Increase In Pore Fluid Pressure Research Articles

A novel stimulation strategy for developing tight fractured gas reservoir

Conventional stimulation methods such as matrix acidizing, acid fracturing, or proppant fracturing have resulted in products that perform poorly and/or fail within months. Other options, such as water fracs with light sand, give better results but are prohibitively expensive. Mineral composition, brittleness index, stress regime, and petrophysical properties, which are favorable for creating complex fracture networks, can be obtained by geochemical and geomechanical analysis. The extended Reshaw and Pollard criterion shows that hydraulic fractures tend to be arrested by pre-existing natural fractures, and complex fracture networks would be created during fracturing. Additionally, the critical stressed faults theory indicates that the pre-existing natural fractures tend to slip with the shear mode as the pore fluid pressure increases. Rotating disk experiments and conductivity tests with artificial sheared plates have shown that flow channels can be etched at the location of scratches on fracture surfaces. Meanwhile, the carbonate cement in natural fractures can be chelated to form wormhole likely flow channels. Complex fracture networks with sufficient acid etched conductivity can be generated by water fracs with acid. A novel and economical volume stimulation strategy known as network acid fracturing has provided Tarim Oil Company the means to develope ultra-deep, ultra-high pressure, high temperature, and ultralow permeability but fractured gas reservoirs. Post-stimulation production performances of numerous wells with network acid fracturing are comparable to those with stimulated reservoir volumes.

Possible correlation between annual gravity change and shallow background seismicity rate at subduction zone by surface load

The Gravity Recovery and Climate Experiment (GRACE) has monitored global gravity changes since 2002. Gravity changes are considered to represent hydrological water mass movements around the surface of the globe, although fault slip of a large earthquake also causes perturbation of gravity. Since surface water movements are expected to affect earthquake occurrences via elastic surface load or pore-fluid pressure increase, correlation between gravity changes and occurrences of small (not large) earthquakes may reflect the effects of surface water movements. In the present study, we focus on earthquakes smaller than magnitude 7.5 and examine the relation between annual gravity changes and earthquake occurrences at worldwide subduction zones. First, we extract amplitudes of annual gravity changes from GRACE data for land. Next, we estimate background seismicity rates in the epidemic-type aftershock sequence model from shallow seismicity data having magnitudes of over 4.5. Then, we perform correlation analysis of the amplitudes of the annual gravity changes and the shallow background seismicity rates, excluding source areas of large earthquakes, and find moderate positive correlation. It implies that annual water movements can activate shallow earthquakes, although the surface load elastostatic stress changes are on the order of or below 1 kPa, as small as a regional case in a previous study. We speculate that periodic stress perturbation is amplified through nonlinear responses of frictional faults.

Characterizing the Potential for Injection‐Induced Fault Reactivation Through Subsurface Structural Mapping and Stress Field Analysis, Wellington Field, Sumner County, Kansas

AbstractKansas, like other parts of the central U.S., has experienced a recent increase in seismicity. Correlation of these events with brine disposal operations suggests pore fluid pressure increases are reactivating preexisting faults, but rigorous evaluation at injection sites is lacking. Here we determine the suitability of CO2injection into the Cambrian‐Ordovician Arbuckle Group for long‐term storage and into a Mississippian reservoir for enhanced oil recovery in Wellington Field, Sumner County, Kansas. To determine the potential for injection‐induced earthquakes, we map subsurface faults and estimate in situ stresses, perform slip and dilation tendency analyses to identify well‐oriented faults relative to the estimated stress field, and determine the pressure changes required to induce slip at reservoir and basement depths. Three‐dimensional seismic reflection data reveal 12 near‐vertical faults, mostly striking NNE, consistent with nodal planes from moment tensor solutions from recent earthquakes in the region. Most of the faults cut both reservoirs and several clearly penetrate the Precambrian basement. Drilling‐induced fractures (N = 40) identified from image logs and inversion of earthquake moment tensor solutions (N = 65) indicate that the maximum horizontal stress is approximately EW. Slip tendency analysis indicates that faults striking <020° are stable under current reservoir conditions, whereas faults striking 020°–049° may be prone to reactivation with increasing pore fluid pressure. Although the proposed injection volume (40,000 t) is unlikely to reactive faults at reservoir depths, high‐rate injection operations could reach pressures beyond the critical threshold for slip within the basement, as demonstrated by the large number of injection‐induced earthquakes west of the study area.

