the effects of thermochemical sulfate reduction upon formation water salinity and oxygen isotopes in carbonate gas reservoirs abstract thermochemical sulfate reduction tsr is a well known process that can lead to sour h2s rich petroleum accumulations most studies of tsr have concentrated upon gas chemistry in this study we have investigated palaeoformation water characteristics in a deep anhydrite bearing dolomite sour gas reservoir of permian age in abu dhabi using fluid inclusion stable isotope petrographic and gas chemical data the data show that low salinity isotopically distinct water was generated within the reservoir by reaction between anhydrite and methane the amount of water added to the reservoir from tsr indicated by reduced fluid inclusion salinity and water 18o values varied systematically with the extent of anhydrite reaction with methane water salinity and isotope data show that the original formation water was diluted by between four and five times by water from tsr thus we have shown that large volumes of very low salinity water were generated within the gas reservoirs during diagenesis following gas emplacement the salinity of formation water in evaporite lithologies is therefore not necessarily high modelling based upon a typical khuff gas reservoir rock volume suggests that initial formation water volumes can only be increased by about three times as a result of tsr the extreme local dilution shown by the water salinity and 18o data must therefore reflect transiently imperfect mixing between tsr water and original formation water the creation of large volumes of water has important implications for the mechanism and rate of thermochemical sulphate reduction and the interpretation of gas volumes using petrophysical logging tools

hydrogen sulfide formation fate and behavior in anhydrite sealed carbonate gas reservoirs a three dimensional reactive mass transport modeling approach a novel hydrogeochemical modeling approach is developed to unravel thermochemical sulfate reduction tsr in hydrocarbon reservoirs our numerical model couples a web of interconnected hydrogeochemical reactions to three dimensional 3 d and reservoir wide diffusive mass transport our modeling approach simulates a semigeneric gas reservoir sealed by anhydrite the calculated diagenetic processes fit the observations in reservoirs affected by tsr formation of water precipitation of calcite metal di sulfides and elemental sulfur as replacements of dissolved anhydrite at the expense of ch4 g as well as formation of hydrogen sulfide h2s by varying input parameters the crucial factors controlling tsr have been identified our results highlight that reservoir wide diffusive mass transport is one prerequisite for tsr an increase in the rate constant of abiotic sulfate reduction asr and in diffusive mass fluxes as well as lack of precursor minerals for metal di sulfide precipitation can increase the souring intensity and accelerate h2s outgassing in contrast precipitation of elemental sulfur which is stable according to the chemical thermodynamics weakens h2s formation our modeling shows that tsr is complex and cannot be represented by the single reaction asr and by simple correlations between the rate constant of asr and the h2s gas content the application of 3 d reactive transport modeling presented here despite its semigeneric nature provides a good example of how such an approach can be used ahead of drilling our modeling helps to investigate tsr in time and space to quantify the mass conversion of all reactants involved within this web and to predict the souring level

dolomitization of gas reservoirs the upper permian changxing and lower triassic feixianguan formations northeast sichuan basin china abstract the lower triassic feixianguan formation and to a lesser extent the upper permian changxing formation contain more than ten giant recently discovered dolomite gas fields on either side of a paleo embayment kaijiang liangping bay sichuan basin china this study documents the origin of the dolomite using a combination of petrology fluid inclusion trace element geochemistry and stable isotope and radiogenic isotope analysis of samples from outcrop and core samples from producing gas fields three stages of dolomitization are revealed dolomicrite from the lagoon facies in the feixianguan formation and minor fabric retentive dolomite from reef facies in the changxing formation on the northeast side of kaijiang liangping bay represent the initial dolomite phase caused by reflux of feixianguan brine with salinity close to gypsum saturation 18 wt salinity during the first stage of diagenesis at 30 c the second and dominant dolomite phase fabric retentive dolomite in both the feixianguan and changxing formations on the both sides of kaijiang liangping bay is the result of reflux of diagenetic fluids with salinity ranging from normal feixianguan seawater 3 5 salinity up to mesohaline water 6 5 salinity during early burial diagenesis at about 35 40 c as shown by the dominance of single phase aqueous inclusions shallow formed stylolites that cut the second phase of dolomite and the early triassic marine carbon oxygen and strontium isotope and rare earth element and yttrium ree y signals the subordinate third dolomite phase fabric obliterative dolomite and dolomite cement was due to burial diagenetic recrystallization between 80 and 140 c in the feixianguan and changxing formations on both sides of kaijiang liangping bay the third phase of dolomitization involved water expelled from the slightly younger lower triassic jialingjiang formation marine evaporites and carbonates on the northeast side of kaijiang liangping bay revealed by the high salinity of the aqueous inclusions that homogenize at 80 to 140 c and elevated strontium isotope data the carbon isotope and ree y signals of the third phase of dolomite are similar to earlier dolomite cements confirming the lack of exotic input future exploration should be focused on dolomitized high energy sediments e g reef and oolitic shoal and bank in similar triassic tethyan paleogeographic settings that are associated with restricted facies but underlying dolomitized high energy permian strata may also be good exploration targets

petroleum geology of the puguang sour gas field in the sichuan basin sw china abstract the puguang sour gas field in northeastern sichuan basin is the largest gas discovery in the paleozoic marine strata in china with at least 9 trillion cubic feet tcf of proven gas reserves it is one of the many anticlinal structures formed during the indo sinian orogeny with gas being trapped in a composite structure lithology pool the presence of the upper permian coal measures lower silurian and lower cambrian marine shales provided excellent hydrocarbon source rocks the lower triassic and upper permian oolitic dolomites serve as high quality reservoir rocks with 10 25 porosity and 1 1000 md permeability largely due to diagenetically produced secondary porosity the lower and middle triassic evaporate beds have acted as the direct top seal for the large lower triassic upper permian gas accumulation the thick upper triassic and jurassic mudstone beds regionally cap the underlying marine carbonate strata the deep seated basement faults acted as vertical hydrocarbon migration conduits during faulting and became effective flow barriers when the faulting ceased the deep burial of a paleo oil accumulation in reservoirs rich in sulfate resulted in extensive thermochemical sulfate reduction tsr this together with the new gas injection and structural reconfiguration transformed a paleo oil pool to a large sour gas pool

impacts of hydrothermal dolomitization and thermochemical sulfate reduction on secondary porosity creation in deeply buried carbonates a case study from the lower saxony basin northwest germany the role of deep burial dissolution in the creation of porosity in carbonates has been discussed controversially in the recent past we present a case study from the upper permian zechstein 2 carbonate reservoirs of the lower saxony basin in northwest germany these reservoirs are locally characterized by high amounts of carbon dioxide co2 and variable amounts of hydrogen sulfide h2s which are derived from thermochemical sulfate reduction tsr and inorganic sources to study the contribution of these effects on porosity development we combine petrography stable isotope and rare earth and yttrium rey analyses of fracture cements with raman spectroscopy and 13c analyses of fluid inclusions it is shown that fluid migration along deep fault zones created and redistributed porosity fluid inclusion analyses of vein cements demonstrate that hydrothermal fluids transported inorganic co2 into the reservoir where it mixed with minor amounts of tsr derived organic co2 the likely source of inorganic co2 is the thermal decomposition of deeply buried devonian carbonates the rey distribution patterns support a hydrothermal origin of ascending iron and co2 rich fluids causing dolomitization of calcite and increasing porosity by 10 16 along fractures this porosity increase results from hydrothermal dolomitization and dissolution by acids generated from the reaction of fe2 with h2s to precipitate pyrite in contrast hydrothermal dolomite cements reduced early diagenetic porosity in dolomitic intervals by approximately 17 however the carbonate dissolution in the predominantly calcitic host rock results in a net increase in porosity and permeability in the vicinity of the fracture walls which has to be considered for modeling reservoir properties and fluid migration pathways

dolomitization history and porosity evolution of a giant deeply buried ediacaran gas field sichuan basin china abstract the deeply buried 7 km upper ediacaran sinian dengying formation ca 551 1 541 ma in the sichuan basin china is the largest precambrian dolostone gas reservoir worldwide gas exploration from the dengying formation however is hampered by a limited understanding of its complex dolomitization history and porosity evolution new petrological geochemical and petrophysical analyses were performed and are discussed here to develop a better understanding of the formation s complexities microbialite dolo grainstone and crystalline dolostone lithologies from the platform margin have high primary porosities relative to dolo mudstone and less common microbialite lithologies from the low energy platform interior spatially variable primary porosity was subsequently overprinted by meteoric dissolution and dolomitization often inducing secondary porosity the various forms of dolomitization sabkha reflux and burial dolomitization and cementation increased the resistivity of these rocks to chemical and physical compaction and porosity destruction in the deep burial domain medium to coarse crystalline dolomite cements and saddle dolomite precipitated at fluid temperatures of 120 160 c and 160 220 c respectively the significance of deep burial dissolution is critically discussed and constrained by i corrosion of late diagenetic minerals ii pores cross cutting or forming around stylolites and iii the occurrence of solid bitumen in the center of secondary pores vuggy inter and intracrystalline pores developed during a late and deep burial stage and comprise ca 20 of the overall dengying reservoir porosity mechanisms which induced corrosion at this burial stage include hydrothermal pulses and thermochemical sulfate reduction similar to many carbonate reservoirs in the phanerozoic this study documents that the platform margin has better porosities and higher gas production compared to the platform interior due to its specific carbonate rock properties and diagenesis data shown here document the complex multiphase dolomitization history and high potential for gas production from deeply buried precambrian dolostone reservoirs

