J. Earth Syst. Sci. (2021) 130:59 Ó Indian Academy of Sciences
https://doi.org/10.1007/s12040-021-01562-w (0123456789().,-volV)( 0123456789().,-vol V)
Palynology, palynofacies and organic geochemistry analysis of the late Eocene shale from Meghalaya, Northeast India
1 1, 2 NRESHMA DEVI ,YRAGHUMANI SINGH *, MARK BABBOTT and 3 ABIJAYALAXMI DEVI 1Department of Earth Sciences, Manipur University, Imphal 795 003, India. 2Department of Geology and Environmental Science, University of Pittsburgh, Pittsburgh, PA, USA. 3Department of Geology, Pravabati College, Manipur, Imphal 795 009, India. *Corresponding author. e-mail: [email protected]
MS received 21 September 2020; revised 13 December 2020; accepted 14 December 2020
Here the depositional environment and hydrocarbon source rock potential of the Kopili Formation is investigated using palynological analysis and Rock-Eval pyrolysis on samples from a borehole section (Borehole BUM14) collected at Umphyrluh area in the Jaintia Hills, Meghalaya. In these Kopili shales, amorphous organic matter is often associated with structural terrestrial organic matter, biodegraded organic matter, charcoal, black carbon debris, dinoCagellate cysts, and spores. The palynotaxa are mainly composed of dinoCagellate cysts comprising eight genera and twelve referable species. Based on the palynological data, the sediments of the study area were deposited in a shallow marine setting under oxygen deBcient conditions in an environment that received a continuous terrestrial inCux throughout the succession. Rock-Eval pyrolysis and Total Organic Carbon (TOC) analysis determine the quantity, type, and thermal maturity of the associated organic matter. TOC values range from 0.03 to 0.45 wt.% (av- eraging 0.28 wt.%) and the Genetic Potential (GP) and Hydrogen Index (HI) values vary from 0.04 to 0.24 mg HC/g rock and 22–100 mg HC/g TOC, respectively. These values imply that all the shale samples have very low TOC values (\ 0.5%), S1, S2, and Hydrogen Index (HI) values. Although most of the samples are in a mature stage as the average Tmax value is 428.16°C and the Production Index (average 0.16) indicates a potential for oil generation, low Genetic Potential (S1 + S2) and TOC con- centrations suggest there is limited potential for oil generation. The HI vs. OI plot and HI vs. Tmax plot show that most of the shale samples fall in the predominantly gas prone domain (mostly Type III and Type IV), because the organic matter is generally derived from a terrestrial source. Thus, the source rock potential for the Kopili shales of the Umphyrluh area is considered to be poor for gaseous hydrocarbons. Keywords. Kopili Formation; Late Eocene shale; dinoCagellate cysts; Rock-Eval pyrolysis; TOC.
1. Introduction particularly at high latitudes. After the early Eocene climatic optimum (*52–49 Ma), one of the Earth’s Palaeoclimatic condition in the Palaeo- most significant transitions in Earth’s climate is cene–Eocene Transition (PET: *55 Ma) is char- represented by the interval from the middle Eocene acterized by globally warm temperatures, to early Oligocene, during which Earth experienced 59 Page 2 of 16 J. Earth Syst. Sci. (2021) 130:59 a long-term global-scale cooling phase leading to regard the formation is characterized by algal, drier conditions at the Eocene–Oligocene Transi- fungal and many diversiBed plant groups such as tion (EOT: *34 Ma). Consequently, greenhouse bryophytes, pteridophytes, gymnosperms, and conditions were succeeded by icehouse conditions angiosperms. of the Quaternary (Zachos et al. 2001). This paper focuses on the major features of the The Kopili Formation, Brst described by Evans formation and their relationship to palynofacies (1932), was dated as Late Eocene (Nagappa 1959; used to identify the depositional environment of Samanta 1971; Mohan and Pandey 1973; Sein and Kopili Formation and organic geochemical analy- Sah 1974; Tripathi and Singh 1984b; Trivedi 1987) sis such as Rock-Eval pyrolysis and TOC analysis based on microCoral and faunal fossil assemblages. of organic matter preserved in the shales to The Formation derives its name from the Kopili evaluate the hydrocarbon potential of the source river in the Kopili–Khorungma area bounded by rocks. latitudes 25°280N and longitudes 92°50E. The type section of the Kopili Formation is best exposed in 2. Geological setting the southern and southeastern parts of Shillong Plateau–Khasi and Jaintia Hills, Meghalaya and The area under study is located in east Jaintia Hills North Cachar Hills, Assam, where it reaches a District, Meghalaya, India (Bgure 1a and b). Out- maximum thickness of about 500 m in its strato- crops of Kopili Formation are featured by a general type situated near Umrongso (previously Garam- extension of E–W tract from the Kopili river val- pani, Lat. 25°230N: Long. 92°420E) in North Cachar ley, the southern fringe of Mikir Hills in Assam Hills District, Assam (Mathur and Evans 1964). through Jaintia and Khasi Hills to Garo Hills of Outcrops of Kopili Formation are found in an Meghalaya. The Kopili Formation represents the extensive E–W tract from the Kopili river valley, uppermost youngest lithostratigraphic unit of the the southern fringe of Mikir Hills in Assam through Jaintia Group of Palaeogene successions in Jaintia and Khasi Hills to Garo Hills of Meghalaya. Meghalaya. It conformably overlies the Prang Baksi (1962) reported palynofossils from the Limestone (Upper Sylhet Limestone Formation) of Kopili Formation exposed along Simsang River Jaintia Group and is overlain by the Laisong For- section, Garo Hills, Meghalaya and the sequence mation of Barail Group. However, the formation is was designated as Simsang palynological zone II. covered by alluvial soil. Generally, the overlying Further, Baksi (1974) proposed eight palynological Kopili Formation is made up of shales alternated zones in the Tertiary sediments of Assam. Subse- with thin bands of siltstone, sandstone, argilla- quently, Sein and Sah (1974) differentiated the ceous limestone and strings of coal, but under Kopili