Late-diagenetic clay mineral assemblages in tuffs of the Vaca Muerta Formation (Neuquén Basin, Argentina): insights into the 2 diagenetic formation of chlorite

CAPELLI, I.; SCASSO, R.A.; CRAVERO, M.F.; KIETZMANN, D.A.; VALLEJO, D.; ADATTE, T.

MARINE AND PETROLEUM GEOLOGY

ELSEVIER SCI LTD

Lugar: Amsterdam; Año: 2021

0264-8172

The mineralogical and geochemical composition of tuffs of the unconventional oil and gas play Vaca Muerta Formation (Tithonian-Berriasian) in the Neuquén Basin of Argentina, allows retracing the diagenetic history and the origin of clay minerals in order to establish the sequence of diagenetic transformations and to explain the presence of chlorite in the tuffs. Thin, calcitized tuff beds, interbedded with the background organic-rich marls and mudstones, were studied on three sections located in the north of the Neuquén Province: El Trapial (ET), Puerta Curaco (PC) and Chacay Melehue (CM). Rock-Eval analyses performed on the background marls revealed that ET section is located in the oil window, whereas PC and CM in the dry gas window. The clays of ET section are mostly constituted by high-ordered mixed-layer illite/smectite (I/S), whereas in PC and CM sections, a highly-crystalline, aluminum-, magnesium-rich chlorite (sudoite) and high-ordered I/S are dominant. The high-ordered I/S (80?90 % illite layers) in the oil window stage indicates that the Neuquén Basin was characterized by a higher geothermal gradient than previously reported, with an estimated value of 57 °C/km. Zr/TiO2 and Nb/Y ratios indicated that the magmatic source of the tuffs was the same for the three sections. The absence of any chlorite precursors in ET section, the lack of correlation between illite and chlorite crystallinities and the pervasive distribution of chlorite´s aggregates in the matrix, suggests that sudoite have precipitated in situ, at a temperature between 180?200 °C. The aluminum, magnesium and silicon, essential to chlorite precipitation were available in the pore waters after the dissolution/recrystallization of minerals from the tuffs. This process was related to the development of the Chos Malal Fold and Thrust Belt, which generated overpressure, deformation and fracturing of the rocks, thus facilitating the circulation of hot waters.