most and least likely locations of large to great earthquakes along the pacific coast of mexico estimated from local recurrence times based on b values we mapped t l and p l local recurrence time and local probability density respectively for m max 7 2 earthquakes along the plate margin of the pacific coast of mexico in order to test two hypotheses the first is that the minima in t l or maxima in p l extracted probabilistically from the frequency magnitude distribution map asperities the second hypothesis is that recurrence times for characteristic events are estimated more correctly by local recurrence times than by the overall recurrence times of the zone comparing these results to the observed recurrence times for events of m 7 0 we find good agreement the coast of guerrero between longitude west 100 5 and 101 5 and the segment off the coast of southern chiapas show the shortest t l estimated as about 20 yr the location at longitude 98 5 near the boundary of guerrero with oaxaca the ometepec segment follows with estimates of about 30 yr as the next shortest t l along the coast of guerrero between 99 and 100 5 longitude t l is estimated as about 40 yr and finally in the segments off the oaxacan coast near 95 7 and 97 7 longitude t l ranges from 40 to 60 yr these volumes we define as asperities long local recurrence times are observed for the areas offshore from the isthmus of tehuantepec on the pacific plate off the coast of guerrero on land along the coast of most of oaxaca and along the northernmost 40 km of the guerrero coast if our ideas are correct then major moment release should emanate from the volumes we defined as asperities and relatively minor moment release should emanate from the volumes identified by the relatively long local recurrence times

exceptional reservoir quality in hpht reservoir settings examples from the skagerrak formation of the heron cluster north sea uk abstract as the exploration of hydrocarbons moves into more complex and deeper basinal settings the need to understand the effect of high temperatures and high pressures on reservoir quality and rock properties becomes even more important the fluvial channel sandstone reservoirs of the skagerrak formation in the central north sea exhibit anomalously high porosities and permeabilities considering their depth of burial 3500 m below sea floor despite the complex depositional setting diagenetic history high overpressure and temperatures encountered in the skagerrak formation hydrocarbons are currently being produced the skagerrak formation reservoirs used in this study have encountered overpressures of 40 mpa and temperatures up to 185 c at present day maximum burial to identify the role played by the high pressure and high temperature encountered in the reservoir sandstones a multidisciplinary approach involving petrographic fluid inclusion and burial history modelling studies has been adopted our interpretation of the results is that the generation of shallow overpressure in these fields limited mechanical compaction and also played an important role in minimizing pressure solution in the chemical compaction regime as evidenced by reduced quartz cementation fluid inclusions found in the quartz overgrowths indicate late stage development with temperatures of formation in the range 130 to 170 c coincident with late stage deeper burial hydrocarbon emplacement occurred after quartz cementation and has had little to no effect on porosity preservation the formation of well developed authigenic chlorite 70 surface coating and to a lesser extent illite clay coats with burial had a positive effect on porosity preservation even though permeability was marginally reduced in the illite rich sandstones a schematic porosity and quartz cement evolution model has been developed which allows for pre drill prediction of reservoir quality in the heron cluster and provide valuable insights for other complex high pressure high temperature reservoirs

mount hood earthquake activity volcanic or tectonic origins on 29 june 2002 a m d 4 5 earthquake occurred 4 6 km south of mount hood oregon more than 200 small aftershocks m d 3 8 occurred between 29 june and 15 august 2002 by which time seismicity returned to background levels we analyze well constrained earthquakes from the summer 2002 sequence near mount hood recorded by the pacific northwest seismic network pnsn and well constrained small earthquakes m d 3 5 recorded by the pnsn between 1986 and 2002 we apply waveform cross correlation to selected events establish a one dimensional p wave velocity model and relocate the entire catalog by using the double difference algorithm of waldhauser and ellsworth 2000 we find that earthquakes before 2002 occur in four distinct clusters a a well defined linear feature striking n15e about 5 km south of the summit b a linear feature trending northwest southeast a few kilometers south of group a c a few tight clusters located about 9 km south southwest of the summit and d a small cluster beneath the summit earthquakes in group d do not appear to lie on a fault and may relate to volcanic activity however almost all earthquakes in the 2002 swarm occur in group a south of the summit at depths similar to earthquakes from previous swarms first motion fault plane solutions from the mainshock and largest aftershocks have normal mechanisms and solutions from well constrained aftershocks have normal to oblique normal mechanisms this is consistent with the apparent strike of the feature south of mount hood s summit and is similar to focal mechanisms of older earthquakes in group a b and p values for the summer 2002 earthquakes resemble a tectonic mainshock aftershock sequence we interpret these earthquakes as the northernmost example of basin and range seismicity yet recorded and conclude that the mount hood earthquakes do not yet suggest potential for volcanic unrest

the earthquake hazard alert of september 1982 in southern tobago abstract an earthquake swarm that occurred in the vicinity of the island of tobago west indies during the latter half of 1982 was monitored in the near field by a five station seismograph network the monitoring of the swarm eventually led to the issuing of a potential earthquake hazard alert 3 days prior to the major energy release earthquake magnitude m b 5 2 we discuss the reasons for issuing this alert in particular daily monitoring of the changing b value and energy release was used to constrain estimates of future earthquake behavior the aftershock seismicity showed activity in a direction trending west to wnw this is in good agreement with the focal mechanism of the main earthquake which showed right lateral strike slip motion along an e w fault plane dipping steeply 71 to the north this active fault appears to form part of the previously unrecognized southern tobago fault system for which there is evidence in the geology of the late neogene rocks of the island

sequence stratigraphy and controls on reservoir sandstone distribution in an eocene marginal marine coastal plain fairway taranaki basin new zealand abstract middle to upper eocene strata in taranaki basin new zealand consist of terrestrial marginal marine shallow marine and offshore sediments deposited in a series of transgressive and regressive cycles within a passive margin setting coastal plain facies are largely confined to eastern and southern sides of the basin and marine mudstones to western and northern parts shoreface to fluvial sandstones within a ne sw oriented coastal facies belt form the pre eminent petroleum reservoir fairway in taranaki basin and provide the bulk of new zealand production the terrestrial coastal plain part of the succession also contains coal rich proven source rocks we have interpreted twelve depositional sequences 3rd and 4th order within the middle to upper eocene strata the base of our study interval marks a relative transgression within the middle eocene and is overlain by composite sequences and parasequences represented by cyclically stacked packages overall regression occurs up to the top of the middle eocene which is overprinted by intermittent shorter term transgressions the overlying upper eocene strata are characterised by overall shoreline retreat with final transgression over the entire taranaki basin in the latest eocene early oligocene stratigraphic cyclicity within the terrestrial to shallow marine facies is noted from paleoenvironmental interpretations determined from well log core biostratigraphic and coal sulphur data whilst eustasy may have played a role in development of the taranaki stratigraphy tectonically induced subsidence was probably the over riding factor especially in the upper eocene petroleum producing reservoirs occur at several distinct horizons within the middle to upper eocene succession interpreted here to represent significant events during sequence development most reservoir sandstones were deposited as channelised facies following 3rd order lowstands sequence boundaries are interpreted at the base of many of these sandstone intervals some of which may represent type 1 boundaries developed as incised valley systems this regional study has used an integrated approach to show how sequence stratigraphy can be applied to a non marine to shallow marine succession to assist with exploration and reservoir development through our recent and multi disciplinary work we have been able to discriminate subtle base level fluctuations and greater marine control of depositional architecture than has previously been recognised with implications for source rock quality fairway exploration reservoir charge and production effectiveness

multiparametric statistical investigation of seismicity occurred at el hierro canary islands from 2011 to 2014 abstract a detailed statistical investigation of the seismicity occurred at el hierro volcano canary islands from 2011 to 2014 has been performed by analysing the time variation of four parameters the gutenberg richter b value the local coefficient of variation the scaling exponent of the magnitude distribution and the main periodicity of the earthquake sequence calculated by using the schuster s test these four parameters are good descriptors of the time and magnitude distributions of the seismic sequence and their variation indicate dynamical changes in the volcanic system these variations can be attributed to the causes and types of seismicity thus allowing to distinguish between different host rock fracturing processes caused by intrusions of magma at different depths and overpressures the statistical patterns observed among the studied unrest episodes and between them and the eruptive episode of 2011 2012 indicate that the response of the host rock to the deformation imposed by magma intrusion did not differ significantly from one episode to the other thus suggesting that no significant local stress changes induced by magma intrusion occurred when comparing between all them therefore despite the studied unrest episodes were caused by intrusions of magma at different depths and locations below el hierro island the mechanical response of the lithosphere was similar in all cases this suggests that the reason why the first unrest culminated in an eruption while the other did not may be related to the role of the regional local tectonics acting at that moment rather than to the forceful of magma intrusion

eustatic variations during the paleocene eocene greenhouse world we reconstruct eustatic variations during the latest paleocene and earliest eocene 58 52 ma dinoflagellate cysts grain size fractions and organic biomarkers in marine sections at four sites from three continents indicate an increased distance to the coast during the paleocene eocene thermal maximum petm the same trend is recognized in published records from other sites around the world together the data indicate a eustatic rise during the petm beginning 20 to 200 ka before the globally recorded negative carbon isotope excursion cie at 55 5 ma although correlations are tentative we recognize other global transgressions during chrons c25n and c24n the latter may be associated with eocene thermal maximum 2 53 5 ma or the x event 52 ma these results suggest a link between global sea level and hyperthermal intervals potentially because of the melting of small alpine ice sheets on antarctica thermal expansion of seawater or both however the early onset of sea level rise relative to the cie of the petm suggests contributions from other mechanisms perhaps decreasing ocean basin volume on sea level rise copyright 2008 by the american geophysical union

reporting history of the central aleutians seismograph network and the quiescence preceding the 1986 andreanof island earthquake the earthquake catalog of the central aleutian seismic network casn was searched systematically for significant rate changes in the period from 1976 through 1985 the magnitude signatures for the nine changes were modeled by synthetic signatures by this method six of the rate changes could be identified as artificial based on the criteria that the rate change was confined to small magnitude events and that magnitude shifts were present for each of these changes an artificial cause existed that affected the network within one to two months of the time of change as estimated from the catalog analysis one of these cases was a rate decrease by a factor of 2 associated with a storm that had put out of operation two thirds of the network another change coincided with the introduction of digital data processing the four remaining cases were artificial reporting rate increases due to installation of several new stations or to repair of several stations that had failed the three rate changes that could not be identified unambiguously as artificial or natural on the basis of magnitude signatures were all rate decreases by factors of approximately 2 two of these were clearly due to disasters that had put out of operation more than half of the network the third was the proposed precursory quiescence to the 1986 andreanof island m 7 9 earthquake from this we conclude that without knowledge of the network history many of the artificial reporting rate changes in earthquake catalogs can be identified as artificial and that their onset time can be estimated to within one to two months an earthquake count independent from the adak catalog was obtained from the helicorder records of the station adak for the years 1978 through 1985 for the vicinity of adak island of a total of 1831 events 335 were not in the adak catalog and 981 were located within the 1986 aftershock area the helicorder earthquake count showed a pronounced decrease by about 50 per cent similar in amount to that in the adak catalog however the decrease started in 1980 at a time when in the central subregion covered by the adak catalog only m 2 9 events decreased while the smaller magnitude bands show a maximum decrease in 1982 we conclude that the possibility could not be ruled out that electronics changes contributed to the reporting rate decrease starting in the years 1980 to 1982 however it seems very unlikely that a network that was at its healthiest during the precursor time would be reduced by an improvement in the electronics to a detection level approximately equal to that in two short periods when only one third of the stations were operating therefore the seismic quiescence reported and interpreted as a precursor by kisslinger et al 1985 was most likely a natural phenomenon

aluminium loss during sandstone diagenesis petrographical observations from a sandstone buried to between 3 2 and 5 7 km in the north sea central graben suggest a significant change in bulk chemistry during burial the sand contained 35 or more feldspar upon deposition but due to extensive dissolution only 5 remains in the deep sandstones this has released much more al than is currently contained as authigenic minerals up to 1460 mol cm 3 of al have been exported this deep secondary porosity is economically important but its recognition by petrographic textures is unreliable as are bulk chemical provenance studies of deeply buried sedimentary and metamorphic strata

diagenetic controls on evolution of porosity and permeability in lower tertiary wilcox sandstones from shallow to ultradeep 200 6700 m burial gulf of mexico basin u s a abstract reservoir quality is a critical risk factor in deep to ultradeep reservoirs at depths 4 5 km analysis of paleogene wilcox sandstones on the upper texas gulf coast provides insight into the evolution of reservoir quality during shallow to ultradeep burial diagenesis reduction of porosity and permeability with burial in wilcox sandstones was evaluated using subsurface samples from 200 to 6700 m at temperatures of 25 230 c diagenesis and petrophysical properties were interpreted from petrographic data and core analyses wilcox sandstones are mostly lithic arkoses and feldspathic litharenites having an average composition of q59f22r19 provenance did not change significantly during wilcox deposition in this area nor does average sandstone composition vary among lower middle and upper wilcox sandstones however composition does vary with sequence stratigraphic position lowstand slope fan deposits contain more rock fragments than do deposits from highstand or transgressive systems tracts given observations from this onshore dataset wilcox sandstones deposited in deepwater environments in the gulf of mexico are likely to contain more rock fragments than their linked highstand equivalents pore types in wilcox sandstones change from a mix of primary and secondary pores and micropores at shallower depths to predominantly secondary pores and micropores in deeper sandstones wilcox sandstones show steady porosity reduction from an average of 33 at 38 c to 12 at 132 c with little additional loss at higher temperatures by 132 c most primary pores have been lost by mechanical compaction or occluded by quartz cementation volume of secondary pores remains nearly constant despite dissolution of 3 5 average potassium feldspar during deep burial volume of this late dissolution of potassium feldspar is offset by precipitation of ankerite albite illite and minor quartz in secondary pores the proportion of microporosity which occurs in authigenic clays altered grains and matrix increases in the deepest sandstones change in proportion of pore types and reduction in total porosity with increasing temperature changes porosity permeability transforms because most deep to ultradeep wilcox sandstones are dominated by secondary pores and micropores porosity permeability transforms have lower slopes than those for shallower sandstones

seismicity and subsidence following the 2011 nabro eruption eritrea insights into the plumbing system of an off rift volcano nabro volcano situated to the east of the afar rift zone erupted on 12 june 2011 eruptions at such off rift volcanoes are infrequent and consequently the plumbing systems are poorly understood we present posteruption synthetic aperture radar sar images from the terrasar x satellite and posteruption continuous seismic activity from a local seismic array interferometric analysis of sar data reveals a circular 12 km wide signal subsiding at 200 mm yr we inverted for the best fit mogi source finding a 4 1 107 m3 yr volume decrease at 7 1 km depth between 31 august and 7 october 2011 we located 658 and relocated 456 earthquakes with local magnitudes between 0 4 and 4 5 seismicity beneath the se edge of nabro at 11 km depth is likely associated with high strain rates from deep magma flow into the modeled reservoir this suggests that magma is supplied through a narrow conduit and then stored at 7 km depth we interpret seismicity at 4 6 km depth as brittle fracturing above the inferred magma reservoir focal mechanisms delineate a thrust fault striking ne sw and dipping 45 to the se across the caldera floor we propose that the crustal response is to slip on this fault which crosscuts the caldera rather than to deform on ring faults the ne sw fault plane is not associated with measurable surface deformation indicating that it does not contribute much to the caldera deformation we show that subsidence of the caldera is controlled by magma chamber processes rather than fault slip

spatial variation in the frequency magnitude distribution of earthquakes under the tectonic framework in the middle east abstract spatial variations of seismic energy released and b value over the middle east region are investigated based on a seismicity catalog from 1995 to 2007 the goal is to use these seismic parameters and based on other geodetic and geophysical observations such as gps measurements strain rate model fault distribution focal mechanism crustal model q model and gravity measurements etc to uncover spatial patterns that seem anomalous areas of high energy released cumulative seem to correspond to the areas of relatively high b values areas of high energy released and high b values seem to correspond very well with the location of continental collision where earthquake activities are high the divergent boundary between arabia and african plates and subduction zone at makran seem to correspond to low to moderate energy release northern pamir azerbaijan caucasus the lower part of zagros mountains eastern turkey owen fracture zone strait of bob el mandeb and south of the sulaiman shear zone seem to correspond to high cumulative energy released high strain rate high b values and high fault density while the central and eastern iran southern zagros northern pakistan gulf of aden alborz southwest of the caspian sea western caucasus and kopeh dagh seem to correspond with lower b values the cross section map for hindu kush shows general decreasing of the b values with depth however a region of high b value is observed underneath pamir at depths from 170 to 230 km this anomaly region can be due to dehydration of pamir crustal slab at depth

mapping the b value anomaly at 100 km depth in the alaska and new zealand subduction zones a positive anomaly in the frequency magnitude distribution b value is detected at approximately 90 100 km depth in two subduction zones the b value in this anomalous zone is 40 higher than in adjacent volumes we use regional catalogs with a magnitude of completeness of 2 6 central alaska and cook inlet 2 0 shumagin islands and 3 5 new zealand to resolve the b value as a function of space in more detail we project all earthquakes onto planes perpendicular to the strike of the slab these cross section views of the b value distribution locate the high b value anomalies at a depth of 90 100 km on the upper surface of the wadati benioff zone at these depths slab dehydration may increase pore pressure thus lowering the effective stress and increasing the b value increased pore pressure would also lower the liquidus in the overlying asthenosphere giving rise to the volcanism that occurs directly above this zone

variations in the frequency magnitude distribution with depth in two volcanic areas mount st helens washington and mt spurr alaska the frequency magnitude distribution of earthquakes characterized using the b value is examined as a function of space beneath mount st helens 1988 1996 and mt spurr 1991 1995 at mount st helens two volumes of anomalously high b b 1 3 can be observed at depths of 2 6 3 6 km below the crater floor and below 6 4 km these anomalies coincide with 1 the depth of vesiculation of ascending magma and 2 the suggested location of a magma chamber at mount st helens study of mt spurr reveals an area of high b value b 1 3 at a depth of about 2 3 4 5 km below the crater floor of the active vent crater peak we propose that the higher material heterogeneity in the vicinity of a magma chamber or conduit due to vesiculation of the ascending magma is the main cause of the increased b value at shallow depths alternatively interaction of magma with groundwater may have increased pore pressure and lowered the effective stress the deeper anomaly at mount st helens is likely caused by high thermal stress gradients in the vicinity of the magma chamber our results indicate that detailed mapping of the frequency magnitude distribution can be used as a tool to trace vesiculation and locate active magma chambers

temporal and three dimensional spatial analyses of the frequency magnitude distribution near long valley caldera california summary the 3 d distribution of the b value of the frequency magnitude distribution is analysed in the seismically active parts of the crust near long valley caldera california the seismicity is sampled in spherical volumes containing n 150 earthquakes and centred at nodes of a grid separated by 0 3 km significant variations in the b value are detected with b ranging from b 0 6 to b 2 0 high b value volumes are located near the resurgent dome and at depths below 5 km at mammoth mountain b values are found to be much lower south of the long valley caldera we interpret this to indicate that an active magma body has advanced from depths below 8 km to depths of 4 to 5 km beneath mammoth mountain in 1989 and that anomalous crust either highly fractured or containing unusually high pore pressure such as is the case in the vicinity of active magma bodies exists north of the seismically active area beneath the resurgent dome at all depths we also investigate the spatial distribution of temporal variations of the frequency magnitude distribution by introducing diverential b value maps b values increased from b 0 8 to b 1 5 underneath mammoth mountain at the onset of the 1989 earthquake swarm and remained high thereafter this suggests that an intrusion permanently altered the average distribution of cracks at 5 10 km depth or that the pore pressure permanently increased we propose that high b values are a necessary but not suycient condition near a magmatic body and therefore spatial b value mapping can be used to aid in the identification of active magma bodies

anomalously high b values in the south flank of kilauea volcano hawaii evidence for the distribution of magma below kilauea s east rift zone abstract the pattern of b value of the frequency magnitude relation or mean magnitude varies little in the kaoiki hilea area of hawaii and the b values are normal with b 0 8 in the top 10 km and somewhat lower values below that depth we interpret the kaoiki hilea area as relatively stable normal hawaiian crust in contrast the b values beneath kilauea s south flank are anomalously high b 1 3 1 7 at depths between 4 and 8 km with the highest values near the east rift zone but extending 5 8 km away from the rift also the anomalously high b values vary along strike parallel to the rift zone the highest b values are observed near hiiaka and pauahi craters at the bend in the rift the next highest are near makaopuhi and also near puu kaliu the mildest anomalies occur adjacent to the central section of the rift the locations of the three major and two minor b value anomalies correspond to places where shallow magma reservoirs have been proposed based on analyses of seismicity geodetic data and differentiated lava chemistry the existence of the magma reservoirs is also supported by magnetic anomalies which may be areas of dike concentration and self potential anomalies which are areas of thermal upwelling above a hot source the simplest explanation of these anomalously high b values is that they are due to the presence of active magma bodies beneath the east rift zone at depths down to 8 km in other volcanoes anomalously high b values correlate with volumes adjacent to active magma chambers this supports a model of a magma body beneath the east rift zone which may widen and thin along strike and which may reach 8 km depth and extend from kilauea s summit to a distance of at least 40 km down rift the anomalously high b values at the center of the south flank several kilometers away from the rift may be explained by unusually high pore pressure throughout the south flank or by anomalously strong heterogeneity due to extensive cracking or by both phenomena the major b value anomalies are located sse of their parent reservoirs in the direction of motion of the flank suggesting that magma reservoirs leave an imprint in the mobile flank we hypothesize that the extensive cracking may have been acquired when the anomalous parts of the south flank now several kilometers distant from the rift zone were generated at the rift zone near persistent reservoirs since their generation these volumes may have moved seaward away from the rift but earthquakes occurring in them still use the preexisting complex crack distribution along the decollement plane at 10 km depth the b values are exceptionally low b 0 5 suggesting faulting in a more homogeneous medium