sediments (late Eocene) from the overlying present investigation, the formation consists of Barail sediments (Oligocene) exposed between greyish-black, friable, splintery weathered shales Lumshnong and Sonapur villages along alternated with thin bands of reddish-brown fos- Jowai–Badarpur road section, Jaintia Hills of siliferous marl. Thin bands of siltstone and Bne- Meghalaya on the occurrence of Monolites mawk- grained, grey, moderately hard sandstone are maensis, Lycopodiumsporite sp. and Tricolpites sp. also associated with the formation. The general characterizing the former and their absence from stratigraphic sequence of the area around Umphyr- the latter. Dutta and Jain (1980) carried out the luh, Jaintia Hills, Meghalaya is shown in table 1. biostratigraphic documentation of acritarchs and microplankton assemblage of dinoCagellate cysts from the Kopili Formation near Lumshnong, 3. Material and methods Jaintia Hills. This assemblage has a predominance of Homotryblium plectilum (97%). A rich and var- The systematic sampling for the present palyno- ied palynological assemblages have also been logical investigation and Rock-Eval pyrolysis reported by many researchers (Sah and Dutta analysis was undertaken from one borehole section 1968; Salujha et al. 1972, 1974; Tripathi and Singh (BUM14) located at latitude 25°190N and longitude 1984a,b, 1985; Singh and Tripathi 1987; Tripathi 92°340E(Bgure 1). The samples had been provided 1989; Trivedi 1985, 1991, 2005, 2009; Kar et al. by the Geological Survey of India (GSI), Shillong. 1994; Mehrotra et al. 2002; Trivedi and Saxena Altogether 30 shale samples were collected sys- 2000, 2009; Trivedi and Ranhotra 2015) from the tematically from different levels to a depth of 59 m Kopili Formation of Assam and Meghalaya, in this from the lower part of the Kopili Formation and J. Earth Syst. Sci. (2021) 130:59 Page 3 of 16 59
Figure 1. (a) Map of Meghalaya and (b) Geological map of Umphyrluh area, Jaintia Hills, Meghalaya (after GSI 2013). represented by KS1 (11 m) to KS30 (59 m). Sam- 3.1 Palynology and palynofacies analysis ple names, depths, and locations are tabulated in table 2. The lithostratigraphic unit of this section For recovery of palynofacies, between 30 and 100 g demonstrating the distribution of organic matter of dry sediments were crushed to 2–5 mm in size and sample position is indicated in Bgure 2. The using a mortar and pastel. The samples were samples were used for the following studies. prepared by the standard palynological acid 59 Page 4 of 16 J. Earth Syst. Sci. (2021) 130:59
Table 1. Generalized stratigraphic successions of Umphyrluh area, Jaintia Hills, Meghalaya (after GSI 2013).
Age Group Formation Member Lithology Recent to Top soil/alluvium Sub-Recent Late Eocene Jaintia group Kopili formation Greyish shale, sandstone, siltstone and ferruginous marl Late Paleocene Shella formation Prang limestone/ Grey, Bne to medium-grained, massive upper Sylhet limestone nummulitic limestone with marly interbands
Table 2. Depth of borehole shale or DPX as mounting media. At least Bve slides per sample were observed. Photomicrography had been Sl. Sample Depth done using a Nikon Eclipse E200 having a built-in no. name (m) digital camera attachment under normal trans- 1 KS1 11 mitted light at the Department of Earth Sciences, 2 KS2 12–13 Manipur University. Most of the photomicrographs 3 KS3 13–14 had been taken by using 409 objective and 109 4 KS4 14–16 eyepiece unless otherwise mentioned. These slides 5 KS5 16–19 were stored at the Museum of the Department of 6 KS6 19–20 Earth Sciences, Manipur University. The preser- 7 KS7 20–21 vation of the palynofossils in this assemblage was 8 KS8 21–22 9 KS9 22–23 rather poor. Out of 30 samples, only Bve yielded 10 KS10 23–24 dinoCagellate cysts in variable abundance and 11 KS11 24–25 diversity. The detailed taxonomy is not included in 12 KS12 26 the present paper. 13 KS13 28–30 To strengthen the interpretation made for the 14 KS14 30–31 depositional environment based on palynofacies, a 15 KS15 31–32 count of the organic matter found in the slides is 16 KS16 32–34 necessary. Percentage data is the most common 17 KS17 36–40 method of palynofacies analyses. According to 18 KS18 40–42 Tyson (1995), a minimum of 300 particles per 19 KS19 42–45 sample were counted using transmitted light 20 KS20 45–46 microscopy and the operational diameter of coun- 21 KS21 46–47 22 KS22 47–48 ted particles and AOM was established as[ 15 lm 23 KS23 48–49 in order to calculate the individual components as 24 KS24 49–50 a percentage of Total Sedimentary Organic Matter 25 KS25 51–52 (TSOM). The ternary diagram proposed by Tyson 26 KS26 52–53 (1995) consisting of AOM, phytoclasts, and paly- 27 KS27 53–54 nomorphs is very eAective for presenting palyno- 28 KS28 54–55 facies percentage data. This plot also reCects 29 KS29 55–57 oxic–anoxic conditions helping to resolve the vari- 30 KS30 57–59 ation in oxygenation status of the depositional environment. maceration technique using hydroCuoric (HF) acid 3.2 Rock-Eval pyrolysis and TOC analysis (40%) and hydrochloric (HCl) acid (35–38%) with oxidation (Funkhouser and Evitt 1959; Staplin Rock-Eval pyrolysis and Total Organic Carbon et al. 1960). The macerated material was siphoned (TOC) analyses were both used to assess the with distilled water through a sieve of 15 lm mesh organic geochemistry of the samples. These anal- to remove all traces of acids. In the end, the water- yses were performed on 25 selected samples using free residue was mixed with polyvinyl alcohol, Rock-Eval 6 Pyrolyser (Make: Vinci Technologies, spread evenly on the coverslips and dried. Perma- France) to obtain their bulk organic properties. nent slides were prepared by using Canada balsam The analyses were carried out at the Oil and J. Earth Syst. Sci. (2021) 130:59 Page 5 of 16 59
Figure 2. Lithostratigraphic section of Kopili Formation of borehole BUM14 showing the percentage of palynofacies parameters with respect to sample position (percentage calculated to total sedimentary organic matter).
