Karst Reservoirs Research Articles

Simulating two-phase flow and geomechanical deformation in fractured karst reservoirs based on a coupled hydro-mechanical model

Two-phase flow in fractured and karstified porous media subject to coupled hydro-mechanical conditions is an important issue for oil recovery in carbonate reservoirs. However, due to the co-existence of porous media flow, fracture flow and free flow, as well as their couplings with geomechanical deformation, modeling the behavior of fractured karst systems remains challenging. In this work, a novel coupled hydro-mechanical model for simulating the complex behavior of fractured and karstified porous media is developed. Two-phase Darcy's equation is used to describe fluid flow in both matrix and fractures, while the free flow in cavities is considered based on an assumption of phase instantaneous gravity segregation. A modified Barton-Bandis's constitutive model is used to mimic the nonlinear fracture deformation. The cavity deformation is solved based on the fluid pressure on the cavity boundaries. A mixed finite volume-finite element method and a fixed-stress iterative splitting method are adopted to numerically solve the coupled system of equations. The model is then applied to a series of 2D and 3D problems to unravel the impacts of fractures and cavities on two-phase flow and geomechanical deformation in fractured karst systems. The results indicate that cavities hinder water breakthrough due to storage effects, while water may quickly migrate through highly conductive fractures. Cavities tend to dominate the flow and mechanical processes even though fractures are present as well. Significant stress concentration is observed around cavities. Furthermore, the results of 3D cases imply that phase gravity segregation in cavities leads to lower water saturation in the area above cavities and delays water breakthrough.

Taking into Account both Explicit Conduits and the Unsaturated Zone in Karst Reservoir Hybrid Models: Impact on the Outlet Hydrograph

The main outlets of karst systems are springs, the hydrographs of which are largely affected by flow processes in the unsaturated zone. These processes differ between the epikarst and transmission zone on the one hand and the matrix and conduit on the other hand. However, numerical models rarely consider the unsaturated zone, let alone distinguishing its subsystems. Likewise, few models represent conduits through a second medium, and even fewer do this explicitly with discrete features. This paper focuses on the interest of hybrid models that take into account both unsaturated subsystems and discrete conduits to simulate the reservoir-scale response, especially the outlet hydrograph. In a synthetic karst aquifer model, we performed simulations for several parameter sets and showed the ability of hybrid models to simulate the overall response of complex karst aquifers. Varying parameters affect the pathway distribution and transit times, which results in a large variety of hydrograph shapes. We propose a classification of hydrographs and selected characteristics, which proves useful for analysing the results. The relationships between model parameters and hydrograph characteristics are not all linear; some of them have local extrema or threshold limits. The numerous simulations help to assess the sensitivity of hydrograph characteristics to the different parameters and, conversely, to identify the key parameters which can be manipulated to enhance the modelling of field cases.

Improving the resolution of impedance inversion in karst systems by incorporating diffraction information: A case study of Tarim Basin, China

Different scales of voids and cavities in karst systems demonstrate considerable potential as exploration targets in the Tarim Basin, northwest China. Numerous diffraction events exist in the seismic data above the karst reservoir in this area because of the strong impedance contrast and irregular shape of voids. The conventional impedance inversion method using migrated data as an input cannot easily estimate the location of voids and the impedance inside the voids. In this case, an alternative approach to impedance inversion that considers the diffractive component of the total wavefield and uses the unmigrated data as an input should be used. The inversion consists of a least-squares minimization of the misfit function between the observed and modeled data. Forward modeling incorporates a combination of reflection and diffraction wavefield components. The adopted method is applied to physical modeling and field data recorded above the karst reservoir. This study’s physical modeling test simulates observed field data and is performed using a high-resolution and high-fidelity 3D modeling system. Inversion results obtained by the proposed and conventional methods are compared. Physical modeling and a field data application show that the adopted impedance inversion method improves the karst location estimation and the acoustic impedance within the voids.

