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

Abstract

The Lajas Formation (Middle Jurassic) of the Neuquén Basin in Argentina, is a renowned unconventional tight gas sandstone reservoir. It has been widely studied from multidiscipline approaches; however, 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 analyzed samples cover a wide compositional spectrum ranging from lithic feldsarenites to feldspathic litharenites, the clay fraction dominated by chlorite or mixed illite-smectite with less than 20 % of expandable layers (I/S), and total porosity ranging from 5 % to 13 %. Inter-crystalline pores, associated to clay minerals, are a key component controlling the pore system of the unit. SEM images and the strong correlation between XRD data, and the clay-bound water derived from NMR T1-T2 maps are clear evidence of this. The analyzed 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.