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.