Natural Gas Corporation (ONGC), Dehradun, which the maximum amount of hydrocarbons are India following the procedures described by Espi- produced in the course of pyrolysis, which is a talie et al. (1977, 1986). Prior to analysis, all the measure of thermal maturity of the organic mat- representative samples were dried at 40°C and then ter), S1 (free hydrocarbons exist in the rock liber- pulverized into a Bne powder passing through a 60 ated at temperatures of about 300°C, expressed in mesh (250 microns) sieve. The technique com- mg HC/g rock), S2 (milligrams of hydrocarbons prised 100 mg of each Bnely homogenized pulver- produced by pyrolytic degradation of organic ized samples were heated initially at 300°Cinan matters in one gram of rock samples during the inert atmosphere by passing a stream of helium, temperature of 300–600°C), S3 (oxidizing carbon with a gradual increase up to 600°C with the rate of produced during thermal cracking of the kerogen temperature rise of 25°C/min. The released and indicates the amount of oxygen in organic hydrocarbons were analyzed with a Flame Ioniza- matters, expressed in mg CO2/g rock) were deter- tion Detector (FID). The samples were then oxi- mined during these analyses. Other secondary dized in an oxidation oven over the temperature parameters such as hydrogen index (HI = S2 9 range of 300–850°C at an increment rate of 20°C/ 100/TOC), oxygen index (OI = S3 9 100/TOC), min. TOC (total organic carbon) content and other genetic potential (GP = S1 + S2), production primary parameters such as Tmax (temperature at index (PI = S1/S1 + S2) had been computed from 59 Page 6 of 16 J. Earth Syst. Sci. (2021) 130:59 the pyrolysis data. The results of the analyses are suggests the site has a high potential to be a good given in table 5. source of rock for oil. However, not all AOM shows good oil-generative potential (Tissot and Welte 1984). 4. Results and discussion
4.1 Characterization of palynofacies 4.1.2 Structural terrestrial organic matter constituents in the Formation Terrestrially derived organic components which Many variables are involved in the deposition and contain an aggregate of preserved structure, such preservation of palynological material (Tyson as root, stem tissues, and unaffected cellular 1995; Batten 1996). The palynological study of the remains of leaf including cuticles, tracheids, and Kopili shales documents the range of different woods are commonly encountered in sediments. types of particulate organic matter that contribute They represent about 0–22% (5.57% on average) to the formation including: amorphous organic of the samples we investigated. Structured wood matter, structural terrestrial organic matter, is dominated by 40 to [ 90 lm sized lath-shaped biodegraded organic matter, dinoCagellate cysts with occasional equidimensional forms having (dinocysts), spores/pollen, charcoal, and black various shades of yellow-brown, dark brown, and carbon debris (Bgure 3). Some samples lack one or brown (Bgure 3). The cuticles are larger and more of these components of organic matter, but all better preserved having various sizes reaching contain amorphous organic matter. Many broken more than 100 lm. They are Cat, platy, relatively and unidentiBable dinocysts were noted. The most thin, and generally well-preserved although some common forms of dispersed organic matter identi- are poorly-preserved and almost degraded. An Bed in our samples are presented in table 3. The abundance of cuticles of special significance, relative percentage of the various palynofacies because their waxy or oily composition means groups are described as follows. that they have significant potential as a source of liquid hydrocarbons, a unique property among phytoclasts (Barker 1974). 4.1.1 Amorphous organic matter (AOM)
Amorphous organic matter (AOM) is the most 4.1.3 Biodegraded organic matter common form of organic matter identiBed in the studied samples, ranging from 51 to 98% with an Structured terrestrial matter forms biodegraded average of 81.63%. Most of AOM exhibits identical terrestrial organic matter of semi-amorphous type diffuse grey to yellow outlines with structureless when aAected by microbial activities, but still features described as granular or Caky of varying shows visual traces of its original cellular structure. sizes ranging from 10 to 70 lm(Bgure 3). In gen- As it is the product of the micro-biological break- eral, AOM sourced from terrestrially-derived down of larger to smaller molecules and impacted materials is darker yellow to amber in colour, while by bacterial and fungal decay producing significant AOM preserved in a marine or non-marine reduc- quantities of protein. This material should have an ing environment is colourless to neutral grey enhanced hydrocarbon source potential when resulting from degradation under the inCuence of compared with structured terrestrial material and anaerobic bacteria, and bacterially degraded should also mature at somewhat lower tempera- amorphous material formed at the same Eh (redox) tures (Masran and Pocock 1981). The frequency of from dominantly terrestrially sourced organic biodegraded organic matter in the study area is material will look similar under the microscope 1–21% (7.33% on average). (Masran and Pocock 1981). The dominance of amorphous organic matter is the result of high 4.1.4 Charcoal/black debris preservation rates in a low-energy environment. The high percentage of identical grey AOM is The abundance of charcoal/black carbon debris strong evidence of a reducing environment, ranges between 0 and 17% (4.57% on average). indicative of a dysoxic–anoxic condition and non- Some opaque particles are characteristic of char- marine origin (Masran and Pocock 1981; Tyson coal produced by natural wildBres. These frag- 1993). The abundance of AOM of marine origin ments are the product of natural pyrolysis of J. Earth Syst. Sci. (2021) 130:59 Page 7 of 16 59
Figure 3. Transmitted light photographs of the palynofacies from the lower section of the Kopili Formation in the Umphyrluh area (Borehole BUM14). Scale bar represents 10 lm. 1. Lingulodinium machaerophorum (DeCandre and Cookson 1955; Wall 1967); 2. Operculodinium microtriainum (Klumpp 1953; Islam 1983); 3 & 5. Operculodinium centrocarpum (DeCandre and Cookson 1955; Wall 1967); 4, 13 & 16. Polysphaeridium subtile (Davey and Williams 1966; Bujak et al. 1980); 6 & 7. Melitasphaeridium cf. seudorecurvatum (Morgenroth 1966a, b; Bujak et al. 1980); 8. Operculodinium major (Dutta and Jain 1980); 9 & 12. Polysphaeridium zoharyi (Rossignol 1962; Bujak et al. 1980); 10. Achmosphaera alcicornu (Eisenack 1954; Davey and Williams 1966); 11. Adnatosphaeridium vittatum (Williams and Downie 1966); 14 & 15. Operculodinium divergens (Eisenack 1954; Stover and Evitt 1978); 17. Homotryblium Coripes (DeCandre and Cookson 1955); 18. Diphyes colligerum (DeCandre and Cookson 1955; Cookson and Eisenack 1955); 19. undetermined dinoCagellate cyst; 20. undetermined spore; 21. amorphous organic matter; 22. charcoal; 23. structural terrestrial organic matter; 24. biodegraded terrestrial organic matter. terrestrial machrophyte material, which are resis- (Cope 1981; Chaloner 1989). Charcoal has no tant to further degradation, i.e., the action of high hydrocarbon source rock potential due to being temperature under conditions of oxygen starvation composed only of carbon. 59 Page 8 of 16 J. Earth Syst. Sci. (2021) 130:59
Table 3. Distribution of organic matter in shale samples to lower part of the Kopili Formation at Umphyrluh area (Borehole BUM 14), Jaintia Hills, Meghalaya.