A Decade of Induced Slip on the Causative Fault of the 2015Mw4.0 Venus Earthquake, Northeast Johnson County, Texas

AbstractOn 7 May 2015, aMw4.0 earthquake occurred near Venus, northeast Johnson County, Texas, in an area of the Bend Arch‐Fort Worth Basin that reports long‐term, high‐volume wastewater disposal and that has hosted felt earthquakes since 2009. In the weeks following theMw4.0 earthquake, we deployed a local seismic network and purchased nearby active‐source seismic reflection data to capture additional events, characterize the causative fault, and explore potential links between ongoing industry activity and seismicity. Hypocenter relocations of the resulting local earthquake catalog span ~4–6 km depth and indicate a fault striking ~230°, dipping to the west, consistent with a nodal plane of theMw4.0 regional moment tensor. Fault plane solutions indicate normal faulting, withBaxes striking parallel to maximum horizontal compressive stress. Seismic reflection data image the reactivated basement fault penetrating the Ordovician disposal layer and Mississippian production layer, but not displacing post‐Lower Pennsylvanian units. Template matching at regional seismic stations indicates that low‐magnitude earthquakes with similar waveforms began in April 2008, with increasing magnitude over time. Pressure data from five saltwater disposal wells within 5 km of the active fault indicate a disposal formation that is 0.9–4.8 MPa above hydrostatic. We suggest that the injection of 28,000,000 m3of wastewater between 2006 and 2015 at these wells led to an increase in subsurface pore fluid pressure that contributed to inducing this long‐lived earthquake sequence. The 2015Mw4.0 event represents the largest event in the continuing evolution of slip on the causative fault.

Distribution, morphology, and morphometry of circular mounds in the elongated basin of northern Terra Sirenum, Mars

An elongated, flat-floored basin, located in the northern part of Terra Sirenum on Mars, holds numerous enigmatic mounds (100 m wide) on the surface of its floor. We investigated their geological context, spatial distribution, morphological characteristics, and morphometric parameters by analyzing a variety of current remote sensing data sets of Mars. Over 700 mounds are identified; mapping of the mounds shows the spatial density of about 21 per 100 km2 and appearances of several clusters, coalescence, and/or alignment. Most of the mounds have smoother surface textures in contrast to the rugged surrounding terrain. Some of the mounds display depressions on their summits, meter-sized boulders on their flanks, and distinct lobate features. We also perform high-resolution topographic analysis on 50 isolated mounds, which reveals that their heights range from 6 to 43 m with a mean of 18 m and average flank slopes of most mounds are below 10°. These characteristics are consistent with the deposition and extension of mud slurries with mud breccia and gases extruded from subsurface, almost equivalent to terrestrial mud volcanism. If so, both abundance of groundwater and abrupt increase in pore fluid pressure are necessary for triggering mud eruption. Absolute crater retention ages suggest that the floor of the basin located among middle Noachian-aged highland terrains has been resurfaced during the Late Hesperian Epoch. Because of cross-cutting relationships with the basin and the mounds superposed on the basin floor, the faults and fissures (part of Memnonia Fossae) are thought to have formed during and/or after the period of mound formation. Compressional stress fields which likely formed Memnonia Fossae and Mangala Valles, expected from the dike emplacement model of Wilson and Head (JGR 107:1-1–1-24, 2002), may have facilitated undercompaction or overpressurization of subsurface fluids, focused pore fluids beneath the basin, and opened conduits along faults for upwelling voluminous sediments and fluids.

