Palaeoclimatic and diagenetic controls based on clay mineralogy and organic matter distribution: The continental rift Cuyo Basin (Triassic), west‐central Argentina

Abstract

The Triassic Cuyo rift Basin is one of the renowned petroleum systems in South America that records distinctive alluvial, lacustrine and fluvial settings with volcanic input. The present study focuses on the clay mineral assemblages that represent the syn‐rift to post‐rift fills. Four clay mineral assemblages are defined based on X‐ray diffraction, optical and electron microscopy analyses. The illite/mica‐dominated assemblage is documented in the syn‐rift I (Cerro de las Cabras Formation). Such mineral phases are inherited and derived from the erosion of pre‐rift units, suggesting that physical weathering associated with a warm and semi‐arid climate played a leading role in their occurrence. Authigenic smectite clays are diagnostic in organic‐lean mudstones of the syn‐rift II (lower Potrerillos Formation) which point to alteration of pyroclastic material under a warm and semi‐arid to humid climate. A warm and humid climate prevailed during the deposition of lacustrine hydrocarbon source rocks (upper Potrerillos and Cacheuta formations). Organic‐rich mudstones show the illite/mica–kaolinite–smectite assemblage in conjunction with amorphous organic matter and restricted terrigenous elements. Diagenetic kaolinite booklets and bipyramidal quartz are attributed to the acidification of pore‐water related to kerogen transformation during burial diagenesis. Vitrinite reflectance values (ca 0.63%) confirm that organic‐rich sediments reached the earliest oil window; however, smectite is devoid of illite layers throughout the Triassic succession, probably related to the absence of potassium in the system and the high crystallinity of the smectite, which delayed the transformation of smectite into mixed layers illite–smectite. Red mudstones of the post‐rift II (the fluvial‐lacustrine Rio Blanco Formation) show the kaolinite‐dominated assemblage where the authigenic kaolinite is interpreted to be derived from the alteration of volcaniclastic detritus and indicative of warm climate with markedly wet periods.

Palynofacies and Organic Geochemistry of Lacustrine Source Rocks: The Potrerillos – Cacheuta Source Rock System in the Triassic Cuyo Basin, West‐central Argentina

“…The organic matter in the studied interval is thermally immature to early oil window mature according to average values of vitrinite reflectance (~ 0.63% R o ), T max (~433 °C) and TAI (2 to 2 + ). Other evidence supporting the relatively low maturity of the analyzed succession is the fact that smectites are pure and illite-smectite (I/S) mixed layers are lacking, suggesting relatively low burial temperatures (Salduondo et al, 2022). The production index (PI) of <0.01 in the studied interval would imply that petroleum generation has not occurred (Peters and Cassa 1994;Peters et al, 2005), but the S1 (~ 0.14 mgHC/g rock) may be affected by weathering of the outcrop samples (Peters et al, 2005, Dembicki, 2016.…”

“…In the Cachetua palaeolake analyzed in this study, an episodic input of pyroclastic material occurred as indicated by the various tuff levels in the studied section (Salduondo et al, 2020(Salduondo et al, , 2022. The input of this material influenced diagenetic processes (Salduondo et al, 2020(Salduondo et al, , 2022 as well as primary productivity in the water column (Pedernera et al, 2021a) and thus the variability of the palynofacies composition. Therefore, the documented vertical and lateral palynofacies changes not only reflect the local depositional setting but also the influence of volcanic activity on the nutrient supply to the water column.…”

“…1). The sucession in this area has previously been investgated using detailed sedimentological (Spalletti et al, 2005) and mineralogical analyses (Salduondo et al, 2020(Salduondo et al, , 2022.…”

“…Sedimentary material derived from an evergreen subtropical seasonal forest, with herbaceous shrub-like palaeocommunities dominated by Umkomasiales, characterizes the uppermost part of the Potrerillos Formation and the Cacheuta Formation, suggesting a warm and humid climate (e.g. Zavattieri and Rojo, 2005;Zavattieri and Prámparo, 2006;Mancuso et al, 2021) consistent with a clay mineralogical assemblage composed of kaolinite, smectite and illite (Salduondo et al, 2022).…”

Analysing of palaeoenvironment and organic matter enrichment: A case study from the Triassic Yanchang Formation in the southern Ordos Basin, China

“…The changes in palaeoenvironment not only regulate the sedimentation process by influencing rainfall, palaeo-water depth, sediment transport, and other factors, but also determine the types and quantities of biological communities, ultimately influencing the accumulation and distribution of organic matter (Burdige, 2007;Salduondo et al, 2022;Spychala et al, 2020; L. L. Xu et al, 2023). Therefore, reconstructing palaeoenvironments is crucial for a better understanding of the formation and evolution of sediments, and can provide an essential scientific basis for oil and gas resource exploration.…”