Ciclos mixtos carbonáticos/silicoclásticos en el Miembro Superior de la Formación Mulichinco (yacimiento Cañadón Amarillo, Cuenca Neuquina central): Implicancias secuenciales y para caracterización de reservorios

Abstract

Con el objetivo de dilucidar relaciones temporo-espaciales y controles entre intervalos carbonáticos y silicoclásticos dentro de sucesiones cíclicas marinas, en este trabajo se presenta un estudio facial, paleoambiental y secuencial de alta resolución del Miembro Superior de la Formación Mulichinco en el subsuelo del extremo austral de la Provincia de Mendoza (yacimiento Cañadón Amarillo). La unidad posee entre 65 y 80 m de potencia en la región y se caracteriza por una recurrente alternancia de escala métrica de intervalos silicoclásticos y otros con dominancia de componentes carbonáticos. El estudio se basa en el análisis sedimentológico de detalle de cuatro testigos de corona que abarcan la totalidad del intervalo estudiado y en total representan 240 m de espesor. En el Miembro Superior de la Formación Mulichinco se identificaron e interpretaron cinco asociaciones de facies, tres dominadas por depósitos silicoclásticos y dos con dominio de componentes carbonáticos. Las asociaciones silicoclásticas se depositaron en un sistema marino dominado por procesos de olas de buen tiempo y tormenta y representan un espectro continuo de condiciones de sedimentación desde ambientes someros de shoreface inferior, pasando por un ambiente intermedio o zona de transición, hasta regiones ubicadas por debajo de la base de olas de tormenta u offshore. Por su parte las asociaciones de facies mixtas y calizas puras representan el desarrollo de un sistema de rampa con alta producción de materiales carbonáticos, en donde se reconocen condiciones de alta energía o rampa somera (grainstones y packstones oolíticoesqueletales), y condiciones más distales de rampa media, con generación de depósitos con alta participación de fango micrítico (wackestones, packstones y floatstones esqueletales). El análisis de diseños estratales, superficies clave y distribución espacial de las asociaciones de facies indica que estos dos sistemas de acumulación no co-existieron, sino que se reemplazaron cíclicamente debido a variaciones de alta frecuencia del nivel relativo del mar. De esta manera los ciclos de pequeña escala (3 a 18 m de espesor) reconocidos en el Miembro Superior de la Formación Mulichinco se interpretan como secuencias de alta frecuencia limitadas por superficies transgresivas de erosión y están típicamente compuestas por hemiciclos transgresivos relativamente delgados y dominados por componentes carbonáticos, seguidos por hemiciclos silicoclásticos más potentes, los cuales representan fenómenos de progradación de una línea de costa. La superficie no erosiva que los limita representa una superficie de máxima inundación. Los resultados de este trabajo contribuyen a avanzar en la caracterización de los niveles productivos alojados en esta sucesión cíclica. Se definieron tipos de facies con buenas perspectivas de constituir reservorio y sus características principales (geometría, espesor, conectividad, heterogeneidades internas), y fue posible elaborar un modelo con carácter predictivo dentro del yacimiento, que explica desde un punto de vista genético, cuál sería la posición más probable de dichas facies dentro de una secuencia de alta frecuencia. La incorporación de estos elementos a un modelado geológico estático puede finalmente contribuir a un mejor y más eficiente desarrollo del campo y sus niveles reservorio.

