Gas Migration Pathways Research Articles

Tectonic Controls on Near-Surface Variations in CH4 and CO2 Concentrations along the Northwestern Margin of the Ordos Block, China

Tectonic controls on near-surface CH4 and CO2 concentrations were investigated by measuring CH4 and CO2 concentrations at the surface and a height of 1.5 m, in the different tectonic units that comprise the northwestern margin of Ordos Block, China, which has a complex tectonic structure and a history of strong earthquakes. CH4 and CO2 concentrations varied from 1905 to 2472 ppb and 397.5 to 458.5 ppm, respectively. Surface CH4 and CO2 concentrations were generally higher than those measured at 1.5 m, but showed similar trends, indicating that the measured CH4 and CO2 predominantly originated from underground gases. The CH4 and CO2 concentrations increased with an increasing strike-slip rate across the faults, and concentrations in the blocks with high internal deformation were much higher than those measured in the stable blocks. Regions of extensional deformation had higher gas concentrations than regions that had experienced compressional deformation. The spatial distribution of CH4 and CO2 at the study site had similar trends to faults associated with the Yinchuan Graben. The results of this study indicated that gas source, gas migration pathway, and tectonic stress were the main factors that influenced gas emission. The key factor is tectonic stress, which controlled the formation of tectonic structures, changed the pathway of degassing, and acted as the driving force for gas migration. The results of this study clarify the mechanism of CH4 and CO2 degassing in faulted regions and suggest that CH4 and CO2 concentrations may be useful precursors in the monitoring of seismic activity. The results may also help inform future assessments of the contribution of geological sources to greenhouse gas emissions.

A Cause Analysis of the High-Content Nitrogen and Low-Content Hydrocarbon in Shale Gas: A Case Study of the Early Cambrian in Xiuwu Basin, Yangtze Region

There are successes and failures in the exploration of marine shale gas in South China. In some shale gas plays with great basis for hydrocarbon generation, a phenomenon exists that gas loggings reflect low gas bearing in some of the wells and the gas is dominated by nitrogen rather than hydrocarbon gas. The study of nitrogen concentration in shale gas contributes to solve the question that how shale gas diffuses in complex tectonic areas, which helps to figure out the preservation requirements and accumulation mechanisms of shale gas and avoid exploration crisis. This study focused on the lower Cambrian shale in Xiuwu Basin, Lower Yangtze Region, with emphasis on the well Jiangye-1, using gas component analysis, stable nitrogen isotope analysis, overburden permeability tests in parallel and perpendicular directions, and FIB-HIM experiments, also combining with core description, outcrop observation, and seismic interpretation to explore the causes of the high-content nitrogen and low-content hydrocarbon in the lower Cambrian shale gas. The results show that the nitrogen of the lower Cambrian shale in Xiuwu Basin is derived from the atmosphere and the deep crust-upper mantle. The bedding planes and the detachment layer at the bottom of the lower Cambrian compose the lateral pathways, and the widespread deep faults are the vertical pathways for shale gas migration and diffusion. Combining these two, an effective pathway network was built, favorable to gas exchange between the shale gas interval and the atmosphere, partly leading to the concentration of nitrogen and the diffusion of hydrocarbon gas. In the Jurassic, the magmatic activities occurred frequently in the surrounding areas, which not only brought nitrogen from the deep crust-upper mantle but also increased the value of paleo-heat flow even though the basin began to uplift, which promoted the graphitization of organic matter and the collapse of organic pores and accelerated the loss of shale gas. Based on the study above, an explanation model was summarized to expound the causes of high-content nitrogen and low-content hydrocarbon in shale gas plays near the plate-active region in Xiuwu Basin, Lower Yangtze Region.