Amorphous Structural organic Biodegraded terrestrial matter Charcoal/black organic organic DinoCagellate Spores/ Sample (AOM) debris matter matter cysts pollen ID (%) (%) (%) (%) (%) (%) KS1 69 7 20 4 0 0 KS2 86 5 2 7 0 0 KS3 75 6 10 6 3 0 KS4 85 2 5 4 3 1 KS5 85 5 2 6 2 0 KS6 80 8 5 7 0 0 KS7 86 3 3 7 0 1 KS8 92 1 6 1 0 0 KS9 73 3 16 8 0 0 KS10 51 5 21 22 1 0 KS11 89 3 6 0 1 0 KS12 82 6 3 9 0 0 KS13 91 2 4 2 0 1 KS14 87 3 8 0 1 1 KS15 84 7 3 6 0 0 KS16 91 0 4 5 0 0 KS17 65 9 18 8 0 0 KS18 67 7 20 6 0 0 KS19 83 0 13 4 0 0 KS20 74 5 2 15 4 0 KS21 80 17 2 1 0 0 KS22 77 8 6 9 0 0 KS23 70 7 15 8 0 0 KS24 87 0 4 9 0 0 KS25 98 0 2 0 0 0 KS26 92 1 3 0 4 0 KS27 92 2 5 0 1 0 KS28 96 1 1 0 1 1 KS29 83 5 4 8 0 0 KS30 79 9 7 5 0 0
4.1.5 Pollen and spores mainly by dinoCagellate cysts with a frequency range from 0 to 4% (0.7% on average). Pollen and spores are known to be enriched in lipids, carotenoids, and waxes, all of which can contribute significantly to the formation of liquid 4.2 Depositional environment hydrocarbons (Masran and Pocock 1981). How- DinoCagellate cysts were used to reconstruct the ever, the abundance of pollen and spores is very depositional environment of the Kopili Formation. low in all the studied samples, ranging between The dinocysts recovered from the lower part of the 0 and 1% (0.17% on average). Kopili Formation consist of 8 genera and 12 referable species. They are Operculodinium cen- 4.1.6 DinoCagellate cysts trocarpum, Operculodinium major, Operculo- dinium microtriainum, Operculodinium divergens, The palynomorph group is the least abundant Melitasphaeridium cf. pseudorecurvatum, Poly- among the palynofacies. However, marine paly- sphaeridium subtile, Polysphaeridium zoharyi, nomorphs recovered in the samples are represented Achmosphaera alcicornu, Adnatosphaeridium J. Earth Syst. Sci. (2021) 130:59 Page 9 of 16 59
Table 4. Shows recalculated percentage of phytoclasts–AOM– (e.g., Brinkhuis 1994). The occurrence of chorate palynomorphs. cysts Adnatosphaeridium vittatum, Polysphaerid- Sample ID Phytoclasts AOM Palynomorphs ium subtile, P. zoharyi, Operculodinium centro- carpum indicate nearshore shallow-marine KS1 31 69 0 environments with open-marine inCuence (Sarkar KS2 14 86 0 and Singh 1988; Saxena and Sarkar 2000; Singh KS3 22 75 3 KS4 11 85 4 and Dogra 2003). Palynotaxa of Operculodinium KS5 13 85 2 and Linugulodinium are dominant species in the KS6 20 80 0 tropic and subtropic waters. Linugulodinium is KS7 13 86 1 most abundant in nearshore estuarine environment KS8 8 92 0 (Wall et al. 1977). As reported by (Mehrotra et al. KS9 27 73 0 2002), Polysphaeridium zoharyi suggests a shallow KS10 48 51 1 inner shelf environment (relatively deeper) KS11 9 89 1 throughout the Eocene. Most of the palynotaxa in KS12 18 82 0 the study area suggests shallow marine and near- KS13 8 91 1 shore, lagoonal environment conditions. KS14 11 87 2 The palynofacies composition and relative KS15 16 84 0 percentage abundances of sedimentary organic KS16 9 91 0 KS17 35 65 0 matter (SOM) in the studied samples are illus- KS18 33 67 0 trated in table 3 and Bgure 2. The recalculated KS19 17 83 0 percentages of AOM, phytoclasts, and paly- KS20 22 74 4 nomorphs are shown in table 4. The percentage KS21 20 80 0 frequencies of sedimentary organic matter (AOM, KS22 23 77 0 phytoclasts, and palynomorphs) are plotted to KS23 30 70 0 show their frequency of occurrence and are com- KS24 13 87 0 pared with the zones of APP ternary plot for- KS25 2 98 0 warded by Tyson (1995) to determine the KS26 4 92 4 palaeoenvironment of deposition (Bgure 4). It has KS27 7 92 1 been observed that the majority of the distribu- KS28 2 96 2 tion frequencies fall under zone IX, but one sam- KS29 17 83 0 KS30 21 79 0 ple falls under zone VI, which indicates respectively proximal suboxic–anoxic basin and proximal suboxic–anoxic shelf deposition of the vittatum, Lingulodinium machaerophorum, Homo- sediments in the studied area (Bgure 4). tryblium Coripes and Diphyes colligerum. An increase in amorphous organic matter Operculodinium-group has extant representa- denotes dysoxic–suboxic conditions and the pres- tives, i.e., Operculodinium centrocarpum, which ence of allochthonous terrestrial phytoclasts and occurs in a wide variety of sedimentary settings, spore grain show proximity to the shoreline. Hence, from restricted-marine to open-marine and neritic the depositional environment of Kopili Formation water masses. Published information suggests that indicates a shallow marine environment (marginal the distribution of the taxa Operculodinium cen- marine), relatively shallow depth, with the trocarpum is a cosmopolitan species that has rela- inCuence of terrestrial inCux in the area under tively high frequencies in oAshore settings oxygen-deBcient conditions. generally due to erosion and transport to these locations (Wall et al. 1977; Edwards and Andrle 1992; Brinkhuis 1994). The common occurrence of 4.3 Hydrocarbon source rock evaluation Polysphaeridium spp. and Homotryblium spp. sug- gest a near-shore, lagoonal environment – the Rock-Eval pyrolysis is used to evaluate oil and gas recent motile stages of the former are commonly potential in sediments by considering the three found in the hypersaline environments (e.g., Wall main factors: (1) quantity or generative potential and Dale 1969; Bradford and Wall 1984), while based on TOC, S1 and S2; (2) quality or type of mass occurrences of fossil Homotryblium are asso- hydrocarbon generated based on HI and S2/S3 ciated by many authors with increased salinity ratio; and (3) thermal maturity or level of thermal 59 Page 10 of 16 J. Earth Syst. Sci. (2021) 130:59
Figure 4. Schematic illustration of organic matter groups used for palaeoenvironmental interpretations for the studied section (after Tyson 1995), where, I. highly proximal shelf or basin; II. marginal dysoxic–anoxic basin; III. heterolithic oxic shelf (proximal shelf); IV. shelf to basin transition; V. Mud dominated oxic shelf (distal shelf);VI. proximal suboxic–anoxic shelf; VII. distal dysoxic–anoxic shelf; VIII. distal dysoxic–oxic shelf; IX. proximal suboxic–anoxic basin.
