Lithostatic Stress Research Articles

Experimental study of the influences of pressure on generation and expulsion of hydrocarbons: A case study from mudstone source rocks and its geological application in the Tarim Basin

Generally, pressure, temperature, depth of organic matter/kerogen burial and organic and inorganic matter interactions can influence the hydrocarbon generation process. However, different understandings were proposed to interpret the role of pressure in hydrocarbon generation. Meanwhile, previous investigations of the influence of pressure on hydrocarbons generation are mainly focused on fluid pressure (FP), only a few work was involved in the effect of lithostatic stress (LS) due to the limitation of experimental techniques. In this study, simulation experiments were conducted to investigate the influence of pressure, including lithostatic stress (LS) and fluid pressure (FP) on yield and process of hydrocarbon generation by a modified simulation instrument. Two systematic comparative pyrolysis experiments, including pressured experiments with LP range from 27.6 to 179.4 MPa and FP range from 12.0 to 109.2 MPa, and non-pressured experiments with LP for 6.0 MPa and FP range from 2.0 to 5.0 MPa were conducted in hydrous experimental condition. The results show that the peak yields of the expelled oil and the liquid hydrocarbons in pressured experiments and non-pressured experiments are 42.48 mg/g·TOC, 52.00 mg/g·TOC, 70.14 mg/g·TOC, and 72.15 mg/g·TOC, respectively. And Ro, ratio of the expelled oil to eluted oil, the accumulated yield of the expelled oil and liquid hydrocarbons, are lower in pressured experiments than that of non-pressured experiments. However, the accumulated yields of eluted oil and residual oil are higher in pressured experiments than that of non-pressured experiments. Additionally, the expulsion efficiencies of the oil range from 20% to 25%, which are lower in pressured experiments than that of non-pressured experiments near the oil generation-window. In addition, the yields of hydrocarbon gas in pressured experiments are greater compared to the yields in non-pressured experiments at the high maturity stage. Based on the experimental results, hydrocarbon generation process can be divided into three stages. Generally, LS and FP could retard the process of mudstone maturation as well as the generation and expulsion of the liquid hydrocarbons. Hence, the residual liquid hydrocarbons in high-pressure condition can act as another source of natural gas generation in the high maturity stage. The results could help us to properly evaluate hydrocarbon generation potentials of sources rocks at petroleum-producing sedimentary basins in China and elsewhere.

Isostatic constraint for 2D nonlinear gravity inversion on rifted margins

We have developed a nonlinear gravity inversion for simultaneously estimating the basement and Moho geometries, as well as the depth of the reference Moho along a profile crossing a passive rifted margin. To obtain stable solutions, we impose smoothness on basement and Moho, force them to be close to previously estimated depths along the profile and also impose local isostatic equilibrium. Different from previous methods, we evaluate the information of local isostatic equilibrium by imposing smoothness on the lithostatic stress exerted at depth. Our method delimits regions that deviate and those that can be considered in local isostatic equilibrium by varying the weight of the isostatic constraint along the profile. It also allows controlling the degree of equilibrium along the profile, so that the interpreter can obtain a set of candidate models that fit the observed data and exhibit different degrees of isostatic equilibrium. Our method also differs from earlier studies because it attempts to use isostasy for exploring (but not necessarily reducing) the inherent ambiguity of gravity methods. Tests with synthetic data illustrate the effect of our isostatic constraint on the estimated basement and Moho reliefs, especially at regions with pronounced crustal thinning, which are typical of passive volcanic margins. Results obtained by inverting satellite data over the Pelotas Basin, a passive volcanic margin in southern Brazil, agree with previous interpretations obtained independently by combining gravity, magnetic, and seismic data available to the petroleum industry. These results indicate that combined with a priori information, simple isostatic assumptions can be very useful for interpreting gravity data on passive rifted margins.