Composition and origin of molecular compounds in the condensate oils of the Dabei gas field, Tarim Basin, NW China

Abstract The Dabei gas field in the Kuqa Depression of the Tarim Basin is the most complex and deep continental condensate gas field in China. Comprehensive two-dimensional gas chromatography-time of flight mass spectrometer (GC×GC-TOFMS) analysis was conducted on five condensate oil samples from this field. The results show that the samples have n-alkane series in complete preservation and rich adamantanes. According to the methyladamantane index, the condensate oil is the product of the source rock with vitrinite reflectance (Ro) of 1.3%–1.6%. According to the gas maturity calculated through carbon isotope and vitrinite reflectance, the natural gas is corresponding to Ro of 1.3%–1.7%, reflecting that the natural gas and condensate oil are basically formed during the same period at the high maturity stage of source rock. The Dabei gas field has favorable geological conditions for hydrocarbon accumulation: thick salt rock in the Paleogene acts as a regional high-quality caprock directly overlying the high-quality sandstone reservoir of the Cretaceous, the coal source rocks have high hydrocarbon generation intensity and provide continuous oil and gas, and the subsalt thrust structures develop in rows with rich faults, providing migration pathways for oil and gas migration. These factors together controlled the formation of the Dabei gas field.

Gas flow through shallow sediments—A case study using passive and active seismic field data

Offshore Quaternary sediment sequences are generally weak and unconsolidated, and fluid flow through these rock formations is hard to predict or model. Surface flow observations, such as gas seeps, often appear random and may be controlled by the architecture and facies distribution within the shallow strata. Using geophysical data acquired before and after a sub-seabed blowout from a 4700m-deep interval in a North Sea hydrocarbon exploration well, we investigate the nature of subsurface gas flow behaviour in shallow sediment sequences. The underground gas blow-out lasted for a period of almost one year, and when the flow from the deep gas reservoir was stopped, we observed that fluids continued to flow in the shallow subsurface and probably continue to this day, almost 30 years later.During the underground blowout phase, passive seismic data revealed episodes of up to 30 min with high seismicity followed by quieter periods of several hours. Time-lapse seismic data revealed that gas had migrated into several shallow sand layers, and that this migration had continued over for at least 25 years. The seismic data also indicated that gas entered into a shallow tunnel valley complex approximately 1–2 years after the blowout, and at a later stage migrated further. The 3D seismic data also shows indications of gas leakage outside the well bore of the relief well, drilled through the overburden sediments 1.2 km away from the main well. This implies that wells drilled through weak overburden rocks can weaken the formations potentially creating vertical pathways for gas migration. The observations from this gas migration case history are used to gain more general insights into the flow of buoyant gases, especially CO2, in shallow unconsolidated sediment sequences.

Innovative Tools for Rapidly Mapping/Quantifying CO2 Leakage and Determining its Origin

Although deployment of onshore CO2 storage will be crucial to reach the EU’s ambitious goal of an 80% reduction in greenhouse gas emissions by 2050, some stakeholders are concerned about potential risks if CCS is situated on land near populated areas. The EU-funded, Horizon 2020 project ENOS (ENabling Onshore CO2 Storage in Europe) is addressing many of these concerns about onshore storage by demonstrating best practices through pilot-scale projects and field laboratories, integrating CO2 storage in local economic activities, and creating a favorable environment through public engagement, knowledge sharing and capacity building/training. As part of this work, ENOS is using sites where natural, geologically produced CO2 is leaking to the surface, to test innovative monitoring tools and to better understand gas migration pathways and early warning signs that could be detected in the unlikely event of CO2 leakage. At least four natural leakage sites are being used in central Italy, including the well-known Latera caldera as well as San Vittorino valley, Ailano, and Fiumicino. All sites exhibit the leakage of almost pure CO2 along bedrock faults and through overlying sediments prior to release to the atmosphere, but each has unique characteristics related to the origin of the leaking gas, the composition of the local bedrock, depth to water table, soil properties, and ground surface conditions. Results from recent ENOS field campaigns at these sites are presented, focusing on data and interpretation related to i) large area, rapid leakage mapping and quantification tools; ii) innovative methods to determine the source of a CO2 anomaly; iii) CO2 leakage style as a function of near-surface conditions.