Velocity model estimation of karstic fault reservoirs using full-waveform inversion accelerated on graphics processing unit

The karstic fault reservoir is a type of hydrocarbon-enriched carbonate reservoir. It is a combination of small-scale caves, vugs, and fractures developing near deep-seated faults, which makes it difficult to delineate using traditional seismic imaging methods. Recent research on geologic characteristics of paleokarst reservoirs facilitates hydrocarbon exploration and exploitation in western China. To interpret karstic fault systems and reservoirs, the seismic image is an effective tool. A high-quality velocity model is essential for high-resolution migration. These seismic results contain crucial information to interpret reservoirs. Full-waveform inversion (FWI) is a powerful method to obtain high-precision velocity models because it makes use of the kinematic and dynamic information in the seismic full wavefield. To image such complicated and irregular fractured-cavity reservoirs accurately, we have applied a multiscale FWI method accelerated on graphics processing unit devices to fault-controlled paleokarst reservoir models. According to the model test results, FWI shows high performance in delineating the boundaries of fault-controlled karstic reservoirs. The proposed method also has the potential to characterize inner structures. Based on the high-precision velocity model provided by FWI, the seismic image obtained by reverse time migration is also improved. These seismic results will show the location, width, and shape of the fault-karst structure, which gives detailed information for fault-controlled paleokarst reservoir interpretation.

Architectural characterization of Ordovician fault-controlled paleokarst carbonate reservoirs, Tahe oilfield, China

Fault-controlled karst carbonate reservoirs are one of the most important reservoir types in the Tahe oilfield of the Tarim Basin. These reservoirs have a large oil reserve and belong to a strongly reconstructed reservoir type with a highly heterogeneous distribution of pores and fractures. This study characterizes a fault-controlled karst reservoir by using integrated methods, including outcrops, well logging, structure interpretation, seismic inversion, and statistical geomodeling. We have established a fault-/fracture-controlling karstic geologic model and classified the internal architectural elements so that we adopted an origin-controlled hierarchical geomodeling strategy based on the fault-controlling characteristics. The results determined that large strike-slip faults provide an important tectonic framework and that its derived fractures act as important channels and spaces for dissolution. Flower structure fault zones and the associated fractures are the main range of karst development, within which a high stress is concentrated during the strike-slip shear process with a high-density fracture development. This is the highly developed karst reservoir, which mainly is concentrated along large faults. The coexistence of fractures and karst dissolution has resulted in a complicated reservoir architecture (karst architecture), which can be classified into four types: (1) large caverns, (2) small caverns and vugs, (3) fractured zones, and (4) matrix (tight limestone). Controlled by the degree of dissolution, the karst architecture is quite different from the sedimentary facies. Large caverns are formed under the strongest degree of dissolution and are the most favorable reservoir type. Small caves and vugs are created under relatively strong dissolution; they are distributed outside large caves and also can act as favorable reservoirs. The fractured zones are not necessarily affected by strong dissolution but have high conductivity and act as important channels for fluid movement. The carbonate matrix is less reconstructed. The architecture development model of the fault-controlled karst carbonate reservoir presented a tree system, within which the karst reservoir caves are connected by the fractures and faults similar to fruits and trunks. The new geomodeling method revealed the constraining characteristics of faults, seismic attributes, and hierarchical architectural elements. Furthermore, we also have built a 3D model of the Tuoputai unit in the Tahe oilfield to show the robustness of this workflow. This research enables us to better understand the structure of fault-controlled karst reservoirs, and it could provide a specified characterization approach that is considered to be theoretically and practically useful.

Recharge, élévation retardée du niveau des eaux souterraines et rendement spécifique au sein de l’aquifère karstique du Trias de la montagne Kopa, dans les Carpates occidentales, Slovaquie

Notable delay of more than 1 year (12.5–15.9 months) between precipitation recharge and groundwater-level upturn was recorded in the Triassic karstic aquifer of the Kopa Mt. (1,187 m asl) in the middle of the West Carpathians in Slovakia. The velocity of recharge transit (0.24–0.70 m/day) was affirmed by borehole hydrographs. The groundwater level here is deep (113–300 m below ground surface) but approximately evenly distributed at the same elevation (~467 ± 10 m asl). Dolomites extend over 78% of the surface area of the karstic reservoir, with a Cretaceous marly aquitard boundary on the perimeter and on its base. The whole area is recharged merely by precipitation and entirely discharged by springs and small surface streams. The water balance between recharged and discharged water, and the groundwater-level fluctuation (shift in time), enabled calculation of stored groundwater volumes. The estimated specific yield of the Triassic dolomitic aquifer was unusually high (7.6–23.4%, mean 16.1%). Karstic cavities were also widely found in the borehole logs of nine boreholes that were 87–490-m deep and drilled in Triassic dolomites and limestones along the axis of a planned highway tunnel. Groundwater level monitoring took place over 40 months, between June 2014 and October 2017. Typical seasonal water-level oscillations were absent in the karstic reservoir, but water-level change was relevant to delayed past recharge events. In a borehole situated in the Cretaceous marly aquitard, good seasonal correlation with effective precipitation and also tidal response was observed.