Nonlinear Rheology at Shallow Depths with Reference to the 2016 Kumamoto Earthquakes

Abstract Strong S waves produce dynamic stresses, which bring the shallow subsurface into nonlinear inelastic failure. We examine implications of nonlinear viscous flow, which may be appropriate for shallow muddy soil, and contrast them with those of Coulomb friction within a shallow reverberating uppermost layer with low‐seismic velocities. Waves refract into essentially vertical paths at the shallow layers and produce tractions on horizontal planes. The Coulomb ratio of shear traction to lithostatic stress for S waves equals the resolved horizontal acceleration normalized to the acceleration of gravity. The ratio of dynamic vertical normal traction to lithostatic stresses is the vertical normalized acceleration from P waves. The predicted viscous inelastic strain rate in muddy soil begins at low normalized accelerations and then increases mildly and nonlinearly with increasing normalized acceleration. Failure is unaffected when P waves decrease the vertical normal traction. Seismic waves recorded at KiK‐net station KMMH16 for the 2016 Kumamoto mainshock and strong foreshock show these effects. Inelastic deformation commences at a normalized horizontal acceleration of ∼0.25 and reduces S‐ and P‐wave velocities within the uppermost ∼15 m reverberating layer. Normalized horizontal accelerations and the Coulomb stress ratio reach ∼1.25. Strong S waves arrived even when strong P waves produced vertical tension on horizontal planes. In contrast, inelastic Coulomb failure commences at a normalized horizontal acceleration equal to the effective coefficient of friction; rapid inelastic strain precludes even higher accelerations. Furthermore, horizontal planes should fail from the stresses of strong S waves during the tensional cycle of strong P waves.

Volume unbalance on the 2016 Amatrice - Norcia (Central Italy) seismic sequence and insights on normal fault earthquake mechanism

We analyse the Mw 6.5, 2016 Amatrice-Norcia (Central Italy) seismic sequence by means of InSAR, GPS, seismological and geologic data. The >1000 km2 area affected by deformation is involving a volume of about 6000 km3 and the relocated seismicity is widely distributed in the hangingwall of the master fault system and the conjugate antithetic faults. Noteworthy, the coseismically subsided hangingwall volume is about 0.12 km3, whereas the uplifted adjacent volumes uplifted only 0.016 km3. Therefore, the subsided volume was about 7.5 times larger than the uplifted one. The coseismic motion requires equivalent volume at depth absorbing the hangingwall downward movement. This unbalance regularly occurs in normal fault-related earthquakes and can be inferred as a significant contribution to coseismic strain accomodated by a stress-drop driven collapse of precursory dilatancy. The vertical coseismic displacement is in fact larger than the horizontal component, consistent with the vertical orientation of the maximum lithostatic stress tensor.

Pressure build-up and stress variations within the Earth\u2019s crust in the light of analogue models

Strength contrasts and spatial variations in rheology are likely to produce significant stress differences in the Εarth’s crust. The buildup and the relaxation of stresses have important consequences for the state of stress of the brittle crust, its deformational behaviour and seismicity. We performed scaled analogue experiments of a classic wedge-type geometry wherein we introduced a weak, fluid-filled body representing a low-stress heterogeneity. The experiments were coupled to direct pressure measurements that revealed significant pressure differences from their surrounding stressed matrix. The magnitude of the pressure variations is similar to the magnitude of the differential stress of the strongest lithology in the system. When rocks with negligible differential stresses are considered, their pressure can be more than twice larger than the surrounding lithostatic stress. The values of the pressure variations are consistent with the stresses that are estimated in analytical studies. This behaviour is not restricted to a particular scale or rheology, but it requires materials that are able to support different levels of stress upon deformation. For non-creeping rheological behaviours, the stress and pressure variations are maintained even after deformation ceases, implying that these stress variations can be preserved in nature over geological timescales.

Analysis of the stress field in the DeSoto canyon Salt Basin for ensuring safe offshore carbon storage

Offshore geologic CO2 storage offers an attractive option to reduce greenhouse gas emissions. Vast CO2 storage capacity exists in Cretaceous-Neogene sandstone in the DeSoto Canyon Salt Basin. Understanding the stress and pressure regimes in the basin can help evaluate the geomechanical integrity of the formations, thus minimizing the risk of CO2 migrating out of the storage complex. Borehole breakouts were identified using four-arm dipmeter logs. Elongation of the breakouts is aligned with the minimum horizontal compressive stress (Shmin), which tends to be oriented northeast-southwest. Vertical reservoir stresses are influenced by rock and fluid density. Lithostatic and hydrostatic stress each have a power-law relationship to depth. The average lithostatic stress (Sv) gradient is ∼21.4 kPa/m. Hydrostatic pressure gradient increases with brine density to a maximum of ∼12.2 kPa/m. Geometric mean of the Shmin-depth values correspond to an effective Shmin - effective Sv quotient of ∼0.5. Injection pressure can be maintained safely below the estimated effective minimum horizontal stress, thereby reducing the risk of cross-formational flow. Future study should focus on further constraint of geomechanical properties, reservoir integrity, and seal integrity.

