Pradesh. Hydrocarbon potential ofthese sediments is estimated on the basis of palynofacies analysis and thermal alteration index (TAI)values based on the fossil spores/pollen colouration. The analyses are based on the classification andhydrocarbon generation potential of plant derived dispersed organic matter present in the sediments.The palynofacies analysis of Subathu Formation in the area reveal moderate to rich organic matter,with amorphous organic matter constituting the bulk of the total organic matter, followed by charcoal,biodegraded organic matter, fungal remains, spores/pollen and structured terrestrial organic matter. TheTAI value for the organic matter in these sediments has been ascertained as 3.00. A dominance of thesapropelic facies (amorphous organic matter) and the measured TAI values for the Subathu sedimentsin the Marhighat area suggests a good source-rock potential for the hydrocarbon generation.

1. Introduction

The Subathu Formation (Late Palaeocene toEocene) represents a sequence of coastal, swampyand shallow-to-open marine sediments in LesserHimalaya. Tectono-stratigraphically, the formationoccurs north of the Siwalik Group and over-lies unconformably Pre-Tertiary Infra-Krol, KrolGroup and Simla slates in different parts of Lesser

Himalaya. Lithologically, the Subathu Formationconsists of limestones, alternating bands of car-bonaceous shales or coal together with limestonebands. In Marhighat area, the upper part of theformation exhibits intercalations of the pale greenfacies with purple facies. The Subathu Formationin Himachal Pradesh occurs in two distinct tectonicunits, viz., the Bilaspur and the Surajpur units(Raiverman and Raman 1971). The geological

set-up of the area investigated during the presentstudy is shown in figure 1 (after Kapoor et al1997). For the palynofacies analysis presented inthis paper, rock samples were collected from theMarhighat section exposed near school located8 km from Sarahan on SarahanNarag road in Sir-maur district of Himachal Pradesh. Litholog of thesection investigated during present study is shownin the extreme left of figure 2. As regards the hydro-

carbon potential of the Subathu Formation is con-cerned, a perusal of the published work revealsthat the Subathu sediments of Jammu foothills andHimachal Pradesh are characterized by a poor tomarginally rich organic carbon content. However,only at few places Subathu sediments are encour-aging inorganic carbon content. The present paperembodies the results of palynofacies variation pat-tern in percentages and thermal alteration index

Keywords. Palynofacies; thermal alteration index; hydrocarbon; source rock; Subathu Formation.

J. Earth Syst. Sci. 120, No. 5, October 2011, pp. 933938c Indian Academy of Sciences 933


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934 O P Thakur and N N Dogra

Figure 1. Geological map of Marhighat area, Sirmaur district of Himachal Pradesh (after Kapoor et al 1997).

(TAI) value estimations to evaluate hydrocarbonsource rock potential of the Subathu Formation inthe area.

Petroleum geologists are well aware of the factthat the dispersed organic matter derived eitherfrom marine or non-marine sediments on reach-ing its maturation level over extended period oftime contributes as source material for the produc-

tion of hydrocarbons. For the study of this dis-persed organic matter, many researchers all overthe world, have proposed different classifications.The important contributions on such classifica-tions are those of Burgess (1974); Tissot et al(1974); Bujak et al (1977); Masron and Pocock(1981); Venkatachala (1981, 1984); Batten (1983,1996); Hart (1986); Pocock et al (1987); Tyson(1994) and Boussafir et al (1995). Further, vari-ous laboratories engaged in the task of petroleumexploration use their own classification of organicmatter. In the present study, the classification of

dispersed organic matter proposed by Masron andPocock (1981) has been adopted with minor modi-fications. For determining thermal alteration ofsediments, many approaches are being used today,most of which require advanced equipments andrelatively expensive analytical techniques. We havehere applied less costly, relatively simple, effectiveand globally accepted approach of visually assess-ing palynomorphs colour to determine TAI (15scale as proposed by Staplin 1969, 1977).

In so far as the detailed palynofacies analysisof Subathu sediments of the area is concerned,much emphasis has not yet been given by the

earlier researchers. However, a few sporadic pub-lished records on palynofacies and hydrocarbonsource rock evaluation studies are of Berry (1989,1994), Misra and Pundeer (1994) and Berry et al(1998).

Berry (1989) carried out palynofacies and ther-mal alteration studies on Subathu sedimentsexposed in Simla Hills from three energy sequences,

viz., Dharampur, Kumarhatti and Makreri exposedin Talheri Ki Nadi, Nora Khondal Chakli Khud,Barog Kumarhatti, Koshalia River and KasauliDharampur traverses. A sapropelic organic mat-ter constituted the dominant type in Dharampurand Kumarhatti energy sequence, while Makreri isdominated by humic matter and TAI values (2.52.75) suggested that the sediments are within thehydrocarbon generation phase.