Stress Distribution and Fracture Development Potential in Rocks around Karst Caves in Carbonate Reservoirs

Karst holes and caves are well developed due to karstification in carbonate reservoirs. Stress distribution and fracture development around karst caves are of great significance to evaluate the storage and migration of hydrocarbons in carbonate reservoirs. In this study, the stress distribution and fracture development of Ordovician buried hills in Jizhong depression in northern China were investigated. The rock mechanical parameters of the target area were determined based on logging data and the boundary loads of the target region were also determined. Next, a numerical stress-analysis model was developed to investigate stress around the karst caves in Jizhong depression. Fracture development in the surrounding rocks of the karst caves was also analyzed. Results show that the principle vertical, maximum horizontal, and minimum horizontal stresses of studied area are all compressive with an approximate value of 100 MPa, indicating that the caves are under a stress environment in which mainly the top and lateral sides of caves bear pressure. The stress in the surrounding areas of the caves are quite homogeneous, and the fracture development index in these areas is low, making it difficult to form pathways for oil and gas migration. However, the stress near the top of the caves is relatively larger and the fracture development index is relatively higher, so that the rocks in this area tend to fracture and form migration pathways. This study is expected to provide scientific basis for the exploitation and development of oil and gas from the carbonate reservoirs in Jizhong depression.

Gas transport modelling at different spatial scales of a geological repository in clay host rock

The work presented in this article is a part of the international European Commission FORGE project which studied the behaviour of repository gases in the context of radioactive waste disposal. Experimental work is essential to understand the main transport mechanisms for gas and to determine the main geophysical parameters. However, while laboratory and rock experiments can help to investigate the short- and partly the middle-term behaviour of gas in a rock formation, long-term predictions have to be based on numerical simulations. Numerical simulation of long-term gas behaviour in a deep geological repository was one of the aims in the FORGE project. The objectives of the FORGE modelling were to describe the state-of-the-art consideration of gas in performance assessment, and to propose an updated treatment of gas issues in long-term safety assessments for high-level radioactive waste and spent nuclear fuel. Three benchmark exercises for a hypothetical geological repository in clay host rock ranging in scale from a single disposal cell to the whole repository were defined. To provide added value to this benchmark, a feature not yet well represented in typical gas simulations was introduced: the explicit representation of the interfaces between the excavation disturbed zone and the engineered elements within the excavation, such as waste canisters, bentonite plugs, and access drifts. In order to assess gas transport behaviour at the whole repository scale, models were developed with ‘mathematical’ or ‘numerical’ upscaling techniques for small-scale features. The assessment across different modelling scales revealed the main long-term gas migration pathways and led to the conclusion that the explicit representation of interfaces is not important.

Advances of formation dynamics of natural gas hydrate

It is of great importance for the estimation of reserve, distribution and feasibility of exploitation to study the formation dynamics of natural gas hydrate. Appropriate low temperature and high pressure are the two key parameters for the formation of natural gas hydrate besides gas supply and migration pathway. Meanwhile, the characteristics of the sediment and the existence of the Chloridionand Sulfate ions are also important for the formation of natural gas hydrate. To analyze correctly the formation of natural gas hydrate in some area, we should first obtain detailed historical data of geology, physics and mechanics by geological survey and then build the proper mathematical model. In this paper, summary on several areas has been first processed based on the enough relative data. Next the general and special characteristics of the formation in these areas have been discussed. Then the previous mathematical models for the formation dynamics of natural gas hydrate have been reviewed. The suggestions about the development of the mathematical model for the formation and the main parameter should be focused in the future exploration have been presented.

National Gas Hydrate Program expedition 02: Identification of gas hydrate prospects in the Krishna-Godavari Basin, offshore India

After completing the first expedition of India's National Gas Hydrate Program (NGHP-01) in 2006, it was concluded that for the next expedition (National Gas Hydrate Program 02; NGHP-02), a new drill site review effort should focus on identifying potential deep-water offshore gas hydrate accumulations in sand dominated depositional environments. Therefore, geological and geophysical data analysis and 3D seismic data interpretation along with associated seismic modeling were carried out in three areas of the Krishna-Godavari Basin: Areas B, C, and E. Conventional petroleum exploration approaches of seismic amplitude evaluation were adapted to prospect for potentially sand-rich depositional systems within the gas hydrate stability zone. Subsequently, these prospective areas were further assessed through the geological and geophysical evaluation of depositional setting, gas sources, and gas migration pathways. In Area B, prospecting focused on a large anticlinal structure with a prominent bottom-simulating reflector and several key horizons that indicated evidence for potential sand-hosted hydrate occurrences. In Area C, the prospects were distributed throughout various settings within a very large deep-water channel-levee-fan system with complex indications of potential gas hydrate occurrence in sand-prone seismic facies. In Area E, prospects were associated with high amplitude events within inferred channel-levee sequences. Based on the pre-expedition/onboard drill-site evaluation, the 22 most promising sites in the Krishna-Godavari Basin were identified and prioritized to investigate and delineate a total of 17 identified gas hydrate prospects. This paper describes the geo-scientific studies carried out prior to NGHP-02 for site identification, evaluation and prioritization. An important outcome of this study is the identification of two potentially producible gas hydrate systems inferred to host significant quantities of gas hydrate in stratigraphic-structural traps.