alteration of the rock relative to oil generation, data and Rock-Eval parameters for the Kopili dependent on Tmax and PI (table 5). Formation are presented in table 5 document the TOC content of the 25 borehole samples ranges from 0.03 to 0.45 wt.% (average value 0.27%), 4.3.1 Organic matter quantity indicating that all samples in the study section Total organic carbon content (TOC) is the basic have a low potential to generate hydrocarbons in quantitative parameter for estimating the source significant quantities. These samples also have low rock potential of sedimentary rock and is usually pyrolysis values S1 (free hydrocarbon) about expressed in wt.%. In marine facies, higher TOC 0.02 mg HC/g rock (average), S2 about 0.10 mg values ([1%) are more commonly associated with HC/g rock (average), S3 about 0.25 mg CO2/g and marine sources, while low (\ 1%) TOC content highly variable HI and OI values. Generally, usually represents intervals comprising primarily organic matter with a genetic potential (GP = terrestrial organic matter (Peters 1986). Peters and S1 + S2) value[ 2 is considered to be moderate to Cassa (1994) state TOC values between 0.5 and excellent source rock with gas and oil generating 1.0% produce a fair source rock generative poten- potential. However, the plot of TOC and S1 + S2 tial, from 1.0 to 2.0% indicate a good generative (Bgure 5) shows poor source potential as all the potential, and TOC value [ 2.0% suggest a very plots fall in the Beld of poor to marginal. good generative potential. Barker (1996) consid- ered a TOC value of 1.0% as the minimum 4.3.2 Organic matter classiBcation acceptable value for an eAective source rock indi- cating good source potential. Peters and Cassa The source rock potential of the Kopili Formation (1994) state TOC values between 0.5 and 1.0% was assessed using several factors including the produce a fair source rock generative potential, quantity and quality of organic matter, the TOC from 1.0 to 2.0% indicate a good generative concentration, and hydrogen and oxygen contents potential, and TOC value [ 2.0% suggest a very (measured as hydrocarbon-type and carbon dioxide good generative potential. Rock-Eval pyrolysis yields, respectively, standardized to TOC, and J. Earth Syst. Sci. (2021) 130:59 Page 11 of 16 59
Table 5. Rock-Eval data of representative samples from the lower part of the Kopili Formation at Umphyrluh area (Borehole BUM 14), Meghalaya.
SN TOC S1 S2 S3 Tmax HI OI PI GP = (S1 + S2) Richness KS1 0.45 0.02 0.12 0.28 439 27 62 0.14 0.14 Poor KS2 0.3 0.02 0.09 0.09 454 30 30 0.18 0.11 Poor KS3 0.37 0.02 0.08 0.06 484 22 16 0.20 0.10 Poor KS4 0.33 0.02 0.09 0.18 443 27 55 0.18 0.11 Poor KS5 0.22 0.02 0.11 0.16 431 50 73 0.15 0.13 Poor KS6 0.31 0.02 0.11 0.13 446 35 42 0.15 0.13 Poor KS7 0.32 0.02 0.09 0.16 440 28 50 0.18 0.11 Poor KS8 0.33 0.03 0.1 0.07 485 30 21 0.23 0.13 Poor KS9 0.37 0.02 0.11 0.14 450 30 38 0.15 0.13 Poor KS10 0.38 0.01 0.1 0.13 445 26 34 0.09 0.11 Poor KS11 0.3 0.04 0.2 0.88 317 67 293 0.17 0.24 Poor KS13 0.16 0.02 0.16 0.20 436 100 125 0.11 0.18 Poor KS14 0.17 0.01 0.06 0.56 342 35 329 0.14 0.07 Poor KS17 0.03 0.02 0.07 0.64 380 23 213 0.22 0.09 Poor KS18 0.06 0.01 0.03 0.16 476 50 267 0.25 0.04 Poor KS20 0.27 0.01 0.09 0.16 455 33 59 0.10 0.10 Poor KS21 0.40 0.02 0.15 0.08 442 38 20 0.12 0.17 Poor KS22 0.44 0.01 0.15 0.21 443 34 48 0.06 0.16 Poor KS23 0.37 0.02 0.12 0.35 451 32 95 0.14 0.14 Poor KS24 0.3 0.01 0.11 0.24 457 37 80 0.08 0.12 Poor KS25 0.38 0.02 0.1 0.2 444 26 53 0.17 0.12 Poor KS26 0.13 0.02 0.06 0.34 385 46 262 0.25 0.08 Poor KS27 0.13 0.02 0.06 0.79 391 46 608 0.25 0.08 Poor KS28 0.22 0.02 0.09 0.09 384 41 41 0.18 0.11 Poor KS29 0.16 0.02 0.07 0.21 384 44 131 0.22 0.09 Poor Explanation. SN: Sample number; TOC (wt.%): total organic carbon; S1: free hydrocarbon present in the rock (mg HC/g of rock); S2: remaining generation potential (mg HC/g of rock); S3: oxidizable carbon (mgCO2/g rock); Tmax: temperature max- imum at which maximum generation of hydrocarbon (S2) from the kerogen in rock sample during pyrolysis (°C); HI: hydrogen index, [(S2/TOC) 9 100 mg HC/g TOC]; OI: oxygen index [(S3/TOC) 9 100 mg CO2/g TOC]; PI: production index, [S1/(S1+S2)]; GP: genetic potential, (S1+S2). presented as hydrogen index (mg HC/g TOC) and zooplankton, and microorganisms deposited in a oxygen index (mg CO2/g TOC). They are plotted reducing environment. Type III kerogen is char- and compared to the HI vs. OI plot (Bgure 6)to acterized by comparatively low HI values (50–200) classify the organic matter types. In immature and is generally derived from higher land plants sediments, organic matter dominated by marine deposited in limited horizons in bioturbated sedi- components commonly has hydrogen index values ments under predominantly dysoxic to oxic con- ranging from 200 to 400 mg HC/g TOC (Stein et al. ditions (Barker 1974). Type IV kerogen has HI 1989; Stein 1991). The kerogen type designations values \ 50 with an increasing percent of terres- are based entirely on the HI (Hunt 1996). Hence, a trial organic matter and corresponds mainly to plot of HI vs. OI (Bgure 6) and HI vs. Tmax plot inertinite often known as dead-carbon. It is derived (Bgure 7) are used in combination to interpret from ligniBed precursors, such as altered woody kerogen type. In these diagrams, four categories of material, and is highly altered mainly by oxidation kerogen are shown classiBed as Type I-very oil processes. It has eAectively no potential for oil and prone, Type II-oil prone, Type III-gas prone, and very little, if any, for gas (Brooks et al. 1987). Type IV-inert (Cope 1981). Type I kerogen has Plotting HI vs. OI (Bgure 6) on a modiBed van high HI values ([ 600) containing algal or Krevelen diagram reveals that the Kopili Forma- cyanobacteria and is produced and deposited tion is dominated by kerogen type III and IV. The mainly in anoxic lacustrine and marine environ- plot of HI vs. Tmax (Bgure 7) also shows that the ments (Peters 1986; Hunt 1996). Type II kero- samples dominantly contain type III and IV gen has medium HI values (300–600) and is kerogens that indicate their capability to generate characterized by mixtures of phytoplankton, gas. 59 Page 12 of 16 J. Earth Syst. Sci. (2021) 130:59
Figure 5. A plot of total organic carbon vs. S1+S2 indicating hydrocarbon generating potential of the analyzed samples Figure 7. Hydrogen index (HI) vs. Tmax plot to estimate the (after Ghori and Haines 2007). type of organic matter and degree of maturation (after Espitalie et al. 1985).
