Dissolution Cavities Research Articles

Numerical simulation of two-phase oil–water flow in fractured-vuggy reservoirs based on the coefficient of porous medium proportion and coupled regions

Based on the flow characteristics of fluids in various reservoir media, fractured-vuggy oil reservoirs can be classified into seepage zones and conduit flow zones. An interface exists between these two regions, where the movement of formation fluid near this interface is characterized by a coupling or transitional phenomenon between seepage and conduit flow. However, the complexity of this coupling interface poses challenges for traditional numerical simulations in accurately representing the intricate fluid dynamics within fractured-vuggy oil reservoirs. This limitation impacts the development planning and production adjustment strategies for fractured-vuggy oil reservoirs. Consequently, achieving accurate characterization and numerical simulation of these systems remains a critical challenge that requires urgent attention. A new mathematical model for oil-water two-phase flow in fractured-vuggy oil reservoirs is presented, which developed based on a novel coupling method. The model introduces the concept of the proportion coefficient of porous media within unit grids and defines a coupling region. It employs an enhanced Stokes–Brinkman equation to address the coupling issue by incorporating the proportion coefficient of porous media, thereby facilitating a more accurate description of the coupling interface through the use of the coupling region. Additionally, this proportion coefficient characterizes the unfilled cave boundary, simplifying the representation of model boundary conditions. The secondary development on the open-source fluid dynamics software is conducted by using matrix & laboratory (MATLAB). The governing equations of the mathematical model are discretized utilizing finite volume methods and applying staggered grid techniques along with a semi-implicit calculation format for pressure coupling—the Semi-Implicit Method for Pressure Linked Equations algorithm—to solve for both pressure and velocity fields. Under identical mechanism models, comparisons between simulation results from this two-phase flow program and those obtained from Eclipse reveal that our program demonstrates superior performance in accurately depicting flow states within unfilled caves, thus validating its numerical simulation outcomes for two-phase flow in fractured cave reservoirs. Utilizing the S48 fault-dipole unit as a case study, this research conducted numerical simulations to investigate the water-in-place (WIP) behavior in fractured-vuggy oil reservoirs. The primary focus was on analyzing the upward trend of WIP and its influencing factors during production across various combinations of fractures and dipoles, thereby validating the feasibility of the numerical modeling approach in real-world reservoirs. The simulation results indicated that when multiple dissolution cavities at different locations communicated with the well bottom sequentially, the WIP in the production well exhibited a staircase-like increase. Furthermore, as the distance between bottom water and well bottom increased, its effect on water intrusion into the well diminished, leading to a slower variation in the WIP curve. These characteristics manifested as sudden influxes of water flooding, rapid increases in water levels, and gradual rises—all consistent with actual field production observations. The newly established numerical simulation method for fractured-vuggy oil reservoirs quantitatively describes two-phase flow dynamics within these systems, thus effectively predicting their production behaviors and providing guidance aimed at enhancing recovery rates typically observed in fractured-vuggy oil reservoirs.

Co-Injection of Foam and Particles: An Approach for Bottom Water Control in Fractured-Vuggy Reservoirs

Fractured-vuggy carbonate reservoirs are tectonically complex; their reservoirs are dominated by holes and fractures, which are extremely nonhomogeneous and are difficultly exploited. Conventional water injection can lead to water flooding, and the recovery effect is poor. This paper takes the injection of foam and solid particles to control bottom water as the research direction. Firstly, the rheological properties of foam were studied under different foam qualities and the presence of particles. The ability of foam to carry particles was tested. By designing a microcosmic model of a fractured-vuggy reservoir, we investigated the remaining oil types and the distribution caused by bottom water. Additionally, we analyzed the mechanisms of remaining oil mobilization and bottom water plugging during foam flooding and foam–particle co-injection. The experimental results showed that foam was a typical power-law fluid. Foam with a quality of 80% had good stability and apparent viscosity. During foam flooding, foam floated at the top of the dissolution cavities, effectively driving attic oil. Additionally, the gas cap is released when the foam collapses, which can provide pressure energy to supplement the energy of the reservoir. Collaborative injection of foam and solid particles into the reservoir possessed several advantages. On one hand, it inherited the benefits of foam flooding. On the other hand, the foam transported particles deep into the reservoir. Under the influence of gravity, particles settled and accumulated in the fractures or cavities, forming bridge plugs at the connection points, effectively controlling bottom water channeling. The co-injection of foam and solid particles holds significant potential for applications.

