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CIG 05423

CENTRO DE INVESTIGACIONES GEOLOGICAS

Unidad Ejecutora - UE

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Título:

Paleoambientes y estratigrafía de la Formación Mulichinco en el Yacimiento Volcán Auca Mahuida (Cuenca Neuquina, Argentina). Implicancias para la caracterización de sellos locales.

Autor/es:

SCHWARZ, E., VEIGA, G.D., VELA, R. Y CANALIS, R.

Lugar:

Mar del Plata

Reunión:

Congreso; VII Congreso de Exploración y Desarrollo de Hidrocarburos; 2008

Institución organizadora:

Instituto Argentino del Petroleo y Gas

Resumen:

ABSTRACT This study presents a detailed facies and stratigraphic analysis of the Mulichinco Formation in the Volcán Auca Mahuida field, located in north-central Neuquén Province. In this region, the unit is about 150 m thick and is encased in black shales and marls. For this study, 160 m of cored intervals, which stratigraphically cover almost the entire unit, were described. Six facies associations, representing continental to marginal-marine depositional environments, were identified: 1) Fluvial channels, 2) Sheet floods, 3) Eolian sandsheets, 4) Playa-Lake, 5) Salt flats, and 6) Coastal sabkha. The Mulichinco Formation sediments were essentially deposited in a continental environment. Sandstone-dominated deposits resulting from unconfined ephemeral floods dominated throughout the unit, but pebbly sandstones and coarse-grained sandstones, interpreted as moderately confined channel-fills, also occur in the lower part of the unit. Additionally, interbedded cross-laminated sandstones and mudstones are interpreted to be the product, respectively, of distal floods and suspension fall-out in the low-relief, downdip region of the fluvial system (playa-lake environment). The episodic changes in fluvial discharge observed through the fluvial section were probably associated with a seasonal semi-arid climate. On the other hand, relatively dry conditions favored aeolian reworking of previously deposited fluvial sands, which were deposited as sandsheets and minor dunes. Besides, silty sandstones having irregular, wavy sedimentary fabric are considered to reflect salt-crust growth and dissolution processes in low-relief areas, probably related to dry conditions and a water table being near or at the surface. These deposits are interpreted to represent deposition within inland salt flats (i.e. evaporite environments where groundwater is dominated by meteoric, continental chemistry). Discrete thin intervals (<3 m) within the Mulichinco Formation are characterized by rhythmic sedimentation together with a trace fossil suite of possible marine affinity. These deposits, which are areally extensive, are interpreted to be related to marine incursions in the area. Sediments and ichnofauna suggest that a marginal-marine setting, most probably comprising upper intertidal and supratidal flats in a coastal sabkha, was developed after marine flooding events. The Mulichinco reservoir sandstones within the Volcán Auca Mahuida field are areally extensive and vertically separated by fine-grained deposits. The detailed facies analysis presented here allowed the discrimination of two types of barrier-prone lithofacies. Playa-lake and inland salt-flat fine-grained deposits could represent important vertical barriers to fluid flow, but given their limited extension and relative low preservation potential, they might not act as effective local seals within the reservoir. On the contrary, mudstone-rich, laminated deposits of coastal sabkha are typically dolomitized and are regionally extensive (even traceable to adjacent fields). This type of barrier deposits acts as local seals, which effectively compartmentalize different flow units within the reservoir sandstones. Based on the vertical distribution of sandstone- and mudstone-dominated associations and on their relative quality as reservoir- or seal-prone facies, an informal stratigraphic subdivision of the Mulichinco Formation is finally presented.