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Argentina is ranked second globally in terms of technically recoverable shale gas, and fourth in shale oil (EIA 2015). The most prolific shale is the Vaca Muerta formation. The objective of this paper is to present geological and reservoir characterization, drilling and production strategies, as well as historical performance and economics of Vaca Muerta. The word petroleum as used in this paper includes oil, natural gas, and natural gas liquids. This paper describes natural fractures and their impact on hydrocarbon productivity. The successful commercial production from this unconventional resource has been driven by many factors, including regional geology, availability of advanced technology such as horizontal drilling and multi-stage hydraulic fracturing, as well as domestic and regional hydrocarbon demand (Sierra 2016). Vaca Muerta itself is very unique with multiple hydrocarbon windows from east to west, ranging from dry gas to wet gas, to light oil and black oil. The productivity of Vaca Muerta is benchmarked to some of the best US shale plays such as the Eagle Ford and the Marcellus. Vaca Muerta contains 1202 Tcf of risked gas in-place and 270 billion barrels of risked oil in-place. It is estimated that 308 Tcf and 16 billion barrels of these resources are technically recoverable (EIA 2015). To date, the total number of horizontal wells exceeds 600, mostly drilled in the black oil window (Secretaria de Energia de Argentina 2020; Wood Mackenzie 2020b). Dubbed the ‘golden goose’ of Argentina, the last decade has seen rapid exploration and development activities. The Argentina state oil company (YPF) leads the development in this region together with its partners. In 2019, production from Vaca Muerta reached 90,000 bbl/d of oil and 1180 MMcf/d of gas, contributing 21% of Argentina's total production (Secretaria de Energia de Argentina 2020; Wood Mackenzie 2020b). YPF predicted these rates would increase by 150% in 2022 (Rassenfoss 2018). Part of this increase will be contributed by La Amarga Chica block, where YPF and its partner, PETRONAS approved their 30-year master development plan in late 2018 to deliver 54,000 boe/d by 2022 (Zborowski 2019). This production increase has obviously been delayed due to the COVID-19 pandemic. The novelty of this paper is integration of geological and reservoir characterization, drilling and production strategies, as well as historical performance and economics of Vaca Muerta. It is concluded that oil and gas potential in the Vaca Muerta shale is significant and rivals the potential of some of the shales widely developed in the Unites States and Canada.
Argentina is ranked second globally in terms of technically recoverable shale gas, and fourth in shale oil (EIA 2015). The most prolific shale is the Vaca Muerta formation. The objective of this paper is to present geological and reservoir characterization, drilling and production strategies, as well as historical performance and economics of Vaca Muerta. The word petroleum as used in this paper includes oil, natural gas, and natural gas liquids. This paper describes natural fractures and their impact on hydrocarbon productivity. The successful commercial production from this unconventional resource has been driven by many factors, including regional geology, availability of advanced technology such as horizontal drilling and multi-stage hydraulic fracturing, as well as domestic and regional hydrocarbon demand (Sierra 2016). Vaca Muerta itself is very unique with multiple hydrocarbon windows from east to west, ranging from dry gas to wet gas, to light oil and black oil. The productivity of Vaca Muerta is benchmarked to some of the best US shale plays such as the Eagle Ford and the Marcellus. Vaca Muerta contains 1202 Tcf of risked gas in-place and 270 billion barrels of risked oil in-place. It is estimated that 308 Tcf and 16 billion barrels of these resources are technically recoverable (EIA 2015). To date, the total number of horizontal wells exceeds 600, mostly drilled in the black oil window (Secretaria de Energia de Argentina 2020; Wood Mackenzie 2020b). Dubbed the ‘golden goose’ of Argentina, the last decade has seen rapid exploration and development activities. The Argentina state oil company (YPF) leads the development in this region together with its partners. In 2019, production from Vaca Muerta reached 90,000 bbl/d of oil and 1180 MMcf/d of gas, contributing 21% of Argentina's total production (Secretaria de Energia de Argentina 2020; Wood Mackenzie 2020b). YPF predicted these rates would increase by 150% in 2022 (Rassenfoss 2018). Part of this increase will be contributed by La Amarga Chica block, where YPF and its partner, PETRONAS approved their 30-year master development plan in late 2018 to deliver 54,000 boe/d by 2022 (Zborowski 2019). This production increase has obviously been delayed due to the COVID-19 pandemic. The novelty of this paper is integration of geological and reservoir characterization, drilling and production strategies, as well as historical performance and economics of Vaca Muerta. It is concluded that oil and gas potential in the Vaca Muerta shale is significant and rivals the potential of some of the shales widely developed in the Unites States and Canada.