Geodynamic of the Cenomanian-Turonian Sedimentary Basin and the Manganese Mineralization Setting (Imini-Boutazoult, Morocco)

The Upper Cenomanian-Lower Turonian of the Imini-Boutazoult region consists of a limestone containing Mn mineralization-rich beds. In order to demonstrate the relation between the mineralization and the sedimentary context, this study was carried out to analyze the evolution of the paleoenvironments during this interval. In the field, the boundaries that allowed the lithostratigraphic division are continuous and are easily recognizable from one section to another, which contributed to simplifying the subdivisions of the Cenomanian-Turonian lithostratigraphy into three units U1, U2 and U3. All the geological data from the field and the sequential, sedimentological and diagenetic analyses made it possible to elaborate correlative diagrams showing the spatio-temporal evolution as well as paleogeographical sketches during the dolomitic deposits of the Cenomanian-Turonian. The Mn mineralization may be linked to the diagenesis in the vadose zone in a very shallow internal platform. It can also be very belated and strongly linked to karstification with areas supporting the circulation of fresh water (dissolution cavities, dissolved fossils, cracks, fractures.

An evaluation framework for production performance of high-temperature fractured and karstified geothermal reservoirs: Production mechanism, sensitivity study, and key parameters ranking

High-temperature (>150 °C) geothermal resources have great potential for sustainable power generation. These geothermal resources can be exploited efficiently through highly permeable geological structures, i.e. tectonic fractures and dissolution caves, which thus are categorized as fractured and karstified geothermal reservoirs. However, the study of long-term cold fluids injection into these high-temperature geothermal systems is faced with significant unsolved challenges, i.e. the simulation of coupled porous media flow and free flow in multiscale and heterogeneous fracture-cave networks with distinct thermo-hydro-mechanical (THM) coupling effects. To address these challenges, we develop a novel coupled THM model based on the discrete fracture-cave network approach. We then apply this model to perform the analysis of THM coupling mechanism, sensitivity study, and key parameters ranking to identify the development characteristics of high-temperature geothermal reservoirs. Results show that with respect to coupled THM processes, the fluid flow and heat transfer are dominated by connected fracture clusters and are strongly influenced by the inclusion of caves. However, the behavior of solid deformation is governed by the distribution of caves. As to the reservoir thermal performance under different working fluids, i.e. water and CO2, the water-based geothermal reservoir exhibits longer service life, greater global thermal power, and higher heat extraction rate than the CO2-based one. Regarding sensitivity analysis and key factors ranking, fracture attributes (fracture length and density) are the two most important parameters in regulating the geothermal extraction performance. Caves fall out as the third most significant parameter. However, the operating parameters (i.e. injection temperature, injection pressure, production pressure, well length, and well distance) play a limited role in influencing thermal performance compared with geometrical parameters of fractures and caves. The developed model and insights obtained from our research have crucial implications for understanding and optimizing the thermal production of high-temperature fractured and karstified geothermal reservoirs.

Structural and textural control exerted on dissolution cavities development, the Furna Feia Karst System case study (Brazil) - Implication for carbonate reservoirs

Structural and textural control exerted on dissolution cavities development, the Furna Feia Karst System case study (Brazil) - Implication for carbonate reservoirs

Stellar calcite aggregates infill dissolution cavities of marcasite in Khoman Chalk near K/Pg boundary: implications of diagenesis in open marine Tethys shelf, Egypt

Stellar calcite aggregates infill dissolution cavities of marcasite in Khoman Chalk near K/Pg boundary: implications of diagenesis in open marine Tethys shelf, Egypt

Formation of Sediment-Hosted Opal-AG at Lightning Ridge (New South Wales, Australia): Refining the Deep Weathering Model

