Overpressure Build-up Research Articles

Seal bypass at the Giant Gjallar Vent (Norwegian Sea): Indications for a new phase of fluid venting at a 56-Ma-old fluid migration system

Highlights: • The Giant Gjallar Vent is still active in terms of fluid migration and faulting. • The Base Pleistocene Unconformity acts as a seal to upward fluid migration. • Seal bypass in at least one location leads to a new phase of fluid venting. The Giant Gjallar Vent (GGV), located in the Voring Basin off mid-Norway, is one of the largest (~ 5 × 3 km) vent systems in the North Atlantic. The vent represents a reactivated former hydrothermal system that formed at about 56 Ma. It is fed by two pipes of 440 m and 480 m diameter that extend from the Lower Eocene section up to the Base Pleistocene Unconformity (BPU). Previous studies based on 3D seismic data differ in their interpretations of the present activity of the GGV, describing the system as buried and as reactivated in the Upper Pliocene. We present a new interpretation of the GGV’s reactivation, using high-resolution 2D seismic and Parasound data. Despite the absence of geochemical and hydroacoustic indications for fluid escape into the water column, the GGV appears to be active because of various seismic anomalies which we interpret to indicate the presence of free gas in the subsurface. The anomalies are confined to the Kai Formation beneath the BPU and the overlying Naust Formation, which are interpreted to act as a seal to upward fluid migration. The seal is breached by focused fluid migration at one location where an up to 100 m wide chimney-like anomaly extends from the BPU up to the seafloor. We propose that further overpressure build-up in response to sediment loading and continued gas ascent beneath the BPU will eventually lead to large-scale seal bypass, starting a new phase of venting at the GGV.

Advanced Workflows for Fluid Transfer in Faulted Basins

The traditional 3D basin modeling workflow is made of the following steps: construction of present day basin architecture, reconstruction of the structural evolution through time, together with fluid flow simulation and heat transfers. In this case, the forward simulation is limited to basin architecture, mainly controlled by erosion, sedimentation and vertical compaction. The tectonic deformation is limited to vertical slip along faults. Fault properties are modeled as vertical shear zones along which rock permeability is adjusted to enhance fluid flow or prevent flow to escape.For basins having experienced a more complex tectonic history, this approach is over-simplified. It fails in understanding and representing fluid flow paths due to structural evolution of the basin. This impacts overpressure build-up, and petroleum resources location.Over the past years, a new 3D basin forward code has been developed in IFP Energies nouvelles that is based on a cell centered finite volume discretization which preserves mass on an unstructured grid and describes the various changes in geometry and topology of a basin through time. At the same time, 3D restoration tools based on geomechanical principles of strain minimization were made available that offer a structural scenario at a discrete number of deformation stages of the basin.In this paper, we present workflows integrating these different innovative tools on complex faulted basin architectures where complex means moderate lateral as well as vertical deformation coupled with dynamic fault property modeling.Two synthetic case studies inspired by real basins have been used to illustrate how to apply the workflow, where the difficulties in the workflows are, and what the added value is compared with previous basin modeling approaches.

Mechanism of Overpressure Formation in Deeply Burial Sandstone

Overpressure is often encountered in the Jurassic tight and the overpressure is closely associated with gas generation. The pressure transfer from the over-pressurized mudstones to adjacent tight sandstones might occur through overpressure induced-fractures. The fine-grained coal containing Jurassic sandstone is sensitive to compaction, and the porosity decreases dramatically with the increase of overlying load. As gas migrates into the tight sandstones, it must overcome the capillary pressure which is greater than the hydrostatic pressure. The gas charging pressure in the tight sandstone must be higher than the capillary pressure, resulting in an overpressure buildup within the tight sandstones. Gas shows, low permeability and strong diagenesis in the overpressure of the tight sandstone system have been observed. Additionally, capillary seals are identified as playing an important role in the mechanism of the overpressure formation in tight sandstone reservoirs. Overpressure might be a driving force to create induced fractures in the interval, which has applications for crossing-formation migration and gas accumulation.

