Porous sandstones are important reservoirs for geofluids.
Interaction therein between deformation and cementation during diagenesis is
critical since both processes can strongly reduce rock porosity and
permeability, deteriorating reservoir quality. Deformation bands and
fault-related diagenetic bodies, here called “structural and diagenetic
heterogeneities”, affect fluid flow at a range of scales and potentially
lead to reservoir compartmentalization, influencing flow buffering and
sealing during the production of geofluids. We present two field-based studies
from Loiano (northern Apennines, Italy) and Bollène (Provence, France)
that elucidate the structural control exerted by deformation bands on fluid
flow and diagenesis recorded by calcite nodules associated with the bands.
We relied on careful in situ observations through geo-photography, string mapping,
and unmanned aerial vehicle (UAV) photography integrated with optical, scanning electron and
cathodoluminescence microscopy, and stable isotope (
Porous rocks, such as sandstone and carbonate, are important reservoirs for
geofluids. Structural and diagenetic processes commonly affect the
petrophysical properties and reservoir quality in these rocks. The
importance of the interaction between deformation and structures, fluid
flow, and diagenetic processes has been emphasized only during the last
2 decades (e.g., see the recently coined term
Granular or porous sediments and sedimentary rocks commonly contain sub-seismic-resolution strain localization features referred to as deformation bands (Aydin, 1978; DBs from now on). The effects of DBs on fluid flow can vary significantly depending on several factors, such as their permeability contrast relative to the host rock, their thickness, density, distribution, orientation, segmentation, and connectivity (Antonellini and Aydin, 1994; Gibson, 1998; Manzocchi et al., 1998; Sternlof et al., 2004; Shipton et al., 2005; Fossen and Bale, 2007; Torabi and Fossen, 2009; Rotevatn et al., 2013; Soliva et al., 2016). In some cases, DBs may act as conduits for fluids (Parry et al., 2004; Sample et al., 2006; Petrie et al., 2014; Busch et al., 2017). In most cases, however, they are associated with significant porosity and permeability reduction relative to the host rock (Antonellini and Aydin, 1994; Fisher and Knipe, 1998; Shipton et al., 2002; Sternlof et al., 2004; Balsamo and Storti, 2010; Ballas et al., 2015; Fossen et al., 2017; Del Sole and Antonellini, 2019), thus inducing permeability anisotropy and reservoir compartmentalization. This might negatively impact production from faulted siliciclastic systems (Edwards et al., 1993; Lewis and Couples, 1993; Leveille et al., 1997; Antonellini et al., 1999; Wilkins et al., 2019) and flow-based models and simulations (Sternlof et al., 2004; Rotevatn and Fossen, 2011; Fachri et al., 2013; Qu and Tveranger, 2016; Romano et al., 2020).
Cement has been found in association with DBs. Localization of cement along these structural features may significantly enhance porosity and permeability reduction caused by mechanical crushing and reorganization of grains, thus increasing their sealing or buffering potential (Edwards et al., 1993; Leveille et al., 1997; Fisher and Knipe, 1998; Parnell et al., 2004; Del Sole et al., 2020). The occurrence, distribution, and petrophysical properties of cement along DBs therefore need to be properly characterized and implemented into reservoir quality modeling to predict porosity, permeability, and their heterogeneity (e.g., Morad et al., 2010).
Models of calcite cementation, in particular, are fundamental for predicting sandstone and fault-rock properties such as porosity, permeability, compressibility, and seismic attributes. Diagenetic processes related to fluid flow mechanisms and evolution within DBs are not fully constrained; in particular, how DBs steer the origin and distribution of calcite cement remains poorly understood. Different processes account for enhanced fluid flow within DBs, such as unsaturated flow relative to the host rock in arid to semiarid vadose zones (Sigda et al., 1999; Sigda and Wilson, 2003; Wilson et al., 2003) and transient dilation in the early stage of DB formation (e.g., Antonellini et al., 1994; Main et al., 2000). Also, these mechanisms have been employed to explain the occurrence of cement and other processes (e.g., cementation, hydrocarbon inclusion entrapment, removal of iron oxide coatings) in and around the band (Fowles and Burley, 1994; Labaume and Moretti, 2001; Ogilvie and Glover, 2001; Parnell et al., 2004; Parry et al., 2004; Sample et al., 2006; Wilson et al., 2006; Cavailhes et al., 2009; Balsamo et al., 2012; Lommatzsch et al., 2015). These mechanisms, however, appear to be limited to specific conditions (e.g., cement precipitation in the early stage of DB formation or in vadose environments) assuming that DBs behave as fluid “conduits” in order to explain the occurrence of cement or other authigenic products within these structures. Nevertheless, a significant number of studies on DBs show that in most cases they are baffle or seals to fluid flow (see Ballas et al., 2015, for a review). Much less attention has been paid to fluid flow and diagenetic mechanisms leading to (post-DB formation) selective cementation in association with low-permeability baffle DBs (e.g., Philit et al., 2015), and models of cement precipitation in these DBs are limited to quartz cement and are mostly experimental (Lander et al., 2009; Williams et al., 2015). Different mechanisms, then, need to be invoked to explain the occurrence of (carbonate) cement in association with DBs in a broader set of conditions.