seismic b values and its correlation with seismic moment and bouguer gravity anomaly over indo burma ranges of northeast india tectonic implications abstract b value is one of the most significant seismic parameters for describing the seismicity of a given region at a definite time window in this study high resolution map of the gutenberg richter b value seismic moment release bouguer gravity anomaly and fault plane solutions containing faulting styles are analyzed in the indo burma ranges of northeast india using the unified and homogeneous part of the seismicity record in the region january 1964 december 2016 the study region is subdivided into few square grids of geographical window size 1 1 and b values are calculated in each square grid our goal is to explore the spatial correlations and anomalous patterns between the b value and parameters like seismic moment release bouguer gravity anomaly and faulting styles that can help us to better understand the seismotectonics and the state of present day crustal stress within the indo burma region most of the areas show an inverse correlation between b value and seismic moment release as well as convergence rates while estimating the b value as a function of depth a sudden increase of b value at a depth of 50 60 km was found out and the receiver function modeling confirms that this depth corresponds to the crust mantle transition beneath the study region the region is also associated with negative bouguer gravity anomalies and an inverse relation is found between gravity anomaly and b value comparing b values with different faulting styles reveal that the areas containing low b values show thrust mechanism while the areas associated with intermediate b values show strike slip mechanism those areas where the events show thrust mechanism but containing a strike slip component has the highest b value

tidal modulation of continuous nonvolcanic seismic tremor in the chile triple junction region 1 we located continuous seismic tremor with coherent amplitude wave trains in the chile ridge subduction region 46 5 s in two clusters north and south of the chonos archipelago between the chile trench and the north patagonian fore arc tremor persisted from december 2004 to february 2007 the entire period of the chile ridge subduction project temporary seismic deployment and lasted 17 h on six occasions tremor in the more active southern cluster reached a maximum duration of 48 h and we observed no more than 3 continuous days without tremor activity the cluster locations coincide with the surface projections of subducted transform faults formed at the chile ridge we also detected simultaneous colocated low frequency microearthquakes with well defined impulsive waves within the tremor signals distributed from the surface to 40 km depth suggesting tremors and earthquakes are part of the same process the periodicity of tremor duration is strongly correlated with semidiurnal diurnal and long period tides m2 n2 k1 o1 p1 and mm 12 421 h 12 000 h 23 934 h 25 819 h 24 066 h and 27 555 days respectively we found a significant correlation between tremor occurrence and earth tides when tidal stress is calculated for the slip plane of a right lateral strike slip fault with strike n95 e which is near parallel to subducted transform faults n78 e of the chile ridge indicating that the very small stresses resulting from the combination of ocean loading and solid earth tides 1 kpa are sufficient to facilitate or suppress tremor production tremors occur when shear stresses are maximum and wane or are low when shear stresses are minimum

spatial variations in the frequency magnitude distribution of earthquakes at mount pinatubo volcano the frequency magnitude distribution of earthquakes measured by the b value is mapped in two and three dimensions at mount pinatubo philippines to a depth of 14 km below the summit we analyzed 1406 well located earthquakes with magnitudes m d 0 73 recorded from late june through august 1991 using the maximum likelihood method we found that b values are higher than normal b 1 0 and range between b 1 0 and b 1 8 the computed b values are lower in the areas adjacent to and west southwest of the vent whereas two prominent regions of anomalously high b values b 1 7 are resolved one located 2 km northeast of the vent between 0 and 4 km depth and a second located 5 km southeast of the vent below 8 km depth the statistical differences between selected regions of low and high b values are established at the 99 confidence level the high b value anomalies are spatially well correlated with low velocity anomalies derived from earlier p wave travel time tomography studies our dataset was not suitable for analyzing changes in b values as a function of time we infer that the high b value anomalies around mount pinatubo are regions of increased crack density and or high pore pressure related to the presence of nearby magma bodies manuscript received 16 december 2002

seismic b value anomalies prior to the 3rd january 2016 mw 6 7 manipur earthquake of northeast india abstract spatial variation of seismic b value is estimated in the indo myanmar subduction zone of northeast ne india using the homogeneous part of earthquake catalogue 1996 2015 recorded by international seismological center isc consisting of 895 events of magnitude mw 3 9 the study region is divided into 1 1 square grids and b values are estimated at each grid by maximum likelihood method in this study the b value varies from 0 75 to 1 54 in the region significant variation of low b value in the respective location may indicate high stress accumulation in that region spatial variation reveals intermediate b value anomalies around the epicenter of the mw 6 7 manipur earthquake which occurred on 3rd january at 23 05 utc 4 january 2016 at 04 35 ist the variations of b values are also estimated with respect to depth the low b value associated with the depth range 15 55 km which may imply crustal homogeneity and high stress accumulation in the crust since ne india lies in the seismic zone v of the country this study can be helpful to understand seismotectonics in the region

organic rich sedimentation in the south pacific ocean associated with late paleocene climatic cooling abstract a distinctive organic rich marine mudstone of late paleocene age occurs in most of new zealand s sedimentary basins and has been identified as a potential source rock for oil and gas identified as the waipawa formation in the east coast basin and the tartan formation in the great south and canterbury basins the unit is a relatively uniform massive mudstone that varies greatly in thickness 2 70 m and grades laterally into distinctive facies equivalents notably greensand and a thin bedded siliceous mudstone all these facies are characterised by relatively high toc 0 5 10 wt and 13 c enrichment 13 c toc 24 and we refer to them collectively as waipawa organofacies our detailed stratigraphic and geochemical studies refine the age 58 7 to 59 4 ma distribution and nature of the waipawa organofacies we have determined that deposition occurred in continental margin settings throughout much of the southwest pacific under cool dysoxic conditions associated with a significant influx of terrestrial organic matter high marine productivity a global fall in sea level and a regional unconformity across shallow and deep marine settings the combination of cool temperatures lowered sea level and bathyal erosion suggests that deposition was linked to short lived growth of an antarctic ice sheet in the earliest late paleocene 59 ma

a geochemical and biostratigraphic approach to investigating regional changes in sandstone composition through time an example from paleocene eocene strata taranaki basin new zealand

inadvertent changes in magnitude reported in earthquake catalogs their evaluation through b value estimates a simple procedure is presented for analyzing magnitudes and seismicity rates reported in earthquake catalogs in order to discriminate between inadvertently introduced changes in magnitude and real seismicity changes we assume that the rate and the frequency magnitude relation of the independent background seismicity do not change with time observed differences in the frequency magnitude relation a and b values between data from two periods are modeled as due to a transformation of the magnitude scale the transformation equation is found by a least squares fitting process based on the seismicity data for earthquakes large enough to be reported completely and by comparing the linear relation of one period to the other for smaller events an additional factor accounting for increased decreased detection is allowed this fitting technique is tested on a data set from parkfield for which two types of magnitudes amplitude and duration were computed for each earthquake we found that the b value fitting technique yielded virtually the same result as a linear regression assuming the same errors in the two magnitudes the technique is also applied to interpret the nature of reporting rate changes in a local guerrero mexico and a regional italy earthquake catalog in guerrero a magnitude change in 1991 37 can be modeled about equally well by m new m old 0 5 or by m new 1 02 m old 0 38 but residuals with the latter transformation are smaller in italy a magnitude change in 1980 21 cannot be modeled satisfactorily by a simple magnitude shift but is well described by a compression of the magnitude scale given by m new 0 67 m old 1 03 the proposed b slope fitting method provides a means to interpret quantitatively and in some cases correct for artificial reporting rate changes in earthquake catalogs

reservoir characterisation of syn rift and post rift sandstones in frontier basins an example from the cretaceous of canterbury and great south basins new zealand abstract a paucity of well data in frontier basins presents serious challenges for reservoir characterisation studies around the world in this paper we provide the first regional study addressing reservoir distribution composition and quality in the frontier canterbury basin cb and great south basin gsb of new zealand and show how the results can be used to help reduce uncertainties associated with reservoir presence and effectiveness cretaceous strata in canterbury and great south basins represent two stages of basin history the oldest cenomanian coniacian mid cretaceous strata are syn rift deposits that developed in response to the breakup of the eastern margin of gondwana while overlying santonian maastrichtian late cretaceous strata are post rift deposits that signify the start of rift drift transgression these cretaceous deposits are characterised by a change in facies from predominantly alluvial fluvial to marginal shallow marine that is recognisable in outcrop and wells and from seismic response based on our petrographic examination of 99 cretaceous samples from well and outcrop locations we conclude that there are significant differences in sandstone texture and mineralogy that are related to differences in depositional facies basin stratigraphy geographic location and degree of diagenetic alteration sandstones from the earliest part of the syn rift succession largely reflect the composition of the underlying basement geology typically granitoid or quartzose western province rocks in gsb and lithic rich eastern province metasedimentary rocks in cb younger strata representing part of the post rift succession are generally better sorted with detrital compositions reflecting sediment mixing of several basement source areas based on available data reservoir quality is directly related to sandstone texture and detrital composition factors of provenance and facies compaction a factor of burial depth temperature and degree or type of diagenetic alteration best quality sandstones are predicted to occur in the late cretaceous post rift succession of gsb and locally in cb either where sediment is derived from local granitoids or where high energy depositional processes have resulted in quartz enriched compositions

mapping active magma chambers by b values beneath the off ito volcano japan the b value of the frequency magnitude relation and thus the mean magnitude in the off ito volcanic area is not uniform based on detailed b value tomography using about 10 000 events in an area of 10 km radius and in the upper 15 km of the crust a high b value anomaly b 15 below 7 km depth and with a radius of about 2 km located below the coast southeast of ito contrasts with lower values of typically b 0 7 north of it and at shallower depths on the basis of surface deformations a tensile crack dislocation source was located above 7 km depth thus we surmise that the magma chamber in the off ito area was located below 7 km depth the correlation of a high b value anomaly with this model supports our hypothesis that active magma chambers may be mapped by high b value anomalies the magma body may be described as consisting of two parts a lower well established chamber below 11 km depth and a shallower part 11 7 km depth the b value in the volume surrounding the magma chamber increased from 0 7 during the 1980s to 1 5 during the earthquake swarms of the 1990s this may reflect a trend of increasing crack density due to the intrusion activity

changes in reservoir heterogeneity and quality as a response to high partial pressures of co2 in a gas reservoir new zealand abstract the kapuni field is the largest onshore petroleum field in new zealand and produces co2 rich gas c 40 45 mol co2 diagenesis within the reservoir is dominated by co2 related reactions that have resulted in localised precipitation of kaolin abundant quartz calcite dolomite and siderite along with localised generation of secondary porosity most of the co2 in the reservoir is thought to have been sourced from intraformational coals with subsequent up dip migration to the crest of the kapuni structure during migration co2 will have dissolved into undersaturated pore fluids and the resulting acidic pore fluids catalysed feldspar and minor carbonate dissolution thereby providing ions for precipitation of authigenic minerals timing of the diagenetic reactions as determined by paragenetic observations and fluid inclusion analysis suggests that both quartz and carbonate formed at a very late stage 100 c corresponding to 0 4 ma which is consistent with modelled maturation and expulsion of co2 from intraformational source rocks 5 ma to present the carbon isotope composition of carbonate cements median 13cpdb 12 4 is similar to that of the reservoired co2 13cpdb 14 5 and this is supportive of late stage cement formation with carbon sourced primarily from thermal maturation reactions the degree of co2 related reactions observed within the kapuni field appears dependent upon grain size lithofacies and stratigraphic position in the reservoir coarse grained high energy facies have undergone the most feldspar dissolution coupled with formation and local preservation of secondary porosity this is interpreted to be due to preferential migration of large volumes of co2 rich fluids through permeable beds thereby further enhancing reservoir quality by comparison much less feldspar reaction but locally significant authigenic mineral precipitation occurs within finer grained lower energy sandstone facies this is considered to be due to relatively low permeability prior to co2 circulation restricting the degree of fluid circulation and resulting in net mineral precipitation authigenic kaolin has precipitated close to the mineral source whilst ions required to precipitate calcite dolomite cements are likely to have migrated further resulting in a complex distribution of carbonate cements with pervasive cements observed at gas water contacts results from this study demonstrate how co2 related reactions are dependent on grain size and mineralogy we show that these reactions can affect vertical heterogeneity of a reservoir by increasing the degree of compartmentalisation through development of sweet spots due to mineral dissolution and tight beds due to mineral cementation

late neogene exhumation patterns in taranaki basin new zealand evidence from offset porosity depth trends taranaki basin new zealand is located adjacent to the australian pacific plate boundary where the tectonic regime changes from dominantly subduction related to the north to transpression related along the alpine fault to the south during the neogene burial and exhumation varied extensively in both time and space in response to subsidence and uplift along this evolving plate boundary zone the basin can be divided into two regions 1 the western platform outside the plate boundary deformation zone where no uplift nor exhumation has occurred and 2 the eastern mobile belt which lies inside the deformation zone and has been variably uplifted and exhumed exhumation magnitudes for sedimentary deposits of the basin are estimated from offset porosity depth trends the analysis is based on correlating sonic log travel times with compensated density logs permitting sonic travel time to be used as a proxy for porosity twelve western platform wells are used to define a standard exponential porosity depth trend with an extrapolated surface porosity of 50 and an exponential decay factor of 2265 m valid for mudstone shale sections spanning a depth range of 300 3000 m this curve is a calibration curve against which offset porosity depth trends from wells in exhumed regions can be compared to determine apparent exhumation amounts porosity depth trends for 43 eastern mobile belt wells are offset 0 2800 m shallower relative to the western platform trend but generally are parallel to it in the southern region of the eastern mobile belt net exhumation amounts which are the sum of the porosity depth trend offset and depth to unconformity range from 850 to 3000 m most of this exhumation occurred on discrete contractional structures in late miocene to early pliocene time probably associated with an increased rate of convergence across the alpine fault system to the south in the eastern region of the eastern mobile belt net exhumation progressively increases from 0 m in the south to 1800 m in the northeast part of the basin consistent with the 2 4 sw structural tilt of onshore strata the regional and consistent pattern of exhumation in the eastern region contrasts with the discrete structure related exhumation in the southern region and is consistent with models of thermal and flexural uplift

spatial variations in the frequency magnitude distribution of earthquakes at soufriere hills volcano montserrat west indies the frequency magnitude distribution of earthquakes measured by the b value is determined as a function of space beneath soufriere hills volcano montserrat from data recorded between august 1 1995 and march 31 1996 a volume of anomalously high b values b 3 0 with a 1 5 km radius is imaged at depths of 0 and 1 5 km beneath english s crater and chance s peak this high b value anomaly extends southwest to gage s soufriere at depths greater than 2 5 km volumes of comparatively low b values b 1 are found beneath st george s hill windy hill and below 2 5 km depth and to the south of english s crater we speculate the depth of high b value anomalies under volcanoes may be a function of silica content modified by some additional factors with the most siliceous having these volumes that are highly fractured or contain high pore pressure at the shallowest depths

stress changes focal mechanisms and earthquake scaling laws for the 2000 dike at miyakejima japan faulting processes in volcanic areas result from a complex interaction of pressurized fluid filled cracks and conduits with the host rock and local and regional tectonic setting often volcanic seismicity is difficult to decipher in terms of the physical processes involved and there is a need for models relating the mechanics of volcanic sources to observations here we use focal mechanism data of the energetic swarm induced by the 2000 dike intrusion at miyakejima izu archipelago japan to study the relation between the 3 d dike induced stresses and the characteristics of the seismicity we perform a clustering analysis on the focal mechanism fm solutions and relate them to the dike stress field and to the scaling relationships of the earthquakes we find that the strike and rake angles of the fms are strongly correlated and cluster on bands in a strike rake plot we suggest that this is consistent with optimally oriented faults according to the expected pattern of coulomb stress changes we calculate the frequency size distribution of the clustered sets finding that focal mechanisms with a large strike slip component are consistent with the gutenberg richter relation with a b value of about 1 conversely events with large normal faulting components deviate from the gutenberg richter distribution with a marked roll off on its right hand tail suggesting a lack of large magnitude events mw 5 5 this may result from the interplay of the limited thickness and lower rock strength of the layer of rock above the dike where normal faulting is expected and lower stress levels linked to the faulting style and low confining pressure

early paleogene temperature history of the southwest pacific ocean reconciling proxies and models we present a new multiproxy tex86 18o and mg ca marine temperature history for canterbury basin eastern new zealand that extends from middle paleocene to middle eocene including the paleocene eocene thermal maximum petm and early eocene climatic optimum eeco in light of concerns that proxy based sea surface temperature sst estimates are untenably warm for the southwest pacific during the eocene we review the assumptions that underlie the proxies and develop a preliminary paleo calibration for tex86 that is based on four multiproxy eocene records that represent an sst range of 15 34 c for the southwest pacific paleogene we show that tex86l exhibits the best fit with the eocene paleo calibration ssts derived from related proxies tex86h 1 tex86 exhibit a systematic warm bias that increases as tex86 values decrease a warm bias of 4 7 c where tex86 0 7 the tex86l proxy indicates that southwest pacific sst increased by 10 c from middle paleocene to early eocene with sst maxima of 26 28 c tropical during the petm and eeco and an sst minimum of 13 16 c cool warm temperate at the middle late paleocene transition 58 7 ma the base of the eeco is poorly defined in these records but the top is well defined in canterbury basin by a 2 5 c decrease in sst and bottom water temperature bwt in the latest early eocene 49 3 ma bwt falls from a maximum of 18 20 c in the eeco to 12 14 c in the middle eocene overall cooler temperatures are recorded in the mid waipara section which may reflect a deeper 500 m water depth and less neritic depositional setting compared with hampden and odp 1172 200 m water depth the high ssts and bwts inferred for the petm and eeco can be reconciled with eocene coupled climate model results if the proxies are biased towards seasonal maxima and the likely effect of a proto east australian current is taken into account

transgressive regressive cycles and the occurrence of coal in some upper cretaceous strata of utah cenomanian santonian strata contain about 60 of the known cretaceous coal resources of utah the areal and temporal distribution of these resources can be related to the transgressive regressive history of the western shoreline of the interior cretaceous seaway two orders of transgressive regressive cycles the larger of which corresponds to third order cycles of vail et al are deemed important in analyzing the distribution of coal in utah the largest coal fields are associated with extensive vertical stacking of the deposits of what are here termed fourth order cycles at the transgressive and regressive maxima of third order cycles deposits of transgressive and regressive phases of third order cycles contain only minor amounts of coal this genetic relationship may prove useful in predicting areas of important cretaceous coal resources in deeper parts of structural basins in the rocky mountain region

systematic destruction of k feldspar in deeply buried rift and passive margin sandstones systematic patterns of detrital k feldspar decline with increasing burial depth are revealed during deep burial 1 5 4 5 km of sandstones in diverse sedimentary basins from three rift and two passive margin settings in the north sea and usa gulf coast k feldspar destruction is predominantly by dissolution though in situ alteration to albite also occurs dissolution can remove up to 15 solid volume of the rock by 4 5 km so that primary arkosic sandstones become diagenetic quartzarenites at depth with increased secondary primary porosity ratios the quantity of k feldspar destroyed during temperature increase is very variable 0 05 1 0 solid volume c compaction continues during deep burial and petrographic textures diagnostic of secondary porosity are destroyed so that many sandstones preserve exceptions to this scenario of detrital k feldspar dissolution are noted proximity of k rich evaporites high sio 2 and k activity due to the breakdown of volcanic clasts or very low p co 2 can stabilize both k feldspar and a suite of k rich cements

anomalously high porosity and permeability in deeply buried sandstone reservoirs origin and predictability porosity and permeability generally decrease with increasing depth thermal exposure and effective pressure however a significant number of deep 4 km approximately 13 000 ft sandstone reservoirs worldwide are characterized by anomalously high porosity and permeability anomalous porosity and permeability can be defined as being statistically higher than the porosity and permeability values occurring in typical sandstone reservoirs of a given lithology composition and texture age and burial temperature history in sandstones containing anomalously high porosities such porosities exceed the maximum porosity of the typical sandstone subpopulation major causes of anomalous porosity and permeability were identified decades ago however quantification of the effect of processes responsible for anomalous porosity and permeability and the assessment of the predictability of anomalous porosity and permeability occurrence in subsurface sandstones have rarely been addressed in published literature the focus of this article is on quantification and predictability of three major causes of anomalously high porosity 1 grain coats and grain rims 2 early emplacement of hydrocarbons and 3 shallow development of fluid overpressure grain coats and grain rims retard quartz cementation and concomitant porosity and permeability reduction by inhibiting precipitation of quartz overgrowths on detrital quartz grains currently prediction of anomalous porosity associated with grain coats and grain rims is dependent on the availability of empirical data sets in the absence of adequate empirical data sedimentologic and diagenetic models can be helpful in assessing risk due to reservoir quality such models provide a means to evaluate the effect of geologic constraints on coating occurrence and coating completeness required to preserve economically viable porosity and permeability begin page 302 in a given play or prospect these constraints include thermal history and sandstone grain size and composition the overall effect of hydrocarbon emplacement on reservoir quality is controversial it appears that at least some cements quartz and illite may continue to precipitate following emplacement of hydrocarbons into the reservoir our work indicates that integration of basin modeling with reservoir quality modeling can be used to quantify prior to drilling the potential impact of hydrocarbon emplacement on porosity and permeability the best case scenario for significant reservoir quality preservation due to fluid overpressure development is in rapidly deposited tertiary or quaternary sandstones our models suggest that significant porosity can be preserved in sandstones that have experienced continuous high fluid overpressures from shallow burial depths the models also indicate that the potential for porosity preservation is greatest in ductile grain rich sandstones because compaction tends to be the dominant control on reservoir quality in such rocks the case for significant porosity preservation associated with fluid overpressures in pre tertiary basins however is more problematic because of the complexities in the history of fluid overpressure and the greater significance of quartz cementation as a potential mechanism of porosity loss

linking the influence of diagenetic properties and clay texture on reservoir quality in sandstones from nw borneo abstract reservoir studies in the sandakan sub basin nw borneo are challenging and inexplicable due to complex diagenetic effects such as compaction cementation dissolution and depositional means including grain size sorting and packing with reference to clay texture these diagenetic effects together impart significant amounts of uncertainty on reservoir quality assessment in this regard we documented the mineralogical diagenetic and reservoir properties of sandstones from the sandakan formation nw borneo through thin sections sem xrd xrf he porosimeter and poroperm in order to analyze the influential parameters we used matrix grain rearrangement and clay coating around detrital grains for the reservoir quality our results show that the sub litharenites to quartz arenite that dominate in the sandstones are drawn from recycled orogenic sand furthermore reservoir properties of the sandstones were ascertained to be controlled by diagenesis micro quartz with clay coatings and pressure dissolution of feldspar with the alteration of lithic fragments the result shows that the physical compaction rearrangements and ductility of grains were the reasons that reduced porosity and permeability the main reason for porosity reduction in the present study is pressure dissolution that resulted in the form of chemical compaction induced by micas with grain contacts nonetheless through sem and petrography clay coatings around the grains and dissolution of feldspar were found to optimally contribute to the preservation of reservoir properties dissolution of feldspar and kaolinization resulted in the formation of secondary porosity that allows meteoric water to percolate through the open and detachment spaces of quartz or other grains this percolation is also responsible in enhancing the pore throat radii in the reservoir media of the analyzed sandstones these traits are obliging to diagenetic effects on porosity and permeability which are linked with the physical compaction and clays coating to reduce and preserve the reservoir properties respectively of same type of reservoirs globally

calculating the primary porosity of unconsolidated sands based on packing texture application to braided river sands abstract primary porosity is an important parameter in studies of reservoir and porosity evolution however the applicable conditions for the few existing mathematical models of primary porosity have yet to be clarified a mathematical model based on the packing texture was proposed by the authors in a previous study however the proposed model was only applicable to well sorted and well rounded modern beach sands in this study the model was modified and applied to poorly sorted and poorly rounded braided river sands the applicable conditions for the proposed models and the previous empirical formula were investigated based on 41 modern braided river sand samples from the kuqa river in the tarim basin and 40 modern beach sand samples from eastern china modern sands can be divided into three types based on the particle size distribution modified mathematical formulas were derived for each type of sands the constant parameters in the mathematical model such as the filling coefficients embedment coefficients and packing void ratios were obtained through experiments and through linear fitting based on the mathematical model the errors between the measured and the calculated primary porosities ranged from 3 to 3 for the modern braided sands and from 2 to 2 for the modern beach sands using the empirical formula the primary porosity errors in the modern braided river sands were large and the formula could not be appropriately applied except to well sorted and well rounded modern beach sands compared with the modified mathematical model the original mathematical model could not be applied well to braided river sands owing to the additional errors caused by ef1 the modified mathematical model based on the packing texture was more credible when applied to poorly sorted and angular braided river sands further it was applicable to all types of unconsolidated sands

nucleation points of recent mainshocks in southern iceland mapped by b values the earthquake catalog in southern iceland between 20 25 w and 21 3 w is complete down to approximately mw 0 and shows no evidence for mag nitude shifts as a function of time since 1991 this means that 6902 earthquakes m 0 1 depth 20 km are available for studies of seismicity patterns preceding the two ms 6 6 mainshocks in 2000 in their source volumes and vicinity 20 25 w and 20 9 w the power law of the frequency magnitude relationship holds down to m0 detailed mapping including 3d mapping of the b value of the frequency magnitude distribution shows that variations from b 0 6 to 1 4 exist over distances of about 2 km this suggests that the tectonic fabric in southern iceland is hetero geneous on a small scale the hypocenters of the two ms 6 6 mainshocks and one additional m 4 5 mainshock are associated with low b values 0 6 0 8 statistically significantly different from volumes in their vicinity therefore one can expect future mainshocks in southern iceland to also emanate from volumes characterized by low b values however the small dimensions of these asperities may render it difficult to identify them before the mainshocks occur low values of estimated local recur rence times 500 2000 years map the two fault zones that ruptured in 2000 against a background of local recurrence times longer than 5000 years the first of the two m 6 6 mainshocks emanated from a volume of short estimated local recurrence time this supports the hypothesis that asperities with short local recurrence times control locations of major ruptures mapping of b values in cross sections shows anomalies of high b at the bottom of the seismogenic crust correlating with the change of its thickness in the middle of the study area these high b value anomalies and other pockets of such anomalies at shallower depths are best interpreted as caused by high pore pressure because evidence of fluids under high pressure is ubiquitous in this area

relocalization and focal mechanisms of volcano tectonic events at colima volcano colima mexico colima volcano 19 51 n 103 62 w 3860 m a s l in western mexico is well known for its frequent activity along with nearby nevado de colima and cantaro volcanoes are part of a volcanic complex rising in the middle of the colima graben a structure on the margin of the jalisco block some one hundred km from the middle america trench the activity of colima volcano is varied and usually accompanied by intense seismicity mostly of volcano tectonic events to investigate several aspects of these earthquakes we analyzed events recorded by the seismological network of colima during the outbursts of 1998 1999 2005 and 2009 those periods of varying intensity lasted from several months to several weeks before and after a significant explosive event the hypocenters located with standard methods appear in a broad region around the volcanic complex to obtain a more accurate location of these events we relocated the hypocenters using the method of double differences a set of 1191 relocated events was obtained providing improved locations which show hypocenters clustered under the edifice of colima volcano and do not seem to be related to any particular geological feature and therefore represent the volume stressed by magma s intrusion the focal mechanism of the relocated events shows a variety of fault plane orientations with a majority indicating dip slip faulting

extensive carbonate cementation of fluvial sandstones an integrated outcrop and petrographic analysis from the upper cretaceous book cliffs utah abstract extensive large scale pervasive cementation in the form of cement bodies within fluvial strata has rarely been documented although fluvial strata commonly act as important hydrocarbon reservoirs as well as groundwater aquifers here we present outcrop petrographic and geochemical data for pervasive ferroan dolomite cement bodies up to 250 m in size from upper cretaceous desert member and castlegate sandstone fluvial strata exposed in the book cliffs in utah these cement bodies are present with coastal plain fluvial strata within both the desert and castlegate lowstand sandstones and are most abundant in the thin distal fluvial strata cement bodies are almost entirely absent in updip thicker fluvial strata petrographic observations suggest a predominantly early diagenetic timing to the mildly ferroan dolomite with a component of later burial origin 13 c values for the cement 4 8 to 5 7 v pdb suggest a marine derived source for the earliest phase with a burial organic matter source for later cement 18 o data 6 3 to 11 8 v pdb suggest precipitation from freshwater dominated fluids it is proposed here that dolomite was derived from leaching of detrital dolomite under lowstand coals and cementation took place in coastal aquifers experiencing mixed meteoric marine fluids as a result of base level fluctuations this data presented here shows that large cement bodies can be an important component within fluvial sandstones with a potentially significant impact upon both reservoir quality and fluid flow within reservoirs especially at the marine non marine interface

factors controlling reservoir properties and hydrocarbon accumulation of lacustrine deep water turbidites in the huimin depression bohai bay basin east china abstract lacustrine deep water turbidite plays are a novel area for exploration in the huimin depression bohai bay basin turbidites in the shang 847 block a typical turbidite play in the huimin depression provide an opportunity to study the factors controlling the reservoir properties and hydrocarbon accumulation in lacustrine turbidite sandstones the reservoir quality of turbidite sandstones very fine grained moderately to well sorted mainly lithic arkose in this study area are mainly controlled by the distribution patterns of carbonate cements and pseudomatrix significant inverse relationships exist between the volume of carbonate cement and both porosity and permeability of the turbidite sandstones carbonate cement is located preferentially near the margins of the sandstone bodies sandstones with distance from the sandstone mudstone contact surface less than 0 7 m or with thickness less than 1 2 m are commonly tightly cemented carbonate cement 15 with low porosity and permeability lower than 10 and 0 1 md respectively the source of carbonate cement was most likely external probably derived from the surrounding mudstone most pore filling carbonate cements occurred during late diagenesis at burial depths greater than 2200 m the petrophysical properties of turbidites have a positive relationship with the content of kaolinite and chlorite but have a negative relationship with the content of illite 2 d and 3 d reconstructions of non oil bearing and oil bearing layers indicate that dissolution of carbonate cement feldspars and unstable rock fragments was more developed in oil bearing layers than in non oil bearing layers and hance oil bearing layers have higher porosity and larger pore sizes petrophysical property appears to have a significant effect on the hydrocarbon accumulation in the turbidite sandstones sandstones with porosities lower than 9 and or permeabilities lower than 0 78 md are not prone to contain oil