Relating vesicle shapes in pyroclasts to eruption styles

Vesicles in pyroclasts provide a direct record of conduit conditions during explosive volcanic eruptions. Although their numbers and sizes are used routinely to infer aspects of eruption dynamics, vesicle shape remains an underutilized parameter. We have quantified vesicle shapes in pyroclasts from fall deposits of seven explosive eruptions of different styles, using the dimensionless shape factor $\Omega $ , a measure of the degree of complexity of the bounding surface of an object. For each of the seven eruptions, we have also estimated the capillary number, Ca, from the magma expansion velocity through coupled diffusive bubble growth and conduit flow modeling. We find that $\Omega $ is smaller for eruptions with ${\rm{Ca}} \ll 1$ than for eruptions with Ca $\gg 1$ . Consistent with previous studies, we interpret these results as an expression of the relative importance of structural changes during magma decompression and bubble growth, such as coalescence and shape relaxation of bubbles by capillary stresses. Among the samples analyzed, Strombolian and Hawaiian fire-fountain eruptions have ${\rm{Ca}} \ll 1$ , in contrast to Vulcanian, Plinian, and ultraplinian eruptions. Interestingly, the basaltic Plinian eruptions of Tarawera volcano, New Zealand in 1886 and Mt. Etna, Italy in 122 BC, for which the cause of intense explosive activity has been controversial, are also characterized by ${\rm{Ca}} \gg 1$ and larger values of $\Omega $ than Strombolian and Hawaiian style (fire fountain) eruptions. We interpret this to be the consequence of syn-eruptive magma crystallization, resulting in high magma viscosity and reduced rates of bubble growth. Our model results indicate that during these basaltic Plinian eruptions, buildup of bubble overpressure resulted in brittle magma fragmentation.

Consequences of Water Level Drops for Soft Sediment Deformation and Vertical Fluid Leakage

Both water level drops and erosion have previously been suggested as causes of fluid overpressures in the subsurface. Quantification of the relevance of these processes to supra-lithostatic fluid pressure formation with a wide selection of input parameters is lacking, and thus desired. The magnitudes and drop times that are required for water level drops to result in supra-lithostatic pore pressures in a variety of situations are calculated. Situations with pore fluids consisting of water, water with dissolved methane, water with a gas hydrate layer and dissolved methane in the underlying sediments, and water with dissolved methane, a gas hydrate layer, and free gas accumulation below the hydrate layer are separately addressed. The overpressure formation from reservoir gas expansion is also simulated. The simulation results demonstrate that high fluid overpressures can develop in a rock as a response to a water level drop without the presence of gas, provided that the rock has a sufficiently low compressibility. The contribution to fluid overpressuring is however dramatically increased if the pore water is saturated with methane prior to the water level drop, and is further amplified by the presence of gas hydrates and free gas accumulations beneath such hydrates. Gas expansion in reservoirs should be expected to significantly increase the fluid overpressures in shallow, sealed pressure compartments that experience erosion or water level drops, even if the water level drop duration exceeds one million years. Enough relationships between the calculated overpressure formation and the main controlling factors are provided in order to enable readers to make inferences about a variety of geological settings. Analyses of simulation results prompt us to suggest that pockmarks are likely to be triggered by gas expansion in vertical fluid migration pathways, that the giant craters at the seabed west of Albatross South in the Barents Sea result from hydrate dissociation, and that overpressure build-up due to gas expansion has contributed to reservoir overpressuring in many eroded basins, including the Hammerfest Basin in the Barents Sea.