Understanding eruptive style variations at calc-alkaline volcanoes: the 1913 eruption of Fuego de Colima volcano (Mexico)

The 1913 sub-Plinian eruption of Fuego de Colima volcano (Mexico) occurred after almost 100 years of effusive and (minor) Vulcanian explosive activity, which modulated dome growth and destruction. Dome extrusion persisted from 1869 to 1913. The transition to explosive eruption started on 17 January 1913, and it progressed in three phases: (1) opening, with the generation of block-and-ash flows, (2) vent clearing, with strong explosions that destroyed the summit dome and decompressed the magmatic system, and (3) sustained column (sub-Plinian fallout) with final collapse producing pyroclastic density currents. Because of this succession of events, the 1913 activity represents an excellent case-study for investigating the eruptive style changes at calc-alkaline volcanoes. We investigated the conditions that led to eruptive style transition from effusive (dome growth) to explosive (the final sub-Plinian fallout) through steady-state numerical simulations, using subsurface data and independently inferred (from volcanological data) mass discharge rates as constraints. Results show good matches for hybrid geometrical settings of the shallow conduit-feeding system (i.e., dyke developing into a shallower cylindrical conduit), and the magma chamber top at 6 km of depth. The fragmentation level was shallower than 2 km, as inferred from the lithics contained in the sub-Plinian fall deposits of Phase (3). The most likely solution is represented by a dyke having major axis between 200 and 2000 m and the minor axis of 40 m. The dyke-cylinder transition was set at a depth of 500 m, with a cylinder diameter of 40 m. It emerges that at least two main mechanisms may be responsible for the effusive to explosive transition that led to the Phase (3) of the 1913 eruption: (i) an increase in magma chamber overpressure (magmatic triggering) or (ii) decrease of lithostatic stress acting on the volcano (non-magmatic triggering). The former implies arrival into the magma chamber of a batch of fresh magma, which can have volume between 10 and 200 × 106 m3, depending on the values of magma and host rock compressibility. The latter requires decompression-induced emptying of at least the equivalent of 1000 m of the magma column to produce the necessary unloading of the conduit-feeding system. A sudden jerk in the lateral spreading of the Fuego de Colima cone would be a reliable mechanism for decompressing the upper conduit and driving fragmentation processes over a time period of few hours. The results are not conclusive on an internal (magma chamber overpressure), external (lowering of lithostatic load), or mixed (internal and external) trigger of the observed eruptive style transition. This work highlights how different processes can have non-linear cascade effects on close-to-equilibrium volcanic systems like Fuego de Colima volcano.

Failure mechanisms of soft rock roadways in steeply inclined layered rock formations

ABSTRACTIn recent years, with the increasing depth of coal mining around the world, traditional shallow geology mechanics are no longer suitable for the research on deep geological problems. In terms of deep mining, soft rock roadways in inclined rock formations show different deformation types compared with soft rock roadways in shallow horizontal strata. Based on the –1000 m track contact roadway in Qishan Mine of Xuzhou, physical model tests on the failure process of 45° and 60° inclined layered soft rock formations are carried out using physical models. The real-time monitoring of strain inside the model during the failure of soft rock formation is conducted to explore the failure mechanisms of 45° and 60° inclined layered soft rock formations in terms of in-depth high self-weight stress and horizontal tectonic stress. FLAC3D is also used for numerical simulation and comparative analysis. The roof and floor of the tunnels through 45° and 60° inclined formations fail on the left, and left and right walls fail near the bottom. The maximum displacement of the roof and floor of the 60° inclined formation is larger than that of the 45° inclined formation, and the maximum amount of deformation of left and right walls of the 45° inclined formation is larger than that of the 60° inclined formation, indicating failure features of different steeply inclined layered soft rocks roadways under high lithostatic stress and horizontal tectonic stress can provide a theoretical basis for mining and support of deep roadways.