pyrolysis treatment (Stein et al. 1989; Stein 1991). The thermal maturity of the shale samples from the Kopili Formation is evaluated by Rock-Eval pyrolysis derived indices, such as Tmax and pro- duction index (table 5). The low Tmax value indi- cates that the organic matter has not produced a substantial amount of hydrocarbon. Tmax values between 430° and 465°C are considered mature and a high Tmax value greater than 465°C indicates a post-mature or over-mature stage (Espitalie et al. 1985). Tmax values between 430° and 460°C rep- resent conditions for mature organic matter in the ‘oil window’. Values between 460° and 475°C are considered wet gas zone whereas Tmax [475°C are in the dry gas zone (Ghori and Haines 2007). Peters (1986) and Espitalie et al. (1985) reported that oil Figure 6. A plot of Hydrogen index vs. Oxygen index for generation from source rocks initiated at Tmax = classiBcation of kerogen types of Kopili shale in the Umphyr- 435°–465°C and production index ‘PI’ between 0.2 luh area (Borehole BUM14), Meghalaya (after Peters 1986). and 0.4, PI and Tmax values less than about 0.2 and 435°C, respectively, indicate immature organic 4.3.3 Organic matter thermal maturity matter and the gas generation from source rocks initiated at ‘Tmax’ 470°C, and production index The Rock-Eval Tmax is the temperature that rep- ‘PI’ more than 0.4. To estimate kerogen quality resents the top of the S2 peak at which the maxi- and degree of thermal maturity of organic matter mum amount of hydrocarbon generation from the of the Kopili Formation, we show HI vs. Tmax plot decomposition of organic matter occurs during (Bgure 7) and PI vs. Tmax (Bgure 8). In the studied J. Earth Syst. Sci. (2021) 130:59 Page 13 of 16 59
Figure 8. A plot of Tmax vs. production index showing maturation potential of the studied rock units (after Ghori and Haines 2007).
samples, Tmax ranges from 317°C (sample KS11) to marine) under a proximal dysoxic–anoxic 485°C (sample KS8). Boreholes KS5, KS11, KS14, condition that is inCuenced by the inCux of KS17, KS26, KS27, KS28, KS29 have Tmax values terrestrial matter. ranging from 317° to 431°CreCecting the occurrence of (3) The Rock-Eval data and TOC analysis demon- immature kerogen, while most of the samples (sample strates that the shale samples are predomi- KS1, KS2, KS4, KS5, KS6, KS7, KS9, KS10, KS13, nantly gas prone (mostly Type III and Type KS20, KS21, KS22, KS23, KS24, KS25) are partly IV). Most of the samples are in a mature stage within the oil window, and three samples (KS3, KS8, and fall partly within the oil window, indicat- KS18) fall in the range of dry gas (Bgure 8). ing their potential for oil generation. However, the samples investigated here fall in the category of poor potential hydrocarbon source 5. Conclusions rock as they have not reached the threshold value of generating potential (GP [ 2 mgHC/ We derive the following conclusions from the gm rock) and TOC is low ([ 0.5%). analysis of our data: (1) Palynotaxa recovered from the lower part of Kopili Formation in the Umphyrluh area of Acknowledgements Meghalaya consist mostly of DinoCagellate taxa, viz., Operculodinium centrocarpum, Oper- We are grateful to Director General, GSI Shillong, culodinium major, Operculodinium microtri- India for providing core samples used in this study. ainum, Operculodinium divergens, Melita- We also express our appreciation to the Head of sphaeridium cf. pseudorecurvatum, Polysphaeri- Geochemistry, ONGC, Dehradun, India for the dium subtile, Polysphaeridium zoharyi, Achmo- analysis of Rock-Eval pyrolysis and TOC. We thank sphaera alcicornu, Adnatosphaeridium vittatum, the Science and Engineering Research Board, New Lingulodinium machaerophorum, Homotryblium Delhi for Bnancial support in form of a project (Grant Coripes and Diphyes colligerum. No. EEQ/2016/000062). RD is also grateful to UGC, (2) Based on evidence from analysis of palyno- (22/06/2014(i) EU-V dated 15th December 2014), fossils, the depositional environment of the New Delhi for Bnancial assistance in the form of JRF. Kopili Formation is interpreted to be a We sincerely acknowledge Shri Ksh Premdas Singh shallow marine environment (marginal for his support during Beldwork. 59 Page 14 of 16 J. Earth Syst. Sci. (2021) 130:59
Author statement Davey R J and Williams G L 1966 The genus Hystrichos- phaeridium and its allies; Bull. Br. Museum (Nat. Hist.) N Reshma Devi collected samples, carried out lit- Geol. Suppl. 3 53–106. erature review, palynofossils identiBcation, result DeCandre G and Cookson I 1955 Fossil microplankton from Australian Late Mesozoic and Tertiary sediments; Aust. interpretation and wrote the manuscript. A Bi- J. Mar. Freshw. Res. 6 242–313. jyalaxmi Devi prepared slides. Prof Y Raghu- Dutta S K and Jain K P 1980 Geology and palynology of the mani Singh participated in sample collection, area around Lumshnong, Jaintia Hills, Meghalaya, India; examined the palynofossils identiBcation and Biol. Mem. 5 56–81. reviewed the manuscript. Prof Mark B Abbott Edwards L E and Andrle V A S 1992 Distribution of selected dinoCagellate cysts in modern marine sediments; In: Neo- reviewed and revised the manuscript providing gene and Quaternary DinoCagellate Cysts and Acritarchs inputs for improvement and prepared the Bnal (eds) Head M H and Wrenn J H, Am. Assoc. Stratigr. version. Palynol. Foundation, Dallas, pp. 259–288. Eisenack A 1954 Microfossilienaus Phosphoriten des sam- landischen Unteroligozans und uber dee Einheitlichkeit der Hystrichosphaeridium; Palaeontographica A105 References 49–95. Espitalie J, Laporte J L, Madec M, Marquis F, Leplat P, Baksi S K 1962 Palynological investigation of Simsang River Paulet J and Boutefeu A 1977 Rapid method for source rock Tertiaries, South Sillong Front, Assam; Bull. Geol. Min. characterisation, and for determination of their petroleum Metall. Soc. India 26 1–22. potential and degree of evolution; Revue de l Inst. Fr. Baksi S K 1974 Significant pollen taxa in the stratigraphical Petrol. Ann. Combust. Liq. 32(1) 23–42. analysis of the tertiary sediments of Assam; In: Aspects and Espitalie J, Dreoo G and Marquis F 1985 Rock-Eval pyrolysis Appraisal of Indian Palaeobotany (eds) Surange K R, and its applications – Part 2; Revue de l Inst. Fr. Petrol. 