Vesiculation in magmas from Stromboli and implications for normal Strombolian activity and paroxysmal explosions in basaltic systems

We performed a series of X‐ray tomographic experiments and lattice Boltzmann permeability simulations on pyroclastic products from explosive activity at Stromboli between December 2004 and May 2006. We reconstructed the 3‐D textures of vesicles to investigate the relationship between the nature of vesiculation in the erupted products and the dynamics of gas transport in the shallow conduit in order to derive implications for the eruptive behavior of basaltic volcanoes. Scoriae from normal Strombolian explosions display remarkably consistent vesicle volume distributions fit by power laws with an exponent of 1 (±0.2). We ascribe the origin of such distributions to the combined effect of coalescence and continuous nucleation events in the steady state, shallow magma system that supplies normal Strombolian activity. Volume distributions and textures of vesicles in pumice clasts from the 5 April 2003 and 15 March 2007 paroxysmal activity are markedly different from those in the scoriae. Besides a power law function with a higher exponent, portions of these distributions can be also fit by an exponential function, suggesting the attempt of the system to reach near‐equilibrium conditions. The investigated pumice clasts also lack the large, connecting vesicles responsible for the development of degassing pathways in the Stromboli magma that erupts the scoriae. This testifies to a decreased degassing efficiency of the magma associated with paroxysmal explosions and potential overpressure buildup at depth. By comparison with degassing experiments on basaltic melts, we derive a time constraint on the order of minutes to hours for the incubation of paroxysms at Stromboli.

Overpressure and slope stability in prograding clinoforms: Implications for marine morphodynamics

[1] Rapid deposition of fine-grained sediments can lead to overpressure buildup along the fronts of prograding sedimentary bodies (clinoforms) ranging from deltas to continental margins. Overpressure reduces shear strength, which can lead to failure of submarine slopes, so that overpressure prediction is potentially of fundamental importance to marine morphodynamics. We generalize one-dimensional overpressure theory to migrating clinoforms where deposition is highly localized in space and time, and show that overpressure is characterized by a dimensionless loading intensity, the Gibson number (given by the product of sediment supply and slope, divided by sediment hydraulic diffusivity). Our results show that compared to terrestrial slopes, slope stability in overpressured clinoforms is especially sensitive to foreset steepness, which directly affects loading intensity. The utility of our theory is demonstrated by application to modern and ancient clinoforms with documented slope failures. This analysis suggests that (1) most muddy clinoforms will have significant overpressure, (2) many delta fronts and continental margins prograde at a limiting slope threshold for deep-seated landsliding, and (3) shallow overpressure and near-surface liquefaction may contribute to mobilization of fluid muds which actively prograde subaqueous deltas. We derive quantitative relationships between sediment supply and slope for clinoforms prograding at limiting equilibrium. Importantly, our results suggest that slope is an inverse function of sediment supply in these systems, implying that commonly used diffusive morphodynamics approaches may be inappropriate.

Overpressure development in rift basins: an example from the Malay Basin, offshore Peninsular Malaysia

The Malay Basin is a Tertiary transtensional rift basin located in offshore Peninsular Malaysia. A study of the subsurface pressure data has revealed at least two major overpressure compartments that are sealed by regional shale units. The main, basin-centre overpressure compartment has a domal shape and, in profile, shows a convex-up top-of-overpressure surface. In the basin centre, the top of overpressure is generally between 1900 m and 2000 m depth, and is confined mainly to a particular stratigraphic unit, the Middle Miocene unit E. It appears that the top of overpressure is influenced by the underlying regional shale seal in unit F, and that the mild overpressure encountered in unit E represents the overpressure transition zone. Due to the convex-upward top-of-overpressure surface, overpressure is generally encountered at shallower depths in the basin centre compared to the basin flanks. A smaller overpressure compartment is also identified on the northeastern flank of the basin. This compartment is sealed by the onlapping, transgressive shale of unit L (Lower Miocene) and occurs at a depth of 2600–3000 m. Hence, regional shale seals have a strong influence on overpressure distribution. Disequilibrium compaction is believed to be the primary causal mechanism for the overpressure in the basin centre. Overpressure development is the consequence of high subsidence and sedimentation rates during basin extension. Low subsidence and sedimentation rates on the basin flanks do not generate overpressure. The domal shape of the top-of-overpressure surface is thus a result of different rates of subsidence and sedimentation across the basin. The basin's overpressure history is simulated by means of a simple model. Modelling results indicate that the basin overpressure developed very early, during the syn-rift phase ( c. 30–21 Ma), when sediment burial rates were very high (>1000 m Ma −1 ). Overpressure build-up occurred rapidly during the syn-rift phase but has started to dissipate gradually since the post-rift phase began 21 Ma ago, when sedimentation rates were well below 1000 m Ma −1 . This suggests that disequilibrium compaction as an overpressure-generating mechanism was effective during only the syn-rift phase of basin development. Sedimentation rates during the post-rift phase (generally less than 500 m Ma −1 ) were not high enough to generate overpressure. The pre-existing overpressure is thus able to dissipate through the sedimentary column, causing the build-up of pore pressure in the post-rift section. Thus, the overpressure in the post-rift strata is probably of secondary origin, derived from excess pressure dissipated from the underlying syn-rift strata. Overpressure generated by disequilibrium compaction during the syn-rift phase has been modified and re-distributed as the basin evolved through the post-rift phase.

