He joined the Regional Geological Laboratories ONGC at CRBC, Calcutta in 1986 to establish Palynology Laboratory and served as Head of  Regional Geological Laboratory. In 1991 he returned to the Central Palynological Laboratory at Dehra Dun and served as Head of the Palynology Laboratory at KDMIPE (1996 – Feb., 2005). The responsibilities at ONGC  labs included palynostratigraphic data backup to exploration efforts in the form of various contract, sponsored, research and essential projects.  It involves preparation of paleogeographic and paleoenvironmental facie's maps; potential source rock facie's delineation; development of sequence biostratigraphy; and data integration to support geological modeling.  Dr Mehrotra has thirty-eight years of professional experience in exploration palynology including systematics and morphology of dinoflagellate cysts and spores and pollen; palynostratigraphic, palaeoenvironmental and source rock evaluation research related to hydrocarbon exploration. In ONGC Dr. Mehrotra rose to the position of Deputy General Manager and Head Palynology.

Major Scientific contributions of N.C. Mehrotra

Stratigraphy of sedimentary basins of India is built upon the fossil data. In the past few decades, palynofossils have played a major role in building the biostratigraphy of petroliferous basins of India, more significantly in areas where micropaleontological data is poor, besides solving many complex stratigraphic problems and suggesting new areas for exploration.
            “Palynology in Hydrocarbon Exploration – the Indian Scenario” (Part-I) is a co-authored memoir in which 50 years of published information on petroliferous basins is detailed. The volume includes numerous scientific contributions which have helped build the palynostratigraphy of Cambay, Mumbai Offshore, Krishna-Godavari, Cauvery and Assam-Arakan basins and their source rock evaluation. This is based on integrated palynological data tied up with lithostratigraphy. The volume serves as a handy database for stratigraphic studies.
            High resolution palynological biostratigraphy is based on phytoplankton. Dinoflagellates, one such important group of microfossils was least understood and documented by most Indian palyologists till mid eighties when a series of joint publications by Mehrotra and Sarjeant (7 Publs.) were brought out from East Coast of India. The latter were the first to document, describe and illustrate their complex morphologies from India. These proved very useful to biostratigraphers, particularly those associated within hydrocarbon exploration because of their immense biostratigraphic value and paleoceanographic significance.
            “Palynology in Hydrocarbon Exploration (Part-II) : Spatial and temporal distribution of selected spores/pollen and dinoflagellate cyst taxa from the Mesozoic – Tertiary sediments of Petroliferous basins of India” includes stratigraphic ranges of palynotaxa; their botanical affinities; ecology; and paleoceanography. Forty-five detailed stratigraphic range tables are included, summarizing the large volume of palynological data. The volume has a concluding chapter dealing with evolution of angiosperms, plate tectonics, migration routes, provincialism and implications in exploration of hydrocarbons.
            Published two atlases covering Mesozoic and Tertiary dinoflagellate cysts from Krishna-Godavari Basin. A maximum sequence stratigraphic resolution of 0.4 to 1 million year has been established for Krishna-Godavari and Mumbai Offshore basins. Also successfully reconstructed the paleogeography and paleoenvironmental models to understand relative sea level changes and develop sequence biostratigraphy. The work on source rock evaluation has helped in identifying depocentres for the development of petroleum source rocks. The above information is of immense value in refining existing geological models for reducing exploration risk.
            The information disseminated will bring out remarkable changes in the field of high-resolution biostratigraphy to give lead in hydrocarbon exploration particularly the stratigraphic traps. The following 5 major publications will serve as a ready reckoner for instant use in interpretation to biostratigraphers and basin modelers for hydrocarbon exploration:

           Initiated development of high impact palynological studies at BSIP, related to fossil fuel exploration and palaeoclimate. Thrust is given to multidisciplinary and multi-institutional integrated approach. First record of Type I matured liptinite organic matter from Late Neoproterozoic sediments is considered significant from hydrocarbon exploration. Establishing an Hydrocarbon Study Centre at BSIP for providing Consultancy Services to operators is in the process.
            Recently established Polar Research Cell and initiated Glaciological Studies at BSIP to fully utilize integrated palaeobotanical data in understanding the past climatic changes especially during the last 10,000 years. Such studies on palaeoclimate would help in better understanding of present climatic changes and conceptualising future predictive models.
            Dr. Mehrotra has to his credit some 53 research papers, published in National and International Journals including 2 monographs (Memoir Geol. Soc. 48 & 61), 2 volumes of atlas on fossil dinoflagellate cysts (ONGC Spl. Publs.) and one Edited Memoir Geol. Soc. 71. Additionally, about 100 Unpublished Technical Reports (ONGC).
            He has following awards and honours to his credit: Awarded Government of India National Scholarship in Geology for post–Doctoral studies at the University of Saskatchewan, Saskatoon, Canada on "Phytoplankton in Oil Exploration" (1982); Birbal Sahni Birth Centenary Award, Indian Science Congress Association for life time achievements in the development of Palaeobotany(2007); S.N. Bhalla Gold Medal-2006, Palaeontological Society of India, Lucknow for outstanding life time contributions (2007); L.R. Rama Rao Centenary Award, Geological Society of India, Bangalore for Life Time contribution to Indian biostratigraphy and Palaeontology (2009); Palaeobotanical Society Diamond Jubilee Award, Palaeobotanical Society, Lucknow – Life time contribution for development of Palaeobotany (2009); President, Palaeobotanical Society, Lucknow; Member, Governing Body, Wadia Institute of Himalayan Geology, Dehradun, Department of Science & Technology, Govt. of India; National Centre for Antarctic & Ocean Research (NCAOR), Goa, Ministry of Earth Sciences, Govt. of India; Chairman, Research Advisory Council (Local), Regional Science Centre, Lucknow, Ministry of Culture, Govt. of India; Member Council and Fellow, Geological Society of India, Bangalore; ONGC honored with several Internal Awards for outstanding contributions in Biostratigraphy during 1987-2000. Provided useful Palynological backup data in hydrocarbon exploration in different prospective basins ; Organised Conferences / Symposium as Chairman – Six (2005-2009), including two international on Palaeobotany; two on Palynology / Micropalaeontology in Hydrocarbon Exploration; one on Polar Research; International Collaborations (Principal Investigator) with Brazil and China on Biostratigraphy / Palaeoclimate.

Some of his recent publications are: