spectrum of pore types and networks in mudrocks and a descriptive classification for matrix related mudrock pores matrix related pore networks in mudrocks are composed of nanometer to micrometer size pores in shale gas systems these pores along with natural fractures form the flow path permeability network that allows flow of gas from the mudrock to induced fractures during production a pore classification consisting of three major matrix related pore types is presented that can be used to quantify matrix related pores and relate them to pore networks two pore types are associated with the mineral matrix the third pore type is associated with organic matter om fracture pores are not controlled by individual matrix particles and are not part of this classification pores associated with mineral particles can be subdivided into interparticle interp pores that are found between particles and crystals and intraparticle intrap pores that are located within particles organic matter pores are intrap pores located within om interparticle mineral pores have a higher probability of being part of an effective pore network than intrap mineral pores because they are more likely to be interconnected although they are intrap om pores are also likely to be part of an interconnected network because of the interconnectivity of om particles in unlithifed near surface muds pores consist of interp and intrap pores and as the muds are buried they compact and lithify during the compaction process a large number of interp and intrap pores are destroyed especially in ductile grain rich muds compaction can decrease the pore volume up to 88 by several kilometers of burial at the onset of hydrocarbon thermal maturation om pores are created in kerogen at depth dissolution of chemically unstable particles can create additional moldic intrap pores

late holocene glacial activity of bridge glacier british columbia coast mountains bridge glacier is a prominent eastward flowing valley glacier located on the east side of the pacific ranges within the southern british columbia coast mountains the terminus of bridge glacier has retreated at rates up to 125 m year over the last 50 years and currently calves into proglacial bridge lake field investigations of the recently deglaciated terrain and moraines led to the discovery of detrital boles and glacially sheared stumps dendroglaciological analyses of this subfossil wood produced five radiocarbon controlled floating tree ring chronologies the relative age and stratigraphic location of these samples revealed that bridge glacier experienced at least four periods of significant advance during the late holocene a tiedemann aged advance ca 3000 14c years bp an unattributed advance ca 1900 14c years bp a first millennium advance ca 1500 14c years bp and a little ice age advance beginning ca 700 14c years bp lichenometric investigations at eight terminal and lateral moraine comple

sandstone porosity as a function of thermal maturity sandstone porosity decreases in the subsurface as a power function of thermal maturity phi a m sup b where phi is porosity and m is a measure of thermal maturity representing integrated time temperature history a and b are constants for a given sandstone of homogeneous properties but vary between data sets the commonly observed exponential dependence of sandstone porosity upon depth follows as a special case from this power function relation when temperature increases linearly with depth the consideration of sandstone porosity in terms of time temperature exposure offers advantages in the comparison of porosity data from diverse geologic settings the recognition of unusual porosity within a sandstone sequence and the prediction of porosity ahead of the drill and at times in the geologic past

porosity evolution of sandstones related to vitrinite reflectance abstract porosity of potential reservoir sandstones undergoes a complex multistage evolution or diagenesis during burial organic matter deposited in shales also changes during burial an irreversible process called thermal maturation measures of thermal maturation such as vitrinite reflectance r o are routinely available for subsurface samples so the question is can vitrinite reflectance be used to predict the evolutionary stage and quality of sandstone porosity prior to drilling no universal quantitative correlation between r o and sandstone porosity can exist for there are too many variables however r o can indicate the stage of sandstone porosity evolution the first stage is marked by progressive destruction of primary porosity and coincides with low levels of thermal maturation of kerogen up to an r o of 0 4 0 5 the second stage involves dissolution and flushing away of cements and detrital minerals by acid solutions to create secondary porosity the acids are produced by decarboxylation of kerogen in shales in the r o range of about 0 4 0 7 the third stage involves destruction of secondary porosity by cementation and compaction or ideally filling of secondary porosity by oil because the oil window begins at an r o of about 0 6 that is about where the third stage begins it continues until porosity and permeability are reduced to low noneconomic levels at which point the diagenetic evolution has reached porosity basement somewhere between r o 1 4 and 2 5 above r o 2 5 are unmistakable indications of incipient metamorphism in the sandstones because r o can be modeled and projected into undrilled parts of a basin it should be possible to predict the stage of porosity evolution of sandstones in advance of drilling a useful capability

