Fluid flow channeling and mass transport with discontinuous porosity distribution

The flow of fluids within porous rocks is an important process with numerous applications in Earth sciences. Modeling the compaction-driven fluid flow requires the solution of coupled nonlinear partial differential equations that account for the fluid flow and the solid deformation within the porous medium. Despite the nonlinear relation of porosity and permeability that is commonly encountered, natural data show evidence of channelized fluid flow in rocks that have an overall layered structure. Layers of different rock types routinely have discontinuous hydraulic and mechanical properties.We present numerical results [1] obtained by a novel space-time method [2] based on a fixed-point scheme inspired by the mathematical analysis [3], combined with a space-time least-squares formulation. This approach can handle discontinuous initial porosity (and hence permeability) distributions. It furthermore exhibits optimal convergence independently of the discontinuities, while standard approximations, as e.g. finite differences, tend to show lower order convergence in discontinuous regimes.The space-time method enables a straightforward coupling to models of mass transport for trace elements. Our results show the influence of different kinds of layering in the development of fluid-rich channels and mass transport [1].

Distribution of incompatible trace elements between the constituents of spinel peridotite xenoliths: ICP-MS data from the East African rift

The challenge of understanding how localized deformation modifies fluid flow in porous rock is addressed. New approaches are presented, based on neutron radiography and digital image analyses, to track fluid flow in rock specimens and to calculate flow velocity fields providing local flow measurements. The results show that neutron radiography, backed up by appropriate image analysis, is a very powerful tool in this context, being far more sensitive to the fluids in the rock than X‐ray radiography. Analysis of neutron radiography images of water imbibition into a laboratory‐deformed sandstone specimen has provided new measurements of local fluid flow velocities within a shear band, indicating that flow is faster and water storage is higher in the band (attributed to higher capillary forces associated with damage).

The mechanisms by which trace and radiogenic elements are held and transported in the Earth's mantle are of prime importance in studies of basalt petrogenesis and the chemical processing of the mantle. While such mechanics include the migration of melts and the convective physical admixture of differentiated material1, the role of fluids as transport agents has attracted much interest, especially, in regions affected by kimberlitic activity2–4, continental basaltic volcanism5,6, and in areas of subduction-related magmatic activity7,8. The observation that many incompatible elements are readily leached from garnet lherzolites by dilute acid treatment3 has led to the assumption that a proportion of these elements is located on grain boundaries, rather than in solution in the various minerals present. This indirect evidence is reinforced by comparison of the compositions of mineral separates with their bulk parental material4–8,11. Although careful electron microprobe analyses have made it possible to detect minor and trace elements within grains12, the low concentrations involved, together with the high X-ray background of the electron microprobe, have made it impossible to investigate the supposed distribution of incompatible trace elements on grain boundaries. We have measured directly, using a high-resolution proton microprobe9,10, the distribution of Sr and other elements in a garnet lherzolite xenolith. The analyses demonstrate that, in this sample, Sr is preferentially concentrated along grain boundaries.

Minerals of the Earth's upper mantle have crystal structures notoriously unfavourable to large ion lithophile elements (LILE, such as K, Rb, Cs, Ba, Th, U), and to some extent to the high field strength elements (HFSE, such as Nb and Ta). Several authors have suggested that, in the convective mantle and the lower lithosphere, these 'highly incompatible elements' are dominantly concentrated in small fractions of partial melt. These melts would be volatile-rich and rather similar in composition to kimberlites and/or carbonatitic magmas (McKenzie, 1989). Alternatively, at shallow depth (lithosphere), the small volume melts could be silica-rich (Schiano and Clocchiatti, 1994; Kelemen et al., 1995). In the lithospheric mantle, LILE and HFSE may also be concentrated in accessory minerals such as amphibole, phlogopite, apatite and titanates, in fluidderived inclusions trapped in minerals, or along grain boundaries (e.g. Zindler and Jagoutz, 1988). Among the predominant rock-forming minerals, only clinopyroxene is considered to play a significant role as a host for incompatible elements. However, very few high-quality are available to constrain these assumptions. Minerals such as orthopyroxene, olivine and spinel have rarely been analysed, and the importance of fluid/melt inclusions and grain boundaries as reservoirs of trace elements remains controversial. For this reason, some important issues such as the behaviour and distribution of the ItFSE in the upper mantle are still a matter of debate. The aim of this paper is to provide quantitative estimates of the distribution of lithophile trace elements between the various constituents of spinel peridotites from the East African Rift. Predominant and accessory minerals were separated in 12 mantle xenoliths from Mega (Sidamo region, South-Eastern Ethiopia). The samples range in composition from fertile, cpx-rich lherzolites to refractory harzburgites Dipartimento di Scienze della Terra e Geologico-Ambientali, Universitg di Bologna, Piazza di Porta S. Donato I, Bologna 140127, Italy