Estimation of radon diffusivity tensor for fractured rocks in cave mines using a discrete fracture network model

This study develops a numerical model for predicting radon effective diffusivity tensor for fractured rocks using a two dimensional discrete fracture network (DFN) model. This is motivated by the limitations of existing techniques in predicting the radon diffusion coefficient for the fractured zones of cave mines. These limitations include access to the fractured zones for the purpose of conducting field studies as well as replication of the degree of fracturing in these zones for laboratory studies. The caving of a rock mass involves the fracturing and breaking of intact and naturally fractured rock, which creates migration pathways for radon gas trapped within uranium-rich rock. Therefore, this study develops a stochastic DFN model with equations derived from radon transport to predict diffusivity. Our simulation results reveal the establishment of a representative elementary volume (REV) for diffusivity tensor; approximately equal principal and cross diffusivity magnitudes for each of the DFN domain; a range of diffusivity with porosity (calculated based on the fractures in the domain); and a significant effect of fracture density on diffusivity tensor. These results are essential in developing proactive measures for mitigation of radon gas in cave mines.

Integration of static and dynamic data and high-resolution sequence stratigraphy to define reservoir architecture and flow units within a ‘super giant’ gas condensate and oil field, Kazakhstan

Abstract The Upper Serpukhovian (Mississippian) interval of the Karachaganak reservoir comprises stratified platform interior carbonates, which are characterised by relatively high porosity and matrix permeability (maximum 23.8% and 2198 mD, respectively), and comprise high frequency (fourth-order) glacio-eustatic cycles. Based on static (geological) and dynamic (well test and production) data, this cyclicity exerts a strong control on gas migration pathways, and reservoir quality is predictable within a third- and fourth-order sequence stratigraphic framework that defines two discrete flow units. The contribution to flow made by fourth-order cycles within flow units is dependent on their position within third-order transgressive or highstand systems tracts. An upper flow unit comprises thick (up to 40 m) fourth-order cycles within a third-order transgressive systems tract. These fourth-order cycles contain large scale, clinoformal bedforms, can be correlated for up to 6 km, and are the main pathways of injected gas through the Upper Serpukhovian reservoir. A lower flow unit comprises fourth-order cycles (within a third-order highstand systems tract) that are thinner, and less laterally continuous, than those in the upper flow unit, and contribute far less to horizontal gas flow. Pressure data indicate that vertical permeability baffles are developed at the third-order sequence boundary that separates the two flow units (linked to diagenetic alteration during prolonged subaerial exposure), and also within the low porosity, mud-rich, transgressive bases of thick fourth-order cycles of the upper flow unit, that formed during periods of increased accommodation space across the platform top during third-order transgression. The scale at which this data set is observed is critical to understanding the dynamic behaviour of the reservoir. Relatively small differences in the composition of microfacies of cycles within both flow units do not explain the variations in dynamic data. This variation is better explained within the broader context of the sequence stratigraphic framework as defined from analysis of cycle stacking patterns. Thicker high frequency cycles deposited in increased accommodation space on the platform top during third-order transgressive intervals are key to developing thick intervals of net pay.