Velocities of deepwater reservoir sands

Deepwater reservoirs, those in water depths ranging from 1000 m to more than 3000 m, often consist of young turbidite sediments associated with early hydrocarbon charge, overpressure buildup, and seal with retarded diagenesis. Deepwater sands maintain shallow properties even at great depths (e.g., 18 000 ft) but these weakly cemented sands—with a history of progressive compaction and cementation—differ from surface sediments.

Giant clastic intrusions primed by silica diagenesis

Postdepositional sediment mobilization is a common phenomenon that occurs over large areas of sedimentary basins, usually within mudstone-rich successions; however, defining the exact mechanism that initiates this process has proved elusive. Here we describe a number of large-scale clastic intrusions (injectites), 50 to >300 m in height, hosted within fine-grained biosiliceous upper Eocene to Pliocene strata in four sedimentary basins. Within these basins diagenetic conversion of opal A into opal CT (cristobalite and tridymite) causes a marked reduction in sediment porosity and the liberation of bound water, often at relatively shallow burial (<0.5 km). We propose that this occurs over geologically short time periods over large expanses of sedimentary basins, causing rapid pore fluid expulsion and overpressure buildup within interbedded sands. The liberated fluids primed these sands, rendering them susceptible to remobilization. This mechanism should have broad applicability.

Brittle–ductile transition, shear failure and leakage in shales and mudrocks

This paper presents an experimental study on brittle–ductile transition, shear behaviour, the formation of shear fractures in shales and mudrocks, and the hydraulic properties of shales and mudrocks with shear induced fractures. The experimental study is based on triaxial tests on different mudrocks and shales from North Sea reservoirs and adjacent areas sheared at different effective confining stresses. It is shown that during burial, mudrocks behave as normally consolidated materials and exhibit ductile response to increased load. However, several processes like uplift, chemical diagenesis and overpressure build-up turn mudrocks into overconsolidated materials which exhibit brittle behaviour during loading. Triaxial tests with different loading conditions have been performed to establish the brittle-to-ductile transition of various mudrocks. Based on the results of the tests, it is shown that the brittle-to-ductile transition can be related to the overconsolidation ratio. A relationship between normalized undrained shear strength and overconsolidation ratio is established. In combination with this relationship, a correlation between compressional wave velocity and apparent pre-consolidation stress, which accounts for both mechanical and chemical diagenesis, may be used as a tool to evaluate possible leakage of hydrocarbon seals.

Dynamics of magma flow inside volcanic conduits with bubble overpressure buildup and gas loss through permeable magma