The aim of our work is to elucidate the influence of DBs on fluid flow and their role in fostering diagenesis and localizing diagenetic products in porous sandstones. The novelty of our work is that by using a multiscalar and cross-disciplinary approach integrating structural and diagenetic analysis, we assess the control exerted by DBs on flow pattern, the origin of diagenetic heterogeneities, and spatial distribution by means of the systematic characterization of the occurrence, as well as the spatial and microstructural relationship between DBs and cement nodules in two porous sandstone reservoir analogs. We examine two field sites in Italy and France where calcite cement nodules are spatially associated with DBs. The comparison between the two locations with different geological settings makes it possible to derive general conclusions that can be extended to other cases in which DBs and diagenetic processes interact. Our study also allows for the evaluation of the impact of both structural and structural-related diagenetic heterogeneities on present-day fluid circulation and on subsequent deformation.
The Loiano study area is in the northern Apennines (Emilia–Romagna region, Italy), 20 km to the south of the city of Bologna (Fig. 1a). The northern Apennines are an orogenic wedge formed in response to the upper Cretaceous–Eocene closure of the Ligurian–Piedmont ocean (Marroni et al., 2017) and the subsequent Oligocene–Miocene convergence and collision between the Adriatic Promontory and the Sardinia–Corsica Block of African and European origin, respectively (Vai and Martini, 2001). Our work focused on the Loiano Sandstones of the Epiligurian Successions (Fig. 1a–c), the middle Eocene to middle Miocene siliciclastic infill of thrust-top, piggyback basins discordant to the underlying Ligurian units, which migrated passively to the NE during the Apennines orogeny atop the entre orogenic wedge (Vai and Martini, 2001). The 300–1000 m thick, late Lutetian–Bartonian Loiano Sandstones are a fan delta to proximal turbidite deposit (Papani, 1998). They are medium- to coarse-grained, poorly consolidated, immature arkosic sandstones and conglomerates deposited in a relatively small lenticular basin (a few tens of kilometers in width and length; Fig. 1a, c). They are composed of 49 %–60 % quartz and 39 %–48 % feldspar, the rest being rock fragments, detrital carbonate clasts, and minor accessories (Del Sole and Antonellini, 2019).
The Bollène site is in the Southeast Basin of Provence (France), 15 km to the north of the city of Orange (Fig. 2a). The Southeast Basin
is a triangular region between the Massif Central to the northwest, the
Alps to the east, and the Mediterranean Sea to the south. It is a Mesozoic
cratonic basin on the edge of the Alpine orogen, approximately 200 km long
and 100 to 150 km wide. Three main tectonic episodes affected the region
(Arthaud and Séguret, 1981; Roure et al., 1992; Séranne et al.,
1995; Champion et al., 2000): SSW–NNE Pyrenean contraction from the Paleocene to
Oligocene, NW–SE Gulf of Lion extension from the Oligocene to early Miocene
(rifting), and, lastly, SW–NE Alpine contraction from the Miocene to Quaternary
(Fig. 2a). The site of Bollène is exposed in a quarry (Fig. 2c) located
in Turonian sand (low-cohesion sandstone) between 10 and 200 m thick and is situated north of the E–W Mondragon anticline (Fig. 2b, c).
The Turonian sands at the Bollène quarry are laminated and fine- to coarse-grained with modal and bimodal grain size distributions; they formed in
deltaic and eolian environments (Ferry, 1997). The host sands are not
cemented. They are composed of 88 % to 92 % quartz, the rest being feldspar.
The median grain diameter (D
The geometry and distribution of DBs and nodules were documented by detailed
field mapping at different scales for both sites. At the Loiano site, a map
(370 m
Polished thin sections of host sandstones, DBs, and nodules were analyzed by
standard petrographic microscopy, cold cathodoluminescence, and
backscattered electron imagery using a JEOL JSM-5400 and an FEI Quanta FEG
200 environmental scanning electron microscope (SEM). These microscopy
techniques were used to examine the textural characteristics (e.g.,
grain size, shape, arrangements, contact relationships) and
microstructures of host rock and DBs, as well as the cement distribution and
texture (e.g., cement type and degree of cementation, cement crystal size and
shape) (Figs. 9–11). In particular, cold cathodoluminescence (CL)
analysis of carbonate cement in nodules was conducted with a CITL cold
cathodoluminescence 8200 Mk5-1 system (operated at 14–15 kV beam energy and
250
Stable carbon and oxygen isotope data from cements from within carbonate
nodules were used to constrain the geochemical environment of precipitation
and possible source of fluids. Powder samples for bulk rock carbon and
oxygen stable isotopes analysis were ground with a dental drill from
unweathered or altered sections of the nodules. A total of 46 sites were
sampled from nodules in Loiano (
Outcrop map that documents the geometry and distribution of DBs and nodules in a portion of the study area in Loiano. The right-hand panel fits on top of the left-hand panel. ZB – zone of bands. The inset (© Google Earth) shows the map location in the study area.