ПРИЧИНА АКТИВНОЇ СЕЙСМІЧНОСТІ В УКРАЇНІ

In the article, the earthquake near Sumy is considered in detail. According to the authors, this earthquake occurred under the influence of several stress regimes. At the regional level: it is related to the transfer of stresses from the Vrancea zone along the mantle lineament of Sollogub; a change in the local stress field, which is created by blocks with different thicknesses of the earth's crust, with the presence of subcrustal thermal anomalies. The area of the location of the earthquake focus near the town of Sumy in tectonic terms is on the southwestern slope of the Voronezh crystalline massif near the northeastern side of the Dnieper-Donets Basin (DDB). The epicenter is confined to the northern extension of the Krivyy-Rig-Kremenchug suture zone. The earthquake focus is confined to the tectonic node formed by the eastern edge fault of the Krivyy-Rig-Kremenchug suture zone and the northern side fracture of the DDB. The main deep faults near the source of the earthquake are the northern side fault of the DDB rift and the northern extension of the Krivyy Rig-Kremenchug fault. The hypocenter of the event is in the area with sharp change in the structural plan of the Moho surface accompanied by a negative mantle gravitational anomaly. In the relief of the Moho surface, a superimposition of the structural plan of the northwestern direction, connected with the Devonian rift of the DDV, on the ancient structural plan of the Ukrainian shield and Voronezh crystalline massive is clearly visible. Within the DDV, the isohypses of the Moho surface clearly delineate the ascent to ~ 35 km in the central part of the rift. The Krivyy-Rig-Kremenchug suture zone is expressed in the Moho surface by a narrow elongated depression with depths of more than 50 km in the central part. Here, in the lower part of the cortex, the development of the crust-mantle mixture is observed. The Krivyy-Rig-Kremenchug suture zone is also distinguished by the development of high-intensity deep magnetic inhomogeneities along it. In addition, here in the lower part of the crust and in the upper mantle a linear object of high electrical conductivity is isolated. Thus, the earth's crust of the region has a pronounced physic-geological heterogeneity, which creates instability of lithostatic stress, and this, in turn, is a prerequisite for the appearance of additional stresses.

Depth dependent stress revealed by aftershocks

Characterising the state of stress in the brittle upper-crust is essential in mechanics of faulting, industrial production processes, and operational earthquake forecasting. Nevertheless, unresolved questions concern the variation of pore-fluid with depth and the absolute strength on tectonically active faults. Here we show that, along the San Andreas fault system, the time-delay before the onset of the power-law aftershock decay rate (the c-value) varies by three orders of magnitude in the first 20 km below the surface. Despite the influence of the lithostatic stress, there is no continuous change in c-value with depth. Instead, two decay phases are separated by an abrupt increase at an intermediate depth range of 2–5 km. This transitional regime is the only one observed in fluid-injection-induced seismic areas. This provides strong evidence for the role of fluid and a porosity reduction mechanism at depth of few kilometres in active fault zones. Aftershock statistics can then be used to predict changes in differential shear stress with depth until the brittle-ductile transition is reached.

PETROGRAPHY AND DIAGENETIC HISTORY OF NAGRI FORMATION SANDSTONE IN DISTRICT BAGH AND MUZAFFARABAD, PAKISTAN

The petrographic analysis of Nagri Formation sandstone based on outcrop study and interpretation of thin sections with special emphasis on the diagenetic history from widely separated areas was accomplished. The calcite concreations, hematite leaching, petrified wood fossils, spheroid weathering, quartz filled joints and the meso scale drag folding has few geological significances. Interpretation of thin sections revealed that the sandstone is lithic arenite and lithic graywackes. The concavo-convex, sutured contacts and muscovite deformations indicates that the compaction of sand under lithostatic stress during early diagenesis. The pore water has widened the intergranular distance in some rocks by dissolving the fractured surfaces of carbonates and other grains, enhancing the concentration of carbonates in water at high pH. On the other hand, the silica dust formation can be attributed to the abrasion of quartz grains during compaction, alteration of feldspar to sericite and overgrowth on quartz by calcite under low pH of solution and pressure solution. The leaching and precipitation of hematite along fractures and thin coating around the grains indicates iron oxides paragenesis from chlorite and biotite in the area. The clay mineral authigenesis includes replacement along the cleaved and fractured feldspar and sericitation. Further elaborations indicate plastic deformations of hydrolytically weak quartz grains and deformations by tectonic upliftment, enhancing the permeability and reservoir quality of sandstone.