early diagenesis of inner shelf phosphorite and iron silicate minerals lower cretaceous of the orpheus graben southeastern canada implications for the origin of chlorite rims wells in the orpheus graben encountered the most proximal part of the deltaic lower cretaceous rocks of the scotian basin more distal sandstones are important gas reservoir rocks with good reservoir quality where fe rich chlorite chamosite rims on framework grains have inhibited quartz cementation cutting samples from the orpheus graben show the presence of fe rich sheet silicates berthierine or chamosite and early diagenetic phosphorite these minerals were analyzed by electron microprobe and their textures were mapped with backscattered electron images studies in the north sea have shown a relationship between high phosphorus and the presence of good chlorite rims in reservoir rocks the mineralization of pore water phosphorus instead of its return to seawater is favored by fe rich sediments and sorption on iron oxides during shallow sea floor diagenesis the fe ti and p contents are uncommonly high in scotian basin shales compared with global average shale compositions the uncommon occurrence of inner shelf phosphorite in this study is interpreted to be a consequence of the same high fe content of the sediment that also favors the formation of fe rich sheet silicates in rapidly deposited deltaic sandstones of the offshore reservoirs the dominance of type 3 kerogen led to sulfate depletion occurring at depths of tens of meters and a corresponding great thickness for overlying eh oxidation potential controlled diagenetic zones the thick fe reduction zone allowed the formation of early diagenetic berthierine which on burial formed the chamosite rims that resulted in the improved reservoir quality in sandstones the distribution of phosphorus minerals may be an indicator of conditions suitable for berthierine formation

pore characterization and shale facies analysis of the ordovician silurian transition of northern guizhou south china the controls of shale facies on pore distribution abstract the ordovician silurian wufeng longmaxi formations in the upper yangtze platform have been considered as the most promising shale play in china however few studies of detailed shale facies and their influence on pore distribution have been conducted therefore it is important to describe the heterogeneity of shale facies in detail and elucidate their relationship to pore networks and development seven distinct facies were identified on the basis of their characteristic physical biological and chemical attributes ion milled samples from all facies were examined with sem for identification of pore types point counting was used to quantify porosity four major pore types appear dominant framework pores phyllosilicate framework pf pores intrapores within inorganic grains and organic matter om pores due to variability in composition and depositional setting of the different facies pore networks are not uniform framework pores between detrital grains and pf pores are widespread in bioturbated claystones pores in black siliceous shales are dominated by framework pores supported by authigenic silica most of these are filled with kerogen bitumen that is host to foam and bubble type om pores intrapores within calcite or dolomite grains are common in fossiliferous mudstones and black dolomitic siltstones but are isolated and do not contribute to productive porosity in muddy siltstones pf pores are well developed within silty bands initial framework pores within silty beds or laminae however were subsequently cemented by calcite or destroyed by compaction as most of primary pf pores and primary frameworks pores are lost during compaction porosity is highly dependent on the population of framework pores between authigenic quartz grains secondary cracking of amorphous organic matter added porosity after initial infilling of porosity by oil and bitumen migration therefore porosity is mainly controlled by abundance of authigenic silica and toc in spite of differences of burial diagenesis among various facies understanding these main controls on porosity should be helpful to predict and find and more porous facies and have a significant impact on future gas exploration in the wufeng longmaxi formations