Controls of hydrocarbon generation on the development of expulsion fractures in organic-rich shale: Based on the Paleogene Shahejie Formation in the Jiyang Depression, Bohai Bay Basin, East China

The development of expulsion fractures in organic-rich shale is closely related to hydrocarbon generation and expulsion from kerogen. Organic-rich shales from the upper part of the fourth member and the lower part of the third member of the Paleogene Shahejie Formation in the Jiyang Depression, Bohai Bay Basin, East China, are used as an example. Based on thin sections, SEM and thermal simulation experiments, the characteristics of hydrocarbon generation and the conditions supporting the development of expulsion fractures were explored. The key factors influencing these fractures include the presence of kerogens, their distribution along laminae and around particle boundaries, their exposure to heat and the build-up in pressure due to confinement by low permeability. The development of excess pore fluid pressures and intrinsic low rock fracture strength are the main influencing factors. Pressurization by rapid generation of hydrocarbon provides impetus for fracture initiation and cause bitumen to migrate quickly. The shale laminae results in distinctly lower fracture strength laminae-parallel than laminae-normal and this directs the formation of new fractures in the direction of weakness. When pore fluid pressure increases, maximum and minimum principal effective stresses decrease by different proportions with a larger reduction in the maximum principal effective stress. This increases the deviatoric stress and reduces the mean stress, thus driving the rock towards failure. Moreover, the tabular shape of the kerogen aids the generation of hydrocarbon and the initiation of expulsion fractures from the tip and edge. The resulting fractures extend along the laminae when the tensile strength is lower in the vertical direction than in the horizontal direction. Particle contact boundaries are weak and allow fractures to expand around particles and to curve as the stress/strength regime changes. When pore fluid pressure fields at different fracture tips overlap, fractures will propagate and interconnect, forming a network. This paper could provide us more detailed understanding of the forming processes of expulsion fractures and better comprehension about hydrocarbon expulsion (primary migration) in source rocks.

Geologic influence on induced seismicity: Constraints from potential field data in Oklahoma

AbstractRecent Oklahoma seismicity shows a regional correlation with increased wastewater injection activity, but local variations suggest that some areas are more likely to exhibit induced seismicity than others. We combine geophysical and drill hole data to map subsurface geologic features in the crystalline basement, where most earthquakes are occurring, and examine probable contributing factors. We find that most earthquakes are located where the crystalline basement is likely composed of fractured intrusive or metamorphic rock. Areas with extrusive rock or thick (>4 km) sedimentary cover exhibit little seismicity, even in high injection rate areas, similar to deep sedimentary basins in Michigan and western North Dakota. These differences in seismicity may be due to variations in permeability structure: within intrusive rocks, fluids can become narrowly focused in fractures and faults, causing an increase in local pore fluid pressure, whereas more distributed pore space in sedimentary and extrusive rocks may relax pore fluid pressure.

Textural variations and fragmentation processes in peperite formed between felsic lava flow and wet substrate: An example from the Cretaceous Buan Volcanics, southwest Korea

Multiple exposures of peperite within the Cretaceous Buan Volcanics, southwest Korea, have been examined in order to determine variations in their textural characteristics and to investigate their mode of formation. Along undulating boundaries between rhyolite (lava flow) and deformed host sediment expressed as a series of load and flame structures, exposures commonly contain two distinct types of peperite. Type-1 peperites are composed mostly of rounded juvenile clasts at their base and polyhedral juvenile clasts at their upper levels, interpreted to have formed via a two-stage process. Firstly, abrasion of juvenile clasts occurred after their fragmentation due to shear stress imparted by the overlying and still-moving lava flow, forming rounded juvenile clasts. Subsequent in situ quenching fragmentation of the lava flow produced clasts with platy to polyhedral shapes immediately after emplacement of the lava flow. Type-2 peperites laterally extend into the interior of featureless rhyolite as layers that decrease in thickness with increasing distance away from the flame zone. These layers exhibit horizontal textural variations, ranging from poorly sorted mixtures of ash- to block-sized angular juvenile clasts in the proximal zone, to closely packed polyhedral and tabular juvenile clasts with jigsaw-crack textures in the middle and distal zones. Type-2 peperite are inferred to have formed due to internal steam explosions that resulted from an expansion of heated pore water (leading to an increase in pore fluid pressure) that had been vertically injected into the interior of the rhyolite from the flame zone. The proximal zone, composed mainly of poorly sorted mixtures of juvenile clasts, represents the explosion sites. Juvenile clasts in the middle and distal zones are interpreted to have formed due to three separate processes: the development of fractures in the rhyolite during the internal steam explosions, injection of the host sediment through the fractures, and in situ quenching fragmentation. Deformation of the host sediment exerted an important control on peperite-forming processes, with the internal steam explosions suggested to have formed the closely packed, juvenile clasts with a jigsaw-crack texture rather than the clasts that are widely dispersed.