Coupling of fluid flow to permeability development in mid- to upper crustal environments: a tale of three pressures

Abstract Orogenic gold systems are open, flow-controlled thermodynamic systems and generally occur in mid- to upper crustal environments where there is strong coupling between fluid flow and dilatant plastic deformation. This paper considers the principles involved in such coupling, with an emphasis on the elastic and plastic volume changes and their influence on the fluid, mechanical and thermodynamic pressures. Some misconceptions regarding the magnitudes of these three distinctly different pressures and their influences on fluid flow and chemical equilibrium are addressed, with examples at both the tens of metres scale and the crustal scale. We show that the mean stress is less than twice the lithostatic stress for Mohr–Coulomb materials with cohesion and the thermodynamic pressure only has meaning under isentropic conditions and hence is less than many previously published estimates based on high mean stresses. At the crustal scale, we also include the role of critical behaviour in influencing the geometry and magnitudes of fluid pressure gradients and fluid flow velocities in open, flow-controlled systems.

Impacts into porous asteroids

Many small bodies in the solar system have bulk density well below the solid density of the constituent mineral grains in their meteorite counterparts. Those low-density bodies undoubtedly have significant porosity, which is a key factor that affects the formation of impact craters. This paper summarizes the results of lab experiments in which materials with porosity ranging from 43% to 96% were impacted at ∼1800 m/s. The experiments were performed on a geotechnical centrifuge, in order to reproduce the lithostatic overburden stress and ejecta ballistics that occur in large-scale cratering events on asteroids or planetary satellites. Experiments performed at various accelerations, up to 514G, simulate the outcomes of impacts at size scales up to several tens of km in diameter.Our experiments show that an impact into a highly porous cohesionless material generates a large ovoid-shaped cavity, due to crushing by the outgoing shock. The cavity opens up to form a transient crater that grows until the material flow is arrested by gravity. The cavity then collapses to form the final crater. During collapse, finely crushed material that lines the cavity wall is carried down and collected in a localized region below the final crater floor. At large simulated sizes (high accelerations), most of the crater volume is formed by compaction, because growth of the transient crater is quickly arrested. Nearly all ejected material falls back into the crater, leaving the crater without an ejecta blanket. We find that such compaction cratering and suppression of the ejecta blankets occur for large craters on porous bodies when the ratio of the lithostatic stress at one crater depth to the crush strength of the target exceeds ∼0.005. The results are used to identify small solar system bodies on which compaction cratering likely occurs. A model is developed that gives the crater size and ejecta mass that would result for a specified impact into a porous object.

Enceladus Plume Structure and Time Variability: Comparison of Cassini Observations.

During three low-altitude (99, 66, 66 km) flybys through the Enceladus plume in 2010 and 2011, Cassini's ion neutral mass spectrometer (INMS) made its first high spatial resolution measurements of the plume's gas density and distribution, detecting in situ the individual gas jets within the broad plume. Since those flybys, more detailed Imaging Science Subsystem (ISS) imaging observations of the plume's icy component have been reported, which constrain the locations and orientations of the numerous gas/grain jets. In the present study, we used these ISS imaging results, together with ultraviolet imaging spectrograph stellar and solar occultation measurements and modeling of the three-dimensional structure of the vapor cloud, to constrain the magnitudes, velocities, and time variability of the plume gas sources from the INMS data. Our results confirm a mixture of both low and high Mach gas emission from Enceladus' surface tiger stripes, with gas accelerated as fast as Mach 10 before escaping the surface. The vapor source fluxes and jet intensities/densities vary dramatically and stochastically, up to a factor 10, both spatially along the tiger stripes and over time between flyby observations. This complex spatial variability and dynamics may result from time-variable tidal stress fields interacting with subsurface fissure geometry and tortuosity beyond detectability, including changing gas pathways to the surface, and fluid flow and boiling in response evolving lithostatic stress conditions. The total plume gas source has 30% uncertainty depending on the contributions assumed for adiabatic and nonadiabatic gas expansion/acceleration to the high Mach emission. The overall vapor plume source rate exhibits stochastic time variability up to a factor ∼5 between observations, reflecting that found in the individual gas sources/jets. Key Words: Cassini at Saturn—Geysers—Enceladus—Gas dynamics—Icy satellites. Astrobiology 17, 926–940.