Hydraulic fracture (HF) in porous rocks is a complex multi-physics coupling process which involves fluid flow, diffusion and solid deformation. In this paper, the extended finite element method (XFEM) coupling with Biot theory is developed to study the HF in permeable rocks with natural fractures (NFs). In the recent XFEM based computational HF models, the fluid flow in fractures and interstitials of the porous media are mostly solved separately, which brings difficulties in dealing with complex fracture morphology. In our new model the fluid flow is solved in a unified framework by considering the fractures as a kind of special porous media and introducing Poiseuille-type flow inside them instead of Darcy-type flow. The most advantage is that it is very convenient to deal with fluid flow inside the complex fracture network, which is important in shale gas extraction. The weak formulation for the new coupled model is derived based on virtual work principle, which includes the XFEM formulation for multiple fractures and fractures intersection in porous media and finite element formulation for the unified fluid flow. Then the plane strain Kristianovic-Geertsma-de Klerk (KGD) model and the fluid flow inside the fracture network are simulated to validate the accuracy and applicability of this method. The numerical results show that large injection rate, low rock permeability and isotropic in-situ stresses tend to lead to a more uniform and productive fracture network.

The solid system in deformable porous media undergoes deformation with the flow of fluid. In this paper, in order to study the micro-mechanism of the deformation, the solid system in the porous media is represented by a pack of spherical particles and simulated by discrete element method. The fluid system in the porous media is also simulated by computational fluid dynamics. To consider the fluid–particle interactions in the porous media, the above techniques are coupled and applied for simulating the solid deformation and fluid flow. Different models consisting of different particle sizes are studied in dry (without the presence of fluid) and wet states (with the flow of fluid). The results show that with the decrease in the particle size, the solid deformation declines, which imitates the actual deformation in the porous media. More importantly, the comparison between the dry and wet models indicates that the effect of the fluid on the particle system is diminishing with the smaller packed particles. The solid deformation tendency is quantified by the reduction in the values of some micro-mechanical properties, such as permeability (absolute and relative), porosity and pore-size distribution.

The bimodal volcanoplutonic (basalt-peralkaline rhyolite with peralkaline granites) association of the Noen and Tost ranges was formed 318 Ma ago in the Gobi-Tien Shan rift zone of the Late Paleozoic-Early Mesozoic central Asian rift system, the development of which was related to the movement of the continental lithosphere over a mantle hot spot. A specific feature of the Late Paleozoic rifting was that it occurred within the Middle-Late Paleozoic active continental margin of the northern Asian paleocontinent. Continental margin magmatism was followed after a short time delay by the magmatism of the Gobi-Tien Shan rift zone, which was located directly in the margin of the paleocontinent. Such a geodynamic setting of the rift zone was reflected in the geochemical characteristics of rift-related rocks. The distribution of major elements and compatible trace elements in the rift-related basic and intermediate rocks corresponds to a crystallization differentiation series. The distribution of incompatible trace elements suggests contributions from several sources. This is also supported by the heterogeneity of Sr and Nd isotopic compositions of the rift-related basaltoids: eNd(T) ranges from 4.4 to 6.7, and (87Sr/86Sr)0, from 0.70360 to 0.70427. The geochemical characteristics of the rift-related basaltoids of the Noen and Tost ranges are not typical of rift settings (negative anomalies in Nb and Ta and positive anomalies in K and Pb) and suggest a significant role of the rocks of a metasomatized mantle wedge in their source. In addition, there are high-titanium rocks among the rift-related basaltoids, whose geochemical characteristics approach those of the basalts of mid-ocean ridges and ocean islands. This allowed us to conclude that the compositional variations of the rift-related basaltoids of the Noen and Tost ranges were controlled by three magma sources: the enriched mantle, depleted mantle (high-titanium basaltoids), and metasomatized mantle wedge (medium-Ti basaltoids). The medium-titanium basaltoids were formed in equilibrium with spinel peridotites, whereas the high-titanium magmas were formed at deeper levels both in the spinel and garnet zones. It terms of geodynamics, the occurrence of three sources of the rift-related basaltoids of the Noen and Tost ranges was related to the ascent of a mantle plume with enriched geochemical characteristics beneath a continental margin, where its influence caused melting in the overlying depleted mantle and the metasomatized mantle wedge. The formation of rift-related andesites in the Noen and Tost ranges was explained by the contamination of mantle-derived basaltoid melts with sialic (mainly sedimentary) continental crustal materials or the assimilation of anatectic granitoid melts.