the influence of variations in illite morphology on the permeability of middle jurassic brent group sandstones cormorant field uk north sea abstract sem examination of core samples from brent group sandstones in the cormorant field reveals that critical point drying preserves delicate illite morphologies that are damaged during air drying this damage may lead to an increase in permeability as defined by the drying effect air permeability air dried air permeability critical point dried the cormorant reservoir sandstones contain four illite morphologies early diagenetic pore lining illite formed before hydrocarbon emplacement and occurs ubiquitously pore bridging illite blooming illite and illite that replaces kaolinite all formed later and occur preferentially at or below the oil water contact damage to these late illites causes an average drying effect of 3 5 and explains why measured permeabilities predict higher water injection rates than occur in the field comparison with data from other reservoirs reveals that the drying effect is influenced by variations in illite particle dimensions as well as illite morphology tem shadowing techniques reveal that illite particle sizes describe a trend from small particles in reservoirs with large drying effects to large particles in reservoirs with small drying effects these size variations probably reflect differing illite growth rates permeability in illite bearing sandstones thus reflects not only the amount of illite present but also the size of the individual particles and their morphology

the physical mechanisms of deep moonquakes and intermediate depth earthquakes how similar and how different abstract we here review the characteristics of deep moonquakes dmq and conclude that they share many common features with certain categories of intermediate depth earthquakes namely the pressure temperature conditions where they occur their occurrence in highly localized nests their tendency to occur repeatedly at the same or nearly the same location and their distribution of sizes b values we thus explore various physical mechanisms proposed for earthquakes that might also explain deep moonquakes these include the possibility that there are partial melts or fluid phases in the lunar mantle that permit dmq to occur at pressures where ordinary brittle fracture is not possible the possibility that stress concentration increases tidal stresses at certain locations where moonquakes occur repeatedly and the possibility that dmq occur at relatively low stresses because rock strength is lowered by a fatigue process induced by the repetitive application of tidal stresses the similarities between dmq and terrestrial earthquakes imply that research focusing on the properties of both phenomena may help us understand the physical origins of both

pre requisites processes and prediction of chlorite grain coatings in petroleum reservoirs a review of subsurface examples abstract deeply buried reservoirs containing chlorite coated quartz sand grains commonly have higher than expected porosity and permeability although prediction of such positive anomalies still remains elusive a total of 54 published examples based on information and data from 62 scientific papers was collated quantification of some of the most common parameters including depositional environment age and latitude of sand deposition effect on reservoir quality and chemical composition of chlorite is presented the dataset indicates that chlorite coats are found in sandstones deposited in a range of depositional environments but most commonly occur in delta related environments 44 with fluvial environments the second most common 19 age relationships indicate that there is an overall exploration bias in published examples with chlorite coats becoming increasingly common through time the latitude at the time of deposition of sands with chlorite coats is wide 60 n 60 s and indicates that temperate and tropical climates are important for the generation of this clay mineral chlorite can have a variable effect on reservoir quality but is typically positive iron rich chlorites occur overwhelmingly in coastal environments while mixed iron and magnesium rich chlorites are principally found in marine and terrestrial sandstones analysis of these factors suggests that hinterland geology basinal soil development and geochemical weathering and proximity to river systems are essential to the formation of chlorite precursor phases these characteristics have been combined to define situations where chlorite coats were more likely or more unlikely to form these parameters will provide insights into the formation of chlorite and to further refine predictive models for the presence or absence of chlorite coated sandstones

review of diagenetic facies in tight sandstones diagenesis diagenetic minerals and prediction via well logs abstract the tight sandstones are characterized by low porosity low permeability complex pore structure and strong heterogeneity due to the extensive diagenetic modifications they experienced understanding of the impact of diagenetic alterations on reservoir quality is crucial to the hydrocarbon exploration and production in tight sandstones diagenetic facies which is the comprehensive description of the diagenesis and diagenetic minerals determines the formation and distribution of sweet spot by correlating the diagenetic facies to well log responses the subsurface distribution of porosity and permeability can be predicted however the prediction of diagenetic facies and reservoir quality via well logs in tight sandstones remains a challenging task this paper critically reviews the impact of diagenesis and diagenetic minerals on reservoir quality in tight sandstones and establishes a model for prediction of diagenetic facies via well logs as assessed from peer reviewed papers in the literature as well as from the authors personal experiences this review begins with reviewing the impacts of compaction cementation dissolution and various types of diagenetic minerals on reservoir quality evolution the definition and classification schemes of diagenetic facies are then discussed and the reservoir quality as well as diagenetic evolution sequence of various diagenetic facies is summarized the same diagenetic facies commonly display similar compositional and textural attributes matrix and cement as well as porosity systems the well log responses gr ac den cnl and rt of various diagenetic facies are summarized by the calibration of log values with cores and related thin sections by translating the diagenetic facies to conventional well logs a predictable model which can be used for subsurface reservoir quality prediction is established then the theory of ecs logs is reviewed and the application of ecs logs in diagenetic facies evaluation is discussed at last the quantitative characterization for various type and degree of diagenesis is reviewed and the subsurface diagenetic facies is predicted by quantitative calculation of the compactional porosity loss cementational porosity loss and dissolution porosity content via well logs correlating the diagenetic facies to well logs provides a powerful tool to predict the distribution of high quality reservoirs in tight sandstones this review will provide insights into the reservoir quality evaluation and sweet spot prediction via well logs in tight sandstones

diagenetic characteristics and reservoir quality of the lower cretaceous biyadh sandstones at kharir oilfield in the western central masila basin yemen the lower cretaceous biyadh formation in the masila basin is an important hydrocarbon reservoir however in spite of its importance as a reservoir published studies on the biyadh formation more specifically on the diagenesis and relate with reservoir quality are limited based on core samples from one well in the kharir oilfield western central masila basin this study reports the lithologic and diagenetic characteristics of this reservoir the biyadh sandstones are very fine to very coarse grained moderate to well sorted quartzarenite and quartzwacke the diagenetic processes recognized include mechanical compaction cementation carbonate clay minerals quartz overgrowths and a minor amount of pyrite and dissolution of the calcite cement and feldspar grains the widespread occurrences of early calcite cement suggest that the biyadh sandstones lost a significant amount of primary porosity at a very early stage of its diagenetic history based on the framework grain cement relationships precipitation of the early calcite cement was either accompanied or followed by the development of part of the pore lining and pore filling clay cements secondary porosity development occurred due to partial to complete dissolution of early calcite cement and feldspar grains in addition to calcite several different clay minerals including kaolinite and chlorite occur as pore filling and pore lining cements kaolinite largely occurs as vermiform and accelerated the minor porosity loss due to pore occlusion chlorite coating grains helps to retain primary porosity a by retarding the envelopment of quartz overgrowths porosity and permeability data exhibit good inverse correlation with cement thus reservoir quality is controlled by pore occluding cement diagenetic history of the biyadh sandstones as established here is expected to help better understanding and exploitation of this reservoir the relation between diagenesis and reservoir quality is as follows the initial porosity was decreased by compaction and cementation and then increased by dissolution of early calcite cement and feldspar grains the reservoir quality is also affected by depositional environment controls of grain size sorting and matrix thus best good quality reservoir rocks were deposited in braided river channels environment where no inhibited authigenic clays and high percentages of detrital quartz

an interdisciplinary approach to reservoir characterisation an example from the early to middle eocene kaimiro formation taranaki basin new zealand abstract the kaimiro formation is an early to middle eocene ne sw trending reservoir fairway in taranaki basin and comprises a range of coastal plain through to shallow marine facies a time of regional transgression is observed across the paleocene eocene transition which is linked to a general global warming trend and to regional thermal relaxation related subsidence in new zealand the earliest eocene transgressive deposits pass upwards into a series of cyclically stacked packages interpreted as 3rd and 4th order sequences maximum regression occurred within the early eocene and was followed by punctuated retrogradational stacking patterns associated with shoreline retreat and subsequent regional transgression in the middle eocene the kaimiro formation is considered a good reservoir target along most of the reservoir fairway which can largely be attributed to a consistently quartz rich lithic poor composition and reasonably coarse sand grain size correlations demonstrate that within the early eocene the main reservoir facies are channel fill sandstones overlying candidate sequence boundaries in paleoenvironmentally landward proximal settings and upper shoreface shoreline sandstones in relatively basinward distal settings middle eocene reservoir facies are not represented in distal wells due to overall transgression at this time yet they form a significant target in more proximal well locations particularly on the taranaki peninsula depositional facies is one of the principal controls on sandstone reservoir quality however while reservoir facies have been proven along the length of the reservoir fairway it is evident that diagenesis has significantly impacted sandstone quality relatively poor reservoir properties are predicted for deeply buried parts of the basin maximum burial 4 5 km due to severe compaction and relatively abundant authigenic quartz and illite in contrast good reservoir properties are locally represented in reservoir facies where present day burial depths are these results illustrate how an interdisciplinary approach to regional reservoir characterisation are used to help reduce risk during prospect evaluation assessment of both reservoir distribution and quality is necessary and can be undertaken through integrated studies of facies sequence stratigraphy burial modelling and petrography

spatial and temporal evolution of b values before large earthquakes in taiwan abstract we investigate the spatial and temporal variations of b values before twenty three earthquakes with ml 6 0 in the taiwan region from 1999 to 2009 we estimate the spatial distribution of b values within a one year period before the occurrence of investigated earthquakes it shows that the epicenters of those earthquakes are located predominately in the regions with low b values relative to the entire study area in terms of temporal evolutions the b values are slightly lower during the year prior to the investigated earthquakes in comparison to the periods two to five years earlier we propose that these anomalies may offer spatiotemporal constraints for the earthquake forecasting in taiwan in the future

are volcanic seismic b values high and if so when abstract the gutenberg richter exponent b is a measure of the relative proportion of large and small earthquakes it is commonly used to infer material properties such as heterogeneity or mechanical properties such as the state of stress from earthquake populations it is well known that the b value tends to be high or very high for volcanic earthquake populations relative to b 1 for those of tectonic earthquakes and that b varies significantly with time during periods of unrest we first review the supporting evidence from 34 case studies and identify weaknesses in this argument due predominantly to small sample size the narrow bandwidth of magnitude scales available variability in the methods used to assess the minimum or cutoff magnitude m c and to infer b informed by this we use synthetic realisations to quantify the effect of choice of the cutoff magnitude on maximum likelihood estimates of b and suggest a new work flow for this choice we present the first quantitative estimate of the error in b introduced by uncertainties in estimating m c as a function of the number of events and the b value itself this error can significantly exceed the commonly quoted statistical error in the estimated b value especially for the case that the underlying b value is high we apply the new methods to data sets from recent periods of unrest in el hierro and mount etna for el hierro we confirm significantly high b values of 1 5 2 5 prior to the 10 october 2011 eruption for mount etna the b values are indistinguishable from b 1 within error except during the flank eruptions at mount etna in 2001 2003 when 1 5 b b 1 our results confirm that these volcano tectonic earthquake populations can have systematically high b values especially when associated with eruptions at other times they can be indistinguishable from those of tectonic earthquakes within the total error the results have significant implications for operational forecasting informed by b value variability in particular in assessing the significance of b value variations identified by sample sizes with fewer than 200 events above the completeness threshold

