Fluid Migration Paths Research Articles

Topological Characteristics of the Pore Network in the Tight Sandstone Using Persistent Homology.

Tight sandstone reservoirs have become important areas for unconventional reservoir development, and their pore network is a key feature for identifying tight sandstone, which affects fluid migration path and reservoir development efficiency. However, the connectivity characteristics of the pore network at different scales have remained unclear owing to the numerous pores and uneven pore shape. Here, using pore size distributions from many hundreds of tight sandstone samples and subsequent topological data analysis, we construct the topological structure of the pore network in the Yanchang Formation tight sandstone of the Ordos Basin in China and visualize the topological characteristics of the pore network with distances. We show that there are three connected groups within the pore structure of the tight sandstone. The topology of the pore network resides on a trident ring manifold, suggesting that the pore network in the tight sandstone encompasses three obvious dominant connection paths. One prominent bar on the H0 dimension in the barcode indicates a two-point connection from nanoscale to microscale in the pore network. Three prominent bars with varying durations on the H1 dimension indicate the presence of three separate multipoint connections within a limited extent in the pore network. Connectivity of combined pores is good and controlled by the topological structure of the pore network. This demonstration of pore connections on a trident ring manifold provides a population-level visualization of the pore network in the tight sandstone.

3D Reconstruction of Pore and Fracture Structures in Crushed Soft Coal with Low Permeability and Its Permeability Analysis: Implications for the Design of CO2 Injection and CH4 Production.

In order to improve the CO2 injection and CH4 production efficiencies during the CO2-ECBM process, it is necessary to clarify the relationship among the complexity of pore and fracture structures, the typicality of the fluid migration path, and the heterogeneity of reservoir permeability. In this study, crushed soft coal with low permeability from Huainan and Huaibei coalfields of China was taken as the research object. First, the three-dimensional (3D) visualization reconstruction of pore and fracture structures was realized. Second, the equivalent pore and fracture network model was constructed. Finally, the permeability evolution and its anisotropy of the coal reservoir were dynamically demonstrated. In this study, the implication of surface porosity on the heterogeneity of pore and fracture structures was first discussed, followed by the implication of coordination number on the anisotropy of fluid flow, and finally, the influence of the anisotropy of fluid flow on the CO2-ECBM process was discussed. The results show that the equivalent pore and fracture network models of the reservoir structure can be constructed based on the digital rock physics technology. The analysis results of porosity, interconnected porosity, typical path of fluid migration, absolute permeability, and surface porosity of each sample have good consistency in characterizing the complexity of pore and fracture structures and the heterogeneity of permeability. The average coordination numbers of RL and LZ samples are 5.99 and 5.78, respectively, and the number of pores and throats is well-balanced, which indicates that LZ and RL collieries are suitable for the development of CO2-ECBM industrial tests. When the interconnected pores and fractures are mainly developed vertically and horizontally, the construction of drilling technology of the CO2-ECBM process should be mainly designed for vertical wells and horizontal wells, respectively. This study has important theoretical and practical significance for the industrial testing and commercialization of CO2-ECBM technology in crushed soft coal with low permeability.

Technology Focus: Reservoir Surveillance (September 2023)