40 Lakhanpal R N and Bharadwaj D C, Birbal Sahni Institute 755–784. of Palaeobotany, Lucknow, pp. 502–515. Espitalie J, Dreoo G and Marquis F 1986 Rock-Eval pyrolysis Barker C 1974 Pyrolysis techniques for source-rock evaluation; and its applications – Part 3; Revue de l Inst. Fr. Petrol. AAPG Bull. 58(11) 2349–2361. 41(1) 73–89. Barker C 1996 Thermal Modeling of Petroleum Generation – Evans P 1932 Tertiary succession in Assam; Trans. Min. Geol. Theory and Applications (Developments in Petroleum Inst. India 27 155–260. Science); Elsevier, Amsterdam. Funkhouser J W and Evitt W R 1959 Preparation techniques for Batten D J 1996 Palynofacies; In: Palynology – Principles and acid-insoluble microfossils; Micropaleontology 5(3) 369–375. Applications (eds) Jansonius J and McGregor D J, Amer. Assoc. Ghori K and Haines P W 2007 Paleozoic petroleum systems of Stratigr. Palynol. Foundation, Dallas, TX, pp. 1011–1064. the Canning Basin, Western Australia; Search and Discov- Bradford M R and Wall D A 1984 The distribution of Recent ery Article 10120. organic-walled dinoCagellate cysts in the Persian Gulf, Gulf GSI 2013 Limestone deposit of Litang Valley, Jaintia Hills of Oman, and northwestern Arabian Sea; Palaeontograph- district, Meghalaya; Bull. Geol. Surv. India Ser. A 631–105. ica B192 16–84. Hunt J M 1996 Petroleum Geochemistry and Geology; 2nd Brinkhuis H 1994 Late Eocene to Early Oligocene dinoCagel- edn, Freeman, New York, 743p. late cysts from the Priabonian type-area (Northeast Italy): Islam M A 1983 DinoCagellate cysts from the Eocene cliA Biostratigraphy and paleoenvironmental interpretation; sections of the Isle of Sheppey, Southeast England; Rev. Palaeogeogr. Palaeoclimatol. Palaeoecol. 107 121–163. Micropaleontol. 25 231–250. Brooks J, Cornford C and Archer R 1987 The role of Kar R K, Handique G K, Kalita C K, Mandal J, Sarkar S, hydrocarbon source rocks in petroleum exploration; In: Kumar M and Gupta A 1994 Palynostratigraphical studies Marine Petroleum Source Rocks, vol 26 (eds) Brooks J and on subsurface Tertiary sediments in Upper Assam Basin, Fleet A J, Geological Society, London, pp. 17–48. India; Palaeobotanist 42(2) 183–198. Bujak J P, Downie C, Eaton G L and Williams G L 1980 Klumpp B 1953 Beitragzur Kenntnis der Mikrofossilien des DinoCagellates cyst and acritarchs from the Eocene of mittleren und oberen Eozan;€ Palaeontographica A103 Southern England; Palaeontol. Assoc Spec. Pap. Palaeon- 377–406. tol. 24 1–100. Masran T C and Pocock S A J 1981 The classiBcation of plant- Chaloner W G 1989 Fossil charcoal as an indicator of derived particulate organic matter in sedimentary rocks; In: palaeoatmospheric oxygen level; J. Geol. Soc. London Organic Maturation Studies and Fossil Fuel Exploration 146 171–174. (ed.) Brooks J, Academic Press, London. Cookson I C and Eisenack A 1955 Microplankton from the Mathur L P and Evans P 1964 Oil in India; In: 22nd Int. Geol. browns creek clays, sw. Victoria; Proc. R. Soc. Vic. 79 Cong., New Delhi, pp. 1–85. 119–131. Mehrotra N C, Venkatachala B S, Swamy S N and Kapoor P N Cope M J 1981 Products of natural burning as a component of 2002 Palynology in hydrocarbon Exploration (The Indian the dispersed organic matter of sedimentary rocks; In: Scenario); Geol. Soc. India, Bangalore. Mem. 48. Organic Maturation Studies and Fossil Fuel Exploration Mohan M and Pandey J 1973 Early Palaeogene ecostratig- (ed.) Brooks J, Academic Press, London, New York, raphy of Upper Assam; Bull. Indian Geol. Assoc. 6(1) pp. 89–109. 47–62. J. Earth Syst. Sci. (2021) 130:59 Page 15 of 16 59
Morgenroth P 1966 Neue in organischer Substanzerhaltene and 659: Implications for the reconstruction of palaeoenvi- Mikrofossilien des Oligozans;€ Neues Jahrbuchfur€ Geologie ronments in the eastern subtropical Atlantic through late und Palaontologie,€ Abhandlungen 127 1–12. Cenozoic times; In: Ocean Drilling Program (ed.) Ruddi- Morgenroth P 1966 Mikrofossilien und Konkretionen des man W F, Proc. ODP, Sci. Results, 108, College Station, nordwesteur opaischen€ Untereozans;€ Palaeontographica pp. 361–386. B119 1–53. Stover L E and Evitt W R 1978 Analyses of pre-Pleistocene Nagappa Y 1959 Foraminiferal biostratigraphy of the Creta- organic-walled dinoCagellates; Geological Sciences, Stan- ceous-Eocene succession in the India–Pakistan–Burma ford University Publications, pp. 15–300. region; Micropaleontology 5 145–192. Tissot B P and Welte D H 1984 Petroleum Formation and Peters K E 1986 Guidelines for evaluating petroleum source Occurrence; Springer, New York, 699p. rocks using programmed pyrolysis; AAPG Bull. 70 Tripathi S K M 1989 Algal and fungal remains from Jowai- 318–329. Sonapur Road Section (Palaeocene–Eocene), Maghalaya; Peters K E and Cassa M R 1994 Applied source-rock Palaeobotanist 37(1) 63–76. geochemistry; AAPG Mem. 60 93–120. Tripathi S K M and Singh H P 1984 Two new pollen genera Rossignol M 