Formation of sedimentary opal-AG in opal fields of eastern Australia has been linked to intensive weathering of their Cretaceous host rocks—the “deep weathering” model. Here we examine possible links between weathering history as recorded in mining exposures and textural observations in thousands of opal nodules from Lightning Ridge (New South Wales, Australia) to further constrain the timing and depositional environment of opal-AG. Satellite imaging identifies river channels—now marked by pedogenic silcrete—associated with an inland river delta as the intermittent source of water that drove localized deep weathering and silicification in reactive volcanogenic sediments. Variably mineralized weathering profiles show evidence for wet/dry cycling that controlled redox and pH fluctuations critical in the conversion of detrital feldspar to kaolinite, release of silica, formation of opaline silica spheres, and opal mineralization during the initial intense phase of weathering. Subsequent less intense weathering under consistently oxidizing conditions modified the weathering profiles but produced little further opal. Textures in oriented opal nodules indicate that cavities filled rapidly under changing Eh-pH. Opal nodules formed when opaline silica spheres, nucleated and grown in perched groundwater bodies, accumulated and drained into cracks and dissolution cavities in underlying claystone. Drier conditions promoted sol-gel ripening processes that produced the solid opal. Patches and bars of precious opal-AG within common opal nodules suggest that it formed through oxidation, diffusion, and leaching in common opaline Si gel during periods of low water flux and was rate limited by the solidification of the Si gel. Ostwald ripening increased silica sphere size to produce the characteristic “play of color.” Opal formation largely ceased once acidification potential was lost or all feldspar had been converted to kaolinite.

The microbial controls on the deposition of Pb‐Zn minerals in carbonate‐hosted Tunisian ore deposits

AbstractThe Ain Allega, Mjar Hannech and Sidi Driss Pb‐Zn‐Ba‐Sr‐As‐(Hg) deposits in northern Tunisia are hosted in a post‐nappe anticline with a core of a Triassic evaporite diapir affected by the NE–SW‐trending Ghardimaou‐Cap Serrat and E‐W lineament. The ore minerals show different styles; particularly, impregnation in dolomite, cement of breccias, replacement ore and open space filling in the dissolution cavities and fractures. Ore minerals include sphalerite, galena, marcasite, pyrite and cinnabar. Principal gangue minerals are composed of barite, celestine, calcite, dolomite and quartz. The orebodies are hosted by Triassic carbonate rocks (Ain Allega ore deposits), Cretaceous carbonate rocks (Mjar Hannech ore deposits) and by the volcanoclastic layers (Sidi Driss ore deposits). These host rocks show hydrothermal alteration, dissolution and brecciation. Sphalerite in the carbonate‐hosted Zn‐Pb deposits in Tunisia occurs as nano‐size microglobular blebs and peloids and forms finely‐laminated bands with wavy and even ripped up features. Peloids are present in collapse breccias of karst cavities. Peloids (5–100 μm) and bacterial filaments (1 μm long) locally form the common micro‐texture of the sulfide mineralization. The core and the shell of peloids are composed of different minerals; there are 1–3 shells and each shell is 1–1.5 μm thick, commonly with coarse crystals. These textures probably represent fossil microbial mats as in‐situ sulfate‐reducing bacteria products and are similar to nano‐textures observed in bacterial biofilms. Electron microscopy, combined with major element changes (Zn, Pb, As, Fe, and Cd) and sulfur isotope values suggest that sphalerite nano‐textures and nano‐spheres are associated with fossilized bacterial‐mat. The δ34S values of sphalerite (−43.9 to +22.38‰) show that the microbial production of H2S controlled sphalerite precipitation. The biogenic nano‐ to macro‐textured sphalerite and sulfur isotope data suggest that microbes had an important role in the genesis of carbonate‐hosted Zn‐Pb deposits in Tunisia during the Miocene.

Responses of multi-scale microstructures, physical-mechanical and hydraulic characteristics of roof rocks caused by the supercritical CO2-water-rock reaction