The presence of carbonate strata within siliciclastic-dominated marine successions (i.e. mixed carbonate-siliciclastic successions) poses serious challenges for palaeoenvironmental reconstructions and reservoir characterization, and unambiguous interpretations can emerge only when the spatial vs. temporal relationship between carbonate production and siliciclastic input is well understood. This study integrates sedimentology and high-resolution sequence stratigraphy in order to better understand the temporal/spatial relationships and controls in the origin of a Lower Cretaceous, mixed carbonatesiliciclastic succession in the subsurface of the Neuquén Basin, west-central Argentina (Figs. 1, 2). This mixed succession, so-called Upper Member of the Mulichinco Formation, accumulated in a shallow, epeiric sea during third-order highstand conditions (Fig. 3), and is presently widely distributed in the central part of the basin, both in outcrops and subsurface (Schwarz et al., 2009; Schwarz et al., 2011). The study area, covering an area of about 80 km2, is located in the Cañadón Amarillo hydrocarbon field (Fig. 4), where this unit has been in production since the 1970’s. This study is based on a detailed description and interpretation of four cored wells (three of them covering the entire unit), complemented with calibrated well-log suites from additional 22 wells (Fig. 5). The Upper Member of the Mulichinco Formation (65-80 m thick) has a cyclic alternation of relatively thick (up to 16 m) siliciclastic packages and thinner carbonate-dominated intervals (Fig. 6). Three facies associations were identified within the siliciclastic packages, and they are inferred to represent shoreface, offshore-transition, and offshore settings in an open marine system influenced by storm- and fair-weather waves. The lower-shoreface facies association (Fig. 7) is composed of amalgamated, siliciclastic sandstone beds (with minor contribution of carbonate particles) mostly having hummocky cross-stratification, horizontal lamination or ripple cross-lamination. Bioturbation varies from low to high and the trace fossil suite (Arenicolites, Gyrochortes, Palaeophycus, Ophiomorpha, Skolithos y ?Macaronichnus) is interpreted to represent an Skolithos ichnofacies (MacEachern et al., 2007). The offshore-transition facies association groups intensely bioturbated muddy sandstones and less bioturbated (i.e. better preserved) sandstone-rich heterolithics (Fig. 8), with occasional medium-bedded sandstone beds (< 15 cm thick) having HCS and rippled tops. Siliciclastics largely dominate within these sediments, but bioclasts (mostly from oysters) can be common locally. The assemblage of trace fossils (Planolites, Palaeophycus, Thalassinoides, Teichichnus, Phycosiphon, Ophiomorpha, Schaubcylindrichnus y Asterosoma) is attributed to represent a Proximal Cruziana ichnofacies (MacEachern and Bann, 2008), and, together with the sediments, reflect a stormdominated offshore-shoreface transition, between the storm and fair-weather wave bases (Reading & Collinson, 1996). In turn, the offshore facies association comprises mostly massive or laminated mudstones, as well as mudstone-dominated heterolithics. In the latter, very thin-bedded siltstone beds with wavy tops are abundant, likely reflecting the distal ends of storm-related flows. Carbonate-dominated intervals are composed of two facies associations that collectively are inferred to represent subenvironments within a carbonate ramp, namely shallow (inner) and middle sectors of it. The shallow-ramp facies association is characterized by cross-bedded ooid-skeletal grainstones/packstones, with subordinated skeletal rudstones and packstones (Fig. 10). Bioclasts derived mostly from mollusks and echinoids, whereas terrigenous material is less than 25%. These sediments deposited in open-marine high-energy settings, likely shoals and intershoals areas (Rankey y Reeder, 2011; Christ et al., 2012). In contrast, the middle-ramp facies association is composed of muddominated textures, mostly skeletal wackestones and floatstones (Fig. 11). They are massive, but argillaceous seams might create a nodular aspect. Bioclasts from epibenthic (oysters) and endobenthic bivalves, as well as from serpulids and echinoids are dominant, but glauconite and intraclasts are locally abundant. Compared to the previous association, these sediments were deposited in lower energy and deeper parts of the ramp. The nature of key stratigraphic surfaces, facies associations distribution and analysis of stacking patterns within this cyclic carbonate/siliciclastic succession suggest that the siliciclastic- and carbonate-dominated depositional systems were not coeval, but replaced over time. Correlation panels show that individual carbonate and siliciclastic hemicycles extend across the entire area and lateral transition between them were not recorded (Figs. 12, 13). Carbonate packages are invariably bounded by sharp, erosive surfaces (Fig. 14a,b), which are interpreted to represent transgressive ravinement surfaces (Swift, 1968; Nummedal and Swift, 1987). Facies associations in these hemicycles suggest a deepening-upward trend, whereas in the overlying siliciclastic packages the stratal patterns indicate normal (i.e. not forced) regressive conditions (Fig. 15). Therefore, the seven small-scale cycles (plus two incomplete cycles) recorded in the Upper Member of the Mulichinco Formation (3-18 m thick) are interpreted to represent high-frequency sequences (nomenclature following Zecchin and Catuneanu, 2013), comprising relatively thin, transgressive, carbonate-rich hemicycles and thicker, siliciclastic regressive hemicycles (Fig 16). The non-erosive boundary between both hemicycles (Fig. 14c) could correlate with the maximum flooding surface (Van Wagoner et al., 1990), or being slightly younger, as it reflects enough terrigenous supply to dilute carbonate productivity (Abbot, 1997). In this context, it is considered equivalent of a downlap surface (Fig 16). These high-frequency sequences were most likely controlled by short-term, low-amplitude, relative sea-level changes, and the thickness of transgressive hemicycles could have been influenced by carbonate productivity and/or rate of transgression. The results of this study provide with a more accurate reservoir characterization of this mixed (and complex) succession. Two reservoir-type facies associations were identified, namely the lowershoreface and shallow-ramp facies associations. The understanding of key reservoir attributes, such as geometry, thickness, connectivity and internal heterogeneity were improved with this study. Additionally, the findings of this work provided with a high-resolution sequence-stratigraphic model that help predicting the occurrence of the reservoirtype facies within a high-frequency sequence. The integration of all these elements within a geological model could contribute to define for example the most efficient development strategy for the reservoir horizons (e.g. vertical versus horizontal wells) which would, eventually, impact in the recovery factor of the field.