Many volcanic eruptions show transitions between extrusive and explosive behaviour. We develop a new generic model that considers concurrence between pressure buildup in the bubbles due to the viscous resistance to their growth and gas escape through the bubble network as they become interconnected. When the pressure difference between bubbles and magma reaches the strength of the material fragmentation occurs. The effect of grain size distribution on the flow in gas-particle dispersion is modelled by two populations of particles which strongly influence the velocity of sound in the mixture. Solutions to the steady-state boundary value problem show non-uniqueness. There are at least two regimes for the fixed parameters in the magma chamber. In the low discharge rate regime, fragmentation does not occur and magma rises with partial gas escape. This regime corresponds to extrusive activity. The upper regime corresponds to explosive activity. The simulations using the parameters defined at the workshop produced the following results for a rhyolitic magma composition: discharge rate 5.5×10 7 kg/s; fragmentation at depth of 2585 m with magma vesicularity of 0.74; exit gas velocity varies from 200 to 450 m/s depending on the mass fraction of small particles in the fragmented mixture; exit pressure is in the range 1.5 to 3 MPa. Variation of conduit diameter d in the range 40 to 70 m gives a mass flow rate Q which depends on the diameter as d 2.8, less strongly than for the case of viscous flow of Newtonian liquid in a cylindrical pipe where Q∼ d 4. With the increase in conduit diameter, fragmentation happens later in the flow and conduit resistance remains high. Changes in magma temperature from 700 to 950 °C lead to increase in discharge rate only by a factor of 4 whereas viscosity decreases by more then 8000 times.

Dome growth behavior at Soufriere Hills Volcano, Montserrat, revealed by relocation of volcanic event swarms, 1995–1996

We have relocated a subset of events from the digital waveform catalogue of ∼17,000 volcanic microearthquakes recorded between July 1995 and February 1996 at Soufriere Hills Volcano (SHV), Montserrat, using a cross-correlation-based phase repicking technique with a joint location method. Hypocenters were estimated for 3914 earthquakes having five or more corrected P-wave picks. The seismic source region collapsed to a volume of ∼1 km 3 from an initial ∼100 km 3. Relocated events represent 36 swarms, each containing nearly identical waveforms, having source dimensions of 10 to 100 m in diameter and spatial separations on the order of 500 m or less. Each swarm occurred over a span of several hours to a few days. Triggered data appear to miss between 65% and 98% of the events that occur within these swarms, based on review of helicorder records. Visual estimates of summit dome growth show a rough correspondence between episodes of intense swarming and increases in extruded magma, although dome observations are too sparse to make a direct comparison for this time period. The limited depth range over which dome-growth-related events occur is consistent with a dynamic model of cyclic plug extrusion behavior in the shallow conduit, governed by magma supply rate, overpressure buildup and physical properties of the magma and conduit geometry. Seismic sources may occur in locally overpressured regions that result from microlite formation in a zone of rapid decompression; we propose that this zone exists in the vicinity of a detachment plane associated with the cyclic plug extrusion.

Isotopic evidence (3He/4He, of fluid‐triggered intraplate seismicity

The aim of this study is to find a link between the occurrence of mantle‐derived CO2 exhalations and earthquake swarms beneath the western Eger rift, central Europe. We monitored the gas and isotope composition of the free gas of the Eisenquelle mineral spring and the Bublak mofette weekly for 2 years. These time series include periods before, during, and after an earthquake swarm event. Shifts in δ13C and 3He/4He were found for several months after the occurrence of the earthquake swarm on 4 and 5 December 1994. The isotopic shifts were explained by the admixture of crustal fluids released at the hypocenter to the “permanent” mantle volatile flux. From the beginning of the earthquake swarm, as starting point for the gas release from the hypocenter and the moment of arriving fluid with changed isotopic signature at the locations, we estimated that the fluid transport velocity in the upper crust ranges between 400 m/day near a center of CO2 emanation (Bublak mofette) and 50 m/day in the periphery (Eisenquelle mineral spring). Fault rupture induced by the buildup of fluid overpressure in the brittle crust (associated with sealing processes surrounding the ascending channels of the permanent mantle flux) seems to be the main reason for periodic/episodic earthquake swarm activity in this area.