carbonate equilibria and ph buffering by organic acids response to changes in pco2 abstract organic acid anions such as acetate a natural product of the thermal breakdown of kerogen may buffer the ph of natural formation waters the extent of ph buffering by acetate should determine whether an increase in the partial pressure of carbon dioxide p co 2 will lead to calcite dissolution porosity enhancement or calcite precipitation porosity destruction successful prediction of porosity therefore requires a quantitative understanding of this process this paper presents computer simulations of the response of the acetic acid carbonate system to changes in p co 2 for typical reservoir fluid compositions and temperatures the effects of variations in temperature ionic strength initial ca 2 concentration and the magnitude of the change in p co 2 were investigated natural formation waters from cenozoic reservoirs in the gulf coast basin and california show maximum organic acid concentrations at 100 20 c at 100 c the simulations show that for concentrations of acetic acid up to 0 06 m 3600 mg l 1 increases in the p co 2 of initially calcite saturated solutions will promote calcite undersaturation at higher temperatures equivalent buffering responses occur at lower acetic acid concentrations but acetic acid concentrations in natural formation waters also decrease rapidly with increasing temperature since present day concentrations of acetic acid in formation waters from cenozoic reservoirs rarely exceed 0 06 m increases in p co 2 will generally promote calcite dissolution by these waters in the gulf coast basin variation in organic acid concentrations as a function of reservoir age suggests that reductions in dissolved organic acid concentrations over geologic time have been relatively small it can be concluded therefore that at any point in the histories of these reservoirs an increase in p co 2 would have promoted calcite dissolution despite the effects of ph buffering by acetate since increases in p co 2 will in most cenozoic basins promote calcite dissolution knowledge of the location and timing of co 2 production in the subsurface can more easily be applied to the prediction of calcite cementation and dissolution

distribution and occurrence of aliphatic acid anions in deep subsurface waters abstract this study reports 144 new analyses of short chain aliphatic acid anions acetate propionate butyrate and valerate in formation waters from eight localities eastern venezuelan basin denver basin eastern green river basin red desert and washakie basins wamsutter arch san juan basin piceance basin raton basin gulf coast basin and the western overthrust reservoir temperature does not predict total or relative abundance of aliphatic acid anions but does predict maximum total concentrations of these species maximum concentrations increase to 90 c above 90 c maximum concentrations decrease above 250 c maximum concentrations should not exceed 1 mg 1 the general order of dominance is acetate propionate butyrate valerate but for coal associated waters is propionate acetate butyrate valerate lack of longer chain aliphatic acid anion dominance over acetate at low reservoir temperatures may suggest hydrologie communication with deeper reservoirs

recent volume loss of british columbian glaciers canada 1 we use the shuttle radar topography mission srtm data and digital terrain models from aerial photography to quantify the change of glacier volume in british columbia bc canada for the period 1985 1999 we note substantial elevation bias in the srtm elevations typically on the order of 12 m km 1 the bias corrected thinning rate is 0 78 0 19 m a 1 which yields an annual volume loss of 22 48 5 53 km 3a 1 this rate of glacier volume loss is 65 of the estimate uncorrected for elevation bias 34 7 km 3a 1 and cautions against the use of uncorrected srtm data for glacier change studies glacier recession in bc could account for ca 0 67 0 12 mm of sea level rise over the period 1985 1999 0 05 0 009 mm yr 1 or about 8 3 of the contribution from mountain glaciers and ice caps the recent rate of glacier loss in the coast mountains 17 0 km 3a 1 is approximately double that observed for the previous two decades

evolution of porosity and pore geometry in the permian whitehill formation of south africa a fe sem image analysis study abstract since the advent of technologically efficient exploitation of economic hydrocarbon reservoirs in shales increasingly more research has been devoted to identifying and characterizing pore systems within shales however only a handful of these studies focused on the development of porosity in thermally mature unconventional reservoirs in this study the evolution of porosity and pore geometry in the permian whitehill formation is addressed with the aid of ultrathin sections 2 3 cm 10 20 m thick and field emission scanning electron microscopy fe sem on samples with mean random vitrinite reflectance values ranging from 1 03 to 4 07 ro we document a strong positive covariation of porosity and total organic carbon content toc in all localities however for samples with vitrinite reflectance values greater than 2 88 ro porosity per unit toc decreased by over 25 relative to samples with lower thermal maturities the positive covariation of thermal maturity and total porosity recorded here is unsurprising and have been documented previously in many gas shales however the dramatic decrease in porosity restricted to samples from localities that experienced advanced maturation ro 2 88 is viewed as an evidence that porosity decrease is directly related to late thermal decarboxylation of organic matter this is supported by the presence of pores and micro fractures infilled by fibrous grains including carbonates clays silicates and phosphates and residual fluid inclusions these grains were likely generated from re precipitation of framework grains previously dissolved by organic acids carboxylic phenolic that were generated during thermochemical decarboxylation of the om our findings do not only fill important gaps in the understanding of organic pore development including processes that create preserve and destroy porosity the porosities described here are also key to gas transfer from shale matrix to induced fractures during fracture stimulation programs