multiphase dolomitization of deeply buried cambrian petroleum reservoirs tarim basin north west china cambrian dolostone reservoirs in the tarim basin china have significant potential for future discoveries of petroleum although exploration and production planning is hampered by limited understanding of the occurrence and distribution of dolomite in such ancient rocks buried to nearly 8 km the study herein accessed new drill core samples which provide an opportunity to understand the dolomitization process in deep basins and its impact on cambrian carbonate reservoirs this study documents the origin of the dolostone reservoirs using a combination of petrology fluid inclusion microthermometry and stable and radiogenic isotopes of outcrop and core samples an initial microbial dolomitization event occurred in restricted lagoon environments and is characterized by depleted 13c values dolomicrite from lagoonal and sabkha facies some fabric retentive dolomite and fabric obliterative dolomite in the peloidal shoal and reef facies show the highest 18o values these dolomites represent relatively early reflux dolomitization the local occurrence of k feldspar in dolomicrite indicates that some radiogenic strontium was contributed via terrigenous input most fabric retentive dolomite may have precipitated from seawater at slightly elevated temperatures suggested by petrological and isotopic data most fabric obliterative dolomite and medium to coarse dolomite cement formed between 90 c and 130 c from marine evaporitic brine saddle dolomite formed by hydrothermal dolomitization at temperatures up to 170 c and involved the mixing of connate brines with sr enriched hydrothermal fluids intercrystalline moldic and breccia porosities are due to the early stages of dolomitization macroscopic intergranular vuggy fracture and dissolution porosity are due to burial related dissolution and regional hydrothermal events this work has shown that old for example cambrian or even precambrian sucrosic dolomite with associated anhydrite buried to as much as 8000 m can still have a high potential for hosting substantial hydrocarbon resources and should be globally targeted for future exploration

diagenetic evolution of selected parasequences across a carbonate platform late paleozoic tengiz reservoir kazakhstan abstract predicting the diagenetic modification of reservoir quality in carbonate fields is poorly understood but of paramount importance to reservoir characterization the diagenetic processes and current pore system are reported for two parasequences from the platform center to the margin of the tengiz reservoir 16 km the late visean lvis13 cbs 900 and bashkirian bash2 5 msf 350 parasequences are separated by 8 million years syndepositional fibrous cement 10 rock volume and aragonite skeletal molds developed in both parasequences on subaerial exposure brown soil related calcite and meniscus and pendant cements were emplaced in both parasequences the application of cl petrography to these rocks for the first time has revealed the depositional components and early marine components are altered developing a mottled pattern implying that their original chemistry is not preserved the principal calcite cement shows a complex growth pattern by cl that is unreported from any other calcite cement cl is also used to divide the main cement into stage 1 nonluminescent and stage 2 orange to brown luminescent calcite stage 1 calcite shows vadose meniscus and pendant fabrics followed by uniform phreatic euhedral terminations stage 1 cement is patchily distributed at a millimeter scale 28 to 40 in the visean and 0 5 to 22 in bashkirian grainstones stage 1 cement is more abundant at the center of the platform in the late visean than at its margin abundant stage 1 calcite creates a framework that prevents later compaction preserving a high porosity at the center of the platform this sweet spot is reinforced by the deposition of late stage bitumen cement that occupies 8 rock volume at the margin of the platform but is almost absent at its center stage 1 13c values 2 4 to 1 7 are derived by dissolution of host rock and their 18o values 6 3 to 3 7 are compatible with precipitation from meteoric or mixed marine meteoric water depositional micrite and micritic grains are transformed by dissolution precipitation processes into porous microrhombic calcite not by ostwald ripening microrhombic calcite is ubiquitous in the tengiz icehouse dispelling the myth that this fabric is restricted to greenhouse epochs at 300 million years before significant burial the study grainstones were patchily cemented and dissolved to reach peak porosity their transformed texture set up flow paths that controlled the distribution of later diagenetic products reservoir compartmentalization did not occur through extensive hardground formation at this time as occurs in some greenhouse sequences but ash bands at parasequence boundaries were turned into tonsteins and wackestones were compacted perhaps sufficiently to influence vertical flow cl examination of stage 2 calcite that adds 0 5 to 16 to rock volume reveals that its precipitation was episodic and switched sites as the pore network became blocked through precipitation and compaction and was opened by new fractures a change from acute to equant habit occurred during stage 2 precipitation causing the change from fibrous to equant crystal fabric the 18opdb values of stage 2 calcite 3 to 9 are linked to precipitation from an 18o enriched fluid that evolved to 6 smow at temperatures of 60 c to 120 c using the equation of kim and o neil 1997 many open macropores are lined with small cement crystals that cl shows have all stages of growth present and are terminated by euhedral surfaces these macropores are only partly filled because of cement inhibition not because they were dissolved during burial as was previously claimed late stage dissolution did create some macropores and rounded micrite crystals but its effect on overall platform porosity is small the cl examination of tengiz platform calcite cement has recognized many diagenetic events that cannot be recognized without cl the new tengiz diagenetic textures recognized here are probably present in other carbonate platforms especially those from icehouse epochs

tsr origin of sulfur in permian and triassic reservoir bitumen east sichuan basin china solid bitumen h2s pyrite and anhydrite samples from the lower triassic feixianguan formation and upper permian changxing formation and source rock kerogen samples from the cambrian ordovician silurian and permian in the east sichuan basin were analyzed for their sulfur content and d 34 s values to determine if the measurements were consistent with a thermochemical sulfate reduction tsr origin of the produced h2s and solid bitumen the results show that the solid bitumen samples have sulfur contents from 2 6 to 12 5 wt and d 34 s values from 13 4 to 22 1 the d 34 s values are significantly heavier than those of potential upper permian source rock kerogens that average 1 3 n 3 suggesting that most of the sulfur in the solid bitumen is derived from isotopically heavy h2s of tsr origin from 12 0 to 13 7 interestingly two gases produced from gas to water transition zones during drill stem testing dst show significantly lighter d

self organized criticality in layered lacustrine sediments formed by landsliding landsliding is the dominant mass wasting process in humid temperate uplands and an important regulator of sediment yield from steep land drainage basins information about the magnitude and frequency distribution of landslides has been derived from aerial photography but it has proved difficult to set limits on the long term scaling behavior of landsliding because the requirements of spatial and temporal coherence and the large number of observations necessary to undertake magnitude versus frequency analyses are not easy to fulfill we use a 2250 yr long record of hillslope erosion associated with extreme hydrologic events preserved in sediments from lake tutira new zealand to investigate scaling in landslide deposits both the magnitude versus frequency distribution of sediment layers attributed to landsliding and the distribution of time intervals between landsliding events take the form of power laws the former with an exponent b 2 06 and the latter with an exponent b 1 4 these results suggest that the erosional events originate from a self organized critical process and are in agreement with observations of scaling in turbidite deposits and grain flows in controlled laboratory experiments the implications are that the aggregate behavior of landsliding at the catchment scale is orderly and that the stratigraphic record preserves a unique long term perspective on a fundamental geomorphic process and the extreme hydrologic events that trigger it

partial dolomitization of a pennsylvanian limestone buildup by hydrothermal fluids and its effect on reservoir quality and performance dolomite was studied in the reinecke field in west texas to determine its origin reservoir characteristics and effect on oil production the reinecke reservoir includes approximately 25 dolomite in an upper pennsylvanian to lowest permian limestone buildup dolomites are fine to coarsely crystalline with undulose extinction petrography indicates that dolomite formed diagenetically late dolomites overgrow stylolites indicating precipitation during deep burial low 18o values are consistent with primary fluid inclusions that indicate dolomite precipitation or recrystallization at 92 to 118 c these temperatures are much higher than those in the existing reservoir or what should have been encountered during regional burial two types of dolomite are observed one type is stratiform replaced micrite rich facies and is interpreted as early dolomite that recrystallized in hydrothermal brines the second type crosscuts stratigraphy and depositional facies and has vertically continuous morphologies that trend northwest southeast the latter dolomite is interpreted as forming in hydrothermal brines derived from latest permian aged highly evaporated seawater these brines descended deep into the basin because of their greater density and then ascended because of heating and thermal convection reinecke dolomites have lower porosity but higher permeability than the surrounding limestones as a result dolomites provide raceways for fluids moving through the reservoir these dolomites helped an artificially enhanced bottom water drive to recover approximately 50 of the original oil in place the dolomite has complicated a crestal co2 flood these hydrothermal reinecke dolomites have lower porosity but much higher permeability than most nearby permian dolomites formed in evaporated seawater during shallow burial arthur h saller is a stratigrapher with chevron energy technology company in houston texas he has worked in the petroleum industry for 27 yr helping with exploration and production projects in west texas canada angola and southeast asia he received his b s degree from the university of kansas m s degree from stanford university and ph d from louisiana state university 1984 john a tony d dickson received his ph d from queen mary college london university in 1967 he has had academic positions in cardiff nottingham and cambridge universities he has worked on smackover andrews horseshoe atoll shu iaba tengiz and other oil reservoirs he retired from academia in 2007 and is now consulting and conducting courses in carbonate diagenesis

