Structural and magmatic controls in reservoirs of the Mulichinco Formation in the Auca Mahuida Volcano field, Neuquén Province, Argentina

CHACCHIERA, S; RIBAS, S.; ORTS, D.L.

Malargüe

Workshop; LASI VI workshop on The physical geology of subvolcanic systems: laccoliths, sills and dykes.; 2019

AGA entre otras

The Auca Mahuida volcano, developed northeast of the Neuquén basin (37º45´S and 68º56´W), is a product of the volcanism that constitutes the volcanic province of Payenia. It was formed between 1.87 y 0.88 My (Rossello et al. 2002) and it is isolated from the Andean zone, about 100 km east of the current deformation front. Also, the volcano is located on the edge of a NNW-SSE Precuyan depocenter (Upper Triassic-Lower Jurasic). The development of the Agrio fold and thrust belt and the eastward progression of the deformation produced subsidence by tectonic load and the inversion of the depocenters in this area. Both, the load related subsidence and the tectonic inversion structures worked as a control for the volcanism of Auca Mahuida (Cristallini et al. 2014). The rise of magma occurred through sedimentary rocks that form a complex petroleum system in the Neuquén basin.This work analyzes a 9 km2 area located northeast to the main crater (Fig. 1) within the oilfield called Volcán Auca Mahuida, concessioned to YPF S.A. The study area has nine wells that have been directionally drilled from existing locations due to topographic, weather conditions and the presence of a natural reserve. The sedimentary column crossed by the study wells is composed of rocks corresponding to Neuquén Group and Rayoso, Centenario Inferior, Centenario Superior Mulichinco and Quintuco formations. Although is known from other neighboring wells that is also composed of the Vaca Muerta, Tordillo and Auquilco formations underneath (Rossello et al. 2002). In the petroleum system, Vaca Muerta formation is the source rock while reservoirs are founded in sandstones of Mulichinco, Centenary Inferior, Centenario Superior and Rayoso formations.The analysis of well data, vertical seismic profiles and geological background information has allowed us the interpretation of structures and the control they exert together with the magmatism in the reservoir conditions of the Mulichinco Formation, main local oil system reservoir.Dykes, sills of different sizes and a 110 m thick laccolith hosted in the Mulichinco Formation are the main intrusive bodies founded. The existence of deeper and bigger sills is known by other authors as Longo (2017).This work shows that borehole imaging and vertical seismic profiles constitutes an essential tool in areas where seismic acquisition is not applicable due to the topography and the high acoustic impedance of the upper basaltic cover.Based on that borehole imaging (Figure 2), it can be inferred that dykes have the same strike and dip that the previous structures proposed by Cristallini et al (2014) for the structural evolution of the region and the same geometry that a vertical seismic profiling carried out in one of the wells in the work area (Figure 3). This is in accordance with Magee et al (2013) which proposes that several fault zones are used for the propagation of the magma. This compartmentalization of the reservoir rocks is called Box-Work type (Holford et al. 2013). In addition, through the analysis of the fluids behavior hydraulic disconnections were observed between them, which show the presence of faults or dikes that act as barriers. As a result of the structural study focused on the Mulichinco Formation, four blocks that disconnect the fluid in the reservoirs were defined (Figure 1).The petrophysical conditions of the reservoirs rocks were not altered by metamorphism, but the presence of hydrothermal activity produced a hydrocarbon remigration. Large amounts of gas indicate that the increased heat generated by the intrusion, increased the maturation in source rocks, going to from the oil window to the gas window. The study of the fluids and their interaction with the subsurface structure allows expanding the knowledge of the structural model and to give a greater precision in the development of the oilfield.