The distribution of incompatible trace elements between clinoand ortho-pyroxenes from spinel peridotite xenoliths has been investigated. The xenoliths came from three localities within the Pannonian Basin, Eastern Central Europe, namely Szigliget, Szentbekalla and Bondoro-hegy (EmbeyIsztin et al., 1989). All upper mantle xenoliths selected for this study are anhydrous and include the following textural types; protogranular, porphyroclastic, equigranular and poikilitic. Both metasomatised and unmetasomatised xenoliths were analysed. However, there were no significant interor intragranular trace element variations in the clinoand orthopyroxenes, and the homogeneity of the major elements in the mineral phases also suggest equilibrium. The four major mineral phases (clinoand orthopyroxene, olivine and spinel) were analysed by laser ablation microprobe inductively coupled plasma mass spectrometry (LAM-ICP-MS) at Memorial University of Newfoundland; for analysis details see Jenner et al. (1993). REE, Sr, Nb, Zr, Hf and Ti are above detection limits for the clinoand orthopyroxenes; however, these elements are generally too low in the olivines and spinels for quantitative analysis. The U, Th and Ta content of orthopyroxene was also below detection limits. The averages and standard deviations of the calculated orthopyroxene/clinopyroxene partition coefficients are given in Fig 1. This figure which is based on the averages of 37 xenoliths strongly confirms the observation of Rampone et al. (1991), McDonough et al. (1992) and Sun and Kerrich (1995) that the orthopyroxene/clinopyroxene partition coefficients of the HFSE elements (which substitute in the M1 sites in the pyroxenes) are significantly higher than those of the neighbouring REE (substituting in the M2 sites). The figure also illustrates orthopyroxene/clinopyroxene partition coefficients from some spinel peridotite xenoliths from British Columbia (Sun and Kerrich, 1995). These later data were calculated using solution ICPMS data on separated clinoand ortho-pyroxenes. The U-shaped pattern for the REE partition coefficients (i.e. the increase of the partition coefficients towards the LREE) is probably the result of impurities in the orthopyroxene separate. Highly incompatible trace elements reside not only in the pyroxene lattice but in fluid inclusions and along the grain boundaries. The REE partition coefficients based on in situ analysis show regular variation with ionic radii, e.g. Fig. 2, where there is illustrated an almost linear trend of the logarithm of the partition coefficients in the D (opx/cpx) versus ionic radii. There is increased scatter of the D values as lower trace element abundances in orthopyroxene are approached. However, the higher scattering in the case of some LREE and Sr (all have relatively large ionic radii) is not simply a result of analytical uncertainty; the orthopyroxene/clinopyroxene partition coefficents of these elements tend to increase with increasing equilibrium temperatures (Fig. 3.). This variation is intrepreted to be the result of a temperature dependence of Ca partitioning between

The Austral Islands, a volcanic chain in the South-Central Pacific Ocean (French Polynesia) are composed mainly of alkali basalts and basanites with subordinate amounts of olivine tholeiites and strongly undersaturated rocks (phonolite foidites and phonolite tephrites). The basaltic rocks have geochemical features typical of oceanic island suites. The distribution of incompatible trace elements indicate that the lavas were derived from a heterogeneous mantle source. The chondrite-normalized patterns of the incompatible elements in basaltic rocks of the Austral Islands are complementary to those of island arc tholeiites. As supported by isotope data, the observed trace element heterogeneities of the source are probably due to mixing of the upper mantle with subducted oceanic crust from which island arc tholeiitic magma was previously extracted.