cyclostratigraphy and the problem of astrochronologic testing abstract when milankovitch cycles are preserved in the geologic record they provide a direct link between chronometer and climate change and thus a remarkable opportunity to constrain the evolution of the surficial earth system the identification of such cycles has allowed exploration of the geologic record with unprecedented temporal resolution and has spurred the development of a rich theoretical framework for climatic change accompanying these successes however has been a persistent skepticism how does one reliably test for astronomical forcing pacing in stratigraphic and paleoclimate data especially when time is poorly constrained from this perspective it would seem that the merits and promise of astrochronology a phanerozoic time scale measured in 20 000 to 400 000 year increments also serves as its achilles heel if the confirmation of such geologically short temporal rhythms defies rigorous hypothesis testing the implications are substantial since much of our understanding of paleoclimate change throughout the cenozoic and beyond is firmly rooted in astrochronologic interpretation in this study a conceptual framework for assessing earth system response to astronomical insolation changes and the propagation of that signal into the geologic record is used as a guide to understand the nature of the problem of astrochronologic testing this framework emphasizes three challenges contamination stratigraphic distortion and temporal calibration a statistical optimization method timeopt meyers 2015 is formulated as a solution to these three challenges providing an approach for astrochronologic testing that objectively evaluates time scale uncertainty while simultaneously identifying an optimal model for climate and depositional system response to astronomical forcing new extensions to the technique are presented allowing explicit reconstruction of distortions to the primary forcing that are known to be omnipresent in the stratigraphic record to illustrate the utility of this approach it is applied to five well studied stratigraphic series throughout the phanerozoic supporting their astronomical origin and yielding constraints on the evolution of the earth system and the astronomical solutions themselves future directions that build on this foundation are discussed including the utility of process based null models approaches for earth system transfer function reconstruction and mapping out ancient solar system behavior and earth moon history using the geologic archive of milankovitch cycles the timeopt approach recognizes astronomy geochronology paleoclimatology and depositional system reconstruction as a unified geoscientific inverse problem

paleocave carbonate reservoirs origins burial depth modifications spatial complexity and reservoir implications paleocave systems form an important class of carbonate reservoirs that are products of near surface karst processes and later burial compaction and diagenesis features and origins of fractures breccias and sediment fills associated with paleocave reservoirs have been studied in modern and ancient cave systems information about such cave systems is used in this paper to reconstruct the general evolution of paleocave reservoirs and their associated scale pore networks and spatial complexities spatial complexities in paleocave reservoirs result from near surface and burial processes near surface processes include dissolutional excavation clastic sedimentation chemical precipitation and localized fracturing brecciation and collapse of cave walls and ceilings burial processes begin as cave systems subside into the subsurface remaining cave passages commonly collapse and early formed breccia clasts are rebrecciated differential compaction of strata around and over collapsed passages produces fractures crackle breccias and mosaic breccias near surface and burial processes combine to produce typically complex reservoirs with several scales of heterogeneity hydrocarbon reservoirs of paleocave origin are commonly the product of coalesced collapsed paleocave systems the coalescing of passages in a cave system into larger connected porosity zones results from a combination of multiple cave forming episodes at composite unconformities and from the collapse of cave systems during burial where surrounding host strata are brecciated and fractured this combination of processes creates spatially complex reservoirs that can be hundreds to several thousands of meters across commonly forming large exploration targets final size pore network types and spatial complexities of coalesced collapsed paleocave systems are products of their evolution from near surface development through burial into the deeper subsurface the coalesced collapsed paleocave reservoir hypothesis explains the scale of reservoirs observed and the spatial complexities involved

overpressure compartments in the central paleo uplift sichuan basin southwest china the sichuan basin is a prime gas producing basin in china besides the giant neoproterozoic weiyuan gas field discovered in the central paleo uplift an extra giant gas reservoir in the cambrian in this area has been confirmed in recent years although the lower paleozoic in the southwest weiyuan area is normally pressured it is overpressured in the central area moxi gaoshiti area combined with the seal distribution five pressure systems including three overpressure systems can be divided based on drill stem test mud weight and sonic transit time data overpressures appear at depth interval of 1500 to 4900 m 4921 to 16 076 ft approximately benefiting from the good sealing capacity of the gypsum in the upper and middle triassic high overpressure pressure coefficient r 2 0 has been preserved in the lower triassic and the upper triassic and cambrian are moderately overpressured 1 3 r 1 7 mechanisms for various overpressure systems are different abnormally high sonic transit time in the permian indicates disequilibrium compaction overpressure the analyses of sonic transit time effective stress suggest that disequilibrium compaction is the primary mechanism for the overpressure in the upper triassic xujiahe formation but the cambrian overpressure system is predominantly associated with fluid expansion which mainly resulted from gas generation furthermore we consider that the late thermal cracking of oil to gas is a key factor for gas and overpressure preservation in old strata the high overpressure in the lower triassic marine carbonate rocks was caused by oil cracking and gypsum dehydration mechanisms combining the origin analysis with the burial and hydrocarbon generation histories we constructed the cambrian pore pressure evolution model which is characterized by roughly normal pressure before 200 ma overpressuring from 200 to 90 ma and overpressure releasing since 90 ma

cyclicity and paleoenvironmental dynamics rocknest platform northwest canada the superbly exposed rocknest formation northwest territories canada contains 140 to 160 shallowing upward cycles 1 to 15 m thick that contain distinctive marker beds and can be correlated for 200 km parallel to strike and 120 km perpendicular to strike cycles are grouped according to cycle base lithology which reflects paleogeographic position on the platform shale based cycles of the distal inner shelf pass westward into dolosiltite based cycles then into tufa based cycles of the shoal complex and then into grainstone based cycles of the outer shelf cycles are bounded by erosional surfaces that become less pronounced away from the shoal complex cycles formed in response to small asymmetric rapid rise slow fall eustatic oscillations in sea level 10 m during sea level rise rapid transgression and submergence of tidal flats occurred along the eastern margin of the shoal complex followed by eastward expansion of the shoal complex and slow progradation of tidal flats over lagoonal facies during sea level fall a glacio eustatic model for cyclicity accounts for the data favoring asymmetric eustatic sea level oscillations subaerial exposure and vadose diagenesis of platform during sea level falls arrested progradation and incomplete shoaling of many cycles and cycle periods of 20 000 to 100 000 yr 18 000 to 30 000 yr for rocknest inner shelf slopes ranged from 8 cm km average at high sea level stand to 2 cm km average at low sea level stand tidal flat progradation rates were 7 to 9 km 1 000 yr interaction of inner shelf slope sea level fall rate and sedimentation rate generates complete condensed or expanded cycle profiles one dimensional computer modeling reveals that sedimentation rates of shoal complex facies were 250 cm 1 000 yr that lag times in sedimentation following transgression may have ranged from zero on the shoal complex to 2 000 yr on the distal inner shelf and that subtidal sedimentation rates may have ranged from 80 cm 1 000 yr for the proximal inner shelf to 20 cm 1 000 yr for the distal inner shelf

concepts and methods of high resolution event stratigraphy modern geological analyses involving sedimentary rocks require a high resolution system of dating integrating and correlating diverse strati graphic geochemical and paleontological data that also takes into account the possibility that short term phenomena 100 kyr or less exercise a strong control on sedimentation such phenomena may be extraterrestrial tectonic volcanic oceanographic climatic sedimentologic and or bio logic in origin some reflect unpredictable perturbations of local to global scale but most short term phenomena are predictable and may be either autocyclic locally regulated with limited stratigraphic continuity or allo cyclic in nature regionally to globally regulated with extensive strati graphic continuity beerbower 1964 a stratigraphic system based on short term phenomena would ideally be chronostratigraphic involving the identification and regional tracing of time lines isochronous surfaces or very thin event deposits and easily integrated with refined independently derived biostratigraphic and geochronologic systems an intensive search for such a new and revolutionary system of stratig raphy has been prompted in many parts of the phanerozoic by attain ment of plateaus in stratigraphic resolution utilizing the more standard tools of magnetostratigraphy biostratigraphy and geochronology most magnetic reversals have been identified biostratigraphic systems can be no more refined than the maximum evolutionary rates of component taxa and their major structures known for many favored groups and geo chronology is regulated by the analytical confidence limits of radiometric