Berry (1994) carried out palynofacies studieson SubathuLower Siwalik subsurface sequencesof Nurpur-A and Suruinsar-A wells of Himachal

Pradesh and Jammu & Kashmir areas respec-tively. The palynofacies maturation studies basedon quality, quantity and TAI values suggestedthat the Lower Dharamsala/Lower Murree andSubathu Formations contained good source poten-tial for hydrocarbons. The Upper Dharamsala/Upper Murree and Lower Siwalik sediments wereevaluated as having poor source potential forhydrocarbons.

Misra and Pundeer (1994) carried out studieson palynofacies and TAI on subsurface sequenceencountered in the well Jawalamukhi-B for sourcerock evaluation of Subathu to Middle Siwalik


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Palynofacies characterization for hydrocarbon source rock 935

Figure 2. Distribution of dispersed organic matter in Marhighat section of Subathu Formation exposed at Marhighat,Sirmaur district, H.P., India.

sediments. They have recorded 12 humic, sapro-pelic and mixed facies zones in the sequence. Biode-graded organic matter had been the dominantorganic matter followed by charcoal, amorphousorganic matter and wood/cuticle. TAI (2.753)values and the type of organic matter recordedbetween 6727 and 5298 m depth suggested verygood source potential for Palaeocene to Lower

Eocene sediments (SubathuLower Dharamsala?).TAI of 2.52.75 was recorded between 928 and5298 m in Middle Eocene to Middle Miocene(Lower DharamsalaLower Siwalik) rocks. Mid-dle Siwalik sediments were rated immature forgeneration of hydrocarbons.

Berry et al (1998) also carried out the studies forhydrocarbon potential of Subathu sediments alongBatalghatBhararighat and SamogMalhoti tra-verses in the Bilaspur district of Himachal Pradesh.The total organic matter they recorded was poorto moderate, however few samples contained goodconcentration of organic matter. This organic

matter is dominated of amorphous organic mat-ter followed by biodegraded organic matter withTAI value as 3.00. The facies recorded from thesetraverses are sapropelicinertinitic and the source-rock potential for hydrocarbon is moderate type.

2. Methodology

The hydrocarbon source rock potential of theSubathu Formation of Marhighat area is evalu-ated on the basis of dispersed organic matter andTAI. The dispersed organic matter in the rock sam-ples is isolated by standard maceration technique(Faegri and Iversen 1989), which involves diges-tion with hydrochloric acid and hydrofluoric acid.The nitric acid treatment is avoided to get unoxi-dised residue. The macerate is thoroughly washedand smeared on zero cover glass and mounted onslides using polyvinyl alcohol and Canada-balsam.The quantitative estimation of the organic matter


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is made by visual estimation and the classificationdescribed by Masron and Pocock (1981) is adoptedhere. TAI values are assessed on the 15 scale ofStaplin (1969, 1977).

3. Hydrocarbon source rock evaluation

Sediments that have been able to generatepetroleum or which may have the potential to doso are called source rocks. Organic rich shales,mudstones and certain very fine-grained carbon-ates deposited under suitable conditions and oxy-gen deficient waters are considered excellent sourcesediments. Both autochthonous and allochthonousorganic debris contribute to the total organic mat-ter (TOM) of a litho-unit. Type of organic mat-ter and its facies are considered very importantwhile evaluating source rock potential. These typesof organic matter must be identified and distin-guished, for different types of organic matter havedifferent hydrocarbon potentials and products. Thedispersed organic matter is classified in to variouscategories depending on the degree of alteration(Masron and Pocock 1981) as shown in table 1.

The different types of organic matter identifiedin the investigated sedimentary sequence of theMarhighat area are broadly classified into: humicand sapropelic. The humic organic matter is land

Table 1. Classification of the particulate organic matter inthe sediments (after Masron and Pocock 1981).

Categories Sub-categories

Sructured terrestrial Leaf

organic matter Root

(Telinite) Stem

Spores and pollen

(Sporonite)

Biodegraded terrestrial Stem and root

organic matter Sheets of cells with wall

(Telo collinite) Sheets of cells lacking walls

Charcoal/black debris Degrado-charcoal

(Fusinite) (Degradofusinite)

Pyrocharcoal

(Pyrofusinite)Fungi (Sclerotinite) Mycelia

Single celled spores

Multicelled spores

Amorphous organic Yellow

matter Grey

Algal

Structured aqueous Dinoflagellates

organic matter Achritarchs

Other

Terms within brackets are the equivalent coal maceralcategories.

or terrestrially derived and is easily recognizable,however, sapropelic organic matter is derived bothfrom marine and terrestrial sources. These broadorganic matter types are further subdivided andclassified in to subfacies (based on Swamy et al1994) to evaluate hydrocarbon source rock poten-tial of these sediments. The different types oforganic matter and their frequency variation pat-

terns as revealed in the Marhighat section of theSubathu Formation is depicted in figure 2 andthe hydrocarbon source rock potential based onthe type of each of the recovered organic mattersubfacies and TAI is discussed as below.