Fluid–rock interaction and evolution of a high-pressure/low-temperature vein system in eclogite from New Caledonia: insights into intraslab fluid flow processes

A complex high-pressure/low-temperature vein system that cross-cuts eclogitic host rocks of the Pouebo Eclogite Melange (northern New Caledonia) records the prograde blueschist-to-eclogite transition and associated formation of garnet–quartz–phengite veins. Geothermobarometry (Grt–Cpx–Ph, Zr-in-rutile) and pseudosection calculations indicate peak metamorphic conditions of ca. 540 °C and 1.9–2.2 GPa. Petrological and geochemical observations as well as pseudosection modelling suggest that the main vein network is formed by dehydration processes that collected internally derived fluids related to the breakdown of hydrous phases (amphibole, chlorite, epidote) during prograde metamorphism. The lower solid volume of the newly formed phases and the associated increase in pore fluid pressure lead to the formation of veins that allowed for accumulation and channelized evacuation of these fluids. Such veins do not show metasomatic alteration selvages because the fluid–rock system had been in chemical equilibrium. A second vein type (transport veins) records the superimposed influx of external fluids with slightly different composition that most likely are related to similar dehydration reactions in other parts of the subducting slab. Due to the source-rock-imposed compositional differences, these fluids are not in equilibrium with the infiltrated rock volume and induce the formation of distinct metasomatic selvages by dissolution–precipitation processes. Mass-balance calculations show that Ca, Na and Li are added to the selvage by the external fluid. LILE and to a lesser extend also HREE are mobilized and removed from the selvage. The LREE are predominantly buffered by newly formed minerals (e.g. epidote). Petrological evidence implies that the studied vein system formed while the sample was still part of a coherent subducting slab. Rb–Sr geochronology indicates that this occurred at 38.2 ± 0.3 Ma. This age is ca. 6 myr younger than the hitherto presumed peak metamorphic age, suggesting a more complex and protracted metamorphic history for the PEM.

Groundwater recharge as the trigger of naturally occurring intraplate earthquakes

Abstract I explore the hypothesis that most intraplate earthquakes and their aftershock sequences are triggered by pore-fluid pressure increases. As proposed in this paper, data from the magnitude 5.7 Virginia earthquake of 23 August 2011 show that this is a two-step process. (1) First, from areas where there is greater than normal meteoric recharge, pore-fluid pressure diffusion by means of Biot slow waves transfers more pore-fluid pressure towards a future hypocentre. Here the cumulation of Biot slow waves produces a steady increase in pore-fluid overpressure until a main shock is triggered. (2) Then, aftershocks occur in the zone reaching from the depth of the main shock to a depth of a few kilometres below the land surface, preferring to localize in a weaker, pervasive anisotropic crustal fabric, in response to locally increased permeability and pore-fluid pressure transients caused by the main shock. The primary corrosive agent responsible for reducing the strength of silicate minerals in this upper crustal zone is water, so that quartz-rich crust tends to have lower values of Poisson's ratio. I show here that increases in pore-fluid overpressure from normal groundwater recharge can start crack dilation leading to fracturing and the creation of new permeability. Previous chemical analyses across the Central Virginia Piedmont that hosted the 2011 Virginia shock show high upper crustal quartz content. This proposed two-step model for a main shock-aftershock sequence explains why intraplate earthquakes are rarely correlated with recognizable brittle faults at the Earth's surface. Supplementary material: A biography of John Costain is available at https://dx.doi.org/10.6084/m9.figshare.c.2854324.v3

