Caprock Research Articles

Sequence stratigraphic framework and sedimentary model of Shanxi Formation in northeast Zhoukou Depression of the North China Plate

ABSTRACT The sequence stratigraphic framework of Shanxi Formation in the northeast Zhoukou Depression was established based on detailed sequence stratigraphical and sedimentological analysis by utilizing the logging and core data of six wells drilled in the eastern tectonic unit of Zhoukou Depression. It was divided into three third-order sequences, namely SQs1, SQs2, and SQs3 from bottom to top. Each sequence can be divided into a transgressive system tract (TST) and a highstand system tract (HST). Furthermore, four sequence boundaries and three maximum flooding surfaces were identified, and they are the bottom interface SBs and maximum flooding surface mfss1 of SQs1, the bottom interface SBs1 and maximum flooding surface mfss2 of SQs2, the bottom interface SBs3 and maximum flooding surface mfss3 of SQs3, and the top interface SBx from bottom to top. Carbonate tidal flat –clastic tidal flat sedimentary system developed in Shanxi Formation in the northeast Zhoukou Depression (also referred to as the study area) under the control of regression. Meanwhile, four sedimentary microfacies were identified in the sedimentary system, which are lime-mud flats, sand flats, mixed flats, and mud flats. The transgression in the study area occurred from the southeast to the northwest. Therefore, the northwestern part is the seaward side, and the southeastern part is the landward side. As revealed by relevant drilling data, SQs1 of the Shanxi Formation is characterized by the development of limestone and carbonaceous mudstone, with limestone, dark mudstone, and carbonaceous mudstone mainly developing. Meanwhile, lime-mud flats were mainly deposited in it. During the periods of SQs2 and SQs3, the sedimentary environment of the study area changed from the carbonate tidal flats to clastic tidal flats as the coastline migrated towards the sea. In these periods, sand flats mainly developed near the maximum flooding surfaces and were relatively continuous in the eastern and southern parts of the transgressive system tract; mixed flats were relatively continuous in the western and northern parts of the transgressive system tract as well as the eastern and southern parts of the highstand system tract; mud flats widely developed in the highstand system tract. Peat flats mainly developed in the period of HSTs2, with coal seams relatively developing in the southeastern part of the study area as revealed by drilling data. The peat flats in SQs2 are favorable hydrocarbon source layers, the lime-mud flats in SQs1 and sand flats formed in the transgression periods of SQs2 and SQs3 constitute favorable hydrocarbon reservoirs, and the mud flats form in the transgressions periods serve as favorable cap rocks. Therefore, the study area features a reservoir-cap assemblage for self-generating and self-storing of hydrocarbon, and the southeastern part of the study area can be taken as a favorable exploration area.

Using NMR, Core Analysis, and Well Logging Data to Predict Permeability of Carbonate Reservoirs: a Case Study

Fluid flow through porous media is a function of permeability for petroleum and gas reservoirs. This study investigated the distribution of permeability in the Mishrif formation in the Nasiriya oilfield. The value of effective porosity was obtained from well log data and used to predict permeability using three methods: applied empirical correlations using interactive Petrophysics (IP) software (Version 3.5), core data measurements, and nuclear magnetic resonance (NMR). Nasiriya oil field is one of the large oil fields in southern Iraq, and the Mishrif formation, the area of the case study, consists of the following units from top to bottom: a cap rocks unit; the Upper Mishrif (unit 1); a shale unit; the first reservoir unit (2), a Barrier rocks unit; and the second reservoir unit (3). A 3D model was built by using Petrel software (2009), and the LAS files used to digitize the contour map of the reservoir layers were obtained from Didger3 software. The total number of 3D grids in the model was 5,394,480, with 336 grids in the I direction, 169 in the J direction and 95 in the k direction. Scaling up was done by using the geometric method for permeability, and sequential Gaussian simulation was used in the distribution of petrophysical properties. The results of distribution showed the importance of each zone and the best permeability value was recorded in the second reservoir layer, which varied from 1 md to 100 md.