Imaging shallow gas migration pathways in a mud‐volcano province using an autonomous underwater vehicle (Malta Plateau, Mediterranean Sea)

ABSTRACTData acquired by an autonomous underwater vehicle (AUV) towing a source (1600–3500 Hz) and a horizontal array of hydrophones have been analysed to image discrete, isolated or even a small cluster of scatterers within the sediment, to determine shallow migration paths of hydrocarbons in a mud volcano system of the Malta Plateau. An algorithm based on a semblance function was applied to the acoustic data to highlight scatterers rather than interface reflections. The resulting scatterer map, obtained along the AUV track, generated a pseudo‐three‐dimensional coverage of the study area, with a horizontal and vertical resolution of roughly 3–5 m and 5–10 m, respectively. This map was combined with high‐resolution bathymetric and backscattering seafloor maps obtained from previous explorations. This integrated dataset provides new evidence for the role of fault zones as a preferential path for gas/fluid migration and reveals the intermittent activity of seeping gas. The data show, in particular, that gas bubble slugs, i.e. discontinuous gas columns, rise through Plio‐Quaternary sediments along a complex system of conduits terminating at the surface into quiescent mud volcanoes. The gas flux is facilitated by the regional stress field that results in dilatant conditions on the mapped fault zones.

Upward migration of gas in an active tectonic basin: An example from the sea of Marmara

Gases of various sources were collected at the seafloor of the Marmara basin suggesting that the gases expelled have experienced multiple sequences of upward migration, from multiple sources. The pathways of upward migration of gas can be reconstructed by considering the distribution of gas seeps with respect to the near-surface geomorphostructure and the regional stratigraphic architecture of the Marmara Basin. Gas seeps appear to be more favourably localized within a 1–2 km swath around active faults where sediment permeability is probably enhanced by deformation. In the fault zones, fault intersections between sets of transtensive and transpressive subsidiary faults, or between subsidiary faults and main faults, are the preferred gas pathways. These subsidiary structures localize methane seeps, observed as elongated black patches with bacterial mats, gas bubble emission sites, and chemoherms associated with buried mud volcanoes. Gas seeps are, however, rare along active faults segments crossing basin depocenters but focus along basin edges and along topographic highs. Considering the role of sedimentary layers as gas migration pathways can explain this characteristic of the distribution of gas seeps. Fault zones that cross updip gas migration pathways will vent comparatively more gas than fault zones on the downdip side. Moreover, gas accumulation and resulting overpressuring along the western fault segment crossing the Western High may be associated with aseismic fault creep and intense gas emissions at the seafloor. In contrast, the poorly focused seepage along the fault segment crossing the Central High may be linked to the locked state of this fault segment.

The Character and Formation of Elongated Depressions on the Upper Bulgarian Slope

Seafloor elongated depressions are indicators of gas seepage or slope instability. Here we report a sequence of slopeparallel elongated depressions that link to headwalls of sediment slides on upper slope. The depressions of about 250 m in width and several kilometers in length are areas of focused gas discharge indicated by bubble-release into the water column and methane enriched pore waters. Sparker seismic profiles running perpendicular and parallel to the coast, show gas migration pathways and trapped gas underneath these depressions with bright spots and seismic blanking. The data indicate that upward gas migration is the initial reason for fracturing sedimentary layers. In the top sediment where two young stages of landslides can be detected, the slopeparallel sediment weakening lengthens and deepens the surficial fractures, creating the elongated depressions in the seafloor supported by sediment erosion due to slope-parallel water currents.

Impact of tectonism on pore type and pore structure evolution in organic-rich shale: Implications for gas storage and migration pathways in naturally deformed rocks

Tectonism significantly affects the pore type and pore structure evolution of organic-rich shale. The Lower Cambrian Lujiaping Shale in the northeastern of the upper Yangtze area is a significant marine shale being studied for shale gas exploration in South China. Here, we use two groups of Lujiaping Shale samples, such as brittle deformation shales (BDS) and ductile deformation shales (DDS), to investigate the relationship between tectonism and pore structure using a combination of mineralogy, organic geochemistry, low-pressure nitrogen adsorption, low-field nuclear magnetic resonance (NMR), optical microscopy, and high-resolution scanning electron microscopy (SEM). Results indicate that the samples are quartz, carbonate, and clay rich. Total organic carbon (TOC) content ranges between 2.27% and 5.88%, and vitrinite reflectance (Ro) measured between 2.95% and 4.11%. Helium porosity lies between 3.02% and 4.66% in BDS and 3.16%-4.29% in DDS. In addition, four major pore types can be identified and classified by their formation mechanism, location, geometry, and connectivity, which we named type I (pore within or between organic and mineral particles), type II (pore in admixtures), type III (micro-channel), and type IV (micro-fracture). Under the BDS, the interparticle (interP) pores, micro-channels, and micro-fractures are more developed, the macropores (＞50 nm) increase, the connectivity and porosity are enhanced. The intraparticle (intraP) pores, micro-channels, and micro-fractures are dominant in DDS, the micropores (< 2 nm) increase and the connectivity is enhanced, whereas the porosity decreases. Meanwhile, pore network within the admixtures of organic matter (OM) and clays can also contribute to gas storage and migration pathways in both BDS and DDS, in addition to other pore types. However, OM pores are poorly-developed in samples. Our findings provide a key component for further evaluation of the exploration and development potential of the Lujiaping Shale in the complex tectonic areas of South China.