At the study site, bedding strikes NW–SE and dips at an average of
38
Relationships between nodules and DBs. Deformation bands occur
either as single structures or organized in clusters (ZB). Nodules along DBs
(or ZB) are isolated
The peculiar characteristic of the Loiano Sandstones is the occurrence of
spatially heterogeneous carbonate cement in the form of isolated or multiple
spheroids or irregular nodules and continuous tabular nodules (Figs. 4 and 5). The nodules weather out in positive relief because they are more
resistant to weathering than the weakly cemented host rock. Isolated nodules
range in diameter (major horizontal axis) from 0.2 to 3 m (Fig. 4c, d),
whereas tabular concretions have a thickness ranging from 0.10 to 0.8 m and
a long axis ranging from 3 up to 15 m in length (Fig. 5a). Generally, the
nodule shape in Loiano is similar to that of an oblate spheroid. There is no
evidence of spherical nodules or prolate spheroids. The volume of the
carbonate nodules ranges from 0.001 m
Bedding-parallel nodules are either isolated (Fig. 3) and multiple but laterally discontinuous (Figs. 3 and 5d, e) or laterally continuous layers with a tabular geometry (Fig. 3; e.g., “nodular beds” in Del Sole et al., 2020). Nodular beds are continuous pervasively cemented layers that extend along the bedding plane for several meters (up to 15 m in length) with a nearly constant thickness of ca. 35–50 cm. Nodules along bedding planes are more rounded gentle boundaries (Fig. 5d, e) than those associated with DBs, which are instead more tabular and exhibit angular and sharp boundaries (Figs. 4 and 5a–c). In some cases, nodule geometry and elongation direction follow both bedding surfaces and DBs (Fig. 3). Nodules, despite being ubiquitous in the sandstone, are mostly observed within coarse levels with a grain size equal to or larger than medium sands (0.25–0.5 mm). We did not observe any nodules in sedimentary rocks with a grain size finer than sand (siltstone and clay). Bedding-parallel nodules are commonly located in sandstone levels confined between clay–silty levels or fine-grained sand levels (Figs. 3 and 5d, e).
A set of joints and veins (Fig. 5a, b) was found exclusively within the carbonate nodules. They postdate DBs and nodules and do not propagate into the surrounding host sandstone.
At Bollène, DBs occur as belonging to three different trends oriented
(i) NW–SE to NNW–SSE (set 1: 334
Calcite cement occurs isolated
Typical relationships between nodules and DBs.
The Turonian Sandstones in the Bollène quarry are characterized by a
spatially heterogeneous cementation (Fig. 6b). These diagenetic
heterogeneities occur as spherical and tabular nodules (Figs. 7 and 8).
Spherical nodules are arranged as isolated bodies within the surrounding
host rock (Figs. 7c, g and 8a, d, e) or aggregated in tabular clusters (Fig. 7a, d). Nodules weather out in positive relief. Spherical nodules range in
diameter from a few millimeters (0.004–0.005 m) to a few tens of centimeters (0.2 m), whereas tabular ones have a thickness ranging from a few centimeters to 0.1 m
and a long axis up to 5 m in length (Figs. 6b and 7a–c). Assessment of
the nodule lateral extension is hampered by the presence of vegetation and
debris cover, whereas subsurface extension cannot be measured because of the
limited vertical exposures of the outcrops. Hence, the values reported here
are minimum values. In general, the nodule shape may be approximated by a
sphere for which length, width, and thickness are “equal” and by an oblate
spheroid for which length and width are larger than the nodule thickness.
Carbonate nodule volume ranges from 10
Host rock (HR) and DB porosity as well as relationships between cement
and DBs in the Loiano Sandstones.
Host rock total porosity (minus cement
Natural- and CL-light photomicrographs showing the
microstructure and cement textures in
Natural- and CL-light photomicrographs showing the internal
texture and microstructure of
Despite the different effects of mechanical and chemical compaction as well as
minor authigenic alterations (refer to Supplement S1 for details), the major
diagenetic components of the Loiano Sandstone are calcite cements. These
cements fill mainly intergranular, and to a lesser extent intragranular
(intraskeletal), pore spaces and intragranular fractures, and they encase the
framework grains and all other diagenetic features. Bedding-parallel nodules
(Fig. 10a–h) are characterized by a mosaic texture of blocky sparite to
poikilotopic bright-orange to orange CL calcite cement. Crystal size is
typically 40–100
To evaluate any sign of dissolution in nodules, we carefully checked cement crystal morphologies adjacent to poorly cemented or non-cemented host rock sectors at the edges of nodules. Here, cement crystal boundaries are regular and sharp (Fig. 10m, n).