Pressure build-up analysis in the flow regimes of the CO2 sequestration problem

In this work we analyse theoretically and numerically the pressure build-up on the cap rock of a saline aquifer during CO2 injection in all flow regimes. Flow regimes are specific regions of the parameter space representing the mathematical spread of the plume. The parameter space is defined in terms of the CO2-to-brine relative mobility λ and the buoyancy parameter Γ. In addition to the known asymptotic self-similar solutions for low buoyancy regimes, we introduce two novel ones for the high buoyancy regimes via power series solutions. Explicit results for the peak pressure value on the cap, which arises in the vicinity of the well, are derived and discussed for all flow regimes. The analytical results derived are then applied for cap integrity considerations in six test cases of CO2 geological storage from the PCOR partnership, most of which correspond to high buoyancy conditions. The validity of the self-similar solutions which are late time asymptotics, is verified with CFD numerical simulations with a commercial software. The comparison between the self-similar solutions and CFD for the pressure estimations are in excellent agreement and the self-similar solutions are valid for typical injection durations even for early times.

Depth Estimation of Geothermal Heat Structures by Euler Deconvolution of Gravity Data at Eburru Area, Kenya

Gravity survey was done at the Eburru area to estimate the source depth locations and delineate the fault boundaries using 3D Euler deconvolution. Gravity data was collected using CG-5 gravimeter. Gravity data reductions were done by applying drift correction, latitude correction, free air correction, Bouguer correction and terrain correction to the observed raw data to obtain complete bouguer anomaly (CBA). The CBA data was transferred to Oasis montaj software for Euler deconvolution processing. The 3D Euler deconvolution was carried out to determine and estimate the depth of the density bodies. Euler deconvolution locates the gravity anomaly source and estimates its depth from the gravity observation level. Euler deconvolution was preferred to other filtering methods in this study as solutions are only determined over identified analytic signal peaks, the window size varies according to anomaly size and the final solution involves only a few more precise depth estimates. The Euler deconvolution was performed using structural indices of 0.5, 1.0 and 2.0. Results from this analysis indicated that the CBA values in this study area range from gravity values of -272 mGal to -286 mGal and residual Bouguer anomaly amplitude range between -3 mGal and 3.4 mGal. The 0.5, 1.0 and 2.0 structural indices generated five solutions at depth range of 433 m - 2269 m, 801 m - 1433 m and 1170 m - 2246 m respectively occurring almost at the same locations on gravity highs. The deep structures were observed to occur in the northern part of the study area, and interpreted to be dense intruding masses likely to be trapped by the overlying cap rock at these depths. These could be geothermal heat sources that can be exploited to generate geothermal energy.

UTILIZING CORE AND NMR DATA TO IDENTIFY ROCK-TYPE AND PORE THROAT RADIUS FOR MISHRIF FORMATION IN WEST QURNA OILFIELD

The middle Cretaceous Mishrif Formation represents the main carbonate reservoir in southern Iraq, charactered by heterogeneity and complexity vertically and laterally, this reservoir encompasses six facies that graduated from mid-ramp facies to supratidal facies. Mishrif Formation in West Qurna oilfield consists of three main reservoir units namely mA, mB1 and mB2 with two cap rocks units CRⅠ and CRⅡ. Rock type and pore throat radius govern the relationship between porosity and permeability. Porosity and permeability are important petrophysical parameters for reservoir evaluation and can be determined directly from core analysis or indirectly using the NMR logging tool. Core data from four wells and NMR data from one well were used to determine the rocks type and the pore throat radius. In this study, Lucia rocks type plot between the porosity and permeability was used to define the classes of Mishrif units’ characteristics. Rocks type of Mishrif Formation was wackestone and packstone in mB2 unit. The mB1unit exhibits increasing in the grain dominated class1. Whereas, unit mA showed a mud-dominated class 3 again. Winland plot was utilized to determine the pore throat radius depending on the mercury injection test with 35% mercury saturation. The unit mB2 marked as mesopores and macropores. Megapores and macropores attribute increased in mB1in the northern part of West Qurna oilfield unit due to increasing shoal and rudist bioherm facies, the mA unit revealed increasing in meso-porous and decreasing in mega-porous. NMR data was correlated with core data and the results supported this reservoir characteristic.

Analytical Model for Leakage Detection in CO2 Sequestration in Deep Saline Aquifers: Application to ex Situ and in Situ CO2 Sequestration Processes.