Potential of porosity waves for methane transport in the Eugene Island field of the Gulf of Mexico basin

The Eugene Island minibasin and hydrocarbon field in the northern Gulf of Mexico basin is a well-documented occurrence of anomalously rapid fluid migration. A variety of evidence suggests that hydrocarbons ascended through kilometers of very low permeability sediments separating source rocks and reservoirs at velocities up to kilometers per year. Previous research has shown that porosity waves could transport petroleum at millimeter per year velocities under a narrow range of low permeabilities. The purpose of the current research was to test the hypothesis that porosity waves could transport methane much more rapidly than oil.To test this hypothesis, a one-dimensional numerical model of porosity wave behavior was constructed for Eugene Island sediments saturated with methane. The results show that gradual rates of pore fluid pressure generation typically caused by diagenesis are too slow for porosity waves to transport methane at kilometer per year rates. Instead, essentially geologically instantaneous pore fluid pressure increases are needed, which then could allow porosity waves to ascend 1–2 km at velocities >10's of m/year. Thus, porosity waves could only reach the lower reservoirs at Eugene Island, but could transport methane orders of magnitude faster than the background Darcian flow regime. Based on their predicted size, 10's of porosity waves would have been needed to charge these lower reservoirs with their original amount of methane. Whether a mechanism for instantaneous pore fluid pressure generation exists at Eugene Island is unclear. Earthquakes are capable of generating essentially instantaneous pore fluid pressure increases of order 1's of MPa or greater, although Eugene Island's seismic history is thought to have been relatively quiet. Thus, despite their high velocities, porosity waves are unlikely to have played a major role in transporting methane at Eugene Island, but could have in other more seismically active locations where required methane transport distances are smaller.

Permeability control on transient slip weakening during gypsum dehydration: Implications for earthquakes in subduction zones

A conflict has emerged from recent laboratory experiments regarding the question of whether or not dehydration reactions can promote unstable slip in subduction zones leading to earthquakes. Although reactions produce mechanical weakening due to pore-fluid pressure increase, this weakening has been associated with both stable and unstable slip. Here, new results monitoring strength, permeability, pore-fluid pressure, reaction progress and microstructural evolution during dehydration reactions are presented to identify the conditions necessary for mechanical instability. Triaxial experiments are conducted using gypsum and a direct shear sample assembly with constant normal stress that allows the measurement of permeability during sliding. Tests are conducted with temperature ramp from 70 to 150 °C and with different effective confining pressures (50, 100 and 150 MPa) and velocities (0.1 and 0.4 μm s−1). Results show that gypsum dehydration to bassanite induces transient stable-slip weakening that is controlled by pore-fluid pressure and permeability evolution. At the onset of dehydration, the low permeability promoted by pore compaction induces pore-fluid pressure build-up and stable slip weakening. The increase of bassanite content during the reaction shows clear evidence of dehydration related with the development of R1 Riedel shears and P foliation planes where bassanite is preferentially localized along these structures. The continued production of bassanite, which is stronger than gypsum, provides a supporting framework for newly formed pores, thus resulting in permeability increase, pore-fluid pressure drop and fault strength increase. After dehydration reaction, deformation is characterized by unstable slip on the fully dehydrated reaction product, controlled by the transition from velocity-strengthening to velocity-weakening behaviour of bassanite at temperature above ∼140 °C and the localization of deformation along narrow Y-shear planes. This study highlights the generic conditions required to trigger instabilities during dehydration reactions. It shows that pore-fluid pressure build-up during dehydration reactions associated with the localization of a velocity-weakening reacting or dehydrated phase along shear planes is necessary for earthquake triggering.