do co2 charged fluids contribute to secondary porosity creation in deeply buried carbonates abstract carbonate dissolution by acidic co2 charged fluids is one possible mechanism leading to the creation of burial porosity the late permian carbonates of the lower saxony basin nw germany are a suitable target to study the influence of such co2 charged fluids on porosity evolution the so called zechstein 2 carbonates ca2 consist of alternating dolomitic and calcitic intervals and contain locally very high amounts of co2 80 and variable amounts of h2s in reservoir gas and vein cement hosted fluid inclusions composition and isotope data from fluid inclusions suggest that the co2 is sourced from the thermal decomposition of underlying devonian carbonates far reaching fault systems connect the devonian co2 source with the permian ca2 reservoir while thick rock salt deposits above the zechstein 2 carbonates prohibit an influence of surface derived meteoric fluids the aim of the study is to evaluate the contribution of co2 on carbonate dissolution seven wells were preferentially sampled along mineral veins which are interpreted as the preferred migration pathways for the present gas classic petrography methods such as transmitted light microscopy and cathodoluminescence were combined with fluid inclusion analysis and porosity data petrographic analysis reveals no great impact of the massive co2 presence on porosity creation as fluids get quickly saturated with respect to ca2 minor vuggy porosity is associated with fractures in only one of the studied wells but the dominant porosity types are early diagenetic fabric selective oomoldic and intercrystal porosity to evaluate whether the porosity values are higher than usual a comparison with a global porosity dataset is included which however shows no increase in average porosities compared to global values the results of this case study suggest that the contribution of co2 alone on carbonate dissolution and therefore reservoir quality is negligible as long as other processes such as late dolomitization or pyrite formation by thermochemical sulphate reduction derived h2s are absent

milankovitch band forcing of sub milankovitch climate variability during the pleistocene climate variability at periods from 10 kyr to 12 kyr that originates from milankovitch band forcing is quantified at three locations for the late pleistocene eastern equatorial pacific ocean drilling program site 846 eastern equatorial atlantic odp site 663 and northeastern atlantic deep sea drilling project site 609 variability at these periods is not present in the primary milankovitch forcing so no linear linkage to milankovitch band variations is possible however these periods are equal to harmonics of precession band oscillations the magnitude of interactions between processes that occur at different timescales can only be resolved in time series data through application of higher order statistics through such an application we demonstrate that up to 75 of the variance in the 10 to 12 kyr band in the sediment records is nonlinearly transferred from precession band 19 23 kyr variations within the millennial to sub milankovitch band defined as the band of variance ranging from 15 to 2 kyr approximately 1 3 of the variability in the records studied is consistent with a low frequency milankovitch band origin this variability may derive from high sensitivity of the tropics to summertime insolation in both hemispheres relative to wintertime insolation a mechanism having equatorial origin and related to low latitude precession variations appears consistent with the observations because the phase coupling between 10 to 12 kyr oscillations and precession is resolved this result has implications for development of models which seek to explain global climate variations on this timescale

evidence of astronomical forcing of the earth s climate in cretaceous and cambrian times abstract we have completed analyses of three sections of shallow marine sedimentary cycles that were deposited at about 80 ma and between about 510 ma and 530 ma in different parts of north america in spite of the antiquity of these strata and the differences in the age location and cycle types power spectra in all of the records have distinct spectral peaks with ratios that are consistent with forcing by eccentricity obliquity and climatic precession for two examples utah 530 ma and the southern canadian rockies 510 ma the spectra reveal prominent peaks corresponding to obliquity with a period closer to that estimated for early paleozoic time than for the modern these are the first results supporting recent calculations of changes in astronomical periods over geological time we were able to extract these intriguing results from rocks as old and as poorly dated as these only after applying a new method the gamma method which we have developed for scaling time in cyclic strata the results are evidence that astronomical forcing has left a robust signal in the geological record and that the astronomical periods observed in the geological data are consistent with recent calculations of the shifts in those periods over phanerozoic times

a 48 m y history of fracture opening temperature and fluid pressure cretaceous travis peak formation east texas basin quartz cement bridges across opening mode fractures of the cretaceous travis peak formation provide a textural and fluid inclusion record of incremental fracture opening during the burial evolution of this low porosity sandstone incremental crack seal fracture opening is inferred based on the banded structure of quartz cement bridges consisting of up to 700 cement bands averaging 5 m in thickness as observed with scanning electron microscope cathodoluminescence crack seal layers contain assemblages of aqueous two phase fluid inclusions based on fluid inclusion microthermometry and raman microprobe analyses we determined that these inclusions contain methane saturated brine trapped over temperatures ranging from 130 c to 154 c using textural crosscutting relations of quartz growth increments to infer the sequence of cement growth we reconstructed the fluid temperature and pore fluid pressure evolution during fracture opening in combination with published burial evolution models this reconstruction indicates that fracture opening started at ca 48 ma and above hydrostatic pore fluid pressure conditions and continued under steadily declining pore fluid pressure during partial exhumation until present times individual fractures opened over an 48 m y time span at rates of 16 23 m m y these rates suggest that fractures can remain hydraulically active over geologically long times in deep basinal settings

limited thermochemical sulfate reduction in hot anhydritic sour gas carbonate reservoirs the upper jurassic arab formation united arab emirates abstract limited thermochemical sulfate reduction tsr in hot 130 160 c and anhydrite rich sour gas reservoir carbonates of the arab formation upper jurassic is manifested by rare calcitization of anhydrite with slightly lower 13cvpdb values 3 2 to 0 1 than calcite precipitated in equilibrium with late jurassic seawater fluid inclusion microthermometry of calcite that has replaced anhydrite indicates that tsr occurred between 130 c and 160 c the lack of evidence for extensive tsr despite the suitable current temperatures and abundant sulfates in the gas reservoir coupled with the relatively more common tsr related calcite in the flanks water zone than crest gas zone indicate that 1 gas emplacement while the reservoir was buried at shallower depth slowed down or inhibited tsr in the crest even when it subsequently reached depths where extensive tsr would occur and 2 h2s up to 38 vol has migrated from the underlying permo triassic and or jurassic sulfate carbonate deposits this study demonstrates that constraining the timing of hydrocarbon emplacement within the context of burial thermal history is crucial for a better understanding of the origin and distribution of h2s in hot anhydrite rich sour gas reservoirs

a coupled non isothermal reactive transport model for long term geochemical evolution of a hlw repository in clay a numerical model of coupled saturated unsaturated water flow heat transfer and multi component reactive solute transport is presented to evaluate the long term geochemical evolution in bentonite concrete and clay formation for a potential geological radioactive waste repository changes in formation porosity caused by mineral dissolution precipitation reactions are taken into account simulations were carried out with a general purpose multicomponent reactive transport code core2d v4 numerical results show that ph in the bentonite porewater can vary from neutral to up to 13 over a time scale of 1 ma although dissolution of silica minerals and precipitation of secondary calcium silicate hydrate csh minerals in bentonite buffer the effect of the hyperalkaline plume mineral precipitation reduces the volume of pore space in bentonite close to the bentonite concrete interface due to the precipitation of csh minerals model results indicate that bentonite porosity decreases less than 25 the hyperalkaline plume from the concrete only extends to a distance of 0 7 m in the clay formation over the time range of 1 ma

porosity loss fluid flow and mass transfer in limestone reservoirs application to the upper jurassic smackover formation mississippi ooid grainstones of the upper jurassic smack over formation are buried to a depth of over 6 km and exposed to temperatures in excess of 200 c at black creek field mississippi combined effects of mechanical compaction intergranular pressure solution and cementation have reduced intergranular porosity of these ooid grainstones to 0 indicating that porosity reduction has gone to completion modal analysis of 24 samples lacking preburial cements indicates that from the original 40 porosity 13 porosity units range 4 to 21 were lost by mechanical compaction 15 porosity units range 8 to 23 were reduced by intergranular pressure solution and 12 porosity units range 1 to 26 were destroyed by cementation intergranular pressure solution caused an average of 28 range 15 to 51 vertical shortening in smackover ooid grainstones under ideal conditions the 28 vertical shortening will generate enough calcium carbonate to precipitate 10 calcite cement this is close to the measured volume of cements in smackover grainstones 12 suggesting that intergranular pressure solution provided most of the calcite cement present no external sources of calcium carbonate are required fine grained samples that experienced high degrees of intergranular pressure solution and contain only small amounts of cement occur at the top of the reservoir whereas coarse grained samples with abundant cement and low degrees of pressure solution occur in the middle and basal parts of the reservoir suggesting that the fine grained intervals acted as sources and the coarse grained intervals as sinks for calcium carbonate mass transfer of pressure solution generated calcium carbonate from the top of the unit to precipitation sites in the middle and basal parts of the reservoir could have occurred by a non rayleigh type convection cell due to calcite s reverse solubility with respect to temperature the cooling upward moving limb of the convection cell would become progressively more undersaturated and hence able to transport more dissolved calcium carbonate released by intergranular pressure solution in the upper portion of the reservoir pore fluids descending in the downward moving limb of the cell would become progressively more supersaturated and calcium carbonate would tend to precipitate as cement in the middle and basal parts of the reservoir as fluids become progressively hotter

modeling 100 000 year climate fluctuations in pre pleistocene time series a number of pre pleistocene climate records exhibit significant fluctuations at the 100 000 year 100 ky eccentricity period before the time of such fluctuations in global ice volume the origin of these fluctuations has been obscure results reported here from a modeling study suggest that such a response can occur over low latitude land areas involved in monsoon fluctuations the twice yearly passage of the sun across the equator and the seasonal timing of perihelion interact to increase both 100 ky and 400 ky power in the modeled temperature field the magnitude of the temperature response is sufficiently large to leave an imprint on the geologic record and simulated fluctuations resemble those found in records of triassic lake levels