One of the prominent methods for carbon dioxide sequestration is disposal into deep saline aquifers. This is mainly because deep saline aquifers provide significant capacity for storage of unwanted fluids underground for a long period. However, saline aquifers may have a leaky cap rock. The sealing capacity of a cap rock must, therefore, be evaluated to ensure the integrity and safety of its storage media; hence robust classifications of the cap rock are required even before starting the storage/disposal operations. Aqueous fluids can be injected into a target storage aquifer, and pressure changes owing to leakage can be monitored in an upper aquifer separated by a cap rock for a short period. The measurement of pressure responses in the monitoring aquifer can be used to identify and characterize any leakage path in the cap rock. This paper provides analytical models in the Laplace domain for both in situ and ex situ CO2 sequestration methods. Using the numerical Laplace inverse method called the “Stehfest” method, the analytical solution in the real domain is calculated. The analytical solutions developed can be used for determining both dimensionless pressure changes in monitoring and storage aquifers due to leakages and dimensionless leakage rates.

Numerical study on the formations of gas channels and subsequent bubbles in unconsolidated sandy seabed sediment using a coupled LBM-DEM method

Carbon dioxide capture and storage is an efficient technology to reduce CO2. A candidate CO2 reservoir is a sub-seabed aquifer under the cap rock. However, there is a risk of CO2 leakage even though such probability is low. Once CO2 seeps at the seafloor, it forms bubbles/droplets and dissolves during their rise. The rising speed and the dissolution rate significantly depend on the bubble/droplet size. Interestingly, past observations of natural seepage of CO2 indicated that the bubble/droplet sizes were not very different, regardless of the seepage depths. In this study, a numerical method to simulate 3D solid-liquid-gas three-phase flow in the unconsolidated particle layers was developed and applied to elucidate the influences of gas flux, porosity, and particle size on the formations of gas channels and subsequent bubbles. The results suggested that the bubble size did not depend on the flux, but on the gas channel size at the level of gas flux expected in possible seepage events.

Faults controlling geothermal fluid flow in low permeability rock volumes: An example from the exhumed geothermal system of eastern Elba Island (northern Tyrrhenian Sea, Italy)

Geothermal fluid circulation and storage within rock volumes are essential conditions for an effective geothermal system. Although several studies examine fluid migration through permeable rock volumes, few papers deal with fluid flow and fluid-rock interaction within the cap rocks of magma-driven geothermal systems. In this paper, I present the results of an integrated study focused on the circulation of geothermal fluids and fluid-rock interaction with clayey-carbonate sedimentary rocks and interbedded tectonic slices of serpentinite, representing the cap of an exhumed geothermal system located in the south-eastern Elba Island (Acquarilli-Norsi area, Tuscan Archipelago).Structural and kinematic data highlight that the prevalent geothermal fluid pathways correspond to NE-trending normal to oblique-slip faults. A Fe-amphibole + quartz + calcite ± chlorite ± epidote ± titanite mineral assemblage filled the fracture networks associated with two different fault generations. Fluid-rock interaction gave rise to skarnoids having three distinct mineralised zones, locally, crossed by the faults.Fluid inclusions data on quartz and calcite coexisting with the amphibole in the first fault generation and calcite of the second fault generation suggest that at least two paleo-geothermal fluids permeated through the fault zones, at a maximum P of about 0.8 kbar. The sequence was (i) first an H2O-rich fluid enriched by a volatile phase mainly made up of CO2 derived from the decarbonation and dehydration reactions of the hosting metacarbonate and metapelitic rocks. This fluid characterised by temperature up to 495 °C and salinity between 1.0 and 6.4 wt.% NaClequiv, records the residual intra-formational fluid (derived from dehydration of phyllosilicate and serpentinite bodies) already present within the host rocks. This fluid was successively mobilised during the development of the fault zones and progressively cooled and CO2-depleted with time; (ii) a second fluid likely of meteoric origin, devoid of CO2 and characterised by relatively low-temperature from 146.5 to 254 °C and salinity between 1.9 and 2.9 wt.% NaClequiv related to the last fluid circulation event. The main conclusions are that: (i) geothermal fluid circulation occurred within the cap rocks of the paleo-geothermal system, along fault zones; (ii) fluid circulation occurred within restricted fault-damaged volumes, developed in an extensional setting; iii) faults dissected rocks that were affected by high-temperature metamorphism induced by the cooling of a magmatic intrusion. The super-hot fluid was produced by metamorphic reactions and only in the latest stage, meteoric water infiltrated in the geothermal system due to the progressive faulting and exhumation.