Pore pressure embrittlement in a volcanic edifice

The failure mode of porous rock in compression—dilatant or compactant—is largely governed by the overlying lithostatic pressure and the pressure of pore fluids within the rock (Wong, Solid Earth 102:3009–3025, 1997), both of which are subject to change in space and time within a volcanic edifice. While lithostatic pressure will tend to increase monotonously with depth due to the progressive accumulation of erupted products, pore pressures are prone to fluctuations (during periods of volcanic unrest, for example). An increase in pore fluid pressure can result in rock fracture, even at depths where the lithostatic pressure would otherwise preclude such dilatant behaviour—a process termed pore fluid-induced embrittlement. We explore this phenomenon through a series of targeted triaxial experiments on typical edifice-forming andesites (from Volcan de Colima, Mexico). We first show that increasing pore pressure over a range of timescales (on the order of 1 min to 1 day) can culminate in brittle failure of otherwise intact rock. Irrespective of the pore pressure increase rate, we record comparable accelerations in acoustic emission and strain prior to macroscopic failure. We further show that oscillating pore fluid pressures can cause iterative and cumulative damage, ultimately resulting in brittle failure under relatively low effective mean stress conditions. We find that macroscopic failure occurs once a critical threshold of damage is surpassed, suggesting that only small increases in pore pressure may be necessary to trigger failure in previously damaged rocks. Finally, we observe that inelastic compaction of volcanic rock (as we may expect in much of the deep edifice) can be overprinted by shear fractures due to this mechanism of embrittlement. Pore fluid-induced embrittlement of edifice rock during volcanic unrest is anticipated to be highest closer to the conduit and, as a result, may assist in the development of a fractured halo zone surrounding the conduit, potentially explaining commonly observed near-conduit outgassing at many active volcanoes. Further, rock embrittlement at depth may create transient outgassing pathways by linking fracture networks near the edifice to larger-scale regional fault systems. Our experimental results affirm that pore pressure fluctuations associated with volcanic unrest may play a crucial role in dictating the evolution of a volcanic system.

A coupled discrete element‐finite difference approach for modeling mechanical response of fluid‐saturated porous materials

SummaryA model of fluid‐saturated poroelastic medium was developed based on a combination of the discrete element method and grid method. The developed model adequately accounts for the deformation, fracture, and multiscale internal structure of a porous solid skeleton. The multiscale porous structure is taken into account implicitly by assigning the porosity and permeability values for the enclosing skeleton, which determine the rate of filtration of a fluid. Macroscopic pores and voids are taken into account explicitly by specifying the computational domain geometry. The relationship between the stress–strain state of the solid skeleton and pore fluid pressure is described in the approximations of simply deformable discrete element and Biot's model of poroelasticity. The developed model was applied to study the mechanical response of fluid‐saturated samples of brittle material. Based on simulation results, we constructed a generalized logistic dependence of uniaxial compressive strength on loading rate, mechanical properties of fluid and enclosing skeleton, and on sample dimensions. The logistic form of the generalized dependence of strength of fluid‐saturated elastic–brittle porous materials is due to the competition of two interrelated processes, such as pore fluid pressure increase under solid skeleton compression and fluid outflow from the enclosing skeleton to the environment. Copyright © 2015 John Wiley & Sons, Ltd.

Long-term evolution of intraplate seismicity in stress shadows after a megathrust

Megathrusts produce large permanent lithospheric displacements as well as strong transient ground shaking up to regional distances. The influence of the 2011 M9.0 Tohoku-Oki earthquake on the seismicity in stable intraplate regions around the Korean Peninsula is investigated. The differential lateral displacements by the megathrust build transient radial tension field over the backarc lithosphere, constructing stress shadows for all types of faults in optimal orientation over the wide backarc region including the southern Korean Peninsula. A characteristic seismic-velocity decrease after the megathrust supports the medium relaxation. The number of earthquakes with magnitudes greater than or equal to 2.5 was increased by 61%after the megathrust. The earthquakes occurred episodically. A series of unusual earthquake swarms and a temporal cluster of moderate-size earthquakes were observed in the Yellow Sea region. The significant seismicity increase since the megathrust may be due to the fluid diffusion during the transient tension field and the pore-fluid pressure increase during the ambient compressional-stress field recovery by tectonic loading. The induced seismicity may continue until the ambient stress field is fully recovered.