In porous sandstone, fluids are guided by major features such as faults or lithologic discontinuities. At the local scale, deformation bands are common structures to baffle fluid flow in such rock. Potential flow-hindering structures less frequently reported of are veins in porous sandstone (Skurtveit et al., 2015, as a rare case), which may root in the circumstance that they do not appear very often and/or have simply been overlooked.  We here present a case where calcite veins formed in the highly porous (up to 25 % porosity) and partially poorly lithified eaolian Lower Cretaceous Twyfelfontein Formation in NW Namibia. This sandstone was buried by the extrusion of voluminous Paraná-Etendeka flood basalts at around 130 Ma and was since then subject to exhumation. Calcite veins occur in roughly half of the visited outcrops of the Twyfelfontein Formation and their dominant parallel trend to the continental margin suggests a tectonic origin. As the host rock is void of carbonate framework material or cement, the veins must have formed through advective fluid circulation. An external source of the calcium may possibly be the alteration of overlying and intercalated basalt. The veins exhibit a remarkable multitude of textures ranging from blocky, colloform, to microcrystalline calcite generations, that have partially experienced brecciation. This argues for highly variable formation conditions, potentially spanning from normal fluid advection to boiling and injection (c.f., Moncada et al., 2012; Salomon et al., 2021).  Preliminary clumped isotope data of the veins indicate a low temperature formation in the range of 19-61°C, which suggests overall shallow burial conditions. This is in agreement with the diagenetic paragenesis of the rock arguing for late stage vein formation, i.e. during exhumation of the rock. Upcoming U/Pb calcite dating is expected to bring greater clarity on this regard. A halo in the host rock surrounding the veins became calcite cemented due to the growth of calcite from the fractures into the sandstone body. This appearance demonstrates the following evolution: (1) fracturing of the sandstone, which enhances advective fluid flow in the rock body; (2) vein precipitation and near-vein host-rock cementation; and consequently (3) reduction of permeability in the fracture and adjacent wall rock.  Due to their potential of forming effective barriers to fluid flow, we stress that their formation needs to be understood in greater detail. The variable vein textures indicate differing formation conditions, which sets the base for a more common occurrence of calcite veins in porous uncemented sandstone. 

CO2-induced dissolution of low permeability carbonates. Part II: Numerical modeling of experiments

Sub-lithospheric origin of Na-alkaline and calc-alkaline magmas in a post-collisional tectonic regime: Sr-Nd-Pb isotopes in recent monogenetic volcanism of Cappadocia, Central Turkey

Summary The ever-growing demand for energy, relatively high price of hydrocarbons, and recent advances in production technologies have brought tight hydrocarbon-bearing reservoirs into attention as a potential source of energy. However, the displacement physics at nano and micro scales and their impact on fluid flow in these rocks is poorly understood. The unconventional rocks, such as shale rocks, are highly heterogeneous, fine-grained, and their representative elementary volume is uncertain. In order to identify flow pathways in the pore network of these rocks, it is essential to characterize nanopores and their connectivity. This can be achieved using high-resolution 3D imaging technique provided by Focused Ion Beam milling and Scanning Electron Microscopy (FIB-SEM). In this technique, a sequence of 2D cross sectional images, spaced evenly through a region of bulk specimen, is acquired. The stack of 2D images is then re-constructed into a 3D digital gray-scale representation of the sample volume. In this study, a reservoir rock sample from a major shale oil reservoir is selected for high-resolution imaging and statistical analysis. Rock specimens, 1 to 2 cm in dimensions, are cut from different locations of the reservoir core from which a high-resolution 2D map and multiple 3D FIB-SEM images are obtained. The digital images are then visualized, segmented, and analyzed to obtain porosity, pore size distribution, pore aspect ratios, spatial distribution of organic/total porosity, and total organic content. We find that the majority of the pores are below 100 nm in radius for this rock. In addition, the total visible porosity and total organic content are in the range of 1 to 2% and 8 to 14 vol.%, respectively. Chemical composition and mineralogy of the samples are also evaluated by Energy Dispersive X-Ray Spectroscopy (EDS) analysis. Furthermore, 3D pore networks are extracted from the FIB-SEM images; pore connectivities are examined; and permeabilies are calculated by solving the Stokes equation numerically using the finite volume method. It is observed that the pore connectivity for these rocks is poor, resulting in low permeabilities ranging from 1 to 6 µD. Finally, the impact of calculated parameters on fluid flow in unconventional rocks is discussed.