The host sands at Bollène are weakly cemented, with the exception of localized carbonate cementation described above. Host rock grains are mostly rounded and lack a fabric (Fig. 11a, f). Here, we describe the microstructure of NW–SE/NNW–SSE normal-sense and strike-slip bands, as well as NE–SW/ENE–WSE strike-slip bands sets. The most recognizable features that characterize both DB sets are the reduction of grain size and porosity, as well as a tighter packing relative to the host rock (Fig. 11). NE–SW strike-slip bands (Fig. 11i–l) have a higher degree of grain comminution, porosity reduction, and tighter packing when compared to NW–SE bands (Figs. 8c and 11g–h). Most grains within the bands are fractured and angular. Despite the strong comminution, a few rounded large survivor quartz grains are preserved in the DB matrix (Fig. 11g, h, k). Fine angular grains that are mostly comminuted feldspar fragments and secondary quartz and minor oxides make up the matrix. We also observed fine particles of crushed calcite cement among the matrix grains within NE–SW bands (Fig. 11k, l). In some cases, the grains in the host rock areas in proximity to the DB are encased by relatively undeformed carbonate cement (Fig. 11i, j). Some grains in the host rock are corroded and partially replaced or coated by calcite cement (Fig. 11c).
The main cement in spherical and tabular nodules is a poikilotopic calcite
that infills intergranular pores (Fig. 11a–e). Most of the cement is
non-luminescent (dark luminescence) under CL (Fig. 11c, d), but a few
crystals show partial overgrowths with a bright-orange CL color (Fig. 11e).
When the crystal has a heterogeneous CL pattern, the non-luminescing zones
are mainly in the crystal core, whereas the luminescing subzones are mostly
at the crystal edges (Fig. 11e). A very thin film (up to ca. 10
Stable isotope analysis results.
Cement from the nodules of the Loiano samples has
Stable isotope analysis of the Bollène samples also defines two groups
of data in the
In the following, we compare the two field sites, highlighting their similarities and differences concerning the interaction between deformation, fluid flow, and diagenesis. We discuss the influence of DBs on fluid flow and their role in enhancing diagenesis and localizing diagenetic products (nodules). Finally, we propose an explanation for the geochemical environment within which fluids were sourced and precipitated the nodule cement. We then explore the implications of SDHs for subsurface fluid flow and reservoir characterization.
The distinctive feature of the Loiano Sandstones is a spatially heterogeneous cementation in
the form of nodules. Field evidence indicates that DB formation predates
calcite cementation. All nodules are spatially related to DBs (Figs. 3 and 5a–c) except for those that are situated along bedding planes
(
Results from microstructural observations show that intergranular cement in the nodules encloses the grains within both host rock and DBs, and it overprints burial-related mechanical and chemical compaction features (Fig. 10a–h). This evidence suggests that the formation of authigenic cements occurred after significant compaction (Cibin et al., 1993; Milliken et al., 1998). Estimated burial depths for the top of the Loiano Sandstones are 800–1000 m (Cibin et al., 1993) and 700–1200 m (McBride et al., 1995). Transgranular microfractures at grain contacts are due to stress concentration at contact points and they are interpreted as load-bearing structures within the granular framework (e.g., Antonellini et al., 1994; Eichhubl et al., 2010; Soliva et al., 2013). In DB-parallel nodule samples the cement that fills the transgranular fractures is in continuity (i.e., same textural and CL characteristics) with the pore-filling cement outside the fractures. The presence of undeformed cement within structural-related features such as microfractures and crushed grains (Fig. 10i–l), both within and outside the DBs, proves that cement precipitation occurred after (at least after the early stages of) deformation.