The integrality of caprock decides the safety of CO2 enhanced coalbed methane recovery (CO2-ECBM), the inferior caprock may reduce the seal performance and induce gas leakage. However, the attribute of caprock can be modified through the supercritical CO2 (ScCO2)-water-rock reaction. To clarify the responses of multi-scale microstructures, physical-mechanical and hydraulic characteristics of caprock after the geochemical reaction, the rock samples of coal seam roof from Qinshui Basin was first collected to simulate geochemical reactions between ScCO2, water, and caprock, then the scanning electron microscope (SEM), low-temperature liquid nitrogen adsorption (LTLNA), mercury intrusion porosimetry (MIP), P-wave velocity, mechanical and permeability tests were conducted. Results show that the ScCO2-water-rock reaction promotes the formation of dissolution caves, which makes the surface rough and uneven and affects the pore distribution through enlarging the volume of transition pores, mesopores, and macropores and reducing the volume of micropores. The P-wave velocity of the sample is decreased nonlinearly with the increasing ScCO2-water-rock reaction time because of the pore structure damage induced by the chemical dissolution effect. The strain energy evolution is also influenced by the ScCO2-water-rock reaction, the long-time geochemical reaction promotes more elastic energy transform to dissipative energy. The peak strength, elastic modulus and Possion's ratio change regularly with increasing reaction time while the failure pattern is independent of the ScCO2-water-rock reaction. The varieties of mechanical parameters can be ascribed to the considerable increase of macropores and can be predicted by the logistic function. The ScCO2-water-rock reaction has not changed the primary permeability while it largely enhances the permeability peak and elevates the infiltration ability of the loaded rock sample. The main leakage form in the study area is the diffusive loss and the CO2-ECBM recovery is relatively safe, however, the CO2 leakage may be exacerbated if the ScCO2-water-rock geochemical reaction is coupled with the local stress concentration or connected with the pre-existing fractures in the roof rocks. This study deepens the understanding of the consequences of ScCO2-water-rock reaction on caprock and provides some help for the site selection and the preliminary safety evaluation of CO2-ECBM.

Late Cretaceous transtensional faulting of the Apulian Platform, Italy

This work focuses on the structural setting of a small area of the Murge Plateau, southern Italy, exposing Turonian bauxites and encompassing Upper Cretaceous carbonates of the Apulian Platform. By combining field geological and structural mapping with mineralogical and petrographic investigations of representative rock samples, we document the control exerted by Late Cretaceous, syn-sedimentary, high-angle faults and dilational fissures striking ca. N105E, N135E, N170E, and N45E. Altogether, these structural elements formed a few km-long, transtensional fault system along which karst-related shafts, dissolution caves, and sinkholes developed. Karst processes first affected the topmost limestone beds of the Valanginian-Cenomanian Bari Fm. Then, during Turonian, in a continental setting, cycles of terrigenous sediment deposition and weathering localized in the karst-related cavities, dilational fissures, and at the top of down-dropped fault blocks. Weathering of the Al–Si rich sediments formed bauxites made up of kaolinite, boehmite, goethite, hematite, and anatase mineral phases. The relative enrichment in kaolinite with respect to boehmite documented for the breccia and conglomerate matrix topping the bauxites is interpreted as due to lower degrees of weathering. Differently, the individual bauxite beds do not show any internal significant variation in terms of their mineral assemblages, but form angular unconformities due to syn-sedimentary faulting. The subsequent marine transgression also occurred under the influence of active tectonics, as displayed by growth folds at the bottom of the Conacian-Early Campanian Altamura Fm. The overall results of this study are in agreement with those previously reported for nearby areas of the Murge Plateau exposing Santonian limestone rocks of the Altamura Fm. Accordingly, we infer that Late Cretaceous transtensional faulting of the Apulian Platform was due to a far field stress state, likely associated to the orogenic processes that involved the northern edge of the Adriatic micro-plate.

Reactive Transport Simulation of Cavern Formation along Fractures in Carbonate Rocks

Karst cavities and caves are often present along fractures in limestone reservoirs and are of significance for oil and gas exploration. Understanding the formation and evolution of caves in fractured carbonate rocks will enhance oil and gas exploration and development. Herein, a reactive transport model was established considering both the matrix and fractures. Different factors affecting the dissolution along fractures were considered in the simulation of matrix–fracture carbonate rocks, including the magnitude and characteristic length of the matrix porosity heterogeneity, intersecting fractures, and complex fracture network. The results show that a strong heterogeneity of the matrix porosity significantly affects the cave formation along the fracture and the existence of fractures increases the heterogeneity due to the high permeability as well as the dissolution area. The characteristic length of the matrix porosity heterogeneity affects the cave location and shape. The larger permeability of intersecting fractures or the matrix greatly increases the cave size, leading to the formation of large, connected cave areas. A complex fracture network leads to more developed karst dissolution caves. The topology of the fracture network and preferential flow dominate the distribution of caves and alleviate the effect of the matrix heterogeneity.