Carbon biogeochemical processes in a subtropical karst river–reservoir system

Cascade damming has seriously affected the hydrologic regimes and solutes transport of the river systems. However, the effect of damming on carbon cycle and the underlying mechanisms are still unclear. In this study, to reveal the effects of damming on riverine dissolved inorganic carbon (DIC) transport, the water chemistry and stable isotopic signature of dissolved inorganic carbon (δ13CDIC) were analyzed in the tributaries and four subtropical karst reservoirs on the Pearl River in southern China. From reservoir epilimnion to hypolimnion, DIC concentrations increased from 183.2 to 229.4 mg L−1, 167.6 to 192.0 mg L−1, 179.1 to 181.5 mg L−1, and 182.4 to 183.8 mg L−1 while δ13CDIC decreased from −6.6‰ to −9.7‰, −7.8‰ to −10.4‰, −9.7‰ to −10.0‰ and −9.9‰ to −10.0‰ in the Chaishitan (CST), Longtan (LT), Yantan (YT), and Dahua (DH) reservoirs, respectively. Our analyses suggest that primary production and organic matter degradation were two dominant processes in carbon evolution in the epilimnion and hypolimnion, respectively. However, there was no obvious spatial heterogeneity of solutes’ concentration in the DH reservoir. Our results also suggested that cascade reservoirs had a regional damming effect and could be significantly weakened without damming or in daily regulated reservoirs. Furthermore, δ13CDIC had a significant positive relationship with the reservoir effect index (Ri), which was affected by hydraulic retention time and water temperature in the water column of reservoirs and is useful to evaluate the thermal stratification strength. This study highlights that dam constructions affected carbon transport and transformation significantly, and the degree of the damming effect on the carbon cycle can be expressed by Ri.

A Multi-Disciplinary Approach to Understand Hydrologic and Geochemical Processes at Koiliaris Critical Zone Observatory

Koiliaris CZO is a European Critical Zone Observatory (CZO) typical of the Mediterranean karstic geomorphology, which represents watersheds affected by humans over the centuries. This study aims to provide information that underpins the hydrologic and geochemical processes functioning at Koiliaris CZO. Linking geomorphologic and tectonic analysis improved the delineation of a karstic area which extends outside of the Koiliaris watershed and identified how structural elements influence the regional hydrology. The fluctuation in the river flow represents processes occurring in the karst and the periodic signal is related to Earth tide stressing of the karstic reservoirs. The conceptualization of a two-reservoir, well-mixed karstic system is confirmed by both the geomorphologic and tidal analysis. The hydrologic response is fast and it is manifested especially during extreme events where 70% of the precipitation becomes surface runoff, creating major flood events. The different sampling sites in the Koiliaris CZO were geochemically clustered and the quantification of the weathering fluxes showed that 25 mm/1000 years and 39 mm/1000 years of carbonate were removed by chemical weathering for the Keramianos ephemeral river and the springs, respectively. These studies illustrate the importance of critical zone science and transdisciplinary studies on water and soil management.