Природный резервуар как геологическое тело для хранения запасов гелия

With the start of the large helium-rich petroleum deposits exploitation in the Siberian platform south, a problem of long-term helium storage arises. Some researchers propose to use reliable underground storage facilities for this purpose. Geological bodies such as rock associations can be used as natural reservoirs for injecting and storing the helium reserves. The saliferous sediments within the natural reservoirs are of particular importance due to their capacity to keep helium for a long time. To determine the paleofacial conditions for the accumulation of the saliferous impermeable layers, the archive and published data on drilling and geophysical studies of the Eastern Siberia fields have been used. The article presents the research results on the cap rocks’ lithological composition and thickness, as well as on the conditions of their formation as areal impermeable beds within the natural reservoirs of the Siberian platform south. The Lower Cambrian deposits of the Usolie suite, a part of the Vendian-Cambrian oil-and-gas complex, can be considered as an areal impermeable bed of this kind.

Magnetotelluric investigation for imaging the subsurface geoelectrical feature of the prospective Sembalun-Propok geothermal zone, Indonesia

This paper discussed the investigation of the Sembalun-Propok geothermal system, West Nusa Tenggara Province, Indonesia. We analyzed the magnetotelluric data and its implication to the subsurface structure related to the system of Sembalun-Propok geothermal reservoir. In this study, information on the dimensionality of regional conductivity structure and its regional strike were analyzed by applying the magnetotelluric phase tensor. By using induction arrows analysis, the influences of the sea toward the magnetotelluric data were also observed. The results show that it could be performing the 2-D modeling for the higher frequency (≥ 1 Hz) magnetotelluric data. The 2-D inversion modeling suggests that the potential geothermal zone is situated in the western-southern part of the study area. In addition, it also can reveal the depth of the unaltered layer (ρ ≥ 500 Ωm). It is probably related to the volcanic rocks in the study area, and its depth is about 0–500 m from the surface. Then, underlying the unaltered layer possibly corresponds to the cap rock layer (ρ = 1–10 Ωm) of the system of the geothermal reservoir.

Distribution and controls of petroliferous plays in subtle traps within a Paleogene lacustrine sequence stratigraphic framework, Dongying Depression, Bohai Bay Basin, Eastern China

The characteristics of petroliferous plays in subtle traps within a sequence stratigraphic framework in the Dongying Depression are investigated in this study. Sand bodies within lowstand systems tracts (LSTs) of sequences, comprising incised-channel fills, sublacustrine fans, deltas in LSTs, controlled by syndepositional normal faults, and sand bodies within transgressive systems tracts (TSTs) to early highstand systems tracts (HSTs), consisting of beach bars, and turbidites, controlled by the prodelta slope, paleorelief, and syndepositional normal faults, are good subtle reservoirs. Mudstones and shale of deep lake subfacies in TSTs to early HSTs of sequences are source and cap rocks. Abnormal overpressure is the dominant dynamic factor for hydrocarbon migration from source rock to the subtle traps. Normal faults, sand bodies, and unconformities function as conduit systems. Sand bodies distributed in the abnormal overpressure source rocks within LSTs to early HSTs are petroliferous plays in lithologic traps. The petroliferous plays in stratigraphic traps are controlled by unconformities at margins of the Depression.

Experimental Investigation on Permeability Evolution of Limestone Caprock under Coupled THM Processes

To investigate the gas sealing capacity of limestone as cap rock for gas reservoir at different burial depths, triaxial compression coupled with permeability tests of limestone subjected to different confining pressures and temperatures were carried out using the improved MTS 815 test machine. During the tests, acoustic emission (AE) monitoring system was also employed to estimate the rock damage. Test results demonstrated a degradation of triaxial compression strength with the increase in temperature and decrease of confining pressure. An empirical equation for the peak stress was established considering confining pressure and temperature. Three different permeability evolution patterns of limestone samples can be proposed based on the experimental results under different coupled thermo-hydro-mechanical (THM) conditions. According to the spatial-temporal characteristics of AE signals, permeability evolution was highly related to the fracture growth and connection in limestone. Ultimately, the experimental results revealed that the permeability of limestone gradually decreased with the increasing confining pressure and temperature. In addition, an exponential relationship between initial permeability and confining pressure was established.