Quantitative Modeling in the North Sea: Towards a More Confident Assessment of the Critical Risks

Quantitative modeling has become a standard part of probability assessment, for both hydrocarbon systems analysis and reservoir quality prediction, undertaken in association with prospectivity evaluation in the Norwegian North Sea and Haltenbanken areas. Presently, the Norwegian North Sea and parts of Haltenbanken can be classified as mature basins. As such, quantitative assessment is focussed towards assessment of prospects lying on basin margins outside known kitchen areas; satellite prospect assessment in and around existing infrastructure; and assessment of deep, often high-pressure, high-temperature (HP/HT) prospects in basin centers. Outside the known kitchen areas, access to a long-distance petroleum migration route is critical for success. In the proximity of existing infrastructure petroleum migration and trapped fluid gas-to-oil ratio (GOR) are often key risks. In deep, HP/HT settings reservoir quality and top-seal integrity are often the most important factors. Application of quantitative models in these environments allows a consistent evaluation methodology, testing of the consequences of geological uncertainty, and an improvement in confidence. We will illustrate the use of 3D migration modeling to assess migration fairways close to existing infrastructure and outside known kitchens; our experience of matching observed fluid GOR using 2D compositional modeling; and how we model porosity evolution in HP/HT areas and the influence that thermally controlled quartz diagenesis has on overpressure build-up and top-seal leakage. These examples show how quantitative modeling tools and new understanding of fundamental processes are being used and developed today in the North Sea and Halten bank.

A quantitative comparison of some mechanisms generating overpressure in sedimentary basins

Expulsion of fluids in low permeable rock generate overpressure. Several mechanisms are suggested for fluid expulsion and overpressure build-up, and some of them have here been studied and compared. These are mechanical compaction, aquathermal pressuring, dehydration of clays, hydrocarbon generation, and cementation of the pore space. A single pressure equation for these fluid expulsion processes has been studied. In particular, the source term in this pressure equation is studied carefully, because the source term consists of separate terms representing each mechanism for pressure build-up. The amount of fluid expelled from each mechanism is obtained from these individual contributions to the source term. It is shown that the rate of change of porosity can be expressed in at least two different ways for dehydration reactions and oil generation. One way in which the reduction in the solid volume enhances the porosity, and another where the porosity remains constant. A gravity number is defined by the permeability and the Darcy flux of the expelled fluid. The gravity number is shown to be a useful indicator for overpressure build-up. The gravity number is also used to estimate the thickness a seal needs for overpressure to rise from almost hydrostatic conditions above the seal to almost lithostatic conditions below the seal. Simple expressions for the Darcy fluxes caused by mechanical compaction, dehydration of clays, hydrocarbon generation and cementation of the pore space are derived. It is shown how these expressions can be combined with the gravity number to obtain an upper bound for the permeability for overpressure build-up to take place. Mechanical compaction and cementation of the pore space are shown to be the strongest expulsion mechanisms. Although most mechanisms can generate overpressure alone given a sufficiently low permeability, it is concluded that cementation of pore space is the most likely mechanism for overpressure generation in deeply buried reservoirs.

How faulting keeps the crust strong: Comment and Reply

[Townend and Zoback (2000)][1] argued that faults keep the crust strong by increasing the permeability and thus preventing a build-up of overpressure. By maintaining hydrostatic pressures, the faults increase their frictional strength and thus make the crust stronger than if the fault were

Generation of overpressure by cementation of pore space in sedimentary rocks

SUMMARY Overpressure build-up is studied when the main cause for porosity reduction is cementation of the pore space sourced locally. The average porosity reduction for siliclastic sediments is modelled with nth-order kinetics. It is shown that the overpressure in one layer at a constant depth will decrease exponentially with time in the case of first-order kinetics for the porosity reduction. The overpressure is studied in one layer subjected to cementation during constant burial along a thermal gradient. A small overpressure build-up is shown above the window for cementation, with a steep rise in overpressure in the upper part of the window. The overpressure build-up is then seen to decrease rapidly towards the end of the window for cementation. Overpressure build-up is also studied when cementation is the main cause for porosity reduction in the entire column of sediments during deposition and burial. The overpressure regime characterized by gravity numbers larger than one is studied. This regime corresponds to low or moderate overpressures in the case of mechanical compaction. Cementation is shown to imply a steep pressure build-up in the window of cementation, which will easily exceed the lithostatic pressure. The porosity loss due to cementation is seen to have a strong impact on the permeability, which leads to the formation of a pressure seal. Although most of the potential for fluid expulsion is exhausted below the seal, because most of the porosity is cemented up, the permeability of the seal is sufficiently low for hydrofracturing to take place. This scenario is consistent with overpressure observations in many wells.