Migration of Natural Hydrogen from Deep-Seated Sources in the São Francisco Basin, Brazil

Hydrogen gas is seeping from the sedimentary basin of São Franciso, Brazil. The seepages of H2 are accompanied by helium, whose isotopes reveal a strong crustal signature. Geophysical data indicates that this intra-cratonic basin is characterized by (i) a relatively high geothermal gradient, (ii) deep faults delineating a horst and graben structure and affecting the entire sedimentary sequence, (iii) archean to paleoproterozoïc basements enriched in radiogenic elements and displaying mafic and ultramafic units, and (iv) a possible karstic reservoir located 400 m below the surface. The high geothermal gradient could be due to a thin lithosphere enriched in radiogenic elements, which can also contribute to a massive radiolysis process of water at depth, releasing a significant amount of H2. Alternatively, ultramafic rocks that may have generated H2 during their serpentinization are also documented in the basement. The seismic profiles show that the faults seen at the surface are deeply rooted in the basement, and can drain deep fluids to shallow depths in a short time scale. The carbonate reservoirs within the Bambuí group which forms the main part of the sedimentary layers, are crossed by the fault system and represent good candidates for temporary H2 accumulation zones. The formation by chemical dissolution of sinkholes located at 400 m depth might explain the presence of sub-circular depressions seen at the surface. These sinkholes might control the migration of gas from temporary storage reservoirs in the upper layer of the Bambuí formation to the surface. The fluxes of H2 escaping out of these structures, which have been recently documented, are discussed in light of the newly developed H2 production model in the Precambrian continental crust.

Characterization of the regional groundwater flow systems in south Transdanubia (Hungary) to understand karst evolution and development of hydrocarbon and geothermal resources

In South Transdanubia (Hungary), the remarkable geothermal and hydrocarbon resources in the Drava Basin and the hypogene caves at the margin of outcropping carbonate hills were usually investigated separately and their interactions were hitherto neglected. The aim of this study is to give all these groundwater-related resources and phenomena a common framework applying the concept of regional hydraulic continuity, and to complete the regional (i.e., basin-scale) hydraulic assessment of the area based on preproduction archival measured data. Pressure-elevation profiles, tomographic fluid-potential maps and hydraulic cross-sections were constructed to determine the vertical and horizontal fluid-flow conditions. As a result, two kinds of fluid flow systems could be identified. Within the gravitational flow systems, horizontal flow conditions are dominant and the regional flow direction tends toward the S–SE. In deeper basin regions, an overpressured flow system is prevalent, where fluids are driven laterally from the deeper sub-basins towards their margins. Based on the regional-scale evaluation of fluid flow systems, conclusions could be drawn regarding the geothermal and hydrocarbon potential of the area. Additionally, local-scale phenomena could be explained, particularly in the southern foreland of the Villány Hills. Cave formation cannot be related to the present-day flow systems here. In the Harkány area, groundwater chemistry could be explained by fluid contribution from the Drava Basin. A comparison with the marginal Buda Thermal Karst area allows for generalized conclusions regarding the connections between marginal karst reservoirs and the Pannonian Basin.

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.

Characterization of paleo-karst reservoir and faulted karst reservoir in Tahe Oilfield, Tarim Basin, China

The Ordovician carbonate reservoir is the most productive deep-earth reservoir in Tahe Oilfield and other oilfields in Tarim Basin. Exploration and production successes in recent years reveal a new reservoir type, namely faulted karst reservoirs, which is closely related to regional strike-slip faults and very different from the well-recognized paleo-karst reservoir. The paleo-karst reservoirs distribute mainly in weathering crust regions in the northern Tahe Oilfield. Their primary reservoir spaces are meter-scale caves and the fluid conduits are predominantly the unconformable surfaces. In production, paleo-karst reservoirs always have sufficient energy, therefore high productivity. The faulted karst reservoirs mainly develop in southern Tahe Oilfield, controlled by the different ordered strike slip faults and related dissolutions. Their reservoir space is smaller than which of paleo-karst reservoirs. The predominant fluid conduits in these reservoirs are the faults. In production, reservoirs along major strike-slip faults have sufficient energy, high productivity and slow watercut increase like paleo-karst reservoirs. While in areas with less strong energy, faulted karst reservoir exhibits weak productivityand rapid watercut increase, implying a rule of “big fault big reservoir, small fault small reservoir, no fault no reservoir. A comprehensive understanding of the geophysical features, distribution characteristics, reservoir property, and production behaviors of the two reservoir types will assist further exploration and production in Tahe Oilfield and other basins containing such reservoirs. Cited as : Lu, X., Wang, Y., Yang, D., Wang, X. Characterization of paleo-karst reservoir and faulted karst reservoir in Tahe Oilfield, Tarim Basin, China. Advances in Geo-Energy Research, 2020, 4(3): 339-348, doi: 10.46690/ager.2020.03.11