The bands are the main controlling factor on the location, geometry, and
elongation direction of DB-parallel nodules. The occurrence and location of
bedding-parallel nodules are instead controlled by grain size and contrast
in grain size within the host rock. Although bedding-parallel nodules are
found in all sands, they are more common within coarse-grained levels (
In the Bollène quarry, all calcite nodules occur in association with the DBs, in particular with the NW–SE/NNW–SSE set (Fig. 6b). At this site, we observe complex relationships among multiple deformational and diagenetic events. Timing of bands and nodules is inferred from crosscutting relationships. There is no evidence of low-angle ESE–WNW reverse-sense DBs crosscutting the cement nodules, whereas NE–SW- to ENE–WSW-trending strike-slip DBs offset the reverse-sense bands, the NW–SE bands, and the NW–SE-trending cement nodules (Figs. 6b and 7). The localization and parallelism between DBs and cement are similar at the two field sites, with the exception that NW–SE-trending nodules and DBs in Bollène are not superposed. Here, DBs are always overprinted by cement but the spatial overlap between DBs and nodules (Fig. 7e) is unusual. Nodules occur in compartments that are spatially confined by DB zones. Tabular nodules and clusters of spherical nodules are oriented with the major axis parallel to the NW–SE DBs (Figs. 6b and 7a). The NW–SE bands do not crosscut the cement; therefore, calcite cementation occurred between the NW–SE band formation (Pyrenean contraction or Oligocene–Miocene extension?) and the NE–SW strike-slip bands (Miocene–Quaternary age Alpine shortening?). Please refer to Supplement S1 for details on how DBs relate to the tectonics of the area. Microstructural observations show that the dominant phase of intergranular calcite cement encloses the grains within the nodules, and it overprints only a proportion of the transgranular microfractures at grain contact points. All microfractures in the nodules are filled by a cement that is in continuity (same texture and CL characteristics) with the pore-filling cement outside the grain. Unfilled microfractures (Fig. 11b) were not connected to the pore network, and they were potentially quickly isolated by the calcite mineral growing in the pore space. It is less likely that they formed after cement precipitation; otherwise, the cement would have been broken.
The localized diagenesis observed in the form of nodules at Loiano and Bollène provides evidence for the effect of structural heterogeneities, such as DBs, on fluid flow in porous sandstones (Eichhubl et al., 2004, 2009; Balsamo et al., 2012; Philit et al., 2015; Del Sole and Antonellini, 2019; Pizzati et al., 2019; Del Sole et al., 2020). The petrophysical properties (porosity, permeability, capillary entry pressure) of DBs influence fluid flow and localize diagenesis and cement precipitation.
Cataclastic DBs increase flow tortuosity in reservoirs and produce capillary barriers that severely baffle the flow at the reservoir scale and limit cross-flow between host rock compartments (Harper and Mofta, 1985; Edwards et al., 1993; Lewis and Couples, 1993; Antonellini and Aydin, 1994; Leveille et al., 1997; Gibson, 1998; Antonellini et al., 1999, 2014; Sternlof et al., 2004; Rotevatn and Fossen, 2011; Ballas et al., 2012; Medici et al., 2019; Romano et al., 2020). Smaller pores within bands result in higher capillary forces than in the host rock. This may cause higher water saturation within the bands with respect to the host rock (Tueckmantel et al., 2012; Liu and Sun, 2020). A higher degree of flow tortuosity (reduction in pore interconnectivity) and lower porosity and permeability within the bands may increase the fluid retention time regardless of the water saturation conditions (Antonellini et al., 1999; Sigda and Wilson, 2003; Wilson et al., 2006). Recently, Romano et al. (2020) documented with single and multiphase core flooding experiments that cataclastic bands can strongly influence the fluid velocity field. Other authors (Taylor and Pollard, 2000; Eichhubl et al., 2004) recognized that a slower rate of solute transport relative to the fluid within the bands causes the formation and local perturbation of diagenetic alteration fronts. In light of these considerations and the temporal and spatial relationships between bands and cements obtained from field and microstructural observations, we discuss a model for selective cement precipitation associated with DBs. In our model we assume a reservoir in saturated conditions (see also Sect. 7.3).
A marked grain surface roughening and reduction of grain size, porosity, and pore size characterize the DBs presented in this work. In Loiano, the combined effect of cataclasis and compaction in the DBs causes porosity reduction by 1 order of magnitude, permeability reduction by 3 orders of magnitude, and advective velocity reduction by 2 orders of magnitude with respect to the host rock (Del Sole and Antonellini, 2019; Supplement S3). Similarly, DBs in the Bollène quarry have lower permeability (up to 3 orders of magnitude) and porosity (up to 50 %) (Ballas et al., 2014; see also Supplement S3) when compared to the host rock. A lower permeability, a higher degree of tortuosity (i.e., lower pore size), and reduced section area available for flow (i.e., lower porosity) in the DBs compared to the host rock may cause a flow “slowdown”. In Loiano, the slowdown effect would be more pronounced when considering the normal-to-DB flow than the parallel-to-DB one given that normal-to-DB permeabilities are lower (1 order of magnitude in average) when compared to parallel-to-DB ones (Del Sole et al., 2020). Cataclasis has competing effects on advective flow velocity; it causes (i) an increase in flow velocity linked to the porosity reduction and (ii) a decrease in the hydraulic conductivity (if the hydraulic gradient does not change). The decrease in hydraulic conductivity (3 orders of magnitude; Del Sole and Antonellini, 2019) dominates over the flow velocity increase caused by porosity reduction (1 order of magnitude; Del Sole and Antonellini, 2019). As a result, there is a net decrease in advective flow velocity in the DBs. A reduction in flow velocity (i.e., slower flow path in the DB with respect to the host rock) might increase the residence time of the fluid migrating through the reactive material (see next paragraph).