Estimation of Shaft Radial Displacement beyond the Excavation Bottom before Installation of Permanent Lining in Nondilatant Weak Rocks with a Novel Formulation

The convergence-confinement method (CCM) applies to circular tunnels in in situ stress fields in which all three principal stresses are equal and the rock mass exhibits elastic-perfectly plastic shear failure. As radial wall displacement cannot be obtained easily using analytical methods, extensive parametric analysis of bidimensional numerical modelling to investigate the strain of the shaft wall close to the excavation bottom was performed. In all, 81 cases were derived from combinations of geometrical parameters and three weak rock categories. Through processing data relating to values of uR0 (radial displacement of the shaft wall at the excavation bottom) obtained by numerical calculation in the different cases studied, it was possible to calculate the uR0/R ratio as a function of the lithostatic stress p0, the lining thickness s, and the shaft radius R. Novel equations were obtained to quickly estimate the value of uR0 given the thickness of the lining concrete, the shaft depth, and the shaft radius for the qualities of rock considered.

Rapid sedimentation and overpressure in shallow sediments of the Bering Trough, offshore southern Alaska

AbstractPore pressures in sediments at convergent margins play an important role in driving chemical fluxes and controlling deformation styles and localization. In the Bering Trough offshore Southern Alaska, extreme sedimentation rates over the last 140 kyr as a result of glacial advance/retreats on the continental shelf have resulted in elevated pore fluid pressures in slope sediments overlying the Pamplona Zone fold and thrust belt, the accretionary wedge resulting from subduction of the Yakutat microplate beneath the North American Plate. Based on laboratory experiments and downhole logs acquired at Integrated Ocean Drilling Program Site U1421, we predict that the overpressure in the slope sediments may be as high as 92% of the lithostatic stress. Results of one‐dimensional numerical modeling accounting for changes in sedimentation rate over the last 130 kyr predicted overpressures that are consistent with our estimates, suggesting that the overpressure is a direct result of the rapid sedimentation experienced on the Bering shelf and slope. Comparisons with other convergent margins indicate that such rapid sedimentation and high overpressure are anomalous in sediments overlying accretionary wedges. We hypothesize that the shallow overpressure on the Bering shelf/slope has fundamentally altered the deformation style within the Pamplona Zone by suppressing development of faults and may inhibit seismicity by focusing faulting elsewhere or causing deformation on existing faults to be aseismic. These consequences are probably long‐lived as it may take several million years for the excess pressure to dissipate.

Influence of Lithostatic Stress on Earthquake Stress Drops in North America

Abstract We estimate stress drops for earthquakes in and near the continental United States using the method of spectral ratios. The ratio of acceleration spectra between collocated earthquakes recorded at a given station removes the effects of path and recording site and yields source parameters including corner frequency for, and the ratio of seismic moment between, the two earthquakes. We determine stress drop from these parameters for 1121 earthquakes greater than M ∼3 in 60 earthquake clusters. We find that the average Brune stress drop for the few eastern United States (EUS) tectonic mainshocks studied (2.6–36 MPa) is about three times greater than that of tectonic mainshocks in the western United States (WUS, 1.0–7.9 MPa) and five times greater than mainshocks potentially induced by wastewater injection in the central United States (CUS, 0.6–5.6 MPa). EUS events tend to be deeper thrusting events, whereas WUS events tend to be shallower but have a wide range of focal mechanisms. CUS events tend to be shallow with strike‐slip to normal‐faulting mechanisms. With the possible exception of CUS aftershocks, we find that differences in stress drop among all events can be taken into account, within one standard deviation of significance, by differences in the shear failure stress as outlined by Mohr–Coulomb theory. The shear failure stress is a function of vertical stress (or depth), the fault style (normal, strike slip, or reverse), and coefficient of friction (estimated here to be, on average, 0.64). After accounting for faulting style and depth dependence, we find that the average Brune stress drop is about 3% of the failure stress. These results suggest that high‐frequency shaking hazard (>∼1 Hz) from shallow induced events and aftershocks is reduced to some extent by lower stress drop. However, the shallow hypocenters will increase hazard within several kilometers of the source. Electronic Supplement: Earthquake catalog.

