Zusammenfassung
We studied the Vaca Muerta (VM) Play (Neuquén Basin, Argentina) focusing on an oil window mature well (VR ~0.9-1.1%) to determine 1) zones of enrichment and depletion, 2) correlation/allocation to produced fluids, and 3) in-situ GOR and PVT characteristics.
Zones of saturation versus depletion, total in-place liquids, oil quality and bitumen enrichment throughout the Upper (U-), Middle (M-), and Lower (L-) VM were documented using screening methods such as Rock-Eval, TOC, thermovaporisation and pyrolysis gas chromatography on 24 original whole rock samples as well as solvent extracted aliquots. For production allocation and API prediction produced fluids were analysed in comparison to extracts from the shale units using whole oil chromatography and stable carbon isotope as well as high resolution mass spectrometry (FT-ICR MS). For prediction of GOR and PVT characteristics a combination of MSSV-pyrolysis and PVT modelling was used in the PhaseKinetics approach (di Primio and Horsfield, 2006) for immature samples and in the PhaseSnapShot approach (Kuske et al., 2019) for matured samples.
Even though the base of the L-VM is richest in oil, oil quality is essentially excellent throughout the entire formation with volatile and thus easy to produce fluids making up 50-80% of the in-situ total petroleum. The VM is overall richer in retained bitumen than other important unconventional resource plays (Barnett, Niobrara, Eagle Ford) showing oil saturations (OSI Index) generally exceeding 100 mg HC/g TOC.
Isoprenoid/n-alkane ratios, stable carbon isotope values for aromatics and saturates, and DBE and aliphatic C-number distributions of selected NSO-compound classes (FT-ICR MS) are identical for produced oils and best fit in the range determined for U-VM extracts.
GOR predictions for in-place fluids fit well to PVT data of produced fluids but are systematically lower (130-150 versus 172 Sm3/Sm3). This is due to production fractionation, i.e., preferential retention of high molecular weight and polar non-hydrocarbons. In line with that, produced oils show an enrichment of N1-containing compounds whereas in-situ fluids are dominated by compounds containing more than one heteroatom (FT-ICR MS). Adapting the approach of Elias and Gelin (2015), who successfully correlated heavy cuts/medium cuts ratios (h/m) of aromatic compounds in oils or extracts to field API, an excellent fit is also achieved using h/m ratios of 9 DBE N1-compounds in solvent extracts and produced oils. Calculated API values are 41° for the U-VM extract, 44° for the L-VM extract, and ~40° for the produced oils closely resembling reported API values of 39-42°.
Especially the methods and experimental protocols used for the predictive assessments (FT-ICR MS; MSSV) are novel, unique, and strongly improve our ability to correctly describe the effects of production fractionation on bulk fluid properties and hence to formulate appropriate production strategies. In general, this study excels by combining results from organic geochemistry with those of PVT modeling and analysis.