Filling characteristics, reservoir features and exploration significance of a volcanostratigraphic sequence in a half-graben basin ---A case analysis of the Wangfu Rift Depression in Songliao Basin, NE China

Abstract It has long been considered that thicker volcanostratigraphic sequences should, in theory, contain more favourable reservoir conditions than in thinner sequences, particularly within large-scale stratovolcanoes found in sedimentary basins. However, drilling results from the Wangfu Rift Depression (WRD) in the Songliao Basin, China, show that the thick intermediate volcanostratigraphic sequence contains only one or two layers of favourable reservoir characteristics over approximately 1000 m of volcanic successions. The volcanostratigraphic sequence that is common in the graben basin has a particular distribution pattern as a result of reservoir and origin processes. The Eruptive Interval Unconformity Boundary (EIUB) is introduced and the burial history is investigated in order to determine the reservoir characteristics and origin. We also identify what controls the gas pool accumulation in the thick intermediate volcanostratigraphy. According to the reworked volcanic rocks and weathered crust patterns, the volcanostratigraphy of the basin has 13 wells containing one EIUB, and 2 wells with two EIUBs. The exposed and burial histories of the volcanostratigraphic sequence have produced a variety of pore space including primary pores, secondary pores and secondary fractures. The secondary pores are the most dominant, followed by primary pores and finally fractures. These pore spaces form a reservoir with moderate to low porosity and permeability and the most productive reservoirs are located 40–80 m below the EIUB. The secondary pores are primarily controlled by weathering and leaching as well as deep-burial alteration. Due to the migration of fluids from the overlying strata into the volcanic rocks of the Huoshiling Formation below, the top most layer dissolves first, which significantly enhances the secondary porosity of this layer found 40–80 m below the EIUB. Thus, the number of EIUBs should control the number of favourable reservoir layers. Based on analysis of the reservoir, source and cap rocks in the study area, we conclude that the best exploration targets in these thick volcanostratigraphic sequences, specifically in half-graben basins, are the volcanic inherited-uplift slope areas, followed by the inherited-uplift of the sag area and the uplifted zones. The findings presented in this work provide an insight of the evolution of porosity in other volcanic systems elsewhere and contribute to the likelihood of locating the favourable exploration targets for thick volcanostratigraphic sequences in half-graben basins.

Applicability of heat-exchanger theory to estimate heat losses to surrounding formations in a thermal flood

Heat losses to cap and base rocks undermine the performance of a thermal flood. As a contribution to this subject, this paper investigates the applicability of the principles of heat exchanger to characterise heat losses between a petroleum reservoir and the adjacent geologic systems. The reservoir-boundary interface is conceptualised as a conductive wall through which the reservoir and adjacent formations exchange heat, but not mass. For a conduction-dominated process, the heat-transport equations are formulated and solved for both adiabatic and non-adiabatic conditions. Simulations performed on a field-scale example show that the rate of heating a petroleum reservoir is sensitive to the type of fluids saturating the adjoining geologic systems, as well as the characteristics of the cap and base rocks of the subject reservoir. Adiabatic and semi-infinite reservoir assumptions are found to be poor approximations for the examples presented. Validation of the proposed model against an existing model was satisfactory; however, remaining differences in performances are rationalised. Besides demonstrating the applicability of heat-exchanger theory to describe thermal losses in petroleum reservoirs, a novelty of this work is that it explicitly accounts for the effects of the reservoir-overburden and reservoir-underburden interfaces, as well as the characteristics of the fluid in the adjacent strata on reservoir heating. These and other findings should aid the design and management of thermal floods.