a new method of testing periodicity in cyclic sediments application to the newark supergroup abstract we present a new quantitative method for analyzing cyclic sediments the gamma method gamma time thickness tests the assumption that the cycles are periodic and provides quantitative estimates for relative accumulation rates of the different sedimentary facies that compose the cycles as such it provides a means of estimating relative time at very high resolution application of the gamma method to hettangian sediments of the newark supergroup indicates that the primary cycles were deposited in response to precessional climatic forcing and that these cycles were modulated by an eccentricity cycle with period ratios very similar to those of today results of gamma analysis also suggest that relative accumulation rates of the laminated deep water lacustrine facies was about six times slower than that of the shallow water facies

geochemistry and origin of sour gas accumulations in the northeastern sichuan basin sw china significant natural gas reserves have recently been discovered in the lower triassic oolitic reservoirs from northeastern sichuan basin sw china in the wake of the december 2003 sour gas well blow out this study presents an overview on the petroleum geology and geochemistry of the sour gas accumulations in the study area two types of natural gas accumulations were identified in the lower triassic oolitic reservoirs both containing highly mature thermogenic gases with their hydrocarbon source rocks in upper permian strata natural gases from the area south of the ancient kaijiang liangping seaway are generally sweet gases formed as the result of thermal maturation whereas those discovered from north of the seaway are products of both thermal maturation and thermochemical sulfate reduction of early accumulated oils in the feixianguan formation reservoirs the proposed origins of the gases are supported by their chemical and stable carbon isotope compositions as well as the presence or absence of pyrobitumens in the reservoir the distribution of gas accumulations is controlled predominantly by the combination of lithologic and structural factors the regional variation in the concentrations of h2s in the gases appears related to the presence and thickness of anhydrite bearing evaporitic rocks interbedded or intercalated with the oolitic reservoirs

thermochemical sulphate reduction can improve carbonate petroleum reservoir quality abstract interest in the creation of secondary pore spaces in petroleum reservoirs has increased because of a need to understand deeper and more complex reservoirs the creation of new secondary porosity that enhances overall reservoir quality in deeply buried carbonate reservoirs is controversial and some recent studies have concluded it is not an important phenomenon here we present petrography geochemistry fluid inclusion data and fluid rock interaction reaction modeling results from triassic feixianguan formation sichuan basin china core samples and explore the relative importance of secondary porosity due to thermochemical sulphate reduction tsr during deep burial diagenesis we find that new secondary pores result from the dissolution of anhydrite and possibly from dissolution of the matrix dolomite assuming porosity before tsr was 16 and the percentage of anhydrite was 6 modelling shows that due to tsr 1 6 additional porosity was created that led to permeability increasing from 110 md range 72 168 md within a 95 confidence interval to 264 md range 162 432 md within a 95 confidence interval secondary porosity results from the density differences between reactant anhydrite and product calcite the addition of new water during tsr and the generation of acidity during the reaction of new h2s with the siderite component in pre existing dolomite in the reservoir fluid pressure was high during tsr and approached lithostatic pressure in some samples this transient overpressure may have led to the maintenance of porosity due to the inhibition of compactional processes an additional 1 6 porosity is significant for reserve calculations especially considering that it occurs in conjunction with elevated permeability that results in faster flow rates to the production wells

tsr versus non tsr processes and their impact on gas geochemistry and carbon stable isotopes in carboniferous permian and lower triassic marine carbonate gas reservoirs in the eastern sichuan basin china abstract the palaeozoic and lowermost mesozoic marine carbonate reservoirs of the sichuan basin in china contain variably sour and very dry gas the source of the gas in the carboniferous permian and lower triassic reservoirs is not known for certain and it has proved difficult to discriminate and differentiate the effects of thermal cracking and tsr related processes for these gases sixty three gas samples were collected and analysed for their composition and carbon stable isotope values the gases are all typically very dry alkane gases being 97 5 methane with low 2 s and co 2 carboniferous gas is negligibly sour while the lower triassic gas tends to be most sour the elevated h 2 s up to 62 is due to thermochemical sulphate reduction with the most sour triassic and permian reservoirs being deeper than 4800 m the non tsr affected carboniferous gas is a secondary gas that was derived from the cracking of sapropelic kerogen derived oil and primary gas and is highly mature carboniferous and non sour triassic and permian gas has unusual carbon isotopes with methane and propane being isotopically heavier than ethane a reversal of typical low to moderate maturity patterns the gas in the non sour triassic and permian reservoirs has the same geochemical and isotopic characteristics and therefore the same source as the carboniferous gas tsr in the deepest triassic reservoirs altered the gas composition reaching 100 dryness in the deepest most sour reservoirs showing that ethane and propane react faster than methane during tsr ethane evolves to heavier carbon isotope values than methane during tsr leading to removal of the reversed alkane gas isotope trend found in the carboniferous and non sour triassic and permian reservoirs however methane was directly involved in tsr as shown by the progressive increase in its carbon isotope ratio as gas souring proceeded co 2 increased in concentration as gas souring proceeded but typical co 2 carbon isotope ratios in sour gases remained about 4 v pdb showing that it was not solely derived from the oxidation of alkanes instead co 2 may partly result from reaction of sour gas with carbonate reservoir minerals such as fe rich dolomite or calcite resulting in pyrite growth as well as co 2 generation

a test of the γ method for evaluating the periodicity of cyclical sediments abstract the method 1 uses thicknesses measured on facies in a set of cyclical sedimentary deposits to determine the facies effective accumulation rates and time intervals of accumulation to accomplish this the assumption is made that every cycle was accumulated over exactly the same interval of time the method has not been tested on a stratigraphic section where the correct answers were known a priori in particular the reliability of the method has not been determined when the assumption does not strictly hold the test constructed here begins with a synthetic time stratigraphic section of cyclical deposits and specified effective depositional rates for each facies generates a thickness section from these parameters and applies the method to the thickness section the method should recover the original time section and accumulation rates but it does not even when the coefficient of variation of the accumulation intervals is only 15

formation and diagenesis of bedding cycles in uppermost cretaceous chalks of the dan field danish north sea metre scale lithologic cycles visible in core and on logs from maastrichtian chalks of the dan field were examined to determine their mechanisms of deposition and relation to hydrocarbon production the lower parts of cycles consist of porous cream coloured largely non stylolitic commonly laminated chalk with limited bioturbation mainly escape burrows cycles are capped by thinner intervals of white to grey hard stylolitic chalk with concentrations of bioclastic material intense burrowing and few preserved primary sedimentary structures the cycle caps contain nearly twice as much mg as compared to the more porous parts of cycles and also have slightly larger 18o values 4 1 for the caps 4 4 for porous zones there is a significant reduction of average cycle thickness as well as total thickness of the maastrichtian chalk section from sw to ne across the dan field the cycle thinning largely results from a reduced thickness of porous chalks from the lower parts of cycles and thus is reflected in lower average porosity and permeability on the ne side of the field these data indicate that episodic winnowing removed fine grained constituents from highstanding northeastern areas porous cycle bases were deposited at relatively high rates that precluded complete bioturbation preserved laminae coupled with escape burrows reflect episodic sediment influx in areas that flank the seafloor highs cycle tops apparently accumulated more slowly throughout the region but especially on seafloor highs perhaps because of reduced productivity of planktic organisms slower sedimentation allowed more complete bioturbation and destruction of sedimentary structures and also led to incipient high magnesium calcite seafloor cementation sufficient to yield firmer sediment and enhanced burrow preservation but not to form true hardgrounds thus the elevated magnesium contents and reduced porosity of the cycle caps reflect very early diagenetic processes that were only partially modified by burial diagenesis rates of chalk deposition as inferred from physical and geochemical evidence appear to be a significant control on reservoir characteristics in north sea chalks the highest average porosities and permeabilities are found in areas with the highest sediment accumulation rates where seafloor diagenesis is minimized topographic depressions at the time of sedimentation can thus be expected to have the best production characteristics and synsedimentary topographic highs should have the thinnest sections and the poorest petrophysical properties

carbon isotope fractionation during methane dominated tsr in east sichuan basin gasfields china a review thermochemical sulfate reduction is considered to result in h2s 10 and high dryness coefficient values in the lower triassic and upper permian carbonate reservoirs in the ne sichuan basin the gases pro duced from gasewater transition and water intervals were measured to have h2s higher than 30 and are calculated to have significantly higher h2s and co2 contents than a gas produced from a gas interval and thus are not used to reflect tsr extents methane and ethane were shown to shift positively in carbon isotopes as a result of tsr however the fractionation has not quantitatively described a loga rithmic relationship is found to give the best fit for methane d 13 c 1 and 1 h2s

carbonate porosity creation by mesogenetic dissolution reality or illusion many authors have proposed that significant volumes of porosity are created by deep burial dissolution in carbonate reservoirs we argue however that this model is unsupported by empirical data and violates important chemical constraints on mass transport because of the ubiquitous presence and rapid kinetics of dissolution of carbonate minerals the mesogenetic pore waters in sedimentary basins can be expected to be always saturated and buffered by carbonates providing little opportunity for the preservation of significantly undersaturated water chemistry during upward flow even if the initial generation of such undersaturated pore water could occur a review of the literature where this model has been advanced reveals a consistent lack of quantitative treatment in consequence the presumption of mesogenetic dissolution producing a net increase in secondary porosity should not be used in the prediction of carbonate reservoir quality