spatial variation of seismic b values beneath makushin volcano unalaska island alaska abstract the frequency magnitude distribution was spatially mapped beneath makushin volcano unalaska island alaska using an earthquake catalog of 491 events that occurred between july 2001 and april 2005 an area of high seismic b values 2 0 is found 4 km east of makushin s main vent at a depth between 4 and 7 km the anomaly is statistically significant based on utsu s p test t utsu on seismicity in report of the joint research institute for statistical mathematics tokyo 1992 139 157 and is not data processing method or parameter dependent interestingly a recent insar interferometric study z lu j a power v s mcconnell c wicks jr d dzurism preeruptive inflation and surface interferometric coherence characteristics by satellite radar interferometry at makushin volcano alaska 1993 2000 j geophys res 107 2002 2266 doi 10 1029 2001jb000970 inferred a surface uplift of about 7 cm during the two year period prior to october 1995 centered approximately in the area with the observed anomalous b values the uplift was caused by the volume increase of an inferred magma chamber at a depth of about 7 km the close correspondence of the seismic and insar observations suggests that the heterogeneous area associated with the observed high b values is most likely the result of increased crack density associated with the magma chamber this study demonstrates the effectiveness of combining insar and seismological observations in locating magma chambers and areas of high heterogeneity in the crust

environmental change across the paleocene eocene transition from eastern new zealand a marine palynological approach abstract recent improvements in the resolution and calibration of dinoflagellate cyst dinocyst bioevents across the paleocene eocene p e transition in new zealand allow better understanding of the relation between dinocyst variations and environmental change in southern mid high latitudes during this greenhouse climate combining information from land spore pollen assemblages with coeval dinocyst records allows evaluation of plant changes in a marine based timeframe palynological associations at tawanui and moeraki hampden eastern new zealand fluctuate across the p e transition and local palynological phases are recognized the onset of major fluctuations in dinocyst assemblages coincide with the intense warmth and excess carbon of the initial eocene thermal maximum ietm changes in sea surface temperature sst appear to have been the main environmental factor that affected dinoflagellates short lived intervals of higher ssts suggested by a high relative abundance of the apectodinium complex are seen during the ietm and lower eocene abundant cysts of probable heterotrophic dinoflagellates suggest surface waters remained relatively eutrophic for up to 0 5 myr after the onset of the ietm perhaps in response to enhanced terrigenous input related to changes in the character and pattern of continental weathering in contrast to dinocysts spore pollen assemblages suggest plant communities remained rather stable across the p e transition vegetation was dominated by a mesothermal conifer dominated multistratal rainforest and low latitude nypa mangroves were established by the uppermost paleocene plants in this region show no clear response to the ietm and the predominant vegetation change may have occurred at least 1 5 myr later characterized by an increase in mesothermal megathermal angiosperm plants

seismicity at the volcanoes of katmai national park alaska july 1995 december 1997 abstract upper crustal seismicity located within katmai national park occurs mainly within four distinct clusters originating near martin mageik mm subnet volcanoes trident volcano katmai caldera and snowy volcano analyses of earthquake frequency magnitude distributions reveal high b values beneath mm subnet 1 5 normal b values at trident volcano 1 0 and intermediate b values at katmai caldera 1 3 for all seismicity occurring between july 1995 and december 1997 detailed analyses of subsets of b values and hypocenter locations at mm subnet reveal a temporal increase in b value associated with an increase in the maximum depth of seismicity the changes occurred during a shallow earthquake swarm beneath mageik volcano between october 16 and 25 1996 and again in november december 1997 before the swarm the weighted least squares b value was 1 01 at mm subnet increased to 1 59 during the swarm and remained anomalously high until april 1997 the corresponding maximum depth of seismicity is generally less than 5 km for well located earthquakes occurring after september 18 1996 but shifted to 10 km during both the october 1996 swarm and the november december 1997 period the november december 1997 event is not associated with an increase in the rate of seismicity or the b value the october 1996 swarm had a cumulative moment release of 5 0 10 20 dyn cm and decayed from a peak rate of 110 events per day with a modified omori law p value of 1 06 0 11 modelling by the flow law with a 10 km depth limit for seismicity suggests that temperature gradients are on the order of 20 40 c km 1 in agreement with the p value estimate we infer that a simple pressurizing intrusion was not associated with the october swarm because higher stresses should increase seismicity and lower the b value the opposite of what we observed alternatively an actively degassing intrusion would reduce the effective stress and increase the b value at shallow depths while the increased stress would induce seismicity at depth surface temperature measurements taken at the mageik crater lake in july 1995 and again in july 1997 revealed an increase of about 10 c in the lake water temperature consistent with a degassing intrusive event

sandstone porosity loss a big picture view of the importance of compaction proper assessment of the relative importance of porosity loss by compaction and cementation is important to the prediction of porosity in sandstones studies of mass transport during diagenesis and the modeling of basin fluid flow quantitative estimation of the amounts of porosity loss by compaction and cementation can be made from standard point count data on cement and pore space abundance this method is discussed and its sources of error are evaluated error in the method arises mainly from uncertainty in initial sandstone porosities and the amount of local grain dissolution during burial data on the porosity of modern sands indicate that assumed initial porosities of 40 to 45 will produce small average errors in the calculation of compactional porosity loss for clean sandstones grain dissolution whether resulting in local porosity enhancement by export in solution of former grain mass or local precipitation of authigenic cement will cause an underestimation of the significance of compaction a large database of point count data from diverse sandstones is used to provide a broad perspective on the importance of compaction and cementation to sandstone porosity loss it is shown that the global importance of compactional porosity loss has generally been under appreciated in fact compaction mechanical and chemical is probably the dominant mechanism of porosity loss in most sandstones the extent of compactional porosity loss is related to sand grain composition and basin thermal structure

diagenesis porosity evolution and petroleum emplacement in tight gas reservoirs taranaki basin new zealand abstract tight gas reservoirs have become popular targets in petroleum exploration in recent years due largely to the increasing market demand for gas and also to technology advances used in extraction reservoir quality is typically poor due to deep burial diagenesis resulting in significant compaction cementation and illitization however analysis of tight reservoirs using integrated techniques can improve our understanding about the controls on reservoir quality and these results can potentially be used to help predict reservoir quality at other sites the use of integrated analysis of diagenetic and burial history is here shown for the eocene k3e kapuni group reservoir in cardiff 1 taranaki basin new zealand the k3e belongs to a deep gas play that was considered uncommercial in the early 1990 s due to poor reservoir quality associated with extensive authigenic illite potassium argon dating has shown that destruction of the reservoir through illitization occurred in the pliocene associated with a late stage heating and or fluid flow event timing of illite authigenesis in the k3e reservoir postdates the likely circulation of co2 rich fluids that caused reaction of feldspar to form kaolinite and quartz homogenization temperatures from two phase aqueous fluid inclusions within quartz cements together with 1d basin models of co2 expulsion from coaly source rocks indicate that feldspar reactions may have started in the mid miocene the co2 rich fluids are thought to have been generated by thermal decarboxylation of intraformational paleocene eocene coals at the same time oil was being expelled from older cretaceous coals and migrated into the mid late eocene reservoirs evidence for oil migration is demonstrated by the presence of residual oil and by the local occurrence of abundant oil bearing fluid inclusions the products of feldspar dissolution appear to be in reasonable balance with the amount of dissolved feldspar suggesting a relatively closed diagenetic system with little improvement in reservoir quality despite the large volume of feldspar dissolved petrographic evidence of reservoir sandstones at cardiff 1 demonstrates the presence of some illite free secondary macropores lined by residual oil this local preservation of porosity may be replicating the original distribution of oil saturated pores in the pliocene at the time when authigenic kaolinite underwent reaction to form illite remobilization of the oil probably occurred by gas flushing after the main phase of illitization at c 5 ma the data presented in this study are consistent with redistributed liquid petroleum into up dip traps and represent the potential for a new exploration play the eocene reservoir at cardiff 1 is currently gas charged with the pore system composed of approximately one third large macropores connected by a tortuous microporous network however it is possible that with the currently buoyant gas market and modern hydraulic fracturing techniques it may now be economic to flow gas from the locally large pores identified in the k3e reservoir

three dimensional p wave velocity structure derived from local earthquakes at the katmai group of volcanoes alaska abstract the three dimensional p wave velocity structure beneath the katmai group of volcanoes is determined by inversion of more than 10 000 rays from over 1000 earthquakes recorded on a local 18 station short period network between september 1996 and may 2001 the inversion is well constrained from sea level to about 6 km below sea level and encompasses all of the katmai volcanoes martin mageik trident griggs novarupta snowy and katmai caldera the inversion reduced the average rms travel time error from 0 22 s for locations from the standard one dimensional model to 0 13 s for the best three dimensional model the final model from the 6th inversion step reveals a prominent low velocity zone 3 6 5 0 km s centered at katmai pass and extending from mageik to trident volcanoes the anomaly has values about 20 25 slower than velocities outboard of the region 5 0 6 5 km s moderately low velocities 4 5 6 0 km s are observed along the volcanic axis between martin and katmai caldera griggs volcano located about 10 km behind northwest of the volcanic axis has unremarkable velocities 5 0 5 7 km s compared to non volcanic regions the highest velocities are observed between snowy and griggs volcanoes 5 5 6 5 km s relocated hypocenters for the best 3 d model are shifted significantly relative to the standard model with clusters of seismicity at martin volcano shifting systematically deeper by about 1 km to depths of 0 to 4 km below sea level hypocenters for the katmai caldera are more tightly clustered relocating beneath the 1912 scarp walls the relocated hypocenters allow us to compare spatial frequency size distributions b values using one dimensional and three dimensional models we find that the distribution of b is significantly changed for martin volcano which was characterized by variable values 0 8 b b b b b b values indicating the robustness of the observations the strong high b value region at mageik volcano is mainly associated with an earthquake swarm in october 1996 that possibly indicates a shallow intrusion or influx of gas the new velocity and spatial b value results in conjunction with prior gravity bouguer anomalies up to 40 mgal and interferometry several cm uplift data provide strong evidence in favor of partially molten rock at shallow depths beneath the mageik katmai novarupta region moderately low velocities beneath martin and katmai suggest that old mostly solidified intrusions exist beneath these volcanoes higher relative velocities beneath the griggs and snowy vents suggest that no magma is resident in the shallow crust beneath these volcanoes

