A multiscale approach to analysing a pore system of tight sandstones: a case study of the Lajas Formation (Middle Jurassic), Neuquén Basin, Argentina

The Lajas Formation (Middle Jurassic) of the Neuquén Basin in Argentina is a renowned unconventional tight gas sandstone reservoir. It has been studied widely from multidiscipline approaches; however, only a few petrophysical studies have been published. The objective of this study is to examine correlations between various porosity measurements obtained through petrographic optical and scanning electron microscopy (SEM), combined with quantitative X-ray diffraction (XRD) mineralogy, and petrophysical laboratory measurements, including nuclear magnetic resonance (NMR at 2 MHz) and gas-filled porosity (GFP). The analysed samples cover a wide compositional spectrum ranging from lithic feldsarenites to feldspathic litharenites, the clay fraction is dominated by chlorite or mixed illite/smectite (I/S) with less than 20% of expandable layers (I/S), and the total porosity ranges from 5 to 13%. Intercrystalline pores, which are associated with clay minerals, are a key component controlling the pore system of the unit. SEM images and a strong correlation between XRD data and the clay-bound water derived from NMR T 1 – T 2 maps are clear evidence of this. The analysed reservoir shows a high variability and complexity in the pore structure related to other textural pores (e.g. non-clay intergranular and intragranular pores), thus reflecting the importance of multidisciplinary and multiscale studies that aim to understand the heterogeneous porosity network of tight sandstone reservoirs.