high resolution early paleozoic ordovician silurian time scales for much of the geologic time scale resolving power appears to be limited by the duration of biostratigraphic zones and subzones yet these zones exploit the appearance and disappearance events of only a small fraction of the species they span computer algorithms can sequence an order of magnitude more events and make explicit the uncertainties that arise when calibrating the resulting time line an illustrative case history builds an ordovician and silurian time scale from the geologic record of the entire graptolite clade over 1900 species from more than 400 localities and sections worldwide the same approach can be applied to any stratigraphic interval with detailed biostratigraphic observations it provides the foundation for time scales and paleobiologic time lines using logic similar to graphic correlation optimizing algorithms search for a composite sequence of species first and last appearance events that minimizes the implied shortcomings of all the field observations the algorithms minimize the number of species coexistences that are implied but never observed and the net adjustments needed to bring local range charts into agreement with a single composite sequence of events after total section thicknesses have been rescaled to help normalize for variation in depositional rate mean stratigraphic thicknesses are used to scale the intervals between adjacent events in the composite sequence the resulting scaled composite sequence is converted to a relative geologic time scale by identifying stage and zone boundaries within the sequence of graptolite events this relative scale is in turn calibrated by dated volcanic ash beds that were incorporated in the search for the optimal sequence these dated events are also used to test for linearity of the scaled composite the final time scale has a potential resolving power of 0 02 0 1 m y more than ten times better than can be achieved by traditional zones graptolite zones vary widely in duration from as short as 0 1 m y to nearly 5 0 m y the mean duration of zones or zonal groupings calibrated here is 1 44 m y in the ordovician and 0 91 m y in the silurian the average uncertainty in locating zone boundaries in a single composite sequence is about one fourth of the mean zone duration the variance resulting from differences in time scales developed from differing numbers of field observations and in response to changes in the optimization criteria gives a very conservative measure of the overall robustness of the method these differences indicate an average uncertainty in the age of graptolite zone boundaries that is more nearly equal to the mean zone duration for zone duration the mean uncertainty amounts to about one half of the length of an average zone

the role of burial diagenesis in hydrocarbon destruction and h2s accumulation upper jurassic smackover formation black creek field mississippi organic inorganic interactions during burial of the smackover formation at black creek field mississippi have resulted in nearly complete destruction of hydrocarbons the formation has been buried to a depth of 6 km has experienced temperatures of over 200 degrees c and presently contains 78 h 2 s 20 co 2 and 2 ch 4 three distinct stages of burial diagenesis correspond to three phases of organic matter maturation pre oil window diagenesis was dominated by precipitation of prebitumen calcite cement diagenesis in the oil window was characterized by precipitation of saddle dolomite and anhydrite in water filled layers and by formation of solid bitumen in the oil column diagenesis in the gas window was dominated by thermochemical sulfate reduction tsr resulting in hydrocarbon destruction anhydrite dissolution large amounts of h 2 s co 2 and s generation and postbitumen calcite cementation during tsr anhydrite reacted with h 2 s to produce s which in turn reacted with ch 4 to generate more h 2 s in a self reinforcing cycle the lack of metal cations to stabilize h 2 s as metal sulfides availability of sufficient sulfate to generate h 2 s and a closed system to prevent h 2 s from escaping resulted in the continuation of the tsr cycle until nearly all hydrocarbons were consumed in mississippi concentration of h 2 s is nearly zero in smackover hydrocarbon reservoirs that have experienced temperatures of 120 degrees c for more than 50 m y suggesting that tsr is not a kinetic time dependent process high h 2 s concentrations initiate at temperatures above 140 degrees c and increase with temperature indicating that tsr is a thermodynamic phenomenon reported high h 2 s concentrations at low temperatures 80 120 degrees c from other locations may be explained by the following processes 1 migration of h 2 s into these reservoirs 2 high geothermal gradients or local thermal perturbations in the past 3 a biochemical origin for the h 2 s or 4 exposure of these reservoirs to temperatures greater than 150 degrees c and a rapid uplift in black creek field burial cementation and pressure solution resulted in total destruction of porosity and permeability in limestone reservoirs but not in dolomite reservoirs which still possess up to 20 porosity and 100 md permeability secondary porosity was not created as a result of hydrocarbon migration abundant co 2 derived during hydrocarbon destruction resulted in calcite cementation rather than carbonate dissolution late secondary porosity development in carbonates may be related to acids generated by metal sulfide precipitation

climate change on tectonic time scales abstract observational and modeling studies relevant to climate change on tectonic time scales are reviewed probably the most significant climate result from this time scale involves inferences about nearly ice free conditions for some intervals of earth history some additional results suggest relatively rapid transitions between climate states modeling studies indicate that although tectonic changes significantly modify the atmosphere and ocean circulation in most cases such modifications do not appear to be sufficient to explain ice free climates increased atmospheric co2 levels 5 10 presentvalues may be needed to completely reconcile models and data although the co2 paradigm has received widespread attention in the last few years the results summarized herein indicate that co2 is only one of several important factors influencing climate i believe the time is appropriate for contemplating climate change on tectonic time scales in terms of a post co2 synthesis in which direct tectonic factors are approximately equal in importance to co2 fluctuations in the last few years modeling studies have progressed from a state of virtually no work on the topic to one where the models are in some respect ahead of the observations in the sense that data have not been compiled to test even some of the most basic model predictions in particular information is needed on past atmospheric co2 levels tropical sea surface temperatures seasonality of continental interiors and locations and production rates of deep waters initial steps are being taken to integrate observational and modeling activities on tectonic time scales and some of these shortcomings may be rectified over the next few years

documenting the reliability and utility of the γ method as applied to cyclic sections using forward modeling abstract the problem of determining if cyclic sections which are older than 5 m y in age were deposited in response to orbital control is discussed gamma analysis a method which uses the assumption that effective accumulation rates are facies dependent to minimize differences in cycle duration is presented as a possible solution to this problem while the assumptions of analysis are generally not met by geologic data the effective accumulation rates estimated may be instrumental in recovering a spectrum which reveals orbital control we address the problem of assessing the reliability of results by performing analyses on three cyclic time series generated by a forward model our results show that exact accumulation rates are not likely to be obtained by the method however the accumulation rates obtained markedly improve the time scale resulting in a spectrum from which one could distinguish orbital forcing if present only when spectra of cyclic data show predicted orbital peaks can analysis be considered to yield effective accumulation rates which are more nearly like the true effective accumulation rates than the assumption of a single effective accumulation rate for all facies

a devonian hydrothermal chert reservoir the 225 bcf parkland field british columbia canada the 225 bcf original gas in place ogip parkland a pool was discovered in 1956 with the drilling of imperial pacific 6 29 81 15w6 this well initially production tested at 19 mmcf gas day and has produced more than 95 bcf of sweet gas the parkland play type as defined by the geological survey of canada consists principally of fracture associated hydrothermally karsted and dolomitized reservoirs hosted in the upper devonian famennian wabamun group medial ramp carbonates resting on the axis of the peace river arch and includes major fields such as tangent teepee and gold creek ironically parkland itself owes little of its production to hydrothermally dolomitized carbonates instead most of the reservoir pore volume can be attributed to microintercrystalline porosity within a pervasive replacement microquartz chert that occurs at the dolomite limestone interface the thickness of this chert zone in the 6 29 well is 35 m and occurs near the top of the wabamun the origin of the chert is problematic it postdates matrix dolomitization and crosscuts the early tournaisian mississippian exshaw ash but is in turn truncated by later saddle dolomite and calcite veins the replacement chert is composed of a microporous up to 30 meshwork of microquartz crystals averaging 5 10 m in size and is texturally distinct from early diagenetic chert nodules also found in the wabamun the two cherts are also distinguishable isotopically with the replacement microquartz showing significantly more depleted oxygen values 22 vs 25 d18o standard mean ocean water smow consistent with precipitation from hot fluids having a temperature range from about 140 to 200 c silicification has had the greatest impact on slightly dolomitized limestone at the outer margin of the hydrothermal dolostone pod entrapped floating euhedral crystals of dolomite within the replacement chert show little evidence of microdissolution suggesting that the silicic begin page 52 acid charged fluid although undersaturated with respect to the microspar lime matrix was saturated with respect to dolomite we postulate that hydrothermal fluids of common parentage only marginally evolved in composition if at all were responsible for both dolomitization and silicification both replacement processes occurred in relatively rapid succession at shallow burial depths and were related to an early tournaisian period of hydrothermal activity associated with both wrench and minor extension structuring linked with the nascent development of the fort st john graben the source of the silica is thought to be the immediately subjacent granite wash or possibly the precambrian basement stratigraphic proximity to the granite wash is likely a prerequisite condition for chert reservoir development within the wabamun and serves to explain along with significantly different hydrothermal fluid temperatures much hotter to the west why parkland is such a distinct field compared to tangent and teepee hydrocarbon reservoirs that are dominantly chert hosted are relatively uncommon but where recognized e g monterey formation california they are related to the redistribution and transformation of biogenic opal a silica derived from diatoms spicules and or radiolarians in deeper water sediments to our knowledge parkland is the only hydrothermal chert reservoir that has been reported anywhere in the world