dendroglaciological investigations of mid to late holocene glacial activity in the mt waddington area british columbia coast mountains canada dendroglaciological investigations near mt waddington in the central british columbia coast mountains provide an enhanced perspective of holocene glacial activity field investigations at confederation franklin and jambeau glaciers led to the discovery of subfossil wood mats encased in glacial deposits and glacially sheared stumps buried beneath till radiocarbon dated wood collected from valley bottom and lateral moraine sites at confederation glacier suggest that an early holocene advance occurred at c 5665 cal yr bp followed by succeeding intervals of glacier expansion at c 3700 and 3500 cal yr bp at jambeau glacier detrital wood mats buried close to the contemporary lateral moraine crests document glacier expansion at c 3000 cal yr bp detrital subfossil wood buried in lateral moraines at the confluence of confederation and franklin glaciers records distinct episodes of little ice age glacier expansion as early as c 1212 cal yr ad and suggests the glacier surface continued to thicken u

carboxylic acid anions in formation waters san joaquin basin and louisiana gulf coast u s a implications for clastic diagenesis abstract carboxylic acid anions caa in formation waters are of interest to studies of clastic diagenesis because of their ability to buffer formation water eh and ph and thus substantially contribute to controls on carbonate mineral stability and their ability to complex and transport al and si from the site of aluminosilicate mineral dissolution during diagenesis carboxylic acid anions are also extremely important to the aqueous geochemistry of ca fe mn pb and zn some formation waters from sedimentary basins contain high concentrations of caas the analyses of 20 formation waters from the san joaquin basin california and 20 formation waters from the louisiana gulf coast basin presented in this study show concentrations as high as 8100 ppm monofunctional and 370 ppm difunctional cca by comparison previously reported analyses indicate monofunctional caa occir in concentrations up to 10 000 ppm and difunctional caa may occur in concentrations up to 2610 ppm analyses of drilling muds and scale soaps presented in this study show that few if any difunctional caa in the study area can be attributed to contamination from these sources additionally aqueous extracts of crude oils contain both mono and difunctional acid anions as do the aqueous and petroleum phases of hydrous pyrolysates previously unreported dissolution experiments equilibrium computer simulations and hydrous pyrolysis experiments support those already published and suggest that caa are generated during thermal maturation of kerogen and expelled from the shale along oil wet microfractures upon entering the water wet sandstone pore the hydrophilic caa partition into the aqueous phase organic inorganic reactions may occur which from caa metal complexes because the complexes are hydrophobic they partition in the petroleum phase where present carboxylic acid anions are of great importance to clastic diagenesis over the temperature range in which they dominate fluid alkalinity certainly no other viable mechanism has been advanced which adequately explains the observed aluminosilicate mineral dissolution with subsequent mass transfer of a1 as well as the carbonate mineral diagenetic successions observed in sand shale systems world wide the utility of modeling these observations of organic inorganic diagenesis is limited only by the ability to model the concentration and distribution of caa through space and time

