Carbonate concretions as proxy for methane-enriched fluid flow in high-porosity sandstone: example from Crotone forearc Basin, Italy

Diagenetic processes exert a strong control on reservoir potential, heavily impacting the exploitation of strategic fossil resources (oil and gas), preservation and management of aquifers, and underground storage of anthropogenic CO₂. Therefore, in high porosity media such as sandstones, the study of selective cementation is crucial to the quantification of reservoir properties and quality. The outcrop-based analysis of cementation types and patterns could unravel fossil fluid flow pathways affecting porous reservoir analogues.

This study is focused on the selective cementation of fluvio-deltaic, Lower to Middle Pliocene age, sandstone to conglomeratic bodies exposed in the Crotone forearc Basin, South Italy. The siliciclastic unit was deposited in a shallow marine setting and reaches a maximum thickness of ~200 m, unconformably overlying the Paleozoic Sila Massif metamorphic basement. The sandstone sequence is almost devoid of diagenetic cement thus preserving most of the original primary porosity. Sandstone beds show a gentle tilting towards SE, with mild brittle deformation in the form of deformation bands and low-displacement faults. Selective cementation of host sandstone can be traced as diagenetic concretions of different shapes and sizes. Concretion types span from tabular-lens shaped with lateral extension up to 10’s m, elongate blade-shaped from 10 cm up to several 10’s meter-long, asymmetric drop-shaped and nodular-spherical bodies. The elongation direction of concretions parallels the southeastward dip of bedding surfaces, while in the vicinity of deformation bands and faults, elongate concretions are parallel to their dip. Pervasive calcite precipitation was responsible for the dramatic porosity loss from 27-32% down to 2-3%, leading to an increase in sandstone cohesion and stiffness. The stiffness increase can be documented in tightly cemented bodies that host 2-3 sets of joints abutting at the concretion-host rock boundary. Cold cathodoluminescence revealed the ubiquitous presence of bright yellow, granular to poikilitic calcite cement in all concretions. Carbon and Oxygen stable isotopes of calcite cement suggest two fluid sources responsible for the selective cementation. The first source can be traced in weakly cemented lens-shaped bodies and along secondary faults and is made of mixed marine-meteoric fluids with contributions from soil percolation. Conversely, the second source can be detected in tightly cemented lens-shaped and nodular to elongate concretions and is given by a mix of marine fluids with contributions from biogenic methane likely related to biological-bacterial activity in a shallow marine setting. The evolution of fluids from meteoric to marine can be associated with a transgressive sea level rise and upward basin-boundary fault propagation that occurred during and after sandstone deposition. The source of methane could be traced in the thick evaporitic (gypsum and anhydrite) sequence underneath the studied sandstone formation, providing large volume of biogenic methane. Methane enriched fluids migrated vertically following major basin-boundary faults permeating the high porosity sandstone and mixed with meteoric to marine fluids providing bed-parallel fluid flow imparted by the hydraulic and topographic gradient.