saddle dolomite as a by product of chemical compaction and thermochemical sulfate reduction petrographic stable and radiogenic isotope trace element and fluid inclusion data from the devonian nisku reef trend in the subsurface of alberta canada suggest that saddle dolomite was a by product of chemical compaction and thermochemical sulfate reduction in subsiding dolomitized rocks furthermore thermochemical sulfate reduction may be representative of other processes that increase the amount of saddle dolomite via an increase in carbonate alkalinity the data also confirm that saddle dolomite is most likely to form at elevated temperatures from hypersaline brines

rare earth element and yttrium rey geochemistry in carbonate reservoirs during deep burial diagenesis implications for rey mobility during thermochemical sulfate reduction abstract the impact of burial diagenesis especially deep burial related processes such as thermochemical sulfate reduction tsr on the distribution of rare earth elements and yttrium rey in diagenetic minerals in carbonate reservoir has gained little attention even though they may be unique indicators of the diagenetic system being closed or open to external influx of material trace element and rey concentrations 87 sr 86 sr 18 o and 13 c have been determined for limestone host dolomite pore filling calcite calcite 2 and late stage fracture filling calcite calcite 3 barite barite 2 and anhydrite anhydrite 3 from lower triassic feixianguan formation reservoirs in northeast sichuan basin china calcite 2 calcite 3 barite 2 and anhydrite 3 precipitated during deep burial from 110 c to 220 c demonstrated by petrology and fluid inclusion thermometry sr isotope analysis revealed that diagenetic carbonate minerals and anhydrite have largely identical 87 sr 86 sr ratios to triassic seawater indicating negligible input of sr from terrigenous sources and probably a relatively closed diagenetic environment for precipitation of these minerals carbon isotope analysis showed that calcite 2 has relatively low 13 c values down to 18 9 v pdb suggesting that they are tsr calcites with the carbonate derived from oxidized isotopically light hydrocarbons unlike the seawater like rey patterns of the limestone and dolomites calcite 3 and anhydrite 3 are enriched in rare earth elements ree and show light rare earth element enrichment and heavy rare earth element depletion exhibiting a chevron like pattern of shale normalized rey trends in contrast tsr calcite calcite 2 has similar ree to the host dolomite but a relatively flat rey sn pattern suggesting strong variations in elemental and ree compositions of the burial fluids significantly tsr calcite shows a prominent positive eu anomaly and an unusually high chondritic y ho ratio both yttrium versus holmium fractionation and eu 2 oxidation to eu 3 must have occurred during thermochemical sulfate reduction hence a positive eu anomaly and an elevated y ho ratio may be used as effective proxies to differentiate calcite resulting from tsr from ordinary calcite cement this is especially useful when carbon isotope analysis cannot be used to give an unambiguous interpretation of the origin of the calcite

evidence for multiple stages of oil cracking and thermochemical sulfate reduction in the puguang gas field sichuan basin china the puguang gas field is the largest gas field found in marine carbonates in china marine carbonate reservoirs in this field were buried to a depth of about 7000 m 22 966 ft and experienced maximum temperature up to 220c before uplift to the present day depth of 5000 5500 m 16 404 18 045 ft with present day thermal maturity between 2 0 and 3 0 equivalent vitrinite reflectance ro sulfur rich pyrobitumens with reflectance up to 3 5 are widespread in the reservoirs and resulted from thermal cracking of crude oils most likely generated from upper permian source rocks and thermochemical sulfate reduction tsr natural gases in the puguang gas field have wide variations in nonhydrocarbon gas contents with h2s contents between 5 1 and 58 3 and co2 contents between 7 9 and 18 0 the hydrocarbon gases originated mainly from thermal cracking of accumulated oil but were altered by tsr thermochemical sulfate reduction in the study area exerted different effects on the isotope compositions of different hydrocarbon gas components at different tsr stages the differential increase of 13c values for different gas components reflects transformation from a heavy hydrocarbon gas dominated tsr stage to a methane dominated tsr stage this caused a decrease of 13cmethane13cethane values and a corresponding conversion from reversed to normal isotope distributions thermochemical sulfate reduction in the study area appears to have been limited by sulfate concentrations in the reservoirs a successive three stage tsr series namely liquid hydrocarbon involved tsr heavy hydrocarbon gas dominated tsr and methane dominated tsr occurred in reservoirs with sufficient sulfate concentration methane can be the dominant organic reactant for tsr but only at elevated temperature and after most c2 hydrocarbons are exhausted

petrological and geochemical constraints on porosity difference between lower triassic sour and sweet gas carbonate reservoirs in the sichuan basin abstract petrographic features carbon and oxygen isotopes and trace elements were determined and fluid inclusions were analyzed on late diagenetic cements and vug fillings from the lower triassic carbonates to account for much higher porosities in the sour dolomite reservoirs than in the sweet limestones in northeastern sichuan basin in the ne side to the kaijiang liangping trough dolostone reservoirs have porosities mainly from 6 to 12 up to 28 and contain 9 20 h 2 s the dolomite includes pre bitumen re crystallized silt sized crystals without occurrence of fluid inclusions and late burial dolomite the late burial dolomite has been measured to have fluid inclusions homogenization temperatures hts from 72 to 128 c part of the dolomites with hts up to 121 c show corroded edges or replacement by tsr calcites tsr calcites show 13 c values from 2 3 to 18 9 feo from null to 810 g g and so 3 from 50 to 18 700 g g vug filling tsr calcites have much lower feo and so 3 than the replacement calcites of anhydrite suggesting that dissolved fe 2 released from the dissolution of dolomite and anhydrite as a result of tsr may have reacted with tsr h 2 s and precipitated as pyrite the dissolution may have released 13 c rich co 2 resulting in the present day co 2 having 13 c values significantly heavier than those of the tsr calcites as light as 18 9 the dolomite and anhydrite dissolution during late stage of tsr may have enlarged or redistributed porosity in the ne side dolostones in contrast oolitic limestone reservoirs in the sw side contain significantly lower sulfates and thus no significant late diagenetic dissolution and tsr the porosity difference in the two sides resulted from original physical property due to early dolomitization and subsequent improvement from anhydrite and dolomite dissolution as a result of tsr

climatic control of fluvial lacustrine cyclicity in the cretaceous cordilleran foreland basin western united states tectono stratigraphic models of foredeep sedimentation have generally presumed a direct link between changing rates of tectonism and concomitant sedimentological response as manifested by change in thickness composition or depositional environment of sediment accumulating in adjacent basins lacustrine limestone units within the early cretaceous fluviamacustrine gannett group of western wyoming exhibit systematic variation in several geochemical proxies of relative rates of precipitation and evaporation indicating that lakewater chemistry was controlled by variation in regional climate change in proportion of allochthonous terrigenous clastic vs autochthonous carbonate deposition as well as carbonate mg ca ratio and stable isotopic composition occurs at two scales metre scale alternation of micritic limestone and argillaceous marl is accompanied by mineralogical and isotopic variation within individual beds indicating preferential carbonate accumulation during intervals of decreased regional meteoric precipitation relative to lake surface evaporation limestone deposition began during intervals of maximum aridity and decreased as increased meteoric precipitation driven flux of terrigenous clastic sediment overwhelmed sites of carbonate accumulation similar upsection variation in limestone mineralogy and isotopic composition at a scale of tens of metres reflects the multiple processes of long term increase in meteoric precipitation and lakewater freshening prior to influx of terrigenous sediment across basin fluvial deltaic progradation and renewed accumulation of riverine terrigenous units such trends suggest that formation scale alternation between fluvial clastic and lacustrine carbonate deposition was controlled by climate change

gas souring by thermochemical sulfate reduction at 140 c reply natural gas in the permian triassic khuff formation of abu dhabi contains variable amounts of h2s gas souring occurred through thermochemical sulfate reduction of anhydrite by hydrocarbon gases sour gas is observed only in reservoirs hotter than a critical reaction temperature 140 c petrographic examination of core from a wide depth range showed that the anhydrite reactant has been replaced by calcite reaction product only in samples deeper than 4300 m gas composition data show that only reservoirs deeper than 4300 m contain large quantities of h2s i e 10 at present day geothermal gradients 4300 m is equivalent to 140 c fluid inclusion analysis of calcite reaction product has shown that calcite growth only became significan at temperatures greater than 140 c thus three independent indicators all show that 140 c is the critical temperature above which gas souring by thermochemical sulfate reduction begins the previously suggested lower temperature thresholds for other sour gas provinces 80 130 c derive from gas composition data that may not allow adequately either for the reservoir temperature history or for the migration of gas generated at higher temperatures into present traps conversely published proposals for higher threshold temperature 180 200 c derive from short duration experimental data that are not easily extrapolated to geologically realistic temperatures and time scales therefore the temperature of 140 c derived from our study of the khuff formation may be th best estimate of temperature required for in situ thermochemical sulfate reduction to produce the high h2s concentrations encountered in deep carbonate gas reservoirs