A first mechanism responsible for cement nucleation in association with DBs would be the presence of highly reactive crushed and pervasive fractured siliciclastic grains within the cataclastic DBs (e.g., Lander et al., 2009; Williams et al., 2015). The comminuted material of the DBs has a large amount of reactive surface area (nucleation spots) and very tiny pore spaces among the crushed grains. With these conditions, cement precipitation requires less free energy to occur (Wollast, 1971; Berner, 1980), whereas greater cement abundances (e.g., Walderhaug, 2000) and faster rates of cement emplacement (Lander et al., 2008; Williams et al., 2015) are promoted. Despite the fact that the role of fracturing in promoting cement precipitation in sandstones has been essentially explored for quartz cement, we think that this mechanism can be applied to calcite cement too. There is plenty of evidence of calcite precipitation over a silica substrate (e.g., Stockmann et al., 2014), but it would require either more time or a higher degree of supersaturation (e.g., Noiriel et al., 2016) to occur (i.e., to lower the energy barrier for nucleation) when compared to a carbonate substratum. This mechanism has already been proposed to explain the presence of (quartz and calcite) cement within the band pore space and contrast in the degree of cementation between the bands and the surrounding rock (Antonellini et al., 1994; Knipe et al., 1997; Fisher and Knipe, 1998; Milliken et al., 2005; Philit et al., 2015; Del Sole and Antonellini, 2019; Pizzati et al., 2019). In this work we show that DBs are a preferred site of cement precipitation. Within the DB there are more nucleation spots for cement nucleation and smaller pores that lead to fast pore clogging. DB pores close faster than host rock ones. Once the cement begins to precipitate, the fresh carbonate substrate (cement) could further enhance precipitation (e.g., Noiriel et al., 2016). This mechanism may be relevant for Loiano where the calcite cement fills small pore spaces among fresh quartz and feldspar surfaces created during fracturing. This process can explain why in most cases DBs are more cemented than the surrounding host rock. On the contrary, this process was less relevant in the Bollène quarry where the bands are not cemented by carbonate and the cementation is localized in compartments between zones of bands rather than within them. The arrangement of nodules in Bollène indicates that the low-permeability DBs hindered the cross-flow, restricted the fluid flow, and focused the diagenesis to parallel-to-bands compartments.
A second mechanism could have worked in combination with the presence of more reactive fine-grained comminution products to promote cementation in the DBs in Loiano. According to their experiments on an analog fault gouge, Whitworth et al. (1999) suggested a membrane behavior for faults in sandstone during cross-fault flow and solute-sieving-aided calcite precipitation. A membrane effect and solute sieving by faults may locally increase the concentrations of components needed for calcite cementation (e.g.. Ca and bicarbonate) on the high-pressure side of the membrane and induce precipitation. The DBs could have acted as a semipermeable membrane in baffling chemically reactive flow and favor cement precipitation. This process may explain a higher concentration of cement along the DBs in nodules and the asymmetric distribution of cement on one side of DBs (upstream side; Figs. 4c, e and 9a, e). An analogous mechanism was proposed by other authors to explain the occurrence of the preferred and asymmetric distribution of the authigenic alterations (carbonate and clay cements, Eichhubl, 2001; hematite bleaching, Eichhubl et al., 2004) on the upstream side of DBs in sandstones.
Other factors that may have locally favored (the initiation of) calcite cement precipitation are the growth of cement on detrital grains (Loiano, Fig. 10e; Bollène, Fig. 11c) and the presence of broken detrital carbonate clasts (e.g., shell fragments) that act as a “seed” (cement nucleation sites) (e.g., Bjørkum and Walderhaug, 1990). The latter case was observed in Loiano, mainly in bedding-parallel nodules (Fig. 9c, d). The mechanisms discussed above explain how and why cement precipitation would occur within the band and in its proximity, as observed on-site. Our field observations confirm the theoretical and flow simulations as well as the analog experiments, which demonstrated that DBs can negatively affect the fluid flow in porous sandstones (e.g., Rotevatn and Fossen, 2011; Antonellini et al., 2014; Romano et al., 2020) and enhance cement precipitation (e.g., Lander et al., 2009; Williams et al., 2015).
