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High-resolution chemostratigraphy, sequence stratigraphic correlation, porosity and fracture characterization of the Vaca Muerta Formation, Neuquén Basin, Argentina
Hernández Bilbao, Eider
Hernández Bilbao, Eider
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2016
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2016-11-19
Abstract
The unconventional resource play potential of the Vaca Muerta Fm. in the Neuquén basin has renewed significant interest from the hydrocarbon industry. A combination of methodologies applied for conventional reservoir characterization, along with recently developed techniques, enables the evaluation of these fine-grained mudrocks. A multi-disciplinary study based on elemental and isotopic chemostratigraphy in a sequence stratigraphic framework allows for a better characterization of the Vaca Muerta reservoir system. The purpose of the first part of this study is the characterization of nano- to microscale pores in the lower Vaca Muerta Fm. and their contribution to the reservoir. The combination of FE-SEM observations of argon ion-milled surfaces with QEMSCAN analysis allows a complete pore characterization in terms of pore distribution, type, shape, and size. The lower Vaca Muerta interval is dominated by interparticle pores, followed by intraparticle pores. Organic matter-hosted pores are the least common at this location due to insufficient thermal maturation. The second part of this study focused on fractures in the Vaca Muerta, and included characterization of bedding-parallel and sub-vertical calcite-filled microfractures and the relative timing of the cement infilling and hydrocarbon generation. CL and carbon and oxygen data support that calcite cementation occurred from a single fluid phase for both types of fractures during a later stage of burial and from fluids derived at greater depths and temperatures than present day burial. Data herein support two fracturing stages: (1) one related to the main stress field during the latter part of the Cretaceous and (2) a second bedding-plane fracturing stage related to the onset of hydrocarbon generation. Open fractures that initially provided flow paths for undersaturated fluids were cemented through calcite precipitation. Lastly, hydrocarbons filled the remaining pore space within the fractures. The third part of the study comprises the elemental and isotopic characterization of the Vaca Muerta Fm. Advanced technologies, such as the handheld energy dispersive x-ray fluorescence spectrometer (ED-XRF), allow for rapid and non-destructive collection of elemental values. The purpose of this part of the study is to combine elemental, and carbon and oxygen isotopic chemostratigraphy with facies interpretation to construct an integrated sequence stratigraphic framework for the Vaca Muerta depositional system. Most suitable elemental proxies for the Formation are detrital proxies (Al, Ti, Si, Ga, Nb, Th, Zr, K, Cr and Rb), carbonate proxies (Ca and Sr), and redox and/or organic matter proxies (Mo, Zn, Ni, V, Cu, As). Stable carbon and oxygen chemostratigraphy can be a powerful technique for mudrock correlation. The interpreted chemostratigraphic units provide chronostratigraphic markers coincident with sequence stratigraphic boundaries. The sequence stratigraphic model developed here shows three sequences that are consistent with 3rd order sequences previously interpreted in the basin. Highstand systems tracts are defined by high Ca peaks in the upper part of the Vaca Muerta section, representing an increase in periplatform sediments. These are composed of bioclastic lime wackestone and the lime mudstone facies. Clay rich transgressive systems tracts are characterized by high values of Mo and Ni, moderate values in detrital proxies, and low Ca values. The Vaca Muerta Fm. in the study area has been previously correlated to the early to late Tithonian time interval. Isotopic values from this study support that the upper Vaca Muerta Fm. was deposited between the mid to late Tithonian. The lower Vaca Muerta however, has depleted δ13C. Isotopic measurements were conducted on whole rock samples from cuttings in organic-rich rocks, rather than from selected carbonate grains or individual bioclastic fragments resulted in, light carbon readings (-8.77‰ and + 0.95‰). The lower Vaca Muerta Fm. thus probably correlates to δ13C values between +2 and +3‰ of the early Tithonian of the global Tethyan curve.
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