correlation of carbon dioxide abundance with temperature in clastic hydrocarbon reservoirs relationship to inorganic chemical equilibrium the mol co2 of gas reservoired in sandstones tends to increase with increasing temperature along both the us gulf coast and the norwegian continental shelf each of these provinces displays identical linear correlations between temperature and the log of the partial pressure of co2 i e pco2 in the reservoir fluid that is the product of the mol co2 the reservoir pressure this correlation is the same both in gulf coast reservoirs younger than 17 ma and in norwegian reservoirs of jurassic to triassic age 144 245 ma indicating that it is not dependent on reaction kinetics norwegian shelf data indicate that the same correlation also applies for gas condensate and oil reservoirs we suggest that the log pco2versus temperature correlation is the result of inorganic chemical equilibria between feldspar clay and carbonate minerals a trend of decreasing ph with increasing temperature is fixed by equilibria between aluminosilicate minerals feldspar kaolinite at lower temperatures followed by various reactions between kaolinite illite chlorite and feldspar as temperatures exceed 110 140 c responding to the ph set by the aluminosilicate system co2 activity in the formation water and thereby co2 partial pressure in any gas phase present is controlled by equilibrium with carbonate minerals this model predicts that co2 released by organic maturation will in general result in precipitation of carbonate minerals below 100 120 c and that there should be widespread carbonate dissolution at higher temperatures

the intercept is a poor estimate of a calibrated radiocarbon age intercept based methods of generating a point estimate of a calibrated radiocarbon date are very popular but exhibit undesirable behaviour they are highly sensitive to the mean of the radiocarbon date and to adjustments of the calibration curve other methods give more stable results the weighted average of the probability distribution function is recommended as the best central point estimate but more consideration should be given to using the full probability distribution rather than a point estimate in developing age depth models

organic acid metastability in sedimentary basins thermodynamic calculations indicate that co sub 2 in sedimentary basins may be in stable redox equilibrium with organic acids found in basinal brines but that ch sub 4 is not these calculations also show that the high concentrations of carboxylic acids are from equilibrium with decarboxylation reactions a compilation of recent analyses of carboxylic acids from aqueous solutions associated with petroleum reservoirs shows that there is no trend in acid concentrations with temperature however inspection of these data at approximately 100 c indicates that acetic and propanoic acids are in homogeneous metastable equilibrium oxygen fugacities indicated by this metastable assemblage are consistently lower than those set by iron oxide assemblages this is not surprising because both hematite and magnetite are unstable relative to siderite in the presence of detectable concentrations of acetic acid

predictive models for sandstone diagenesis abstract factors governing the evolutionary path that sandstone and shale sequences follow during burial diagenesis include provenance and depositionally controlled compositional and textural elements near surface redox reactions organic inorganic interactions within the zone of intermediate burial diagenesis deep diagenetic reactions which include abiotic thermal sulfate reduction carbonate mineral alteration and quartz cementation and the potential for meteoric influx and renewal of near surface processes due to uplift exposure and or base level fluctuation as a consequence the use of thermal exposure indicators to describe the extent of diagenesis such as r0 or time temperature history indicators such as tti must be used with caution though useful in predicting general trends in porosity loss due to simple compaction they have little utility in making forward predictions of anomalously high porosity e g preserved and or enhanced porosity or anomalously low porosity a more useful approach whereby the processes controlling porosity evolution are evaluated in terms of the depositional hydrologic burial and thermal histories is presented here

fluid rock interactions during continuous diagenesis of sandstone reservoirs and their effects on reservoir porosity the cretaceous sandstone reservoir in the kuche depression tarim basin western china was investigated with reference to its reservoir property evolution during diagenesis six general diagenetic stages were recognised through petrographic mineralogical and geochemical analyses fluid rock interaction experiments were conducted under these six continuous diagenetic conditions that is with a simulation sequence of compaction early diagenesis organic acid incursion i elevated temperature pressure organic acid incursion ii late diagenesis corresponding to these six experimental stages a total of six models were constructed finally an extended model of fluid rock interaction during diagenesis at a geological timescale from 30 ma to present was constructed after various parameters had been validated results demonstrate the diagenetic stages from both the experimental and numerical simulations generally matched findings obtained from the petrographic and geochemical analyses i with compaction becoming weakened cementation by various minerals was gradually increased ii quartz overgrowth occurred because the contemporaneous sedimentary water was alkaline iii most minerals for example calcite and feldspar minerals displayed dissolution owing to the first organic acid incursion resulting in the visual porosity to increase to 29 26 iv increases of temperature and pressure caused a minor fluctuation of the porosity change v the cement that formed during earlier stages largely dissolved with the second organic acid incursion vi during the last stage the reservoir fluid was diluted by sedimentary alkaline water and most minerals precipitated under an alkalic environment the present porosity simulated is about 11 4 comparable with the actually measured data this study demonstrates that the combination of petrographic observations laboratory experiments and numerical simulations can not only reconstruct the diagenetic process but also provide a quantitative evaluation and prediction of reservoir petrophysical properties this article is protected by copyright all rights reserved