We integrate petrographic observations and the stable isotope characterization of cements here with the mesoscale spatial organization and microscale textural relationships between nodules and DBs to discuss the geochemical conditions and potential fluid sources that controlled the formation of carbonate nodules in the studied areas. In Loiano, the first calcite cement to precipitate was the intraskeletal and pore-lining cement associated with bioclasts in bedding-parallel nodules (Fig. 9c–d). The cement fabric and textures, circumgranular dogtooth and void-filling drusy mosaic, suggest a phreatic environment (Longman, 1980; Moore, 1989; Adams and Diamond, 2017). Drusy calcite spars can result from replacement of aragonite in bioclasts in meteoric environments (Flügel, 2013). The second, more pervasive phase of cementation is documented by the intragranular cement observed in all the nodules. The mosaic of blocky sparite with coarse crystals and homogeneous distribution also point to phreatic conditions (Longman, 1980; Flügel, 2013; Adams and Diamond, 2017). The intergranular cement pattern is analogous in DB-parallel nodules and bedding-parallel nodules, meaning they probably formed in a similar phreatic environment.
Oxygen isotope data suggest a meteoric environment (Fig. 12a). According to
the compilation made by Nelson and Smith (1996), the moderately depleted
Cementation patterns can be used to infer the paleo-fluid flow direction at the time of calcite precipitation (Mozley and Goodwin, 1995; Mozley and Davis, 1996; Cavazza et al., 2009; Eichhubl et al., 2009; Balsamo et al., 2012). The different spatial arrangements between DBs and nodules in Loiano make the paleo-fluid flow direction reconstruction challenging. The asymmetric distribution of cement in some nodules associated with DBs can be explained by lateral fluid circulation (Fig. 13a), and cement would accumulate on the upstream side of the DBs (Figs. 4c, 9, and 13a). In other cases, cement is roughly symmetrical with respect to the bands, or it is placed where conjugate bands intersect (Figs. 4b, 5c, and 13a). The most likely interpretation is that both lateral flow under saturated conditions and “direct” meteoric infiltration from the surface, with percolation through the rock, contributed to the formation of nodules in Loiano (Fig. 13a).
Calcite (i.e., diffusive supply of Ca
In the Bollène quarry, the relative timing of DB formation and cementation in the Turonian
Sandstones is complex to unravel. Carbonate cementation occurred between
distinct deformation phases with multiple DBs forming (see Sect. 7.1). The
dominant dark cathodoluminescence pattern and homogeneously distributed
poikilotopic spar texture could suggest an oxidizing (high
Field evidence suggests that clusters of low-permeability DBs in
Bollène impeded cross-fault flow since no cement was found in superposition with the
DBs. The presence of nodules between the DB clusters indicates that the
DBs forced the fluid flow and localized the diagenesis in parallel-to-band
compartments. This evidence and the fact that nodules are homogenous along
their elongation direction discredit the hypothesis of lateral flow. The
cement could have been originated from (i) downward fluid flow directly
from infiltration of meteoric waters or (ii) upward flow of basinal
fluid (pressurized aquifer) along fractures and fault pathways in the
carbonate rocks (Fig. 13b). In both cases, the water flow was potentially
driven by the vertical continuity of DB clusters that have acted as
propagation features of faults in overlying (i) or underlying (ii) series
and aquifers (Fig. 13b). This scenario might explain why the cement is found
only in association with the NW–SE DBs. In both cases (i and ii), the
constituent necessary for the precipitation of cement in nodules (i.e., Ca
and bicarbonate) would come from the surrounding carbonates. Above the
Turonian Sandstones there are several carbonate layers in the upper Turonian
and Santonian interval (Fig. 2c; Ferry, 1997), whereas below there are
carbonates belonging to the Jurassic and Cretaceous series (Fig. 2b, c;
Debrand-Passard et al., 1984). In the first case (i), continental meteoric
waters saturated with meteoric carbon dioxide have dissolved the necessary
constituents along their path through the rock succession toward the
high-porosity Turonian Sandstones. The water percolation through the soil
favored fluid acidification. Similar depleted
Generalized conceptual model for calcite nodule precipitation in
the two study areas:
Models for calcite cementation are of fundamental importance for predicting
sandstone and fault-rock properties such as porosity, permeability,
compressibility, and seismic attributes. In Loiano, DBs have acted as
fluid flow baffles. First, they buffered the fluid flow and localized cement
precipitation, acting as areas of preferential cementation in otherwise
excellent-porosity sandstones. The resulting diagenetic products enhance
porosity and permeability reduction caused by cataclasis, further affecting
subsequent fluid circulation. The presence of structural-related cement in
the form of concretions (i) strengthens the rock volume, (ii) degrades porosity
and permeability, thereby increasing the buffering effect or sealing capacity of DBs, and (iii) imparts mechanical and petrophysical anisotropy to the host
rock (Del Sole et al., 2020). We think that it is important to consider the
possibility of concretions to form in association with faults within
siliciclastic reservoirs, especially where these structures (DBs) are below
seismic resolution (e.g., Del Sole et al., 2020). It is also critical to
understand SDH spatial organization, extension, continuity, density,
hydraulic role in terms of fluid flow circulation, and
mechanical influence on the host rock. This information should be included
in a robust reservoir characterization, and, in general, it is beneficial
during geofluid exploration and energy appraisal, resource development
strategies (groundwater, geothermal, hydrocarbon), well production,
reservoir simulation modeling, geomechanical evaluation of a drilling site,
and other environmental and industrial operations (e.g., waste fluid
disposal; groundwater contaminants; geologic CO
In this contribution, we present two examples of structural control exerted
by DBs on fluid flow and diagenesis recorded by calcite nodules strictly
associated with DBs. The objective of this research was to constrain the
role of DBs in affecting the flow pattern and in localizing cement
precipitation in porous sandstones, as well as to elucidate the mechanisms
involved in these processes. The major results of our study can be
summarized as follows.