Characteristics, influencing factors, and prediction of fractures in weathered crust karst reservoirs: A case study of the Ordovician Majiagou Formation in the Daniudi Gas Field, Ordos Basin, China

The study of fractures in karst reservoirs is a hot spot in the field of geosciences. In this article, we conducted a systematic study on the characteristics, influencing factors, and prediction methods of karst reservoir fractures of the Ordovician Majiagou Formation in the Daniudi Gas Field, China. Both weathering and tectonic fractures are mainly developed in the Ma 5 member of the Majiagou Formation. The former is categorized into surface weathering, steep wall tensile and karst collapse fractures, and the latter is made of tensile and shear fractures. The palaeo‐temperatures of the fracture filling inclusions range from 90 to 140°C, peaking at 111–120°C, which are derived from oxygen isotopes of −7.9 to −17.5‰, with average −14.1‰ from the same filling inclusions. Integrations of regional tectonic activities, inclusion tests, and stable isotope analysis recognizes four formation periods of fractures since the Caledonian. The factors controlling the development degree of the karst reservoir fractures include lithology, petrophysical properties, rock layer thickness, structural deformation strength, and karst palaeomorphology. The buckling thin plate simulation results show a good positive correlation between the maximum principal stress and tensile fracture density. The same positive correlation is also observed between the shear stress and fracture density. The ancient karst landforms have strongly affected the distribution of weathering fractures. Surficial weathering fractures occur typically in the weathered residual monadnocks, while steep wall tension ruptures in areas near geomorphic steep ridges, and karst collapse fractures in karst residual monadnocks and karst pit areas.

Hydrocarbon reservoir development in reef and shoal complexes of the Lower Ordovician carbonate successions in the Tazhong Uplift in central Tarim basin, NW China: constraints from microfacies characteristics and sequence stratigraphy

The analyses of hydrocarbon reservoirs in the high-frequency reef and shoal facies of the Lower Ordovician carbonate successions in the Penglaiba Formation (O1P) and Yingshan Formation (O1–2Y) in the Tazhong (Katake) Uplift in central Tarim basin showed three types of hydrocarbon reservoirs: type 1 associated with synsedimentary facies, type 2 developed by hot active basinal dissolution fluids [mesogenetic fluids, geothermal/hydrothermal fluids and thermochemical sulfate reduction-related fluids] and tectonic activity, and type 3 are hypogenic–epigenetic karst reservoirs formed by meteoric and mesogenetic dissolution fluids. Porosity and permeability development in the reef and shoal facies in the highstand system tracts (HST) of the third-order sequence cycles (SQ1–SQ3) correlates well with regression peaks in O1P and O1–2Y, mostly related to multiphase tectonic activity, long-term exposure, denudation and dissolution in near-surface to shallow burial settings. The carbonate successions in the O1P and O1–2Y are composed of seventeen microfacies (MF-1 to MF-17) deposited on a shallow marine platform. Six main groups of microfacies associations (MFA-1 to MFA-6) were recognized, with each microfacies association corresponding to the transgressive system tracts and HST of SQ1–SQ4 in O1P and O1–2Y. The microfacies represent specific sedimentary environment including platform interior and platform margin deposited under particular hydrodynamic conditions. Tectonics and frequent relative sea-level fluctuation associated with rapid transgression and slow regression favored the deposition of the large-scale vertical stacking, high-frequency reef and shoal facies across the carbonate platforms in this uplift. The hydrocarbon reservoirs in this study have relatively moderate porosity and low permeability, and are somewhat potential reservoirs for hydrocarbon exploration in the Tazhong Uplift.