The energy transition and global geopolitical situation present numerous oil and gas operators with opportunities to accelerate production growth and monetize hydrocarbon resources. While rapid production growth comes with proportional risks, it becomes crucial for reservoir surveillance engineers to quickly identify, derisk, and mitigate subsurface performance uncertainties. In this demanding scenario, the need for high-frequency reservoir performance surveillance is more critical than ever. Fortunately, advanced technologies have emerged as a refuge for subsurface engineers facing these challenges. One such technology is fiber optics, which has matured significantly over the past two decades, allowing for real-time well-performance overviews in various applications. This capability enhances well-management efficiency and enables prompt corrective actions, as demonstrated in papers SPE 200088 and SPE 200806. Along similar lines, the dynamic nature of the industry has also driven subsurface practitioners to think innovatively and apply existing technologies for out-of-the-box solutions. For instance, tracers, traditionally used for understanding subsurface connectivity, are now being used to calibrate surface networks and optimize gas lift injection performance. Such unconventional approaches have been explored in paper SPE 207431, highlighting their effectiveness. In addition to tracers, technologists are turning rock cuttings, often considered mere waste, into valuable information. As demonstrated by paper SPE 206214, these insights aid in elucidating depositional sequences and fluid-migration paths, thereby facilitating better field development decisions. Furthermore, technology groups are working on innovative solutions that use advanced sensing and artificial-intelligence (AI) -based technologies. As a result, interventionless downhole pressure and temperature surveying are no longer distant dreams (paper IPTC 22255). In another example, the power of AI has elevated sensing intuition, optimizing reservoir and fluid characterizing operations and leading to improved drilling-cost efficiency and the quality of well delivery (paper SPE 210091). Embracing advanced technologies is vital for oil and gas operators to optimize production, reduce risks, and make informed decisions in the evolving energy landscape. From real-time fiber optic monitoring to unconventional tracer applications and AI-driven solutions, reservoir surveillance engineers now have an array of tools at their disposal to drive the industry toward a more efficient and sustainable future. Recommended additional reading at OnePetro: www.onepetro.org. SPE 200806 An Industry Overview of Downhole Monitoring Using Distributed Temperature Sensing: Fundamentals and 2 Decades Deployment in Oil and Gas Industries by Mohammad Soroush, RGL Reservoir Management, et al. SPE 206214 Reservoir Architecture and Fluid Connectivity in an Abu Dhabi Oil Accumulation by Erik Tegelaar, Triple EEE, et al. IPTC 22255 Sensor Ball: Field Deployment of Autonomous and Untethered Surveillance by Mohamed Larbi Zeghlache, Saudi Aramco, et al. SPE 211543 The Sensor Ball Revolutionizes Wireline Operations by Nasser M. Al-Hajri, Saudi Aramco, et al.

Uncovering soil piping vulnerability using direct current geophysical techniques in Awka, Anambra State, Nigeria

In recent times, there have been increasing reports of soil subsidence occurring in various parts of Anambra State, Nigeria. The formation of soil pipes in the subsurface, which has been reported by many researchers globally, is the chief cause of these subsidence incidents. Thus, this research aims to use Direct Current (DC) geophysical techniques to delineate the spatial distribution, pattern, and characteristics of these soil pipes at two sites: Awka Site 1 and Awka Site 2 both in Awka South LGA of Anambra state, Nigeria. Two electrodes array configurations - the dipole-dipole array for analyzing the Electrical Resistivity Tomography (ERT) and the Schlumberger array for analyzing the Vertical Electrical Sounding (VES) - were employed. Nine ERT profiles and eighteen soundings were carried out. The results from the ERT survey divided the subsurface into six distinct structures. The high-resistivity 3000-30000Ωm, the eroded structure with dry pore spaces, which mainly occur at the top of the profile, was interpreted as having the right conditions for the formation of soil pipes. The result from the VES survey revealed two to five different geo-electrical sections and fourteen distinctive sounding curves that are characterized by the vertical changes in the subsurface. The weathered soil with resistivity ranging from 1200-30000Ωm, which aggregates very close to the surface 0-5m of the study areas, was interpreted as having the best soil formations that allow the building of soil pipes. The 2D and 3D iso-resistivity maps obtained from the data of the VES show that soil pipes follow the NW-SE direction. This coincides with the stress direction, fluid migration paths, and sloppy terrains of the study areas.

Experimental study on compatibility of reservoir and polymer gel profile control agent

The coupling relationship between polymer gel system and reservoir pores is an important basis for scientific formulation of tertiary oil recovery programs. The polymer gel profile control agent can significantly modify the fluid migration path inside the reservoir. In this study, artificial cores were used to carry out indoor water flooding experiments after polymer gel injection that considering formation permeability, polymer concentration and polymer injection rates. Based on the accurate records of the breakthrough pressure and injection volume during water flooding, as well as a large number of observations of the microscopic morphology of the cores after the polymer gel profile control and displacement, a set of criteria for determining reservoir-matched polymer gel profile control agents is proposed. The experimental results show that a complex spatial network structure is formed after the gelation of the polymer gel system. As the polymer concentration increases, the network structure of the gel system becomes denser. At the same time, the unit network structure is becoming smaller, and the order of the grid chain structure becomes clearer. Under the condition of low polymer concentrations, the backbone structures of the gel are obvious, but there are a lot of fine pores. When the polymer concentration is greater than 1,500 mg/L and the injection rate is less than 0.15 ml/min, the water flooding breakthrough pressure is greater than 0.2 MPa, the plugging rate is greater than 95%, and a good plugging effect is obtained.

