Organic petrology of the Potrerillos-Cachueta source rocks in the Triassic Cuyo Basin, west-central Argentina

PINEDA JUAN; ERRA GEORGINA; SALDUONDO, J.; COMERIO M.; OTTONE, G.

Congreso; 37 TSOP ANNUAL MEETING; 2021

The Triassic Cuyo Basin is a prolific petroleum basin in Argentina with high rates of oil production. It corresponds to a passive continental rift (Barredo, 2012), and extends over more than 60,000 km2, including the provinces of San Juan, Mendoza, and San Luis (30º - 34º LS). It is characterized by several depocenters that follow a NW-SE trend departing from the N-S trend of the present-day Andean chain. The Triassic infilling of Tupungato-Cacheuta depocenter is composed of, from base to top, by the Rio Mendoza, Cerro de Las Cabras, Potrerillos, Cacheuta and Río Blanco (Fig. 1). Most of the organic petrology studies of the basin are unpublished (e.g. Ottone, 2015) or are based on transmitted light microscope (e.g. Rojo & Zavattieri, 2006; Zavattieri & Rojo, 2006). The present contribution provides an integrated petrology organic study, including paleoenvironmental and thermal maturity evaluation of the Potrerillos-Cachueta sedimentary succession. The study area is located in the Cerro Bayo (32º 57´8,1´´S 69º 12´48,4´´W), Potrerillos locality, west of Mendoza city (Fig. 1). A detailed sedimentological section of 1130 m thick was logged in the studied section, from which seven outcrop samples were petrographycally analyzed (Fig. 2), five samples from the Cacheuta Fm (P26, P15, P17, P18, P19) and two samples from the upper Potrerillos Fm (P4A and P7B). Lacustrine deposits are recognized in the upper interval of Potrerillos Formation (Kokogián & Mansilla, 1989; Uliana et al., 1999) which exhibit similar characteristic as the overlaying Cacheuta Formation. The latter is composed of black laminated shales interbedded with tuffs, fine-grained sandstones and grey claystones (Spalletti et al., 2005; Salduondo et al., 2020).The palynofacies analyses were carried out by quantitative and qualitative methods (Tyson, 1995) using microscope Zeiss M2m for the transmitted white light (TL) and fluorescence light (FL). For the determination of vitrinite reflectance (%Ro), whole rock samples were prepared following the standard methods for preparation (ASTM, 2015). The vitrinite identification and reflectance (Ro%) (ASTM, 2014) was made under reflected light microscope (RL), using CRAIC PV 508 microspectrophotometer and CoalPro program. Two organic assemblages, CCB (P26, P15, P17, P18, P19) and PCB (P4A and P7B), are recognized based on the palynofacies and maceral analysis. The CCB palynofacies is dominated by amorphous organic matter (AOM) (ca. 66%, Fig. 2) with the occurrence of subordinate phytoclast (ca. 22%) and palynomorphs (ca. 12%). The PCB palynofacies is characterized by the predominance of terrigenous elements, phytoclast group (ca. 41 %, Fig. 2). The AOM represent the ca. 32%, followed by the ca. 27% palynomorphs. The palynological content of terrestrial origin is poorly preserved and includes mainly haploxylononoid disaccate pollen (Alisporites), followed by diploxylononoid disaccates and monosulcates pollen, and trilete spores. The freshwater algae colonies (Botryococcus) are well-formed in PCB, but poorly preserved and smaller in size at CCB. The coloration of these algae varies from light to dark-brown under LT with yellow coloration under FL (Fig. 2). Organic petrographic results indicate a clear predominance of AOM (Bituminite) at CBA that occurs as a slightly fluorescent groundmass (Fig. 2), while the terrestrial macerals (inertinite, vitrinite, sporinite) are predominant at PCB. The telalginite (botryococcus) is present in both associations, and its fluorescence is low yellow while lamalginite fluorescence was orange. Vitrinite reflectance histograms reveal three vitrinite populations: suppressed, primary/suppressed?, and recycled vitrinite. Measured Ro% values between 0.52 and 0.60 are from CCB, while in PCB varies from 0.69-0.73. These values indicate a low to middle degree of thermal maturity; however, these values may be affected by the suppression effect as it was reported in the subsurface of Cacheuta Fm. in the Cuyo Basin (Laffite, 1987). The low maturity stage also was confirmed by fluorescence of Botryococcus and TAI values (2.0 to 2.5). The high abundance of AOM at CCB could indicates a low-energy environment with anoxic bottom water. Some intensively fluorescing AOM could be derived by bacterial activity, which agrees with the poor preservation state of the palynomorphs. The increase of phytoclasts at PCB would be associated with high fluvial discharge, however the reduction condition may have persisted due to the presence of AOM and well-preserved botryocococus alga. The kerogen is type I/II (CCB) and III (PCB) according to its composition (mostly AOM with terrigenous contribution), and as for its maturity, it is immature and early window. Cautions should be taken with Ro% thermal estimation, and calibration with other optical and geochemical parameters are necessary.