Origin of the carbonate-hosted Danaopo Zn-Pb deposit in western Hunan Province, China: Geology and in-situ mineral S-Pb isotope constraints

The western Hunan-eastern Guizhou (WHEG) Pb-Zn metallogenic belt hosts around 300 carbonate-hosted Pb-Zn deposits (total Pb-Zn metal reserve: >20 Mt), accounting for a major proportion of Pb-Zn resource in China. The origin and ore-forming process of these Pb-Zn deposits are still under debate. The newly-discovered Danaopo Zn-Pb deposit (~150 Mt ore @ 2.58% Zn and 0.48% Pb), which was formed during the Early Devonian orogenic event (~410 Ma), is one of the largest Zn-Pb deposits in the region. In this study, LA-MC-ICP-MS S-Pb isotope analyses were performed on sulfides (pyrite, sphalerite, and galena) and sulfate (barite) from the Danaopo Zn-Pb deposit. δ34SVCDT values of the diagenetic and late hydrothermal barites are of +36.5 to +36.9‰ and +27.1 to +28.1‰, respectively. The sulfides δ34SVCDT values (+23.5 to +35.5‰) imply a thermochemical sulfate reduction (TSR) origin from coeval seawater sulfates, as supported also by the dissolution cavities of diagenetic barite and the nodular and stripe-textured calcite. Furthermore, in-situ sulfide Pb isotopic ratios of different ore-forming stages and altitudes are highly similar: 206Pb/204Pb = 18.144–18.254, 207Pb/204Pb = 15.718–15.797, and 208Pb/204Pb = 38.378–38.605. Such narrow Pb isotopic ranges suggest well-mixing of multiple Pb sources, which likely include the Lower Cambrian Niutitang Fm. (LCNF) black shale, Proterozoic Banxi Group metamorphic basement rocks, and the Lower Cambrian Qingxudong Fm. (LCQF) reef limestone. Considering the deposit geological and S-Pb isotopic features, we suggest that the hydrothermal fluids may have circulated and extracted metals from various sequences in the basin. The fluids then ascended along the Huayuan-Zhangjiajie fault zone and mixed with reduced sulfur-bearing fluids from ore-bearing strata and precipitated the sulfide ores. Mineralization at the Danaopo deposit is thus best classified as Mississippi Valley Type (MVT), which is controlled by the ore-hosting reef limestone and various fault structures.

Structural and sedimentary discontinuities control the generation of karst dissolution cavities in a carbonate sequence, Potiguar Basin, Brazil

Epigenetic karstic systems in carbonate rocks commonly result from progressive dissolution by acidic meteoric waters over thousands to millions of years. The generation of secondary porosity and permeability improvement due to dissolution in carbonate reservoirs of geofluids (e.g., groundwater, hydrocarbons, and CO2) can profoundly impact reservoir storage capacity and subsurface fluid flow. This study investigates the control of structural discontinuities such as stylolites, fractures, and primary sedimentary discontinuities on the generation of multiscale karst dissolution cavities by epigenetic fluid percolation in a Late Cretaceous carbonate sequence (Jandaíra Formation) in the Potiguar Basin, Northeastern Brazil. The study relies on micro- and macroscale analyses such as stratigraphic logs, field structural investigations, rock strength data collected in the field (Schmidt hammer), microtomographic and drone images, thin section analyses, porosity and permeability laboratory measurements. The results show that bed-perpendicular stratabound and non-stratabound stylolites and fractures can be enlarged due to meteoric water percolation until they merge and form a single channel system that crosscuts all sedimentary multilayers. Bed-parallel stylolites are ubiquitous in carbonate sequences overprinting bed interfaces and layers. Where not dissolved, bed-parallel stylolites have low porosity and permeability and thus can act as barriers to vertical fluid flow. Where dissolved, such stylolites can contribute to horizontal fluid flow and form channel porosity. The results of this study led to a formulation of a conceptual model of rock dissolution along structural and sedimentary discontinuities that affects carbonate rock successions in the subsurface.