Effect of Pore Structure and Capillary Number on Gas-Water Flow Patterns in Carbonate Rocks

Summary Rock pore structure and capillary number determine gas-water flow through limestone. Limestone is of key importance for natural gas production and gas storage; however, there is still limited direct evidence on the precise influence of the pore space morphology (vugs, fractures, and homogeneous pore matrix) and capillary number. Here, we thus studied gas-water flow patterns in various limestones via in-situ X-ray microtomography combined with numerical flow simulations. Pore structure heterogeneity significantly affected the fluid migration path. Gas flowed rapidly through large pores and vugs but flowed slowly through microfractures. In contrast, water flowed through microfractures and small pores but did not enter large pores at low capillary number conditions. Water flow simulations [performed for different capillary numbers directly on microcomputed tomography (μ-CT) images] demonstrated that snap-off and dead-end corners controlled the distribution of residual gas, consistent with the experiments. In addition, the simulations showed that less residual gas distributed around dead-end corners at a low capillary number, and a part of the residual gas can be displaced by increasing viscous forces. Moreover, a power-low relationship between gas cluster volume and surface area was observed, and the gas cluster size distribution could also be fitted with a power-law correlation. In all types of tested limestones, the power-law exponents (p ≈ 0.77, τ ≈ 0.86) were lower than that predicted by simple percolation theory (which predicts p ≈ 1, τ ≈ 2.189). There was evidence that a simple percolation model was unlikely to provide reliable predictions in homogeneous porous media, and we further extended the application scope of this conclusion to heterogeneous porous media. This work therefore provides fundamental data and improves fundamental understanding of gas-water flow through limestones and aids in the further advancements of improved hydrocarbon recovery and gas storage in limestone reservoirs.

Interpretation of Subsurface Fault Through Multi-Level Second Vertical Derivative Gravitational Data in Bittuang Geothermal Working Area, South Sulawesi, Indonesia

The research was conducted in Bittuang, Tana Toraja Regency, South Sulawesi Province, as one of the geothermal prospect areas and targets for the initial stage of the Government exploration drilling program for the 2020-2024 period. One aspect of geothermal is the manifestation control structure as a fluid migration path from below the surface. Therefore, identification of existing structures in the Bittuang geothermal area was carried out and confirmed the surface geological structure contained in the Bittuang geothermal geological map. In determining the presence of a fault and knowing its characteristics such as the type of fault, the direction of the dip, and the magnitude of the dip of the fault, the gravity data is processed using the multi-level second vertical derivative (ML-SVD) method. To strengthen the interpretation, the results from the ML-SVD were matched with the data from the horizontal gradient (HG) method and the geological data of the structure of the study area. From this process, there are 27 faults in the Bittuang geothermal area, two of which are indicated as controlling faults for the manifestation of the Balla group and the Cepeng group. This research is expected to describe faults in the Bittuang geothermal area, which can support detailed exploration activities.

Spatial and temporal multiplet analysis for identification of dominant fluid migration path at The Geysers geothermal field, California

Multiplet analysis is based on the identification of seismic events with very similar waveforms which are used then to enhance seismological analysis e.g. by precise relocation of sources. In underground fluid injection conditions, it is a tool frequently used for imaging of subsurface fracture system. We identify over 150 repeatedly activated seismic sources within seismicity cluster induced by fluid injection in NW part of The Geysers geothermal field (California). Majority of multiple events (ME) occur along N–S oriented planar structure which we interpret as a fault plane. Remaining ME are distributed along structures interpreted as fractures, forming together a system of interconnected cracks enabling fluid migration. Temporal analysis reveals that during periods of relatively low fluid injection the proportion of ME to non-multiple events is higher than during periods of high injection. Moreover, ME which occur within the fault differ in activity rate and source properties from ME designating the fractures and non-multiple events. In this study we utilize observed differences between ME occurring within various structures and non-multiple events to describe hydraulic conditions within the reservoir. We show that spatial and temporal analysis of multiplets can be used for identification and characterization of dominant fluid migration paths.