Las Pailas geothermal field - Central America case study: Deciphering a volcanic geothermal play type through the combination of optimized geophysical exploration methods and classic geological conceptual models of volcano-tectonic systems

Sustainable exploitation strategies of high-enthalpy geothermal reservoirs in a volcanic geothermal play type require an accurate understanding of key geological structures such as faults, cap rock and caldera boundaries. Of same importance is the recognition of possible magmatic body intrusions and their morphology, whether they are tabular like dikes, layered like sills or domes. The relative value of those magmatic bodies, their age, shape and location rely on the role they play as possible local heat sources, hydraulic barriers between reservoir compartments, and their far-reaching effect on the geochemistry and dynamics of fluids. Obtaining detailed knowledge and a more complete understanding at the early stages of exploration through integrated geological, geophysical and geochemical methods is essential to determine promising geothermal drilling targets for optimized production/re-injection schemes and for the development of adequate exploitation programs. Valuable, extensive geophysical data gathered at Las Pailas high-enthalpy geothermal field at northwestern Costa Rica combined with detailed understanding of the geological structures in the underground may represent a sound basis for an in-depth geoscientific discussion on this topic. Currently, the German cooperation for the identification of geothermal resources in Central America, implemented by the Federal Institute for Geosciences and Natural Resources (BGR), supports an international and interdisciplinary effort, driven by the Instituto Costarricense de Electricidad (ICE) with different international and national research institutions, including the Leibniz Institute for Applied Geophysics (LIAG). The discussions and joint studies refer to the optimized utilization of geophysical and geological methods for geothermal exploration in the Central American region, using the example of Las Pailas Geothermal Field. The results should contribute to a better understanding of the most appropriate geothermal exploration concepts for complex volcanic field settings in Central America.

The effect of temperature on 3D rock structure of andesite rock sample from potential geothermal area, Sulawesi-Indonesia

An andesite rock sample was taken from potential geothermal area on Lili-Sepporaki, west Sulawesi. The sample is an impermeable cap rock with a high silica mineral at alteration zone around a manifestation. Its Three-dimensional rock structure was analyzed to see its deformation due to temperature change. Change of temperature from 24˚C to 100˚C, 150˚C, 200˚C, and 250˚C was applied to the sample. Subsequent to each change of temperature, the sample was scanned using a micro-Computed Tomography Scan (μ-CT Scan). From the scanning process, a sub-volume 300 pixels were reconstructed, and its 3D pore and mineral structure were characterized. It is found that porosity {φ (T)}, specific surface area {SsA (T)} decrease polynomial for pore structure and increase polynomial as temperature increases. This result show that as temperature increases the volume of mineral is bigger make the pore volume smaller. The Fractal dimension for pore structure (2.65 ± 0.02) and for mineral structure (2.89 ± 0.015).

The effect of uniaxial pressure on development on 3D fracture and its properties of clay cap rock on one potential geothermal area in Candi Umbul-Telomoyo, Semarang-Indonesia

A Sample of clay cap rock on geothermal area in Candi Umbul-Telomoyo, Semarang has been digitized to the observe 3D fracture development due to the influence of uniaxial pressure. The 3D fractures were analysed using digital image processing and analysis method. The sample is an altered cases andesite breccia with diameter of 4.4 cm and height of 4.3 cm. The rock sample was collected from well at 590 m. The rock sample was given uniaxial pressure treatment of 59 bar, 75 bar, 91 bar, 107 bar, 123 bar and 156 bar. The results show that 3D fracture develops as pressure increase are total orientation {θ (P)}, aperture {e (P), fracture density {Φ (P)}, fracture intensity {I (P)} and fractal dimension {D (P)}. Total orientation and fracture intensity have polynomial to relative to the given pressure. Aperture and fractal dimension are linearly proportional to the give pressure, while the fracture density is exponentially proportional to the give pressure. The effect of increasing pressure to the safety of this geothermal clay cap was studied by conducting a continuous uniaxial compressive test on the sample. From this test, it is observed that the sample broken at the pressure of 478 bar.

