Análisis mineralógico integrado (QEMSCAN y DRX) de lutitas negras: los depósitos tithonianos basales de la Formación Vaca Muerta (Cuenca Neuquina, Argentina)

Abstract

Se estudia la composición por difracción de rayos X y QEMSCAN (19 muestras)de fangolitas y margas de la Formación Vaca Muerta, acumuladas durante latransgresión tithoniana de la Cuenca Neuquina. Se definen importantes variaciones composicionales entre tipos litológicos y facies sedimentarias. Las rocas del sector marginal de la cuenca muestran fuerte influencia de los aportes terrígenos (cuarzo, feldespatos, illita e illita/esmectita). En las sedimentitas del sector depocentral facies de pelitas grises oscuras) son importantes los indicadores de productividad orgánica (carbonatos y cuarzo biogénico), de condiciones anóxicas (pirita, siderita) y de un lento ritmo de acumulación sedimentaria. Los depósitos depocentrales de la transgresión tithoniana corresponden a la cocina de hidrocarburos de la Formación Vaca Muerta. Sus amplias variaciones mineralógicas ejercen fuerte impacto en la conversión de materia orgánica y en la liberación de gas y petróleo, así como en las propiedades petrofísicas y la fragilidad de rocas que constituyen la fuente principal para la explotación no convencional de hidrocarburos en la Cuenca Neuquina.

Recent studies have demonstrated that the mineralogical composition of shales plays an important role in unconventional hydrocarbon production (Chen et al., 2014). Mineralogy may influence the nature of the pore structure, the fracktability of these fine-grained deposits and pyrolysis reactions, all of them essential in the stimulation and extraction processes of low-permeability reservoirs (Karabakan and Yürüm, 2000; Jarvie et al., 2007; Ross and Bustin, 2009). This contribution describes and analyzes the mineralogical composition of the Tithonian basal deposits of the Vaca Muerta Formation, which resulted from two independent methodologies, QEMSCAN and DRX. The datasets comprises 19 samples distributed from the austral to the central sectors of the Neuquén Basin (Fig. 1a). The sampled sediments were deposited during the marine transgression of the early Tithonian (Fig. 1b) and accumulated under bottom conditions that favored the preservation of organic matter. The studied interval is the most important source rock of the basin. Previous geochemical studies (Spalletti et al., 2014) showed that the basal interval comprises fine-grained sediments with a very variable composition (Fig. 3), ranging from pure siliciclastic to mixed (carbonate/ siliciclastic) mudstones. Despite this compositional variability, macroscopically in the field only three main facies were recognized: greenish mudstones, yellowish mudstones and dark grey mudstones. The first two facies are commonly distributed in the marginal areas of the basin, whereas the latter is more characteristic of basinal regions (Fig. 2, Table 1). The samples were analyzed with conventional optical methods and by X-ray diffractometry (whole rock and <2 μm fraction), as well as by a combination of SEM (Scanning Electron Microscopy) and EDS (Energy Dispersive Spectrometry). This technique is known as QEMSCAN, which stands for Quantitative Evaluation of Minerals by SCANning electron microscopy. The integration of analytical methods revealed significant compositional variations between different lithologic types and lithofacies (Figs. 4-6, Table 2). Hybrid mudstones (especially marls and calcareous marls) show high calcite contents, whereas more siliciclastic deposits are dominated by quartz and feldspar, with clay minerals as illite and interstratified I/S dominant in the mudstones, together with minor contents of kaolinite and analcime. The mineralogical composition of identified lithofacies also shows changes, even among samples of the same lithofacies (Fig. 6). Greenish mudstones are characterized by illite, smectite and quartz, with subordinated contribution of kaolinite and interstratified I/S. For yellowish mudstones there are no clear trends, with a wide spectrum of quartz/calcite relationships (Fig. 6) and variable content of Illite, interstratified I/S and analcime. In turn, dark grey mudstones, which are typical of the depocentral sectors, have minerals which are indicative of low oxygenation (pyrite, siderite), but a broad compositional range in terms of calcite, quartz and clay minerals. This study has allowed establishing a significant equivalence between the information presented here (QEMSCAN and DRX), and the one gathered by means of inorganic geochemical analysis (Spalletti et al., 2014). The mineralogical composition of the sediments located toward marginal settings during the Tithonian transgression reflects a strong influence of terrigenous supply from hinterland (Fig. 7). In contrast, the sediments that accumulated in more basinal locations of the marine setting were heavily influenced by biogenic productivity (intrabasinal concentration of carbonate- and silicarich biota), anoxic conditions, and more likely, lower sedimentation rates (Fig. 7). These basinal, basal deposits (dark grey facies) of the Vaca Muerta Formation correspond to the highest total organic concentration across the basin (kitchen) and this interval was responsible for the expulsion of large quantities of hydrocarbons during different geological times (Villar et al., 1993, 2006). The mudstone mineralogy exerts a strong control in several processes such as organic matter conversion (pyrolysis-related reactions), expulsion of hydrocarbons, petrophysical properties and geomechanical attributes, that in turn influences the reservoir properties and extraction processes (cf. Patterson and Henstridge, 1990; Patterson et al., 1990). Therefore, the wide range of compositional variability that is inherent in the dark grey mudstones of the Vaca Muerta Formation (so-called "black shales") is key in order to maximize its exploration and exploitation as an unconventional resource.