ДИНАМІКА РОЗВИТКУ ЛІТОФЛЮЇДНИХ НАФТОГАЗОВИХ СИСТЕМ У КРЕЙДОВИХ ВІДКЛАДАХ ЗАХІДНОЇ ЦЕНТРИКЛІНАЛІ ПРИЧОРНОМОРСЬКОГО МЕГАПРОГИНУ

On the basis of modeling the dynamics of the catagenesis of the Cretaceous deposits, the lateral heterogeneity of the history of their catagenesis within the western part of the Black Sea megadepression, caused by the tectonic regime of the blocks separated by the Golitsyn, Sulino-Tarkhankutsky, Gubkino-Donuzlavsky sublatitudinal faults, was established. The spatio-temporal features of the development of reservoirs and migration paths of hydrocarbon fluids have been established, the predicted zones of oil and gas accumulation have been localized. There are four types of lithogenetic reservoirs, which differed in the time of formation, structural features, fluid regime and the nature of oil and gas saturation.In the first cycle of catagenesis, the generation center was localized mainly in the sediments of the Lower Albian (t 80–100 оС), in the second – the Middle Albian (t 100–120 оС), in the third – the Upper Albian (t 130–150 оС).Given a certain difference in the situations of sedimentation of these stratigraphic units, the composition and content of scattered organic matter in the sediments were different. This, as well as the different generation temperatures, obviously caused the specific composition of hydrocarbon fluids formed during individual cycles of catagenesis. Along the migration pathways and during structural changes, these fluids were mixed, multicomponent systems were formed, and the composition of hydrocarbons changed.The possibility of the existence of subvertical "vein" areas of oil and gas accumulation with a width of up to 10 km and a height of 1,5–2,0 km is argued. Such reservoirs, on the one hand, may contain deposits and, on the other hand, supply hydrocarbons to overlying traps. Given the nature of oil and gas accumulation, the potential resources and, consequently, the prospects for oil and gas in the region can be much higher.

Design and plugging property of composite pressure activated sealant

Pressure activated sealant (PAS), as a novel kind of sealing fluid, can provide rapid and self-adapting repair on the tiny damage of static seals (e.g., screws and pipelines) while encountering pressure difference of leakage sites, but its sealing property is affected more by the size of leakage site, so it exhibits limited application in the production and wellbore integrity recovery of oil & gas wells. To extend PAS's applicability, this paper put forward a composite PAS technology involving the cooperativity of solid and liquid sealing materials for enhancing large pore plugging based on the phase state transition and sealing behaviors of sealing fluid under differential pressure. In this technology, two types of solid sealing materials (bridging rigid particle and tension reinforcing fiber) were used jointly to modify the geometries of leakage pores and the migration paths of fluids, which can promote the formation of effective solid barrier from the PAS to fill in large leaking pores, so as to perform sealing repair. And the following laboratory experimental results were obtained. First, the newly prepared PAS is a kind of polydisperse system, and the core particles are regular in shape with the size distribution mainly in a range of 300–400 μm. Second, in the range of evaluation pressure, sepiolite fiber, as the structure promoter, reinforces the integrity of sealing solids. Third, mussel powder, as the pore structure modifier, improves the retention ability of the sealing fluid in the limited leakage space. These two sealing materials not only cooperatively improve the structure of solid barrier, but also markedly enhance the plugging capability of composite PAS in large pores. In conclusion, these results can provide theoretical and technical support for the development and application of novel multifunctional sealants in oil & gas wells.

High-resolution Integrated Geophysical Investigation at the Lancaster Gold Mine, Krugersdorp, South Africa

An integrated geophysical approach using seismics and geoelectrical techniques was employed to investigate the architecture of historic narrow-reef workings and a proposed open-pit mine at Lancaster Gold Mine, near Krugersdorp, South Africa. The mining activities in the area were mainly carried out within the Kimberley Reef Package in the upper Central Rand Group of the Witwatersrand Supergroup, which hosts several gold-bearing conglomerates (locally known as reefs). The reefs are generally thin (≤ 2 m thick) and dip between 28° and 32° south. The low-velocity weathered layer introduces significant static shifts in the reflection seismic data. Moreover, environmental noise from drilling and trucking, and the prominent bedrock-overburden contact that produces various wave conversions (P-S conversion), caused undesirable noise that contaminates the shot gathers as high-amplitude, source-generated and monochromatic noise. The noise was removed from the shot gathers using frequency and velocity filtering techniques. The final depth-migrated sections are characterised by high-resolution images of the subsurface from ~10 to ~150 m depth, which are constrained by the borehole information. The reflection seismic data delineate the interfaces between different rock layers and the stopes. The refraction and resistivity tomograms, on the other hand, provide more detailed images of the top 20–50 m of the subsurface and depict the approximate shallow geometry of fluid migration paths, mined-out areas, and bedrock-overburden boundaries. The integrated results indicate that the study area is characterised by a weathered surface layer with variable low P-wave velocity (400–1200 m/s) and resistivity (150–800 Ωm). The deeper layer reveals an increase in resistivity and velocity, and it’s characterized by discontinuities, weak zones, cavities or water-bearing zones due to the mining activities. The combined borehole and geophysical data provide valuable information regarding the physical characteristics of the subsurface and can be helpful for future risk management decisions, environmental and engineering studies in the area.