Logging identification and evaluation of vertical zonation of buried hill in Cambrian dolomite reservoir: A study of Yingmai-Yaha buried hill structural belt, northern Tarim basin

Buried hill reservoir is indispensable part of karst reservoirs in northern Tarim basin, China, which is resulted from the long-term exposure of Cambrian carbonate rock due to meteoric freshwater. Four types of zones could be divided in the buried hill of Upper Cambrian Xiaqiulitage Formation in Yingmai-Yaha structure, including surface karst zone, vertical vadose zone, horizontal underflow zone and deep slow flow zone from up to bottom. Firstly, the conventional and image log response characteristics of different types of vertical karst zonation were summarized through cores, thin section and well log-calibration. The surface karst zone is close to the weathering crust, showing breccia and residual characteristics, and is recognized as bright spots appearances on the image logs. Due to the vertical movement of atmospheric fresh water, a large number of high-angle fractures and dissolution caves are developed in the vertical vadose zone, and the vertical vadose zones are characterized by high-angle sine curve appearances on the image logs. The horizontal under flow zone has a horizontally connected caves or vugs due to the horizontal movement of groundwater, and the image logs are recognized as dark bands. The conventional logs also show the decrease of resistivity and the increase of gamma ray (GR). The deep slow flow zone is less affected by karstification, and there are fewer dissolution caves and fractures, but there is also a certain weak dissolution and may contain small vugs. The image logs show isolated dark spots. The Vertical zonation of buried hill can be divided using well logs by calibrating cores, and thin sections. Finally, the core analysis, and the oil test data are used to determine the zoning of the high-quality reservoir. The results show that the four zones of the buried hills have obvious well log response characteristics. The karst zoning can be well divided by the image logs and conventional well logs when calibrated with cores. The data of core analysis and oil testing show that although the reservoir quality in the horizontal underflow zone is the best, the study area is a bottom-water reservoir, and the reservoirs in the horizontal underflow zone are mainly water-producing. On the contrary, in the vertical vadose zone, a large number of high-angle fractures communicated with small pores, which not only improved the quality of the reservoir, but also the oil test data proves that the reservoir is mainly producing oil and gas. Therefore, compared with the horizontal underflow zone and deep slow flow zone, the vertical vadose zone is favorable to the enrichment and preservation of oil and gas. The research results will provide important insights in the hydrocarbon exploration and exploitation in dolostone reservoirs of Tarim Basin.

Characteristics and genesis of carbonate rocks’ hydrothermal dissolution in Laiwu Basin, Shandong Province, China

Through analysing the chemical composition of rock samples, investigating of hydrogeological points and hydrothermal dissolution geological points, the characteristics and mechanism of hydrothermal dissolution in Laiwu Basin were studied. The results show that the high temperature and high pressure magma intrusions resulted in hydrothermal metamorphic marbling of carbonate rocks in the contact zone. Hydrothermal fluids migrating along permeable faults, fractures can cause acidic hydrothermal dissolution and mixed dissolution in carbonate formation and formed dissolution cavities of different scales. The cavities are constituting groundwater enrichment space at present.

The nature and origins of decametre-scale porosity in Ordovician carbonate rocks, Halahatang oilfield, Tarim Basin, China

At >7 km depths in the Tarim Basin, hydrocarbon reservoirs in Ordovician rocks of the Yijianfang Formation contain large cavities ( c . 10 m or more), vugs, fractures and porous fault rocks. Although some Yijianfang Formation outcrops contain shallow (formed near surface) palaeokarst features, cores from the Halahatang oilfield lack penetrative palaeokarst evidence. Outcrop palaeokarst cavities and opening-mode fractures are mostly mineral filled but some show evidence of secondary dissolution and fault rocks are locally highly ( c . 30%) porous. Cores contain textural evidence of repeated formation of dissolution cavities and subsequent filling by cement. Calcite isotopic analyses indicate depths between c . 220 and 2000 m. Correlation of core and image logs shows abundant cement-filled vugs associated with decametre-scale fractured zones with open cavities that host hydrocarbons. A Sm–Nd isochron age of 400 ± 37 Ma for fracture-filling fluorite indicates that cavities in core formed and were partially cemented prior to the Carboniferous, predating Permian oil emplacement. Repeated creation and filling of vugs, timing constraints and the association of vugs with large cavities suggest dissolution related to fractures and faults. In the current high-strain-rate regime, corroborated by velocity gradient tensor analysis of global positioning system (GPS) data, rapid horizontal extension could promote connection of porous and/or solution-enlarged fault rock, fractures and cavities. Supplementary material: Stable isotopic analyses and the velocity gradient tensor and principal direction and magnitude calculation are available at https://doi.org/10.6084/m9.figshare.c.4946046 Thematic collection: This article is part of the The Geology of Fractured Reservoirs collection available at: https://www.lyellcollection.org/cc/the-geology-of-fractured-reservoirs

