Evaluation of the black shales of the Los Monos Formation (Devonian) as an unconventional shale oil and shales gas reservoir, Balapuca, southern Bolivia

Abstract

The object of this study is the evaluation of the black shales of the Los Monos Formation as an unconventional shale oil and shale gas reservoir. To this end, the sedimentology and mineralogy of Los Monos Formation at Balapuca (22°31?00"S, 64°26?00"W), in southern Bolivia immediately north of the limit with Argentina is studied. A sedimentological profile was taken from this unit (140 m) and a total of 48 samples were analyzed by X-ray diffraction on total rock and clays fraction. Los Monos Formation (late Eifellian-early Frasnian) is the typical mother rock of the north Argentine and south Bolivian Andean environment. Disalvo and Villar (1998) postulated the division of this unit according to its geochemical characteristics. They determined a lower section with a greater content of badly preserved amorphous kerogene (type III/IV), a probable gas generator, and an upper section with predominance of type II/III kerogenes of mixed marine-continental origin generating gas and oil. However, other studies (Cerqueira and Schulz, 1998; di Pascuo, 2007) indicate different distributions. Furthermore, the kerogene dilution in the rock (1% total organic carbon TOC, Disalvo and Villar, 1998) and its fair quality (average original S2 pyrolysis peaks estimated at no greater than 400mg HC/g TOC) would indicate that expulsion and migration have not been efficient. Therefore, there are still important volumes of generated hydrocarbons dispersed within the shales, a phenomenon also evinced by super pressure and manifestations during perforation. The poor quality of mother rock generation is compensated by the potent subsoil (700-1000 m) and broad area distribution (Disalvo and Villar, 1998). In the area of interest, Los Monos Formation is characterized by decimetric to centimetric intercalations of laminated, micaceous black shales and fine to medium grained grayish psammites in small waves and HCS (hummocky cross stratification), in tabular and lenticular banks. There are also thin layers of brown limolite intercalation. Abundant vegetable fragments, bioturbations and badly preserved plants are visible in both shales and psammites. On the basis of sedimentological characteristics, these would correspond to distal platform facies (Disalvo and Villar 1998).