Tracing deep fluids in the Bozhong Sag: Evidence from integrated petrography and geochemistry data

Abstract The Bozhong Sag is a mature exploration area for petroleum in the Bohai Bay basin, China, which is a typical oil-bearing basin and hosts the Penglai (PL) 19-3, and Qinhuangdao (QHD) 32-6 giant oil fields (Xue, 2018). Despite 60 years of hydrocarbon exploration in the basin, only few large gas fields have been discovered. The discovery of the Bozhong (BZ) 19-6 gas field not only indicated that the Bozhong Sag has great potential for natural gas exploration, but also raised questions regarding the formation of large-scale natural gas fields in typical oil-prone basins and in sapropelic source rocks. The present study therefore performs a detailed investigation of the sources and migration pathways of the deep fluids in the Bozhong Sag area as a means of shedding light into this scientific challenge. The Bozhong Sag is located in the thinnest crust and tectonic transition zone of the Bohai Bay basin. Intense extension and thinning of lithosphere led to the upward ejection of deep crust and mantle materials under diapirism. Improvements in this tectonic setting and the fluid migration path occurred as a result of early faults that were formed by compressive thrust in the Indosinian-Yanshan orogenies in addition to a process of extensional development and reactivation of Himalayan faults. We obtained cores from 13 wells in the Bozhong Sag, and subsequently observed their petrography and analyzed samples for fluid inclusions. Also, volcanic rock samples were analyzed for major and trace elements, and carbonate rock samples were analyzed for δ13C, δ18O, and 87Sr/86Sr. Previously reported δ13CCO2 and He isotopic data were are also discussed. Our results showed that petrography was characterized by thermal fading and hydrothermal breccia around fractures filled with quartz and calcite. Deep fluid activity was mainly reflected by petrographic assemblages of quartz, ankerite, pyrite, plus calcite and volcanic eruptive rocks that were i) enriched with large ion-lithophile elements (LILE), ii) enriched with light rare-earth elements (LREE), iii) relatively deficient in high-field strength elements (HFSE), and iv) relatively deficient heavy rare-earth elements (HREE). Thermal anomaly events were reflected by the homogenization temperature of fluid inclusions, and characteristics of magmatic hydrothermal fluids were exhibited by δ13C, δ18O, and 87Sr/86Sr. Our findings indicated that deep fluids were mainly derived from the deep parts of the upper mantle, and that they were slightly contaminated by crustal materials. Combining our results with those of previous studies on deep xenoliths and xenolithic fluid compositions in eruptive rocks of eastern China (Zhang et al., 1999; Han et al., 2008), we interpreted that deep fluid properties in the study region were characterized by high H2 and low total volatile gases. We conclude that the deep fluids in the Bozhong Sag was mainly derived from deep material in the upper mantle, and that the fluid activity pattern was aided by the deep fracture-fault systems in addition to central and fissure-type wide-area eruption activities.

Hydrothermal origin of early Permian saddle dolomites in the Junggar Basin, NW China