Geochronology and Stable Isotope Analysis of Fracture-Fill and Karst Mineralization Reveal Sub-Surface Paleo-Fluid Flow and Microbial Activity of the COSC-1 Borehole, Scandinavian Caledonides

The deep biosphere hosted in fractured rocks within the upper continental crust is one of the least understood and studied ecological realms on Earth. Scarce knowledge of ancient life and paleo-fluid flow within this realm is owing to the lack of deep drilling into the crust. Here we apply microscale high spatial-resolution analytical techniques to fine-grained secondary minerals in a deep borehole (COSC-1) drilled into the Silurian-Devonian Scandinavian Caledonide mountain range in central Sweden. The aim is to detect and date signs of ancient microbial activity and low-temperature fluid circulation in micro-karsts (foliation-parallel dissolution cavities in the rock) and fractures at depth in the nappe system. Vein carbonates sampled at 684 to 2210 m show a decreased C isotope variability at depths below 1050 m; likely due to decreased influence of organic-C at great depth. Micro-karsts at 122–178 m depth feature at least two generations of secondary calcite and pyrite growth in the voids as shown by secondary ion mass spectrometry analytical transects within individual grains. The younger of these two precipitation phases shows 34S-depleted δ34Spyrite values (−19.8 ± 1.6‰ vs. Vienna-Canyon Diablo Troilite (V-CDT)) suggesting microbial sulfate reduction in situ. The calcite of this late phase can be distinguished from the older calcite by higher δ18Ocalcite values that correspond to precipitation from ambient meteoric water. The late stage calcite gave two separate laser ablation inductively coupled mass spectrometry-derived U-Pb ages (9.6 ± 1.3 Ma and 2.5 ± 0.2 Ma), marking a minimum age for widespread micro-karst formation within the nappe. Several stages of fluid flow and mineral precipitation followed karst formation; with related bacterial activity as late as the Neogene-Quaternary; in structures presently water conducting. The results show that our combined high spatial-resolution stable isotope and geochronology approach is suitable for characterizing paleo-fluid flow in micro-karst; in this case, of the crystalline crust comprising orogenic nappe units.

Pore structure characteristics of an ultradeep carbonate gas reservoir and their effects on gas storage and percolation capacities in the Deng IV member, Gaoshiti-Moxi Area, Sichuan Basin, SW China

Abstract Evaluating reservoir storage and percolation capacities, which are closely associated with pore morphology and connectivity, is significant for the exploration and exploitation of oil and gas fields. However, ultradeep carbonate gas reservoirs have extremely high heterogeneity and complexity compared to other relatively shallow reservoirs. Therefore, investigations into all aspects of ultradeep carbonate face enormous challenges. To the best of our knowledge, no systematic study on the pore structure characteristics of ultradeep carbonate gas reservoirs has been conducted to date. In this paper, the pore structure characteristics and their effects on the gas storage and percolation capacities of the Deng IV Member gas reservoir were investigated using conventional petrophysical measurements, casting thin section analysis, scanning electron microscopy (SEM), high pressure mercury intrusion (HPMI) tests, and multiscale CT scanning analysis. The results show that the lithology of the Deng IV Member reservoir is dominated by algal clotted dolomite, algal stromatolite dolomite and algal doloarenite. The conventional petrophysical measurements indicate that the average porosity and permeability are 3.91% and 1.02 mD, respectively. However, the high porosity (8-4%) and high permeability (10–10,000 mD) intervals are developed locally. The casting thin sections, SEM and two-dimensional CT analyses identify six main types of reservoir spaces in the Deng IV Member: interparticle dissolution pores, intercrystal dissolution pores, fenestriform cavities, dissolution cavities, structural fractures and dissolution fractures. Furthermore, three types of throats, that is, necking, tubular and lamellar, were also detected. Four types of reservoirs, namely, matrix type, pore type, cavity type and fracture-cavity type, were classified according to the comprehensive analysis of HPMI data and curves. Three-dimensional pore throat network topologies were reconstructed for four types of representative core samples based on the multiscale CT scanning data and “maximum-ball” method. The distribution characteristics of pore throat radii, volumes and coordination numbers reveal the following: the reservoir space of fracture-cavity type is dominated by cavities and multiscale pore throats with the best pore-throat connectivity; the reservoir space of cavity type is primarily controlled by cavities and large-scale pore throats with moderate connectivity; the reservoir space of pore type comprises multiscale pore throats with moderate connectivity; and most areas of matrix type reservoir are occupied by rock matrix, and the pore-throat connectivity is the worst. In conclusion, the development of fractures and bedding cavities considerably improves the reservoir percolation capacity, and the development of cavities is an important supplement to the reservoir storage capacity. The reasonable combination of cavities and fractures is the basis for the efficient development of ultradeep carbonate gas reservoirs.