The influence of faults and intraplate stresses on the overpressure evolution of the Halten Terrace, mid-Norwegian margin

The Halten Terrace is a structural element of the Meso-Cenozoic mid-Norwegian margin. The pore fluid pressure distribution in the faulted Jurassic formations on the Halten Terrace is characterized by significant lateral variations. In general, the fluid overpressure increases stepwise across faults from east to west, from zero (hydrostatic fluid pressure) to about 30 MPa. Fault-bounded pressure cells can therefore best explain the fluid pressure distribution. The results of analyses of log-derived porosities indicate that the high overpressure in the westernmost pressure cell was built up recently. However, despite the high sedimentation rates during Plio-Pleistocene, the high overpressure cannot be explained by local mechanical compaction. Alternative explanations for the high overpressure proposed by other authors are based on pore fluid volume increase (e.g. hydrocarbon generation). We propose that the high overpressure is caused by fluid flow from the deep Rås Basin to the western part of the Halten Terrace, through fractures in the Mesozoic, deep seated Klakk Fault Complex. Opening of fractures in this fault zone by seismic and static mechanisms is possible in the present-day intraplate stress field, which is characterized by a NW–SE oriented maximum horizontal stress direction. During Miocene, the maximum horizontal stress was E–W oriented, which implies a stress rotation during Pliocene. The E–W orientation of the maximum horizontal stress has impeded the initiation and opening of fractures in the N–S striking Klakk Fault Complex during Miocene. Fluid flow from the Rås Basin through faults of the Klakk Fault Complex can therefore have occured since Pliocene. Thus, the rotation of the intraplate stress directions can explain why the build-up of overpressure on the western part of the Halten Terrace occured recently, as indicated by the results of porosity analyses. Understanding the overpressure evolution of the Halten Terrace is important for exploration in that area, as hydrocarbons have been found in the hydrostatic pressure cells, whereas they are absent in the high overpressure cells.

Gas Generation and Accumulation in the West Siberian Basin

We appreciate the discussion by Murris (2001) and take the opportunity to clarify some of his objections and discuss some of the still open questions in more detail. Murris's main objections to the conclusions of Littke et al. (1999) concern, first, the influence of overpressure in the Lower Cretaceous and Upper Jurassic on hydrocarbon distribution and, second, the model of gas release by exsolution (degassing) from deep ground water as the main mechanism for accumulating the huge gas fields of northern West Siberia. The first objective of Murris's discussion is how the overpressure in Jurassic strata might have influenced the distribution of hydrocarbons in the overlying Cretaceous sediments. Two crucial points regarding this question are the reason and timing of overpressuring and its influence on hydrocarbon movement. We agree with Murris that the effect of compaction disequilibrium is not sufficient to explain the extent of overpressure in the Urengoy area and that hydrocarbon generation in the Bazhenov Formation has contributed to the buildup of overpressure. Two-dimensional basin modeling along a seismic line through the Urengoy field, however, showed that hydrocarbon generation in Jurassic source rocks (Bazhenov and the Tyumen formations) did only generate sufficient overpressure to explain the recent situation if the models were laterally completely sealed. This sealing appears to be highly unlikely. Furthermore, hydrocarbon generation ceased during the Miocene, when the tectonic movement started to lift the source rocks into shallower depths, where lower temperatures were reached. Thus, any overpressure caused by petroleum generation must have been preserved over a period of about 5-20 m.y. In addition, the overpressure is not restricted to the Jurassic Bazhenov Formation but extends into the Cretaceous, including the Achimov sands. Uplift might have contributed to the overpressure, however. Maximum pore pressure from the stage of deepest burial might have …