Hydrothermal-related saddle dolomites widely exist in the lower Permian Fengcheng Formation in the northwestern Junggar Basin, and this formation is a set of carbonate-rich deposits in an alkaline saline basin. Core and thin section observations, cathodoluminescence (CL) imaging, and carbon and oxygen isotope composition analyses have been used to study the origins of saddle dolomites with various occurrences. Hydrothermal fluids include Si-rich and Si-poor fluids, and saddle dolomites occur, along with siliceous bands, as debris and assemblages in fragmented rocks and as cements filling pores and microfractures. Different types of dolomites in the Fengcheng Formation have similar δ13C values, while the δ18O values differed from each other, which demonstrated that the dolomites originated from the same primary carbonates and underwent multiple stages of recrystallization. The δ18O values were extremely negative, which was caused by fractionation at high temperatures from hydrothermal fluids. Dolomites are directly or indirectly affected by hydrothermal diagenetic fluids. According to the diagenetic stages, the types of migration paths for hydrothermal fluids, and the patterns of hydrothermal fluids acting on the dolomites, a model was proposed to illustrate the origins and evolution of the dolomites, and three paths were utilized. Path I occurred during the contemporaneous diagenetic stage, when the carbonate particles were still free and could be captured by high-density bottom hydrothermal flows and distorted. Path II occurred during the long diagenetic history after the carbonate-bearing sediments had consolidated. Saddle dolomite assemblages and saddle dolomite debris occurred with the aid of compressive tectonic forces. Path III also occurred in the long diagenetic history, and the host rocks were not fragmented. The saddle dolomites reprecipitated as cements in the pores, stylolites, fractures/microfractures or laminated seams.

Insight into diagenetic processes from authigenic tourmaline: An example from Carboniferous and Permian siliciclastic rocks of western Poland

Authigenic tourmaline is a rare and minor component of siliciclastic rocks, however, once it occurs, it can be used to interpret a number of subsurface parameters and processes, such as fluid composition, redox state, salinity, pH, and fluid migration paths. Notwithstanding, the conditions of tourmaline authigenesis still need to be recognize with greater clarity. Carboniferous and Permian siliciclastic rocks in the subsurface of western Poland in the vicinity of the Dolsk Fault show unusually high boron contents, at places surpassing 1000 μg/g, which are associated with the presence of authigenic tourmaline. The mineral forms fine, elongated crystals, of nano- to micrometer width, grouped in radiating or semi-parallel aggregates, which are dispersed in the clay matrix of shales and graywackes, coat grain surfaces and line secondary pores in porous sandstones, replace some grains in volcaniclastic breccia, and overgrow detrital tourmalines in quartzarenites. Regardless of host rock compositions, the tourmaline is Si-deficient, moderately Al-rich, and shows significant X-site vacancies and highly variable Mg/(Mg + Fetot) ratios. In rare zoned tourmaline, the Mg/(Mg + Fetot) ratios distinctly decrease and vacancies slightly increase towards the growth direction. The results suggest that the tourmaline nucleated rapidly under a strong driving force at high boron concentration and high temperature. The fluids which enabled the process were acidic, reducing, B- and Mg-bearing, and of low Na and Ca contents. The tourmaline grew primarily at the expense of kaolinite, possibly of other clay minerals and K-feldspar, in the temperature range ~150–200 °C. Its crystallization was a short episode in the diagenetic history of the host rocks. Tourmaline authigenesis across the Carboniferous/Permian unconformity, in the proximity of a prominent fault zone, is evidence for fluid and heat migration along faults, and for temporarily open geochemical system during late diagenesis. The source of boron has been unproven, yet the circulation of bitterns originated from Upper Permian evaporites was highly probable.

Hybrid multiplicative time-reversal imaging reveals the evolution of microseismic events: Theory and field-data tests

The generation of microseismic events is often associated with induced fractures/faults during the extraction/injection of fluids. A full characterization of the spatiotemporal distribution of microseismic events provides constraints on fluid migration paths in the formations. We have developed a high-resolution source imaging method — a hybrid multiplicative time-reversal imaging (HyM-TRI) algorithm, for automatically tracking the spatiotemporal distribution of microseismic events. HyM-TRI back propagates the data traces from groups of receivers (in space and time) as receiver wavefields, multiplies receiver wavefields between all groups, and applies a causal integration over time to obtain a source evolution image. Using synthetic and field-data examples, we revealed the capability of the HyM-TRI technique to image the spatiotemporal sequence of asynchronous microseismic events, which poses a challenge to standard TRI methods. Moreover, the HyM-TRI technique is robust enough to produce a high-resolution image of the source in the presence of noise. The aperture of the 2D receiver array (azimuth coverage in 3D) with respect to the microseismic source area plays an important role on the horizontal and vertical resolution of the source image. The HyM-TRI results of the field data with 3D azimuthal coverage further verify our argument by producing a superior resolution of the source than TRI.