Spatial association of mud volcano and sandstone intrusions, Boyadag anticline, western Turkmenistan

The mud volcano and sandstone intrusions complex occurring in the Boyadag anticline, western Turkmenistan, is the only well-documented example of co-existing, but not synchronous, mud volcanism and sand intrusion. Integrated field and laboratory evaluation investigates the spatial and genetic relations between the mud extrusion and the later sand intrusion. A sandstone dike and a pillar pierce mud volcano deposits on the crest of Boyadag anticline. Two more dikes occur near the escarpment caused by a crestal normal fault. The rising of mud and hydrocarbons from the Oligocene Maykop Fm. fed the mud volcano after the exposure of the Lower Pleistocene units at the core of the anticline. The main physical process that later led to sand fluidisation is identified as the progressive increase in pore fluid pressure, during a stage of reduced or null activity of the mud volcano, caused by the up-dip migration of hydrocarbons from the deep basin into the Pliocene sandstone reservoir within the Boyadag anticline. The hydrocarbons generated in the source rock levels of the Maykop Fm., whereas the saline water involved in the sand fluidisation is identified as the connate water of the Pliocene Red Beds Fm. The pressure rise was responsible for the fracturing of the sealing units, already weakened by the crestal normal fault and the mud volcano feeding system. The sand intruded into sealing units and mud breccia deposits, also using the mud volcano conduit and the crestal normal fault as preferential pathways. During the present-day background activity, the outcropping sandstone intrusions provide high permeability pathways for continuous fluid leakage, precluding the re-establishment of high pressures in the reservoir.

Natural hydraulic fracturing of tight-gas sandstone reservoirs, Piceance Basin, Colorado

Natural fractures form preferred pathways for basinal fluid flow and associated heat and mass transport. In gas sandstone reservoirs with low matrix permeability, fractures provide flow pathways between organic-rich source and reservoir layers during gas charge, and between matrix pores, hydraulic fractures, and the well bore during production. While the formation of natural fractures has previously been associated with gas generation and pore-fluid pressure increase through a process referred to as natural hydraulic fracturing, other driving mechanisms such as stress changes by tectonic or exhumation processes remained viable alternatives. To test whether these mechanisms contributed to fracture development, we investigated the spatial and temporal distribution of fracture formation and its relationship to gas generation, migration, and charge in sandstone of the Cretaceous Mesaverde Group across the entire production interval on a basinwide scale. Using fluid inclusion microthermometry of crack-seal fracture cement formed concurrently with fracture opening, we observed temperature trends that, when compared with temperature evolution models of the formation, date fracture formation between 41 and 6 Ma in the northern and between 39 and 6 Ma in the southern Piceance Basin. The onset of fracture formation 20–30 m.y. prior to maximum burial eliminates changes in stress state associated with exhumation as a mechanism for triggering the onset of fracture formation. Instead, calculated paleo–pore-fluid pressures of 40–90 MPa (5800–13,000 psi) during fracture opening and the presence of methane-rich inclusions in fracture cement suggest that fracture formation was aided by high pore-fluid pressures during gas generation in organic-rich shales and coals and associated charging of adjacent and interlayered sandstone reservoirs. A 10–20 m.y. age progression in the onset of fracture formation from deeper to shallower horizons of the Mesaverde Group is consistent with gas generation and onset of fracture formation activated by burial temperature with limited upward migration of gas at this stage of reservoir evolution. This age progression with depth is inconsistent with fracture formation triggered by changes in stress conditions associated with tectonic or structural processes expected to affect the entire formation synchronously. Our observations are thus most consistent with fracture formation by natural hydraulic fracturing in response to gas generation in interbedded source layers and reservoir charge. Based on widespread observations of fractures with similar structural and diagenetic attributes, we consider natural hydraulic fracture formation in response to thermocatalytic gas generation to be a fundamental mode of brittle failure in otherwise structurally quiescent basins.

