Palynology and organic geochemistry of an Early to Middle Devonian succession from borehole TCB X-1001 ? Tacobo, Tarija Basin, southern Bolivia

MARTÍN PEREIRA; VICTORIA JOSEFINA GARCÍA; CLAUDIA VIVIANA RUBINSTEIN; GUSTAVO VERGANI

Salvador de Bahía

Congreso; XIV International Palynological Congress (IPC) and the X International Organisation of Palaeobotany Conference (IOPC); 2016

Asociación Latinoamericana de Paleobotánica y Palinología

Palynological, palynofaciesand organic geochemical analyzes have been performed on an Early to MiddleDevonian clastic marine sedimentary succession of the borehole TCB X-1001 ?Tacobo, in the Foothills, Tarija Basin (southern Bolivia), located between theSubandean and the Chaco-Plains. TheFoothills isconsidered one of the areas of the country boasting best exploration potentialfor gas and oil. The studied section ranges, from the upper to the lower part,between 4520 m and 5780 m in depth, mainly encompassing shallow marine depositsof the Los Monos, Huamampampa and Icla formations. The Los Monos Formation,considered the main source beds for oil and gas in the area, is mostly composedof dark grey to black shales. The Huamampampa Formation is characterized bygrey to greenish grey fine-grained sandstones intercalated with light grey siltstonesand sandy-siltstone beds. The Icla Formation consists of alternations of darkgrey shales, thin-bed sandstones and gray siltstones. Eighteen from the twenty-threecutting samples available for this study were productive, yielding variable abundance,diversity and preservation of organic-walled phytoplankton and miospores. Therelative abundance of marine and terrestrial palynomorphs fluctuates throughoutthe section with an increasing trend of terrestrial palynomorphs transpiring towardsthe younger beds. Archaeozonotriletes variabilis,Chelinospora timanica, Convolutispora subtilis, Cristatisporites triangulatus, Geminospora lemurata, Cymbosporites cyathus, Dibolisporites uncatus, Dibolisporites farraginis, Grandispora libyensis, Grandispora permulta, Grandispora verrucosa, Samarisporites eximius, Verrucosisporites premnus, cf. Chelinospora ligurata, Arkonites bilixus, Crucidia carminense, Maranhitesmosesii, Stellinium comptum and Polyedryxium talus are among the mostrelevant taxa of the Los Monos Formation. The Huamampampa Formation comprises Acinosporites acanthomammillatus, Acinosporites eumammillatus, Polyedryxium sp. cf. condensum and Umbellasphaeridium deflandrei. The Icla Formation contains Bimerga bensonii, Cordobesia cf. uruguayensis,Duvernaysphaera angelae, Dibolisporites echinaceus, Emphanisporites annulatus, Acinosporites sp. and Grandispora sp. Based on the miosporesand the organic-walled phytoplankton of the studied section, the Los Monos, Huamampampaand Icla formations are interpreted to be Givetian, Eifelian and Pragian? -Emsian in age, respectively. The palynofacies analysis evidences a strongpredominance of AOM (amorphous organic matter) in the majority of the samples. Considering TOC, RockEval pyrolysis, HI and Tmaxdata, all the source rocks in the Los Monos, Huamampampa and Icla formationspresent poor to regular quality with a mixed III/IV kerogen type. Although theshale intervals in the Los Monos Formation reached initial to advance oilwindow maturity; similar lithology from the Huamampampa and Icla Formationshave reached a thermal evolution from advance oil to condensate window.