The structural stability of lunar lava tubes

Mounting evidence from the SELENE, LRO, and GRAIL spacecraft suggests the presence of vacant lava tubes under the surface of the Moon. GRAIL evidence, in particular, suggests that some may be more than a kilometer in width. Such large sublunarean structures would be of great benefit to future human exploration of the Moon, providing shelter from the harsh environment at the surface—but could empty lava tubes of this size be stable under lunar conditions? And what is the largest size at which they could remain structurally sound? We address these questions by creating elasto-plastic finite element models of lava tubes using the Abaqus modeling software and examining where there is local material failure in the tube's roof. We assess the strength of the rock body using the Geological Strength Index method with values appropriate to the Moon, assign it a basaltic density derived from a modern re-analysis of lunar samples, and assume a 3:1 width-to-height ratio for the lava tube. Our results show that the stability of a lava tube depends on its width, its roof thickness, and whether the rock comprising the structure begins in a lithostatic or Poisson stress state. With a roof 2m thick, lava tubes a kilometer or more in width can remain stable, supporting inferences from GRAIL observations. The theoretical maximum size of a lunar lava tube depends on a variety of factors, but given sufficient burial depth (500m) and an initial lithostatic stress state, our results show that lava tubes up to 5km wide may be able to remain structurally stable.

Development of Helium-Mass-Spectrometry-Permeameter for the Measurement of Permeability of Near-Impermeable Rock

A helium leakage detection system was modified to measure gas permeability on extracted cores of nearly impermeable rock. The Helium-Mass-Spectrometry-Permeameter (HMSP) is duplicating the classic Darcy’s experiment with a constant pressure differential and steady-state flow through a sample using helium gas. Under triaxial stress condition, the newly developed HMSP can measure hydraulic permeability of rocks and geomaterials down to the nanoDarcy scale (10−21 m2). The extension of measuring the lower end of the permeability scale may help answer important questions regarding the permeability of rock at great depth where fractures may close completely under high lithostatic stress.

Effect of lithostatic stress on methane sorption by coal: Theory vs. experiment and implications for predicting in-situ coalbed methane content

Recent research has demonstrated that confining stresses applied to the solid framework of coal can reduce its gas sorption capacity by several percent to perhaps several tens of percent. To evaluate the magnitude of this effect more rigorously in relation to predicting in-situ coalbed methane (CBM) content, a better understanding of the effects of stress on methane sorption by coal is needed. In this paper, a previous thermodynamic model for the effects of stress on CO2 sorption by coal is revised and applied to CH4. The revised model predicts that in-situ CBM content is indeed determined not only by the geological factors generally considered, such as coal rank, coal composition, moisture content and temperature, but also by lithostatic or confining stress, which is usually ignored. This prediction is tested by means of experiments performed on a composite cylindrical sample of Brzeszcze 364 high volatile bituminous coal subjected to 10MPa methane pressure at a temperature of 40°C, varying the hydrostatic stress or confining pressure in the range 11-43MPa. In these experiments, we determined if CH4 was desorbed as confining pressure was increased by subtracting the poroelastic expulsion of CH4 from the total CH4 expelled, assuming the former to equal the gas volume expelled in control experiments performed using Helium. The experimental results show that the equilibrium sorption capacity for CH4 at 10MPa gas pressure and 11MPa confining pressure (1MPa Terzaghi effective stress) was 0.808mol/kgcoal. This was reduced by at least ~6% by increasing the confining pressure to 43MPa (33MPa effective stress), confirming the validity of our model. We apply our model to predict in-situ CBM concentration as a function of coal seam depth for dry, high volatile bituminous coal, assuming a geothermal gradient of 32°C/km. The results indicate a maximum CH4 concentration of ~0.76mol/kgcoal at a burial depth of ~900m, which is ~3% lower than conventional predictions. This reduction is minor but helps to explain why gas saturation is generally lower than expected from conventional sorption measurements on unconfined coal powders. More importantly, our results confirm that there is an intimate coupling between in-situ stress, strain and sorption in coal that needs to be considered in developing gas-enhanced CBM strategies.