Paleodepositional and Hydrocarbon Source-Rock Characteristics of the Sonari Succession (Paleocene), Barmer Basin, NW India: Implications from Petrography and Geochemistry

Abstract

Lignite deposits, associated with Akli Formation (Paleocene), from the Sonari mine of Barmer Basin, Rajasthan, were investigated by applying organic petrography, palynofacies, and geochemistry in order to understand the origin, nature, and character of these lignite-bearing deposits and to assess their thermal maturation and hydrocarbon generation potentiality.The studied samples contained an abundance of huminite group of macerals (av.54.0 vol.%) and relatively higher abundance of C27 and C29 n-alkane hydrocarbons.High carbon preference index (CPI: 5.03-9.44) and high terrigenous aquatic ratio (TAR: 5.09-20.01), together with the liptinite macerals (av.10.3 vol.%), inform the prevailing contribution of higher plants.Besides, the significant amount of detrohuminites (av.26.8 vol.%) and non-biostructure phytoclasts (av.42.25%), along with hopanoids, denote a meaningful herbaceous plants input and/or high level of tissue destruction (bacterial activity).The terpenoid composition was mainly constituted by pentacyclic triterpenoids and A-ring-degraded angiosperm-derived compounds and diterpenoids.The inertinite contents (av.22.3 vol.%) and the pristane/phytane (Pr/Ph) ratio imply the variation in the redox conditions during the accumulation.The petrographic indices revealed that the paleo-flora were accumulated in a limno-telmatic condition, with fluctuating groundwater level.Likewise, the palynofacies data displayed that the peat was deposited in dysoxic-suboxic settings under proximal condition.Subsequently, the incidence of dinoflagellate cysts in the studied samples suggests a marine intrusion.The considerable total of pyrite (up to 16.7 vol.%, comprising framboidal) suggests a coastal swamp condition (marginal marine).The thermal alteration index (TAI: av.2.15), Tmax (av.411 �C for lignite and 414 �C for associated shale) and the gross calorific values (av.4601 cal/g) showed the immaturity of the studied samples.The lignites contained low to moderate ash yields (av.12.57 wt.%) and moisture (av.12.79 wt.%) contents, whereas the carbondaf (daf?=?dry ash-free basis) contents were high (av.67.22 wt.%) and corroborated well with the inertinite group of macerals.The fuel ratio varied from 0.77 to 1.32.The volatile matter yielddaf (av.51.09 wt.%), fixed carbondaf (av.48.91 wt.%), and the oxygendaf (av.22.16 wt.%) contents were moderately high.The total organic carbon contents (TOC: 1.17-54.84 wt.%, av.24.78 wt.%) and hydrogen index values (HI: 32-361 mg HC/g rock) exhibit that the studied samples mostly have type III kerogen and show an excellent potentiality to generate gaseous hydrocarbons.