Elongate fluid flow structures: Stress control on gas migration at Opouawe Bank, New Zealand

High-resolution 2D and 3D seismic data from Opouawe Bank, an accretionary ridge on the Hikurangi subduction margin off New Zealand, show evidence for exceptional gas migration pathways linked to the stress regime of the ridge. Although the ridge has formed by thrusting and folding in response to a sub-horizontal principal compressive stress (σ1), it is clear that local stress conditions related to uplift and extension around the apex of folding (i.e. sub-vertical σ1) are controlling shallow fluid flow. The most conspicuous structural features are parallel and horizontally-elongated extensional fractures that are perpendicular to the ridge axis. At shallower depth near the seafloor, extensional fractures evolve into more concentric structures which ultimately reach the seafloor where they terminate at gas seeps. In addition to the ridge-perpendicular extensional fractures, we also observe both ridge-perpendicular and ridge-parallel normal faults. This indicates that both longitudinal- and ridge-perpendicular extension have occurred in the past. The deepest stratigraphic unit that we image has undergone significant folding and is affected by both sets of normal faults. Shallower stratigraphic units are less deformed and only host the ridge-parallel normal faults, indicating that longitudinal extension was limited to an older phase of ridge evolution. Present-day gas migration has exploited the fabric from longitudinal extension at depth. As the gas ascends to shallower units it ‘self-generates’ its flow pathways through the more concentric structures near the seafloor. This shows that gas migration can evolve from being dependent on inherited tectonic structures at depth, to becoming self-propagating closer to the seafloor.

Gas migration pathways and slope failures in the Danube Fan, Black Sea

Abstract A large geophysical dataset, including bathymetry, and 2D and 3D P-cable seismic data, revealed evidence of numerous gas flares near the S2 Canyon in the Danube Fan, northwestern Black Sea. This dataset allows us to investigate potential relationships between gas migration pathways, gas vents observed at the seafloor and submarine slope failures. Vertical gas migration structures as revealed in the seismics appear to be concentrated near submarine slope failure structures. Where these seismically defined features extend upwards to the seafloor, they correlate with the location of gas flares. However, not all these structures reach the seafloor, in some cases because they are capped by overlying sediments. A strong correlation is inferred between gas migration pathways, heterogeneous mass transport deposits and contacts between adjacent units of contrasting lithology. Although missing age constrains prevent a final judgement, we discuss the potential relationship between submarine slope failures and gas migration in order to determine if gas migration is a precursor to failure, or if the presence of slope failures and associated mass transport deposits facilitates the migration of gas. Our observations indicate that lithological heterogeneity, mass transport deposits and minor sediment deformation control gas migration pathways and the formation of gas chimney-like features. Gas migration is focused and gradual, resulting in gas flares where the chimney-like features extend to the seafloor, with no evidence of erosive features such as pockmarks.