Structured terrestrial (wood/cuticle): Thisorganic matter comprises unaltered cellularremains of leaf, root and stem tissues and isrepresented in low frequency, i.e., 01% (avg.0.3%) in the study area. It mainly contributes forgaseous hydrocarbons.

Biodegraded organic matter: This is the

plant derived organic matter that has undergonea considerable amount of biodegradation, butstill shows visual traces of its cellular structure.The frequency of biodegraded organic matter inthe investigated area is 036% (avg. 4.9%). Thisorganic matter is considered to possess enhancedhydrocarbon source potential than structuredterrestrial organic matter.

Amorphous organic matter: This organicmatter is completely structureless and is theend product of structured and biodegradedplant parts. Amorphous organic matter in the

present section is of yellowish to grey in colourwhich indicates the presence of reducing envi-ronmental conditions during deposition (Masronand Pocock 1981). The frequency of this organicmatter is quite high (avg. 75%) in the stud-ied sediments and the range varies from 199%.Amorphous organic matter is considered to be anexcellent source for liquid hydrocarbons.

Black debris/charcoal: This is the productof oxidation of structured materials. Occurrenceof black debris in the above sediments rangesbetween 1 and 65% (avg. 16.6%). Black debrissuggests oxidizing environment during deposi-tion. It has no/negligible hydrocarbon sourcerock potential except for dry gas.

Spores/pollen: Spores and pollen are knownto be rich in lipids, which contribute mostlyto liquid hydrocarbons. The frequency of sporesand pollen is quite low (


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Palynofacies characterization for hydrocarbon source rock 937

rich in proteins and lipids and could be a mod-erately good source of hydrocarbons.

Structured marine (phytoplankton): Struc-tured marine organic matter in the sedimentsis mainly contributed by dinoflagellates and itsfrequency ranges from 01% (avg. 0.1%).

The overall dominance/trend of different types

of organic matter in the Marhighat section is:Amorphous organic matter> black debris>biodegraded terrestrial organic matter> fungalremains> spores/pollen> structured terrestrialorganic matter> structured marine (phytoplank-ton). The dominance of amorphous organic mat-ter and biodegraded terrestrial organic matter inthe Subathu Formation of the area indicates goodsource-rock potential for hydrocarbons.

The distribution of dispersed organic matter (fig-ure 2) clearly reveals that the lower and middleparts of the Marhighat section are dominated byamorphous organic matter whereas, the upper part

of the section is dominant in black debris (char-coal) facies. The lower and middle parts of the suc-cession, therefore, can be categorized as sapropelicorganic matter facies with adequate maturity (TAI3.0) to generate liquid hydrocarbons, while theupper part of the succession has sapropelic humic-charcoal facies with TAI 3.0, which is consideredto be moderate to good for gaseous hydrocarbongeneration.

3.1 Thermal alteration index (TAI)

For evaluating organic maturation level of thesediments vis-a-vis hydrocarbon potential, TAI tothe 15 scale as proposed by Staplin (1969, 1977);has been used for the present study. The TAI isbased upon the maturation colour of spores-pollen,cuticles and phytoplankton. Also the humic andamorphoussapropelic matters were used in thepresent study for determining TAI values in theabsence of palynomorphs. The TAI values workedout for the Subathu sediments of area are measuredas 3.0, which is an optimum maturation level of avery good hydrocarbon source rock potential.

4. Conclusions

The occurrence and relative abundance of dis-persed organic matter types, the thermal matu-ration levels as evaluated by TAI values and thedominance of organic facies have been workedout to interpret the hydrocarbon source poten-tial of the Subathu Formation of Marhighat area.The palynofacies data generated during the analy-sis is quite significant in understanding hydrocar-bon generation potential of these sediments. The

present study reveals a moderate to fairly richorganic matter in the sequence. The lower and mid-dle part of the Marhighat succession is dominant inamorphous organic matter whereas the upper partis dominant in black debris (charcoal). TAI val-ues of these sediments are measured as 3.0, whichindicate well-matured facies. The lower and middlepart of the sequence has sapropelic organic matter

facies with adequate maturity (TAI 3.0) to gener-ate liquid hydrocarbons, while the upper part ofthe sequence has sapropelic humiccharcoal facieswith TAI 3.0, which is considered to be moderateto good for gaseous hydrocarbon generation.

Acknowledgements

The first author O P Thakur is grateful toDr N C Mehrotra, Director, Birbal Sahni Instituteof Palaeobotany for providing the necessary

research facilities and permission to publish thepaper. Authors are also thankful to Chairman,Department of Geology, Kurukshetra University,Kurukshetra for providing research facilities.Authors thankfully acknowledge anonymous refer-ees for their suggestions in improving the qualityof research paper.

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MS received 7 August 2010; revised 4 May 2011; accepted 24 May 2011

Palyno Facies Subathu 2011

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