diagenesis and reservoir quality evolution of the eocene sandstones in the northern dongying sag bohai bay basin east china abstract the eocene sandstones in the northern dongying sag bohai bay basin china are reservoirs for large accumulations of hydrocarbons the sandstones are mainly lithic arkoses and feldspathic litharenites texturally and compositionally immature these sandstones have a wide range of porosity 0 4 37 and permeability 0 004 6969 md and show an overall decrease in reservoir quality from 1500 m to 5000 m below sea level the reduction in reservoir quality is a product of several digenetic processes these include compaction precipitation of dolomite and calcite in eodiagenetic stage compaction feldspar dissolution precipitation of quartz cements and clays kaolin and illite and precipitation of ferrocalcite and ankerite in mesodiagenetic stage mineral distribution pattern and isotopic composition suggest carbonate cements in sandstones originate from sources outside the sandstones carbonate cementation together with compaction reduced the sandstones porosity and permeability significantly in a sandstone bed marginal sandstones with distance to sandstone mudstone interface less than one meter always have lower porosity than central sandstones as burial depth exceeds 4000 m marginal sandstones have very low porosities 2 m can be potential effective reservoirs feldspar dissolution and precipitation of clays and quartz cements have little impact on absolute porosity mineral distribution pattern and quantitative data show that leached feldspars are the internal source of authigenic quartz and clays in sandstones and the volume difference between feldspar secondary porosity and related authigenic cements is generally less than 0 25 however although there is little or no net import of matter to the sandstones the pore architecture changes dramatically primary macropores are lost as clays and quartz precipitate while the proportion of microporosity increases occurring mainly between clay crystals the overall result is that permeability is significantly degraded

spatial mapping of the b value at mount etna italy using earthquake data recorded from 1999 to 2005 1 the spatial pattern of the b value of the frequency magnitude relation has been analyzed using gridding techniques beneath mount etna italy a regional data set of 2900 events with md duration magnitude 1 5 up to 15 km depth occurring between august 1999 and december 2005 has been used two regions with an abnormally high b value have been found one centered beneath the southern part of the valle del bove above the 6 km below sea level bsl deep basement and the other beneath the summit region 2 km bsl east of the central craters we can infer that these high b value anomalies are regions of increased crack density and or high pore pressure related to the presence of nearby magma storage this interpretation is supported by all the available geophysical evidence such as tomographic studies and geodetic deformation measurements the data set has also been subdivided into five periods corresponding to different phases of volcanic activity 2001 preeruption 2001 eruptive 2002 2003 preeruption 2002 2003 eruptive and 2002 2003 posteruption the minimum magnitude of completeness mc and the b value were computed for each period a volume of anomalously high b values can be observed in each of these periods except for the 2002 2003 preeruption interval this approach has allowed the detection of the transient presence of magmatic intrusions during the various periods evaluated

transgressive changes from tidal estuarine to marine embayment depositional systems the lower cretaceous woburn sands of southern england and comparison with holocene analogs the lower cretaceous woburn sands lower greensand group in southern england constitutes one of the most intensively studied tidal sandstone outcrops for sedimentological and reservoir analog studies most recent workers have interpreted the whole 30 60 m 100 200 ft thick succession around leighton buzzard as representing an ancient tide dominated estuary however unequivocal estuary characteristics are limited to the lowermost part about 15 20 m 50 66 ft we suggest that a significant portion of the woburn sands and most of the middle part was formed in a tide dominated marine embayment hence the vertical facies change from the lower to middle part of the woburn sands is interpreted as a change from 1 a narrow estuary to 2 a broad marine embayment the wash embayment in eastern england is a striking modern analog it receives most of its sediments and waters from marine sources and is largely filled with nondiluted seawater moreover the holocene transgressive history of the wash is remarkably similar to the transgressive evolution of the woburn sands early estuarine sequence models predict landward translation of facies zones along the valley thalweg during transgression but eventual facies translation in the strike direction has not been fully documented or discussed an embayment facies that is not commonly confined in the early incised valley can occur vertically between the estuarine valley fill and marine shelf deposits and is probably underrepresented in current models however this segment of the transgression comprising along strike bay expansion and the development of a broad marine embayment may be more important than previously thought

a brittle failure model for long period seismic events recorded at turrialba volcano costa rica a temporary seismic network consisting of 23 broadband and six short period stations was installed in a dense network at turrialba volcano costa rica between 8 march and 4 may 2011 during this time 513 long period lp events were observed due to their pulse like waveforms the hypothesis that the events are generated by a slow failure mechanism based on a recent new model by bean et al 2014 is tested a significant number 107 of the lps are jointly inverted for their source locations and mechanisms using full waveform moment tensor inversion the locations are mostly shallow with depths 800 m below the active southwest crater the results of the decompositions of the obtained moment tensor solutions show complex source mechanisms composed of high proportions of isotropic and low but seemingly significant proportions of compensated linear vector dipole and double couple components it is demonstrated that this can be explained as mode i tensile fracturing with a strong shear component the source mechanism is further investigated by exploring scaling laws within the data the lps recorded follow relationships very similar to those of conventional earthquakes exhibiting frequency magnitude and corner frequency versus magnitude relationships that can be explained by brittle failure all of these observations indicate that a slow failure source model can successfully describe the generation of short duration lp events at turrialba volcano

overpressures in the taranaki basin distribution causes and implications for exploration analysis of subsurface pressure data from taranaki basin using direct e g repeat formation tester and indirect measurements drilling parameters and wireline log data such as sonic and resistivity indicates the presence of three pressure zones a near hydrostatic regime zone a that extends across the entire basin and to varying depths an underlying overpressured regime zone b with pressures approximately 1100 psi 7 584 mpa above hydrostatic that extends throughout the manaia graben and north along the eastern basin margin at depths of 1900 to 4100 m 6234 13 451 ft and a third regime zone c with approximately 2100 psi 14 479 mpa overpressure that directly underlies zone a and zone b in different parts of the basin although well penetrations are limited the primary cause of overpressure is interpreted to be disequilibrium compaction preserved in upper eocene and oligocene marine shales in parts of the basin hydrocarbon generation and in particular cracking to gas at high maturities is interpreted to contribute to overpressures the overpressures drain laterally and vertically into permeable units intervening transition zones seals comprise lithologic boundaries diagenetic zones and fault planes oligocene carbonates although commonly thin provide an effective barrier to vertical hydraulic communication over much of the basin the manaia graben is a partially closed system with overpressures retained by a complex combination of a top shale seal overlying a regional sequence boundary lithologic barriers within fault compartments fault planes and subcropping sequences episodic fault breach enables vertical transfer of fluids from zone b to zone a in a dynamic fault valve process to date all oil reserves have been found in zone a a large proportion of gas condensate reserves are within zone b and no commercial reserves have been established within zone c the spatial definition of these zones and the appropriate pressure regime is important for well design drilling safety determining hydrocarbon column heights and gas expansion factors and for exploration migration analysis regional analysis of pressure regimes can identify subsurface barriers and seals faults in particular are key elements in fluid migration and the focusing of liquids at abrupt pressure transitions the strength of fault planes and diagenetic zones is the likely control on dynamic fluid release zone c has been very lightly explored and may represent a potential for large dry gas accumulations the zone may be sealed by a diagenetic zone crosscutting lithologic boundaries conventional mapping horizons

diagenesis and reservoir quality of the sherwood sandstone triassic corrib field slyne basin west of ireland abstract the triassic sherwood sandstone in the corrib field slyne basin west of ireland on the european continental margin is a dry gas reservoir with a mercia mudstone top seal petrographic analysis combined with x ray diffraction stable isotope fluid inclusion and core analysis have been used to assess timing of growth and origin of mineral cements whether sandstone diagenesis involved mass flux the controls on reservoir quality and how reservoir quality is likely to vary in more deeply buried sections depositional and early diagenetic characteristics of the sherwood in corrib are typical of a semi arid fluvial environment containing groundwater of meteoric origin early diagenesis included the development of copious dolomite cement in the form of dolocrete as well as the formation of abundant clay while less volumetrically important grain coating haematite and k feldspar cement also grew burial diagenesis witnessed the initial growth of minor chlorite and albite quartz overgrowths and ankerite followed and are the most important burial cements growing over a temperature interval between 100 and 1165 c albite commenced growth at about 90 c quartz cement at 100 c and ankerite at 110 c these cements reached the zenith of their development at 105 110 c for albite 125 130 c for quartz and 135 145 c for ankerite siderite and anhydrite are relatively minor late stage cements the formation water has been consistently highly saline during burial and in terms of mineral cement derived oxygen stable isotope values is likely to be a diagenetically evolved version of the initial depositional water suggesting long term stagnation of the pore fluids the diagenetic reactions that formed burial cements all seem to be essentially isochemical including the ankerite that has carbon isotope characteristics of the cannibalisation of dolocrete reservoir quality is mainly controlled by early diagenetic dolomite and clay although grain size is also important because only sandstones with 200 m grain size have 50 md permeability both shallower and deeper sections than corrib may have similar porosity and permeability since temperature dependent diagenetic controls on reservoir quality e g quartz and illite cement are peripheral

spatio temporal evolution of frequency magnitude distribution and seismogenic index during initiation of induced seismicity at guy greenbrier arkansas abstract in this study we carry out detailed analyses of the spatio temporal variations of seismic b value during the onset of the potentially induced earthquake sequence between guy and greenbrier arkansas to investigate correlations with pore pressure change the range of b values suggests that the seismicity in the guy greenbrier area is mostly a result of the activation of pre existing faults the spatial distribution of b value correlates with modeled pore pressure changes in the northern segment of the fault b value increases with depth due to large pore pressure changes and opening of new fractures in the deeper part and stress relaxation in the shallower parts whereas in the southern segment the shallower part shows higher b values due to higher pore pressure fluctuations but the deeper part in the crystalline basement has low b value due to higher confining stress the correlation between the temporal variation of b value and hypocentral depth explains a previously observed temporal drop of b value this suggests that temporal variations should be interpreted along with the spatial variations estimation of the seismogenic indices for the guy greenbrier fault is provided our analysis suggests that monitoring changes in b value and seismogenic indices during an injection period might be used to avoid the occurrence of significant events if injection volume is reduced at critical times