Correlations Between Nano-Scale Geology and Porosity Logs of Vaca Muerta Shales

Abstract

The porosity and geological texture of tight oil and shale gas samples have significant effects on reservoir production and oil/gas recovery. Classical measurements, such as porosity logs, alone cannot accurately predict the amount of oil and gas storage and final recovery ability because of aspects, such as the rough resolution of the technique and indirect measurement methods; therefore, the corresponding calibration is a critical step. Additionally, the presence of nanopores and micropores within organic matter or on the boundaries between mineral components affect formation properties, and logs cannot resolve at this scale. This paper presents the internal structure imaging and analysis studies of a series of Vaca Muerta Shale samples from different depths and basin locations. Methods include the nanoscale scanning electron microscope (SEM) imaging, pore size distribution analysis, various imaging based simulations and calculations, and field logging data comparisons. As stated in the literature, the Vaca Muerta Shale is a highly promising continuous tight oil and shale gas reservoir and is an important petroleum source bed for oil production. The results indicate that the trend of petrophysical properties, such as porosity as a function of depth, is consistent with logs; but, lab measurements suggest a relative shift of log values, which demonstrates the importance of lab calibration. The internal structures of samples have a stable trend as the depth increases in the way most pores exist within the organic matter at deep depths, while the position of pores gradually moves to the boundaries of mineral components at shallow depths. The shape of the pores varies as the depth increases. This is one of the first papers discussing combining novel digital rock imaging techniques with traditional logging methods. Efficient and accurate digital rock studies can help provide a good calibration using logging analysis, which can eventually benefit reservoir evaluation and oil/gas production. Comparison between direct observations/image based calculations and logging data analysis provide explanations of changing trends of pore and organic matter. Other important petrophysical and geological properties, such as permeability, total organic carbon (TOC), and so on, through lab measurements are also discussed. This study provides insight into the development of predictive stratigraphic framework and a conceptual reservoir model for the Vaca Muerta interval and also helps improve the understanding of its storage and flow capacity.