oilfield waters and sandstone diagenesis abstract the evolution of the organic geochemistry and carbonate alkalinity of oilfield waters is apparently regular and predictable this evolution can be typified by five generalizations 1 at or near 80 c there appears to be an exponential rise in the concentration of organic acid anions 2 the maximum concentration of organic acid anions occurs over approximately the 80 100 c temperature interval 3 the highest concentrations of difunctional acid anions are associated with the other organic species maxima 4 difunctional acid anions are the first to be decarboxylated typically at temperatures of 100 110 c 5 with increasing temperature 110 130 c monofunctional acid anions begin to become decarboxylated resulting in a fluid alkalinity dominated by bicarbonate dissolution experiments using artificial and natural oilfield waters demonstrate that mono and difunctional carboxylic acid anions and hydroxybenzoic acid anions present in both oilfield waters and the aqueous phase of hydrous pyrolysates are capable of greatly enhancing al si fe and ca concentrations in solutions from dissolution of minerals by organometallic complexation this enhancement of mineral solubility has been called upon to explain aluminosilicate dissolution porosity which is quantitatively important in many subsurface reservoirs certainly no other viable mechanism has been proposed to explain aluminum transport in the subsurface when integrated into basin models the regular evolution of organic and carbonate alkalinity in oilfield waters and the changing mineral stabilities that accompany that evolution help explain commonly observed diagenetic sequences in clastic systems

marcasite in black shales a mineral proxy for oxygenated bottom waters and intermittent oxidation of carbonaceous muds abstract black shales in the geologic record have long been considered to reflect deposition under anoxic or even euxinic conditions although bottom water anoxia favors deposition of black shales certain biological indicators such as benthic fossils and bioturbation suggest that bottom water conditions may not have been as persistently anoxic as commonly presumed examination of twelve marine carbonaceous shale bearing units ranging in age from middle proterozoic to cretaceous suggests that in addition to paleontological clues early diagenetic marcasite formed in uncompacted surficial sediments is a reliable indicator of oxygen bearing bottom waters marcasite requires low ph values to form and oxidation of already existing sedimentary pyrite provides a plausible chemical scenario to achieve that condition in the studied examples degraded pyrite framboids and corrosion features on pyrite grains indicate partial pyrite destruction in surface sediments and suggest that intermittent reoxidation of earlier formed iron sulfides may indeed have generated the low ph values required for marcasite formation thus observations of textures spatially associated with secondary marcasite growth are consistent with a scenario whereby oxygen in overlying sea water drove downward migration of oxidation fronts pervasive marcasite in black shales from a wide range of occurrences suggests multiple reoxygenation events during the deposition of these shale units and calls for more dynamic ocean circulation than previously assumed the latter consideration may for example prove important for those instances where a black shale is associated with an oceanic anoxic event oae because these are commonly presumed to have been deposited under fully anoxic conditions

late holocene glacial activity in manatee valley southern coast mountains british columbia canada the dendroglaciologic and lichenometric research methodologies employed in this study provide a perspective of glaciological conditions from 5 ka to present in a remote headwater area of the british columbia coast mountains since holocene ice fronts of four glaciers at this site periodically extended below treeline previous glacier advances overrode and buried forests beneath till deposits this study suggests that glaciers were expanding into standing forests at 4 76 and 3 78 ka following glacier expansion at 3 78 ka a period of recession ensued when glaciers withdrew upvalley long enough for the development of deep pedogenic surfaces and the growth of trees exceeding 300 years investigations at beluga and manatee glaciers benchmark a subsequent episode of significant glacial expansion at 2 42 ka referred to as the manatee advance this advance has regional correlatives and is distinguished from the tiedemann advance at manatee glacier by documentation of substantive ice front retreat between the