Deformation band formation as a function of progressive burial: Depth calibration and mechanism change in the Pannonian Basin (Hungary)

Abstract Deformation bands (DB) are ubiquitous structural elements that can be found in Miocene pre- and syn-rift sediments of the extensional Pannonian Basin, central Europe. Across the field sites we see examples of disaggregation to cataclastic DB sets and evidence of frequent reactivation by discrete faulting. Thin section analysis, cross-cutting relationships and well-defined time constraints of analysed brittle structures demonstrate that with increased burial depth, DB deformation mechanisms progressed from granular flow to cataclasis. The DB sets were classified into 10 deformation phases based on formerly published independent fault-slip analysis and structural mapping. Subsidence curves were constructed for each stratigraphic level involved in deformation and were used along with the intersection of deformation episodes to calculate the depth intervals of DB generations and subsequent structural elements. The DB formation depths obtained were transferred to the depth range of the related mechanisms. This combined methodology permits a more quantitative approach to determine the changes in the deformation mechanisms with depth. Our results show that granular flow (disaggregation bands) dominates down to 100–150 m as the earliest deformation structure, followed by weak then moderate cataclasis. The transition between weak and moderate cataclasis is at approximately 300 ± 100 m for host rock rich in feldspar or fragile tuffitic components and from around 900 ± 100 m in quartz-rich sediments. In addition, deformation by frictional sliding concentrates on discrete fault planes at the margin of cataclastic bands or on new fracture planes from ∼500 ± 100 m in volcanoclastic or feldspar rich host rocks, and 1000 ± 100 m in quartz-rich host rocks. We suggest that burial-induced diagenetic processes is dependent on subsidence history, and partly in connection with regional fluid migration path, control the transition from moderate or advanced cataclasis to discrete fault slip. All these changes affect the pore structure and porosity that contribute to rheological changes, and hence change in deformation mechanism of coeval fracturing events.

Multidisciplinary investigation on cold seeps with vigorous gas emissions in the Sea of Marmara (MarsiteCruise): Strategy for site detection and sampling and first scientific outcome

MarsiteCruise was undertaken in October/November 2014 in the Sea of Marmara to gain detailed insight into the fate of fluids migrating within the sedimentary column and partially released into the water column. The overall objective of the project was to achieve a more global understanding of cold-seep dynamics in the context of a major active strike-slip fault. Five remotely operated vehicle (ROV) dives were performed at selected areas along the North Anatolian Fault and inherited faults.To efficiently detect, select and sample the gas seeps, we applied an original procedure. It combines sequentially (1) the acquisition of ship-borne multibeam acoustic data from the water column prior to each dive to detect gas emission sites and to design the tracks of the ROV dives, (2) in situ and real-time Raman spectroscopy analysis of the gas stream, and (3) onboard determination of molecular and isotopic compositions of the collected gas bubbles. The in situ Raman spectroscopy was used as a decision-making tool to evaluate the need for continuing with the sampling of gases from the discovered seep, or to move to another one. Push cores were gathered to study buried carbonates and pore waters at the surficial sediment, while CTD-Rosette allowed collecting samples to measure dissolved-methane concentration within the water column followed by a comparison with measurements from samples collected with the submersible Nautile during the Marnaut cruise in 2007.Overall, the visited sites were characterized by a wide diversity of seeps. CO2- and oil-rich seeps were found at the westernmost part of the sea in the Tekirdag Basin, while amphipods, anemones and coral populated the sites visited at the easternmost part in the Cinarcik Basin. Methane-derived authigenic carbonates and bacterial mats were widespread on the seafloor at all sites with variable size and distributions. The measured methane concentrations in the water column were up to 377 μmol, and the dissolved pore-water profiles indicated the occurrence of sulfate depleting processes accompanied with carbonate precipitation. The pore-water profiles display evidence of biogeochemical transformations leading to the fast depletion of seawater sulfate within the first 25-cm depth of the sediment. These results show that the North Anatolian Fault and inherited faults are important migration paths for fluids for which a significant part is discharged into the water column, contributing to the increase of methane concentration at the bottom seawater and favoring the development of specific ecosystems.