Dewatering structure and soft-sediment deformation controlled by slope instability: examples from the late Miocene to Pliocene Miura–Boso accretionary prism and trench-slope basin, central Japan

Abstract Systematic arrays of dewatering structure and soft-sediment deformation (e.g. convolute laminations and flame structures possibly associated with dewatering) were examined in the late Miocene to Pliocene Miura–Boso accretionary prism and Plio-Pleistocene trench-slope basin, central Japan, with respect to paleoslope inclination. Numerous preferred orientations of these structures strike perpendicular to the slumping direction. The observed features, including the measured orientations and field observations that dewatering-induced foliations terminated by the slip plane of a slope failure deposit, indicate that the structures formed under the influence of slope instability. The foliations from both the upper part of the accretionary prism and the slope basin correspond to plausible “pseudo-hydraulic fracture” planes, parallel to σ 1 and perpendicular to σ 3 , and are associated with slope instability. Pore fluids or liquidized silt escaped along the foliations until pore fluid pressures along the uppermost and lowermost levels of the foliated zone were balanced. Consequently, pore fluid pressure increased just below the relatively impermeable shear planes. This pore fluid migration and increase in pore fluid pressure would have been critical for slope instability and could have triggered failures, especially within sediments close to the critical state. The findings indicate that pore-fluid behavior associated with shear stress controls the failure patterns and their locations.

Pressure constraints on the CO 2 storage capacity of the saline water-bearing parts of the Bunter Sandstone Formation in the UK Southern North Sea

The Bunter Sandstone Formation (BSF) in the UK sector of the Southern North Sea is thought to have a significant potential for the injection and storage of anthropogenic CO 2 within periclines that lie above salt domes and pillows formed by halokinesis in underlying Zechstein strata. During the formation of the periclines, the BSF and its overlying top seals were subjected to extensional stresses and, in consequence, are commonly cut by seismically resolvable faults that present a risk to the containment of gas and buoyant fluids such as supercritical CO 2 . Although most of the closed structures in the BSF are saline water-bearing, eight gas fields (total gas initially-in-place >72 bcm (billion cubic metres)) have been discovered to date. The seismically resolved structure of these gas fields demonstrates that two different top seals, the Haisborough Group and the Speeton Clay, can seal gas columns of up to 128 and 104 m respectively, despite the presence of faults with small displacements above the field gas–water contacts. The observed gas columns are equivalent to CO 2 columns of up to around 100 m in height. Simple geomechanical modelling suggests that existing optimally orientated faults may dilate or be reactivated if the pore-fluid pressure increase as a result of CO 2 injection exceeds a gradient of about 13.4 MPa km –1 , potentially resulting in loss of storage integrity.

Changing fluid conditions during folding: An example from the central Appalachians

Fluids inclusion microthermometry and stable isotope analyses of vein minerals in the central Appalachian Patterson Creek anticline record dynamic changes in fluid temperature, pressure, and chemistry during folding. These changes resulted from variations in fracture connectivity and fluid communication between stratigraphically restricted fluid reservoirs. Minerals in early-formed bed parallel and ENE–NNW striking cross-fold veins show that prior to folding, fluids in Silurian through lower Upper Devonian rocks were well stratified based temperature, salinity and isotopic composition. Stable isotopes data indicates that the fluid systems were essentially closed, with no communication between layers. As folding commenced, early minerals in NE–NW striking veins in the Siluro-Devonian carbonate rocks record 2km of burial by syn-tectonic sedimentation, with corresponding increases in fluid temperature and pore-fluid pressure. An additional 2km of burial along with the influx of low-salinity, methane-saturated brines into the Middle and Upper Devonian clastic rock sequence is recorded by later quartz in the NE–NW striking veins. At some point during folding, Stable isotope data indicate that fluid communication increased along fracture and faults between major stratigraphic fluids reservoirs. The culmination of this process was the breaching of the stratigraphic seal at the base of the Devonian shales, allowing fluid infiltration from the Devonian shale sequence into the upper portion of the Helderberg Group. This resulted in the local precipitation of quartz and possibly the enhancement of solution structures.