A Preliminary Organic Geochemical and Petrographical Investigation of Neyveli lignite (mine-1) to Understand the Depositional Environment and Hydrocarbon Source Characteristics.

Abstract

The lignite deposit of Neyveli mine-1 associated with the Cuddalore formation (Miocene) of Cauvery Basin, South India is studied using a combination of organic geochemical (FTIR spectroscopy and GC-MS) and organic petrographical analyses to identify the composition and kerogen type to reconstruct its palaeoflora and palaeodepositional setting. Also, its rank, thermal maturity, and hydrocarbon source potential are identified. The huminite macerals (78.11 av. vol.%) were found to be the most abundant. The petrographical facies indices (GI-TPI, GWI-VI), Paq values, and Pr/n-C₁₇ vs. Ph/n-C₁₈ plot indicate a mixed type of organic matter deposited in a limno-telmatic regime under mesotrophic to rheotrophic hydrological conditions.  The presence of herbaceous plants is supported by a high percentage of detrohuminite, VI values, and mid-chain alkanes. The presence of ulminite, resinite, sporinite, suberinite and cutinite, fluorescing huminite, high CPI (avg. 3.16) and TAR values (avg. 17.27), high P_wax ratio (avg. 0.81) and high abundance of longer chain alkanes (n-C₂₇ and n-C₂₉), supports the presence of higher plants. Short-chain alkanes and alginite support algal input. The percentage of funginite (3.18 avg. vol.%) and the ACL values represent a warm and humid palaeoclimate. The Pr/Ph ratio and Pr/n-C₁₇ vs Ph/n-C₁₈ plot indicate the suboxic to anoxic redox conditions (transitional environment). Minimal peat fires and periodic flooding events contributed to the low rank. Hydroxyl (3656-3226 cm^-1), aliphatic stretching and bending (2847-2954 cm^-1, 1377-1451 cm^-1), aromatic (1513-1615 cm^-1) and carboxyl groups (1701-1709 cm^-1) are obtained from FTIR spectra. The huminite reflectance measurement gives a rank of low-rank B (av. 0.37% R_r), and OEP values indicate immaturity. The different ternary diagrams and pseudo-Van Krevelen diagram indicate this lignite possesses admixed type-III/II kerogen which can generate heavy hydrocarbons. By also considering the presence of pre-oil solid bitumen, the Neyveli mine-1 lignite is identified as gas-prone and capable of oil generation.