3D seismic analysis of the gas hydrate system and the fluid migration paths in part of the Niger Delta Basin, Nigeria

Comprehensive qualitative and semi-quantitative seismic analysis was carried out on 3-dimensional seismic data acquired in the deepwater compressional and shale diapiric zone of the Niger Delta Basin using an advanced seismic imaging tool. The main aim of this work is to obtain an understanding of the forming mechanism of the gas hydrate system, and the fluid migration paths associated with this part of the basin. The results showed the presence of pockmarks on the seafloor and bottom simulating reflectors (BSRs) in the field, indicating the active fluid flux and existence of gas hydrate system in the area. In the area of approximately 195 km2 occupying nearly 24% of the entire study field, three major zones with continuous or discontinuous BSRs of 3 to 7 km in length which are in the northeastern, southern and eastern part of the field respectively were delineated. The BSR is interpreted to be the transition between the free gas zone and the gas hydrate zone. The geologic structures including faults (strike-slip and normal faults), chimneys and diapirs were deduced to be the main conduits for gas migration. It is concluded that the biogenic gases generated in the basin were possibly transported via faults and chimneys by advection processes and subsequently accumulated under low temperature and high pressure conditions in the free gas zone below the BSR forming gas hydrate. A plausible explanation for the presence of the ubiquitous pockmarks of different diameters and sizes in the area is the transportation of the excessive gas to the seafloor through these mapped geologic structures.

Discrete Fracture Network modelling of a hydrocarbon-bearing, oblique-slip fault zone: Inferences on fault-controlled fluid storage and migration properties of carbonate fault damage zones

Discrete Fracture Network (DFN) modelling is a useful tool to assess both fluid storage and migration properties of naturally fractured carbonate rocks. This work presents the results of multiple DFN models, which represent the structural network associated with a ca. 100 m–offset, oblique-slip, normal fault zone crosscutting Miocene ramp carbonates of the Majella Mountain, Italy. Specifically, by taking advantage of a previously published conceptual model of fault nucleation and growth, the DFN models are aimed at computing the amount of fracture porosity and equivalent permeability of representative portions of the evolving fault damage zone. In fact, by first considering only background structural elements, those pre-dating the fault nucleation stage, all fault-related structural elements such as joints, small and medium faults are added, one by one, according to their relative time of formation. Small faults are characterized by several cm-offset, whereas the latter ones have several 10's of cm-offset. The following fracture parameters are inputted for each structural element: (i) length; (ii) aspect ratio; (iii) mechanical and hydraulic aperture; (iv) fracture intensity and (v) orientation. A sensitivity analysis is carried out in order to test the seeding procedure of the software, and the input fracture aperture values. Results of DFN modelling are consistent with the small faults forming the main repository for underground geofluids. Differently, the fluid migration paths through the fault damage zone are mainly controlled by opening-mode-related fractures, which mainly form clusters at the extensional quadrants of sheared structural elements. Due to their attitude and distribution, these features mainly enhance fault-parallel fluid migration paths. In fact, the computed values of horizontal equivalent permeability obtained along this direction are about one order of magnitude higher than those along the orthogonal direction. The approach and methodologies applied in this work, together with the results of the multiple DFN model configurations show the relative influence and role of the different structural elements on fluid storage and migration properties of limestone-hosted, fault damage zones.

Mechanisms leading to potential impacts of shale gas development on groundwater quality

The development of shale gas resources was made possible by the combination of horizontal drilling and high‐volume hydraulic fracturing (fracking). Environmental concerns have been raised relative to shale gas production, especially potential impacts on groundwater. Fluids related to unconventional oil and gas (O&G) operations contain chemical compounds that can impact groundwater quality. Such impacts can occur due to (1) the infiltration of surface contaminant releases, (2) failures of the integrity of O&G wells, and (3) upward fluid migration from a shale/tight reservoir along preferential paths that can be natural (faults or fracture zone) or man‐made (O&G wells). Surface releases represent the most probable mechanism leading to groundwater contamination. Improvements in O&G drilling operations under stringent regulations can minimize this risk. Experts identify O&G well integrity as the most challenging issue that may lead to groundwater contamination. Failure of casing and cement can lead to upward fluid flow within or outside O&G wells, especially of methane. Integrity failures leading to fluid migration to shallow fresh water aquifers or to the surface are well understood and can be detected and repaired, but this can be complex and costly. A few regulators now impose groundwater monitoring to detect impacts from integrity failures. Occurrences of communication with existing O&G wells from fracking operations have also led some regulators to impose rules aiming to avoid such potential fluid migration paths. There is an ongoing scientific debate regarding the potential for fluids to migrate upward from exploited shale gas units to aquifers through natural preferential paths. WIREs Water 2017, 4:e1188. doi: 10.1002/wat2.1188This article is categorized under: Engineering Water > Sustainable Engineering of Water Science of Water > Water Quality