碱性造岩矿物能够记录碱性岩源区特征、岩浆演化以及晚期成矿的重要信息，是开展碱性岩成岩成矿研究的有效手段。波孜果尔碱性花岗岩型铌-钽-锆-铷-稀土矿床位于塔里木北缘-中亚南天山晚古生代造山带，是塔里木地块北缘铌成矿带中典型的碱性岩型矿床。本文通过对含矿岩体中的霓石和钠铁闪石开展矿物学研究，结合全岩成分揭示波孜果尔稀有-稀土金属矿床含矿岩体的岩石类型、演化特征、构造背景及成矿条件。研究发现，含矿碱性岩体由霓石钠铁闪石碱长花岗岩、霓石钠铁闪石石英碱长正长岩、霓石钠铁闪石碱长正长岩组成。不同岩相岩石均表现出相似的稀土和微量元素配分模式，以富集轻稀土元素和高场强元素，亏损大离子亲石元素和重稀土元素为特征，具有显著的负Eu异常，指示三种岩石类型是同源岩浆演化的产物。含矿岩体中的辉石为霓石-霓辉石，角闪石为钠铁闪石。霓石和钠铁闪石稀土和微量元素配分模式相似且含量都较低，富集重稀土、亏损轻稀土，具有显著的负Eu异常，并富集Zr、Hf等高场强元素，亏损Ba、Sr等大离子亲石元素。霓石和钠铁闪石微量元素特征指示三个岩相单元的演化程度由低到高依次为霓石钠铁闪石碱长花岗岩→霓石钠铁闪石石英碱长正长岩→霓石钠铁闪石碱长正长岩，且结晶分异作用控制了岩体的形成。同时，早期霓石钠铁闪石碱长花岗岩的钠铁闪石呈现Ce正异常，晚期岩相中则无异常，指示演化的早期阶段氧逸度较高，随着磁铁矿等氧化物的结晶熔体趋于还原。波孜果尔含矿岩体成矿元素除Rb外分布并不均匀，表现为演化早期的岩相富集Nb，而演化晚期的岩相更富集稀土和Zr。进一步研究发现，塔里木北缘碱性岩带发育的铌、稀土、钽、锆、铷、铀等稀有稀土金属矿化与地幔柱引起的幔源岩浆底侵有关，其构造背景可能为地幔柱对造山带的叠置。;Alkaline minerals can record the important information of source area characteristics, magmatic evolution and late mineralization of alkaline rocks, which is an effective means for the study of diagenesis and mineralization of alkaline rocks. Boziguoer Nb-Ta-Zr-Rb-REE deposit is a typical deposit related to alkaline rocks, which is located in the Late Paleozoic orogenic belt of the southern Tianshan-the northern margin of Tarim block. Based on mineralogical study of aegirine and arfvedsonite in the ore-bearing intrusion, combining with its whole rock compositions, this paper reveals the rock types, evolution characteristics, tectonic setting and metallogenic conditions of the intrusion in the Boziguer rare-earth metal deposit. The rock types of ore-bearing intrusion are aegirine arfvedsonite alkali feldspar granite, aegirine arfvedsonite quartz alkali feldspar syenite and aegirine arfvedsonite alkali feldspar syenite. All of the three types of rocks show similar REE and trace element distributions, with enriched LREE and HFSE, depleted HREE and LILE, and a significant negative Eu anomaly, indicating that they are the products of homologous magmatic evolution. The pyroxene group minerals are aegirine, and the amphibole group minerals are arfvedsonite. Aegirine and arfvedsonite show similar distribution patterns of trace and rare earth elements, which are characterized by enriched HREE and depleted LREE with a significant Eu negative anomaly. They all show enrichment in HFSE (Zr, Hf), depletion in LILE, such as Ba and Sr. Trace element and REE distributions of these minerals indicate that the degree of their evolution from low to high is: aegirine arfvedsonite alkali feldspar granite→aegirine arfvedsonite quartz alkali feldspar syenite→aegirine arfvedsonite alkali feldspar syenite, showing the formation of the intrusion is controlled by crystallization and differentiation. The arfvedsonite of aegirine arfvedsonite alkali feldspar granite shows a positive anomaly of Ce, indicating a high oxygen fugacity environment at the early stages of its evolution. The crystallization of magnetite in the early lithofacies make the evolution environment of the melt tend to be reduced. The Rb in all rock types is equally distributed, and the rocks of early evolution stage is more enriched in Nb, while that of the late-stage more enriched in REE and Zr. The mineralization of rare earth metals such as Nb, REE, Ta, Zr, Rb and U developed in the alkaline rock belt in the northern margin of Tarim basin is related to the mantle magma underplating caused by mantle plume, and its tectonic setting may be the superposition of mantle plume to orogenic belt.