Internal Structure of a Seafloor Massive Sulfide Deposit by Electrical Resistivity Tomography, Okinawa Trough

AbstractAlthough seafloor massive sulfide (SMS) deposits are crucially important metal resources that contain high‐grade metals such as copper, lead, and zinc, their internal structures and generation mechanisms remain unclear. This study obtained detailed near‐seafloor images of electrical resistivity in a hydrothermal field off Okinawa, southwestern Japan, using deep‐towed marine electrical resistivity tomography. The image clarified a semi‐layered resistivity structure, interpreted as SMS deposits exposed on the seafloor, and another deep‐seated SMS layer at about 40‐m depth below the seafloor. The images reinforce our inference of a new mechanism of SMS evolution: Upwelling hydrothermal fluid is trapped under less‐permeable cap rock. The deeper embedded SMS accumulates there. Then hydrothermal fluids expelled on the seafloor form exposed SMS deposits.

Timing and origin of natural gas accumulation in the Siljan impact structure, Sweden

Fractured rocks of impact craters may be suitable hosts for deep microbial communities on Earth and potentially other terrestrial planets, yet direct evidence remains elusive. Here, we present a study of the largest crater of Europe, the Devonian Siljan structure, showing that impact structures can be important unexplored hosts for long-term deep microbial activity. Secondary carbonate minerals dated to 80 ± 5 to 22 ± 3 million years, and thus postdating the impact by more than 300 million years, have isotopic signatures revealing both microbial methanogenesis and anaerobic oxidation of methane in the bedrock. Hydrocarbons mobilized from matured shale source rocks were utilized by subsurface microorganisms, leading to accumulation of microbial methane mixed with a thermogenic and possibly a minor abiotic gas fraction beneath a sedimentary cap rock at the crater rim. These new insights into crater hosted gas accumulation and microbial activity have implications for understanding the astrobiological consequences of impacts.

The cap rocks sealing the reservoir in the Węglówka oil field, sub-Silesian unit, Polish outer Carpathians: Petrographical approach

The Weglowka oil field is located in the outer Carpathians. The outer Carpathians are a region where hydrocarbons were discovered and exploited at the end of the nineteenth century in several dozen oil fields, which are relatively small. The Weglowka oil field is one of the largest in this region. In the 150 yr or so of hydrocarbon exploration in the area, more than 1 million t (>1,237,000 tons [>8,841,000 bbl]) of oil have been produced. Hydrocarbons are concentrated in Lower Cretaceous sandstones (Grodziszcze and Lgota sandstones) that form an anticline sealed by Upper Cretaceous marls called the Weglowka marls. These cap rocks are up to 600 m (2000 ft) thick. Because of the thrust-related exhumation, they were exposed at the surface and represent the youngest deposits in the region. The present work is focused on a detailed petrographic characterization of the Weglowka marls. This study allows petroleum geologists to better understand the evolution of porosity in these cap rocks and can serve as a foundation for the prediction of their sealing properties. The marls appear as a succession of interbedded red and green varieties, which occur in up to 2-m (6-ft)-thick beds. These beds are nonarenaceous, soft, and bioturbated. Grain size corresponds to approximately 80% clay and less than 20% silt fractions. X-ray diffraction (XRD) reveals that the marls contain, on average, 54% clay, 28% calcite, 16% quartz, up to 3% feldspars and, in red marls, 3% hematite. The XRD patterns of clay are typical of mixed-layer illite–smectite ([I–S]; 40% illite in I–S). The clay structures are dioctahedral with similar octahedral Mg and relatively high Fe3+ contents both in the red and green intervals. As revealed by standard petrography combined with high-resolution petrography performed through the use of a field emission scanning electron microscope, the marls have mudstone textures according to Dunham’s (1962) classification and are mostly composed of coccoliths and clay with rare nanoquartz. This rock may be considered an impure chalk. Sealing properties of the Weglowka marls are indicated by the specific surface area, porosity, pore size, and permeability, calculated using N2 gas adsorption, helium, and mercury porosimetry. The sealing potential is postulated to result from a combination of the following: (1) origin of components (i.e., deposition of minute calcareous bioclasts and volcanic material as a source for clay); (2) oxygenated sedimentary environment (as a result of the presence of oxygen in the sediments, burrowing caused the rocks to be homogenized); and (3) tectonic-induced clogging of pore space because of reorganization of clay flakes (the rocks were strongly tectonically deformed, which resulted in reduction of porosity in clay aggregates).