Vadose Zone Gas Migration and Surface Effluxes after a Controlled Natural Gas Release into an Unconfined Shallow Aquifer

Core Ideas Subsurface gas migration results in localized surficial CH4 releases. Surficial CH4 emissions show pronounced temporal variations. Methane concentrations in soil gas exceed lower explosive limits at low leakage rates. Increasing CO2 effluxes and stable C isotope signatures indicate vadose zone CH4 oxidation. Instantaneous surficial effluxes do not indicate the magnitude of subsurface gas leakage rates. Shale gas development has led to concerns regarding fugitive CH4 migration in the subsurface and emissions to the atmosphere. However, few studies have characterized CH4 migration mechanisms and fate related to fugitive gas releases from oil or gas wells. This paper presents results from vadose zone gas and surface efflux monitoring during a natural gas release experiment at Canadian Forces Base Borden, Alliston, Ontario, Canada. Over 72 d, 51 m3 of natural gas (&gt;93% CH4) was injected into a shallow, unconfined sand aquifer at depths of 4.5 and 9 m. Methane and CO2 effluxes in combination with soil gas concentrations and stable C isotopic signatures were used to quantify the spatiotemporal migration and fate of injected gas. Preferential gas migration pathways led to vadose zone hot spots, with CH4 concentrations exceeding the lower explosive limit (5% v/v). From these hot spots, episodic surface CH4 effluxes (temporally exceeding 2500 μmol m−2 s−1 [3465 g m−2 d−1]) occurred during active injection. Higher injection rates led to increased average CH4 effluxes and greater lateral migration, as evidenced by a growing emission area approaching 25 m2 for the highest injection rate. Reactive transport modeling showed that high CH4 fluxes resulted in advection‐dominated migration and limited CH4 oxidation, whereas lower CH4 effluxes were diffusion dominated with substantial CH4 oxidation. These results and our interpretations allowed us to develop a conceptual model of fugitive CH4 migration from the vadose zone to the ground surface.

Effects of early oil emplacement on reservoir quality and gas migration in the Lower Jurassic tight sand reservoirs of Dibei gas field, Kuqa Depression, western China

It is common for many tight gas sandstone reservoirs to have experienced an early oil charge before gas invading. To determine the effects of early oil emplacement on reservoir quality and gas migration has important role in predicting “sweet spots” of gas production in tight sand reservoirs. We investigated the palaeo and current fluids contacts accurately due to parameters from quantitative grain fluorescence in the Lower Jurassic Ahe Formation of Dibei gas field. The porosity and permeability values in palaeo-oil leg are totally higher than in palaeo-water leg, especially there being a wide gap of permeability with an order of magnitude. The variation of reservoir quality derives from the early oil emplacement, which restrained clay conversions from kaolinite or illite-smectite mixed-layer into fibrous illite that dramatically increasing flow-path tortuosity in sandstones, according to core analysis and X-ray diffraction. The early oil preserved penetrating quality of palaeo-oil leg, but the sandstones that never experienced early oil emplacement contains much more fibrous illite. It made most of early oil pathways subsequently act as the migration pathways for late gas and less than 50% of the migration pathways for gas were caused by microfractures due to quantitative grain fluorescence. Only the sandstones with medium early oil saturation did become sweet spots for gas in the tight sand reservoirs. Too much and too little oil once saturated in pores maybe adverse to the late gas migration and accumulation.

Methods and challenges to locating legacy wells in western Pennsylvania: Case study at Hillman State Park

This study demonstrates the application of aeromagnetic surveys for locating late 1800s-era oil and gas wells in Hillman State Park. The study area in southwestern Pennsylvania offered several unique challenges to locating legacy wells. Location records for many of Pennsylvania’s legacy wells do not exist. Those that do exist are often incomplete and inaccurate, and old wells were commonly abandoned without effective plugging. Now, unplugged legacy wells may serve as vertical migration pathways for fluids and gas associated with modern oil and gas operations. Wells in Hillman State Park were abandoned in the early 1900s, leaving little evidence of a well site. However, the steel well casing commonly remained at the site. Between 1940 and 1960, 50% of the land area at Hillman State Park was surface mined for coal. The removal of coal overburden also removed the upper well casings in surface-mined areas to the depth of the coal. The wells were then buried under mine spoil during regrading operations. Today, much of Hillman State Park is covered in trees and dense vegetation, and locating wells with ground-level searches is difficult, time consuming, and often futile. The airborne magnetic survey used in this study identified well locations, including buried wells in mined areas, based on the unique magnetic signature of vertical, steel well casing. The results of the aeromagnetic survey were combined with aerial photography, historic maps, and high-resolution topographic data in a geographic information system to refine well locations prior to verification with a ground search.