At both study sites, one or more sets of DBs precede and control selective
calcite cement precipitation in the form of nodules. The later localization
of cementation along these structural features results in a complex and
spatially heterogenous cementation pattern (SDHs). Selective cementation of nodules associated with DBs indicates interaction
between deformation structures, fluid flow, and chemical processes. The
volumetrically significant presence of cement (10 %–25 % of the exposed
outcrop volume) indicates that fluid flow and mass transport have been
strongly affected by the presence of low-permeability DBs. Two main processes are discussed to explain selective carbonate cementation
associated with DBs in Loiano. (i) The high concentration of nucleation sites on
fine-grained comminution products with increased reactive surface area
of the pore–grain interface and small pore throats in the DB trigger cement
precipitation and fast pore clogging with respect to the host rock. (ii) Solute sieving across the DB (membrane effect) promotes Ca and bicarbonate
concentration increase on the upstream side. In Bollène no clear superposition among bands and cement was observed. Here, the
clusters of bands acted as hydraulic barriers to cross-flow, thus
compartmentalizing fluid circulation and localizing diagenesis in volumes
arranged parallel to the bands. In both areas, cement textures, cathodoluminescence patterns, and their
isotopic signature suggest that the cement in the nodules precipitated in a
phreatic environment from fluids of meteoric origin. In a framework of late-stage diagenesis (post-DB formation) and saturated
conditions (phreatic environment), the processes commonly employed to
explain focused fluid flow and preferential cement precipitation associated
with DBs, such as “transient dilation” and “capillary suction” (see
Sect. 1), appear not to be pertinent. In Bollène and Loiano the DBs
buffered and compartmentalized fluid flow and localized diagenesis. Further analyses, such as flow simulations and cement precipitation
modeling, are deemed necessary to further explore microscale fluid flow and
diagenetic mechanisms that drove preferential calcite cement precipitation
along DBs in the studied porous sandstones. DBs control flow pattern and affect how diagenetic heterogeneities are
distributed within a porous sandstone. The association of diagenetic
cementation with DBs further increases the flow-buffering potential of these
structural features. It also creates SDHs that impart a mechanical and
petrophysical anisotropy to the host rock volume and can seriously affect
the subsurface fluid circulation in porous sandstones. These features should
be considered during reservoir characterization, especially where SDHs are
below seismic resolution.
All the data produced and used to write the paper are contained in it and in the Supplement. More detailed information will be made available on request by contacting the corresponding author.
The supplement related to this article is available online at:
MA and LDS conceived the paper. LDS collected and processed field and laboratory data, provided their interpretations, drew the figures, wrote the paper, and did the revisions. RS and GB contributed to fieldwork at the Bollène study site and to cathodoluminescence data interpretation. MA, FB, and GV participated in fieldwork at the Loiano study site. All authors actively participated in discussing the results and drawing the conclusions, as well as critically revising the paper.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Faults, fractures, and fluid flow in the shallow crust”. It is not associated with a conference.
Leonardo Del Sole kindly acknowledges Natalia A. Vergara Sassarini for fruitful discussion concerning the interpretation of cathodoluminescence imaging data and Mattia Pizzati for technical support during sampling for stable isotope analysis. The Laboratoire Géosciences Montpellier is acknowledged for hosting Leonardo Del Sole as a visiting PhD student in the period between April and July 2019, during which the cathodoluminescence analysis and the fieldwork in the Bollène quarry were carried out. The authors also wish to thank Paola Iacumin, Enricomaria Selmo, and Antonietta Di Matteo for stable isotope analysis in the SCVSA Department (University of Parma). Constructive criticism and comments by James P. Evans and Geoffrey C. Rawling greatly improved our paper. This research is part of a PhD project of the first author.
Leonardo Del Sole dedicates this work to the loving memory of Antonio Del Sole.
This paper was edited by Randolph Williams and reviewed by Geoff Rawling and James Evans.