TOC evaluation in Vaca Muerta is challenging due to its complex mineralogy and depositional settings. Laboratory measurements can be affected by sample preparation, especially for wells drilled with oil-based mud. Empirical methods like Passey ∆log R relies on resistivity as one of the inputs, which can be affected by clay and mineralogy in shale. In this study, an improved TOC prediction using a multiple regression equation is proposed. The findings reflect the vertical variability of TOC. The method developed in this study first evaluates the TOC correlation with available electrical logs from a vertical well that includes spectral gamma ray, neutron porosity, density and resistivity. It also assesses the correlation with clay and inorganic mineralogy available from X-ray diffraction. This study also incorporates for the first time, thin bed heterogeneity that comprises calcite beef, ash beds and nodules. They make up a considerable portion of the facies, especially in the organic-rich unit of Lower Vaca Muerta (LVM). Despite the complexity, the modelled TOC calibrate well with the laboratory-measured TOC. The TOC regression equation is developed based on two key findings. First, the TOC is positively correlated with uranium and resistivity; and negatively correlated with dolomite and calcite. High TOC is observed in low Ca (calcite, dolomite and ankerite) and high QFP (quartz, k-feldspar and plagioclase) intervals, and vice versa. This negative correlation is unique to Vaca Muerta, which is attributed to the mixed carbonate-siliciclastic depositional system (Kietzmann et al., 2014). Second, the TOC is also strongly affected by thin bed heterogeneity that is identified through micro-resistivity image log and high-resolution logs. Their effect is more pronounced on resistivity log; therefore, an adjustment factor is applied to the regression to account for their presences. Results show that the modelled TOC match well the core TOC as compared to Passey ∆log R method. An important observation is that the Passey ∆log R technique would overestimate the TOC at the top of Upper Vaca Muerta due to high and resistive Ca content; and underestimate it in LVM due to conductive clay in the argillaceous ash beds. Consequently, it would mislead the estimation of reservoir thickness, identification of sweet spot for landing zones, as well as resource estimation in Vaca Muerta shale. This paper develops an original regression equation that models TOC in the presence of thin bed heterogeneity in Vaca Muerta. The results compare well with the laboratory-measured TOC. The study reveals the vertical variability of TOC across the five stratigraphic units in a vertical well. More importantly, it highlights potential TOC discrepancy by Passey ∆log R technique that could mislead reservoir thickness estimation due to the effects of mineralogy and thin bed heterogeneity on resistivity.
Sw modeling in Vaca Muerta using ‘resistivity-based’ methods has been recognized as challenging due to the complex mineralogical and depositional settings of Vaca Muerta. Consequently, ‘resistivity-free’ methods are commonly preferable. In this study, Sw modeling using Archie's equation (1942) and Pickett plots (1966, 1973) have been used with the inclusion of thin bed heterogeneity. Despite the complexity, the analysis reveals the variability of both naturally fractured and oil-wet shale characteristics of Vaca Muerta. Several published studies have used Archie's equation to model Sw in Vaca Muerta, but often assumed that the porosity exponent, m is equal to the water saturation exponent, n. In this study, a new approach is presented using Archie's equation, but this time with the m and n being determined from Pickett plots for each stratigraphic unit. Although this method is simple, no such application has been published for the studied area. This technique is very powerful as it helps to relate the vertical variation of m and n to the complex pore system and wettability characteristics in Vaca Muerta. The Sw analysis using Archie's equation and Pickett plots shows vertical variability in m and n values in each stratigraphic unit of a vertical well. The decreasing m and increasing n values with depth are corroborated by the increasing natural fractures intensity and oil wettability towards the Lower Vaca Muerta (LVM), as indicated by the cuttings descriptions, micro-resistivity images and a published Scanning Electron Microscopy (SEM) study. Better reservoir quality lies in the deeper section, especially the LVM with lower Sw, higher Total Organic Carbon (TOC) and porosity. The organic rich unit also has a higher intensity of thin bed heterogeneity, which comprises bedding parallel calcite-filled microfractures (beef), ash beds and calcite nodules. This emphasizes the criticality of including them in petrophysical evaluation. The most pronounced effect of thin bed heterogeneity is on the resistivity log. Despite the complexity, the modelled Sw matches well the Sw determined from retort and Dean Stark measurements. This shows that resistivity and porosity-based techniques, such as Archie's equation and Pickett plots are applicable in the complex Vaca Muerta shale. Through analysis of Sw modeling using Archie's equation and Pickett plot, the variability of naturally fractured and oil-wet shale characteristics are revealed in each stratigraphic unit of a vertical well in Vaca Muerta. Despite its complexity, the analysis also includes, for the first time, thin bed heterogeneity. These challenges do not hinder the application of the above resistivity and porosity-based techniques which are proven to be powerful tools for characterizing the complex Vaca Muerta shale.
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