1962 Analyse pollinique de sediments marins from the lower tertiary sediments of Meghalaya; Palaeob- quaternaires en Israel, II Sediments Pleistocenes; Pollen otanist 32(2) 153–157. Spores 4(1) 121–147. Tripathi S K M and Singh H P 1984b Palynostratigraphical Sah S C D and Dutta S K 1968 Palynostratigraphy of the zonation and correlation of the Jowai–Badarpur Road Tertiary sedimentary formations of Assam: 2. Stratigraphic Section (Palaeocene–Eocene), Meghalaya, India; In: Pro- significance of spores and pollen in the Tertiary succession ceedings of the 5th Indian Geophytological Conference, of Assam; Palaeobotanist 16(2) 177–195. Lucknow, 1983, Special Publication (eds) Tiwari R S, Salujha S K, Kindra G S and Rehman K (1972) Palynology of Awasthi N, Suresh C, Srivastava, Singh H P and Sharma B the South Shillong Front, Part I. The Palaeogene of Garo B, The Palaeobotanical Society, London, pp. 316–328. Hills; In: Proceedings of the Symposium on Palaeopalynol- Tripathi S K M and Singh H P 1985 Palynology of the Jaintia ogy and Indian Stratigraphy (eds) Ghosh A K, Chanda S, Group (Palaeocene–Eocene) exposed along Jowai-Sonapur Gosh T K, Baksi A K and Banerjee M, Bot. Dept., Calcutta Road, Meghalaya, India Part I. Systematic palynology; Univ, Calcutta, pp. 265–291. Geophytology 15 164–187. Salujha S K, Kindra G S and Rehman K 1974 Palynology of Trivedi G K 1985 Palynology of the Kopili Formation (Upper the South Shillong Front Part II. The Palaeogenes of Khasi Eocene) exposed along Jowai-Badarpur Road, Meghalaya; and Jaintia Hills; Palaeobotanist 21(3) 267–284. J. Indian Bot. Soc. 64 66–72. Samanta B K 1971 Early tertiary stratigraphy of the area Trivedi G K 1987 Palynostratigraphy of the Upper Eocene around Garampani, Mikir–North-Cachar Hills, Assam; J. sediments in Meghalaya; Ph.D. Thesis, Lucknow University. Geol. Soc. India 12 318–327. Trivedi G K 1991 Reworked Gondwana palynomorphs from Sarkar S and Singh H P 1988 Palynological investigation of the the Kopili Formation (Late Eocene) of Jaintia Hills, Subathu Formation (Eocene) in the Banethi-Bagthan area Meghalaya; Geophytology 20(1) 66–68. of Himachal Pradesh, India; Palaeontographica B209(1–3) Trivedi G K 2005 Palaeopalynology of Kopili Formation – A 29–109. review; In: Applied Botany (ed.) Trivedi P C, Aavishkar Saxena R K and Sarkar S 2000 Palynological investigation of Publishers, Distributors, Jaipur, India, pp. 183–189. the Siju Formation (Middle Eocene) in the type area, South Trivedi G K 2009 A palynoCoral diversity in the Kopili Garo Hills, Meghalaya, India; Palaeobotanist 49 253–267. Formation (Late Eocene) from North-East India; Acta Sein M K and Sah S C D 1974 Palynological demarcation of Musei Natl. Pragae Ser. B Hist. Nat. 65(1–2) 9–24. the Eocene–Oligocene sediments in the Jowai–Badarpur Trivedi G K and Ranhotra P S 2015 PalynoCoral evidence for Road Section, Assam; In: Symposium on Stratigraphical palaeoecology and depositional environment of the Kopili Palynology, Spec. Publ. 3, Birbal Sahni Institute Palaeob- Formation (Late Eocene), Jaintia Hills, Meghalaya; J. otany, Lucknow, pp. 99–105. Geol. Soc. India 86 33–40. Singh Y R and Dogra N N 2003 Age and Palaeoenvironmental Trivedi G K and Saxena R K 2000 PalynoCoral investigation constraints of Subathu Formation of Dharampur and Koti of the Kopili Formation (Late Eocene) exposed near Areas of Solan District, Himachal Pradesh – a Palynological Umrongso in North Cachar Hills District, Assam, India; approach; Gond. Geol. Mag. 6 195–205. Palaeobotanist 49(2) 269–280. Singh H P and Tripathi S K M 1987 Palynology of the Jaintia Trivedi G K and Saxena R K 2009 Palynological investigation Group (Palaeocene–Eocene) exposed along Jowai-Sonapur of the Kopili Formation (Late Eocene) in North Cachar Road, Meghalaya, India Part II. Data analysis and inter- Hills, Assam, India; Acta Palaeobot. 49(2) 253–277. pretation; Palaeobotanist 35(3) 301–313. Tyson R V 1993 Palynofacies analysis; In: Applied Staplin F L, Pocock S J, Jansonius J and Oliphant E M 1960 Micropalaeontology (ed.) Jenkins D G, Kluwer Academic Palynological techniques for sediments; Micropalaeontology Publishers, Amsterdam, pp. 153–191. 6(3) 329–331. Tyson R V 1995 Sedimentary Organic Matter – Organic Stein R 1991 Accumulation of carbon in marine sediments; In: Facies and Palynofacies; Hapman and Hall, London, 615p. Lecture Notes in Earth Sciences (ed.) Bhattarcharji S, Wall D 1967 Fossil microplankton in deep-sea cores from the Springer, Berlin, 217p. Caribbean Sea; Palaeontology 10 95–123. Stein R, TenHaven H L, Littke R, Rullkotter J and Welte D H Wall D and Dale B 1969 The hystrichosphaerid resting spore 1989 Accumulation of marine and terrigenous organic of the dinoCagellate Pyrodinium bahamense Plate 1906; J. carbon at upwelling site 658 and non-upwelling sites 657 Phycol. 5 140–149. 59 Page 16 of 16 J. Earth Syst. Sci. (2021) 130:59
Wall D, Dale B, Lohmann G P and Smith W K 1977 The and Cainozoic DinoCagellate Cysts (eds) Davey R J, environmental and climatic distribution of dinoCagellate Downie C, Sarjeant W A S, Williams G L, Bull. British cysts in modern marine sediments from regions in the North Museum (Natural History) Geology, Supplement 3 and South Atlantic Oceans and adjacent areas; Mar. 215–235. Micropaleontol. 2 121–200. Zachos J, Pagani M, Sloan L, Thomas E and Billups K 2001 Williams G L and Downie C 1966 Further dinoCagellate Trends, rhythms, and aberrations in global climate 65 Ma cysts from the London Clay; In: Studies on Mesozoic to present; Science 292 686–693.
Corresponding editor: SANTANU BANERJEE