Abstract

In specific subsurface conditions, the sandstones of the Lajas Formation represent a tight gas reservoir in a deltaic mouth-bar system. Therefore, understanding the diagenetic processes that affected these sandstones is a first step towards reservoir quality prediction and production optimization.¹⁴C-PMMA autoradiography can be used to characterize the porosity distribution at centimeter-to-micrometer scales. The mineral phases can be associated with specific porosities using scanning electron microscopy (SEM) in the backscattered-electron imaging mode (BSEi) and can be combined with elemental mapping using energy-dispersive X-ray spectroscopic (EDS) analysis. For bulk analysis, porosity mapping is useful for characterizing the constrictivity and the tortuosity of the porous network created by the geological processes related to the basin’s evolution (e.g., compaction, dissolution, fracturing, cataclasis). ¹⁴C-PMMA autoradiography offers a reliable porosimetry method which is adaptable to most rocks. In our study, the superimposition of porosity maps and mineral maps revealed that: 1) secondary porosity, deriving from feldspar dissolution in medium- to coarse-grained sandstone, and the varying degree of dissolution are both influenced by grain size and degree of cementation, 2) mudstone rip-up clasts have homogeneous porosities (3–4%), 3) the host rock’s porosity varies widely (1–8%), 4) bioturbated siltstone and sandstone present higher porosities within the trace fossils, mainly associated with organo-pores (primary porosity in the organic matter), mineral formation (pyrite precipitation), and clay development (microporosity). Integrating these petrophysical and mineralogical observations with the interpretation of the sedimentary and structural processes that affected the lithologies analyzed allows the following generalizations: 1) the highest porosity and permeability occur in samples representing high-energy environments of deposition (deltaic channel infills and mouth bars), 2) deformation bands cannot be considered fairways for fluid migration, as they have a lower porosity than the sandstone matrix, mainly because dilational shear bands underwent cementation and compactional shear bands underwent cataclasis.