Electrical resistivity imaging and engineering geological investigation of Gereb Segen dam, northern Ethiopia

The paper presents results of electrical resistivity imaging and engineering geological investigations conducted at the Gereb Segen dam site and reservoir to evaluate water tightness and stability. A programme of 2D resistivity imaging, discontinuity surveying, core drilling and Lugeon testing was conducted to assess the engineering geological performance of the dam and reservoir area. The geology of the dam site comprises alluvial deposits, dolerite, micritic limestone and marl-limestone-shale intercalation units. The investigations revealed dissolution cavities up to meters in size showing a preferred alignment along the valley; the separate lines of investigation correlated well to provide confidence in the findings. A karstified zone was identified at the base of the marly limestone unit at the dam site. A layer of gypsum found at shallow depth (9.5 m) along the dam axis may pose a dissolution hazard to water tightness and ground quality. Lugeon test results define very-low- to low-permeability units (shale with gypsum lenses and shale-mudstone) and high-permeability units (marly limestone and dolerite) and very-high-permeability zones associated with the karstic features. The presence of discontinuities and/or weak zones at the abutments and cavities at the dam foundation will affect the stability of the dam and facilitate leakage. Multiple line grout curtains for the karst zones and grouting for the jointed limestone at the dam axis and both abutments are recommended to reduce the possible leakage that will occur at the dam.

Diagenetic processes associated with unconformities in carbonate successions on isolated oceanic islands: Case study of the Pliocene to Pleistocene sequence, Little Cayman, British West Indies

The youngest part of the Neogene carbonate succession on Little Cayman is a thin (<3 m), unconformity bounded package of friable Pleistocene grainstones that belong to Unit D of the Ironshore Formation. The basal unconformity separates it from the underlying Pliocene grainstones of the Pedro Castle Formation, whereas the upper bounding unconformity is the present-day land surface. The limestones in both formations have undergone extensive diagenesis with micritization and variable cementation of the grainstones in both formations, the creation of dissolution cavities and tunnels that are commonly filled with internal sediments (including terra rossa) or flowstone in the Pedro Castle Formation, rootcrete-lined plant-generated cavities in both formations, and terra rossa-filled fractures that cut through the entire succession. The negative δ18O and δ13C values of the grainstones and the diagenetic fabrics indicate diagenesis mediated by CO2-charged groundwater. Rare earth element (REE + Y) concentrations are low in the Pedro Castle Formation, higher in the limestones of the Ironshore Formation, and highest in the terra rossa. The REE profiles indicate that the REE in the limestones were derived from the overlying terra rossa via downward-percolating groundwater. Collectively, the diagenetic fingerprints found in this succession provide clear evidence of freshwater-mediated diagenesis, at all scales, that occurred during the sea level lowstands. Critically, the unconformity at the top of the Pedro Castle Formation acted as an aquitard that resulted in the REE being concentrated in the limestones of the overlying Ironshore Formation. These results highlight the complex processes that control the depositional and diagenetic evolution of limestones on isolated oceanic islands.