Later fluid alteration of eogenetic karst spaces in carbonate: insights from the Cambrian Longwangmiao Formation, Northwestern Sichuan Basin, China

Pore-cave systems formed by karstification in the eogenetic stage of carbonate rocks provide abundant potential reservoir space for hydrocarbons. However, whether these dissolution pore-caves can become effective reservoir spaces during the later burial period, serving as the key to the success of hydrocarbon exploration. Therefore, it is important to explore the fluid activities and their alteration effects on eogenetic karst reservoirs during the later burial. Focusing on the Cambrian Longwangmiao Formation in the northwestern Sichuan Basin, this study systematically analyzed the formation of reservoir space in the eogenetic stage and the reworking of the system by fluids in the later stages, based on petrology, geochemistry, burial history, and tectonic evolution data. Results showed that many millimeters to several centimeters scale of pores and caves in the Longwangmiao Formation were produced by eogenetic karstification. These pore-caves underwent by two episodes of dolomite infilling in the shallow burial stage (D1) and in the Caledonian–Hercynian period (D2). Geochemical parameters indicate that D1 and D2 were both affected by meteoric water. In the early shallow burial stage, the dolomitic fluid was enriched in a relatively closed, reducing environment, whereas in the later stage, the fluid was affected by a relatively open oxidizing environment due to Caledonian–Hercynian fractures. Both D1 and D2 took place before the massive hydrocarbon migration from the Cambrian source rocks in the Middle Permian to those of the Middle Triassic. After the formation of the dissolution pore-caves, the precipitation from two episodes of dolomitic fluids led to the degradation of the Longwangmiao Formation carbonate reservoir space in the northwestern Sichuan Basin. In the southern part of the Shatan section-Well MS1, closed to the paleo-uplift of the central Sichuan Basin, where eogenetic karstification was superimposed by Caledonian–Hercynian supergene karstification, may be form effective reservoir and is a significant prospect for exploration.

Contribution of Full Wave Acoustic Logging to the Detection and Prediction of Karstic Bodies

A 3D seismic survey was done on a near surface karstic reservoir located at the hydrogeological experimental site (HES) of the University of Poitiers (France). The processing of the 3D data led to obtaining a 3D velocity block in depth. The velocity block was converted in pseudo porosity. The resulting 3D seismic pseudo-porosity block reveals three high-porosity, presumably-water-productive layers, at depths of 30–40, 85–87 and 110–115 m. This paper shows how full wave acoustic logging (FWAL) can be used to validate the results obtained from the 3D seismic survey if the karstic body has a lateral extension over several seismic. If karstic bodies have a small extension, FWAL in open hole can be fruitfully used to: detect highly permeable bodies, thanks to measurements of acoustic energy and attenuation; detect the presence of karstic bodies characterized by a very strong attenuation of the different wave trains and a loss of continuity of acoustic sections; confirm the results obtained by vertical seismic profile (VSP) data. The field example also shows that acoustic attenuation of the total wavefield as well as conversion of downward-going P-wave in Stoneley waves observed on VSP data are strongly correlated with the presence of flow.

Sources, variations, and flux of settling particulate organic matter in a subtropical karst reservoir in Southwest China

The biological carbon pump (BCP) effect is a key mechanism in karst surface freshwater ecosystems, and can transform inorganic carbon from carbonate weathering into organic carbon via photosynthesis and sedimentation. In the process, an understanding of particulate organic carbon sources, their flux, and their variability is important for understanding BCP effects, but these relative processes are still poorly understood. Therefore, we used a sediment trap experiment to collect settling particulate organic carbon (SPOC) and conducted a systematic study to understand the sources and factors that influence SPOC, as well as determine the amount of SPOC generated by the BCP effect by combing meteorological data monitoring and modern hydrochemical monitoring in the Dalongdong (DLD) reservoir, a karst groundwater-fed reservoir. Based on δ13C, δ15N, and molar C/N data, as well as relative models, the SPOC sources came from algae, resuspened matter, soil, and domestic sewage. Among these, resuspension (45.27%) and algae (31.57%) were the main sources for SPOC. Thus, the seasonal variations in different sources were mainly affected by water movement caused by climate changing with respect to air temperature and rainfall. In addition, domestic sewage discharge in the DLD reservoir also affected the sources for SPOC. The results suggested that the SPOC from algal sources in the DLD reservoir was 17.07 gC/m2/a at the surface and 9.26 gC/m2/a at the bottom via the BCP process. The SPOC deposition efficiency (OCDE) of algae was 54.25%. Organic carbon from algal source via the BCP effect at the surface of water column was finally deposited and buried in sediment. According to estimation of historical data, the stored organic carbon through assimilating DIC from carbonate rock weathering by BCP effect was as much as 10.84 tC/a. The quantity of organic carbon stored and fixed by BCP effect cannot be ignored in the reservoirs.

Characteristics and controlling factors of dolomite karst reservoirs of the Sinian Dengying Formation, central Sichuan Basin, southwestern China

The Sichuan Basin is a typical superimposed oil- and gas-bearing basin with abundant hydrocarbon resources in deeply buried marine carbonate rocks. The petrological characteristics, origin, diagenetic evolution and controlling factors of the high-quality Sinian Dengying dolomite reservoirs were investigated based on outcrop and core observations in addition to data from various analyses, including thin section, casting thin section, scanning electron microscopy, cathodoluminescence, carbon and oxygen isotope, major and trace elements, and X-ray diffraction analyses. The main lithologies of the Dengying reservoirs are microbial dolomite, crystalline dolomite, granular dolomite and karst breccia dolomite. The reservoir spaces include microbial pores, interparticle dissolution pores, intraparticle dissolution pores, intercrystalline pores, karst caves and fractures. Three models to explain the origin of Dengying dolomites are proposed: (1) syngenetic microbial dolomitization, (2) penecontemporaneous reflux dolomitization and (3) burial dolomitization. Development of the high-quality Dengying dolomite reservoirs in the central Sichuan paleo-uplift was essentially controlled by two factors: sedimentation and supergene karstification. High-quality reservoirs were developed mainly in algal mound and grain-shoal sediments deposited on the platform margin and intraplatform margin. Affected by the multistage Tongwan movement, long-term weathered crust karstification resulted in the formation of a large number of dissolution-enlarged pores and caves in the Dengying reservoirs. Multistage fractures, especially the unfilled tectonic fractures that formed during the late Yanshanian–Himalayan period, connected the relatively isolated pores and karst caves and effectively improved the permeability of the reservoirs. The results of this study provide a deeper understanding of the origin and preservation mechanism of deeply buried paleocarbonate reservoirs and can also effectively guide future exploration in the Sinian Dengying Formation of the Sichuan Basin.

Karstification of Ordovician carbonate reservoirs in Huanghua depression and its control factors

With extensive ancient karsts and abundant oil and gas resources, the Ordovician carbonate reservoirs in the Huanghua depression have broad prospects of exploration and development. The types, characteristics, distribution and controlling factors of karsts were investigated by means of the analysis of cores, thin slices, logging, seismic, and geochemical data in this study. The study reveals that the Ordovician carbonate reservoirs in the Huanghua depression falls into three main categories, pore type, pore-vug-fracture type and fracture type, of which, pore-vug-fracture reservoirs account for 62%. The epigenetic uniform karst mainly developed in the Late Ordovician to Early Carboniferous, while the differential karst developed in the Mesozoic. The uniform karstification mainly include chemical dissolution of the original carbonate rock, while the differential karstification not only include chemical dissolution but also hydraulic erosion and biochemical dissolution by organic acid. The burial karstification mainly involved the dissolution of the Ordovician reservoir by large amount of acidic fluids generated by the overlying Upper Paleozoic coal measure source rock while generating hydrocarbon. The dissolution caused by upwelling magmatic-tectonic hydrothermal fluids along faults also contributed to the formation of burial karst reservoirs. The sedimentation-diagenesis and paleogeomorphology-tectonic movement have certain control effects on the karstification of the Ordovician reservoirs in the Huanghua depression, determining the reservoir quality.

Diatoms from the Holocene sediments of lakes Pustoye and Chernoe (Nizhny Novgorod region)

The diatom analysis of the Holocene sediments of Pustoye and Chernoe lakes revealed 37 taxa of diatoms of two classes, five orders, eighteen families and twenty-five genera with a low degree of similarity of species composition (39%) and differences in the ecological priorities of the diatoms according to the leading abiotic factors – salinity and pH. Lake Chernoe is characterized by a greater number of species (26), the presence of halophilic species, as well as the prevalence of indicators of alkaline reaction of the environment. A feature of Lake Pustoye is the complete absence of halophilic forms and the predominance of species that prefer an acidic water reaction. A greater number of planktonic species was characteristic of Lake Chernoe due to the greater depths of this karst reservoir. According to the geographical location in these lakes, arcto-alpine species are noted.