periodicity of mesoscale phanerozoic sedimentary cycles and the role of milankovitch orbital modulation data on more than 200 mesoscale sedimentary cycles indicate that phanerozoic cycle periods are randomly distributed with respect to the four major milankovitch orbital parameters except late mississippian through late pennsylvanian cycles which show positive clustering about the 413 000 yr eccentricity period cycle periods calculated from average thicknesses and accumulation rates are strongly dependent on the former correlation coefficient 0 802 but relatively independent of the latter correlation coefficient 0 360 hence calculated period is largely a function of average cycle thickness and milankovitch range periods will be preferentially calculated for cycles with average thicknesses between 1 m and 20 m regardless of actual causes of cyclicity for individual cyclic sequences arguments for milankovitch control based solely on average period are unfounded comparison of short term holocene sedimentation rates and long term phanerozoic accumulation rates for cyclic peritidal and delta

the influence of rock fabric and mineralogy on thermochemical sulfate reduction khuff formation abu dhabi abstract thermochemical sulfate reduction tsr is the reaction between anhydrite and petroleum fluids at elevated temperatures to produce h2s and calcite in this study of the dolomite hosted hydrocarbon gas reservoirs in the permo triassic khuff formation abu dhabi a geochemically well constrained rock gas system we demonstrate for the first time a clear influence of rock texture and mineralogy on the rate and extent of tsr reactions and thus on h2s concentration in the gas phase the controls on the rate on h2s accumulation were tsr reaction kinetics tsr became significant as temperature exceeded 140 c a critical threshold temperature for chemical reaction between aqueous sulfate and aqueous methane anhydrite dissolution rate once initiated the subsequent reaction rate was controlled by the rate of supply of aqueous sulfate to the reaction site sulfate limitation is indicated by the lack of fractionation of sulfur isotopes between sulfate and sulfide i e suggesting total reaction for each unit of sulfate that dissolves also finely crystalline anhydrite began reacting at a lower temperature than coarse crystalline anhydrite likely a result of relative surface area suggesting that anhydrite dissolution was the rate limiting step transport rates are unlikely to have been rate limiting at the earliest stage of reaction because anhydrite was replaced in situ by calcite on the very edges of anhydrite nodules and crystals transport rate later as tsr proceeded it became transport controlled as calcite growing on the surface of anhydrite crystals began to isolate them from dissolved methane tsr ceased once calcite had effectively armor plated totally isolated the remaining anhydrite finely crystalline anhydrite underwent more extensive and more rapid tsr than coarser anhydrite crystals because these had a greater ratio of surface area to volume allowing more and faster dissolution and requiring more calcite to isolate them from methane localized loss of h2s occurred in the reservoir by reaction with indigenous fe bearing clays consequently reservoirs with a relatively high siliciclastic content have less h2s than would be expected from the advanced state of anhydrite replacement by calcite in order to predict tsr related h2s concentration in hydrocarbon gases it is thus important to understand the diagenetic and textural characteristics of the reservoir as well as the thermal and petroleum emplacement history

pangaean climates megamonsoons of the megacontinent the climate for a highly idealized geographic representation of the megacontinent of pangaea corresponding to the period 250 200 m y ago is explored using a low resolution atmospheric general circulation model that is coupled to a static mixed layer ocean of constant depth with a provision for sea ice formation and decay soil moisture is included as a predicted variable on the continent because there are large uncertainties about key factors that might influence the pangaean climate sensitivity experiments are used to examine the climate for two different mean elevations 0 and 1000 m two different snowcover prescriptions and two different levels of implied greenhouse heating and solar luminosity modern co2 concentration and modern solar luminosity fivefold increase in modern co2 concentration and a 1 decrease in modern solar luminosity in all of the experiments the large bipolar land masses of laurasia and gondwanaland joined near the equator in the west and separated by the tethys sea in the east exhibit extreme continentality with hot summers cold winters and large scale summer and winter monsoon circulations year round or seasonal aridity is simulated for all but the eastern coastal regions the tropical western coast and the regions poleward of 40 latitude the simulated climate of the idealized pangaean continent has many features that correspond qualitatively with geologic evidence for tropical warmth and aridity in the late permian and triassic however the model results serve mainly to indicate the importance of various climate forming factors geography topography heating detailed comparisons will require new experiments with better models and more accurate geologic boundary conditions

the marl limestone rhythmites from the lower kimmeridgian platynota zone of the central prebetic and their relationship with variations in orbital parameters abstract the origin of the rhythmic marl limestone bedding recognized in lower kimmeridgian platynota zone succesions of the central prebetic southern spain is interpreted a detailed analysis was carried out on the basis of field observations and mathematical procedures of two sections characterized by the variable alternation of marly and limestone horizons the study primarily reveals signs of a possible secondary imprint in the stratification however we conclude that this imprint cannot have been sufficiently important to cause the recorded stratification or significantly alter the existing primary bedding the mathematical procedure applied to the thicknesses of the beds reveals the existence of different orders of cyclicity that are not recognizable in the field we were able to date these cycles by detailed biostratigraphy and the application of fourier spectral analysis the close relationship between the timing calculated for the marl limestone rhythms recognized in the lower kimmeridgian platynota zone successions and the periodicity assumed for milankovitch cycles has been shown for the first time in the prebetic zone and leads us to accept the influence of orbital variations on sedimentation in the sector that was studied we have also been able to recognize the influence of local factors which determined differences in the depositional organization in the sections studied

open or closed geochemical systems during diagenesis in sedimentary basins constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs descriptions of mineralogy and textural relationships in sandstones and limestones have been used to establish a sequence of diagenetic events epigenesis involving mineral dissolution and precipitation which have been interpreted to have occurred during the burial history published epigenetic sequences commonly imply a geochemically open system with very significant changes in the bulk chemical composition of the sediments during burial near surface diagenetic reactions may be open involving significant changes in the sediment composition and formation of secondary porosity caused by high pore water flow rates of meteoric water or reactions with sea water near the sea floor calculations show that the bulk chemical composition of the sediments below the reach of high pore water flow rates of meteoric water or hydrothermal convection should remain nearly constant during progressive burial because of limited pore water flow mass transport between shales and sandstones is also limited because the pore water is in most cases buffered by the same minerals so that the concentration gradients are low recent studies show that silica released from clay mineral reactions in mudstones has been precipitated locally as small quartz crystals and not exported to adjacent sandstones if the geochemical constraints for mass transfer during burial diagenetic reactions are accepted the chemical reactions involved in diagenesis can be written as balanced equations this offers the possibility to make predictions about reservoir quality based on assumptions about primary sediment composition related to facies and provenance large scale changes in the bulk composition of sandstones and mudstones during burial diagenesis have been suggested but because such changes cannot be explained chemically and physically no predictions can be made burial diagenetic processes are in most cases not episodic but occur as slow adjustments to increased stress and temperature driving the sediments toward increased mechanical and thermodynamic stability as a result the porosity of a single lithology must decrease during progressive burial but each lithology has a different porosity curve this article discusses quantitative calculations and estimates that show clearly that burial diagenesis must represent geochemically nearly closed systems where mineral dissolution and precipitation must be balanced this provides a theoretical basis for the modeling and prediction of reservoir quality

whither cyclostratigraphy testing the gamma method on upper pleistocene deep sea sediments north atlantic deep sea drilling project site 609 1 abstract gamma analysis a procedure for tuning cyclic sediments in which gamma is defined as 1 sedimentation rate was applied to the upper pleistocene gray scale record of deep sea drilling project site 609 from the north atlantic the spectrum of the gamma tuned time series produces eccentricity obliquity and precession index signals when the primary periodicity is tuned to the precessional beat frequency however the timescales are significantly too long suggesting that some of the primary cycles used in the gamma tuning procedure have higher frequency than the precessional beat by filtering the specmap calibrated time series at i 0 12 kyr the spurious cy cles are removed from the tuning procedure in this case the resulting gamma corrected spectra look much more like that of the specmap tuned time series however the time series remains slightly too long forward modeling is consistent with the results from modeling the core that is the gamma method is quite robust at revealing orbital periodicity in an orbitally controlled re cord for which sedimentation rates are facies dependent however the method does not produce an accurate time series criteria for complete rejection of gamma results are clarified as either 1 least squares results include negative gamma values or 2 the resulting spectrum does not show an or bital signature variation on sedimentation rates greater than 5 result in incorrect time series however variations on sedimentation rates as high as 15 generally reproduce strong orbital spectra and occasionally the method can also generate an orbital spectrum with up to 25 noise the method is not as robust in predicting gamma values or sedimentation rates although it generally distinguishes facies with relatively high sedimentation rates from facies with relatively low sedimentation rates the values predicted may be incorrect by a factor of2 thus while gamma tuning of the time series is an improvement over observed thicknesses the resulting timescale is not correct the implications of these results for cyclostratigraphy are significant the generation of an orbital spectrum from a time series that is demonstrably incorrect casts doubt on cyclicity as a dating tool

generation of isotopically and compositionally distinct water during thermochemical sulfate reduction tsr in carbonate reservoirs triassic feixianguan formation sichuan basin china abstract thermochemical sulfate reduction tsr the reaction of petroleum with anhydrite in reservoirs resulting in the growth of calcite and the accumulation of h2s has been documented in the feixianguan formation dolomite reservoir in the sichuan basin china fluid inclusion salinity and homogenization temperature data have shown that tsr results in a decrease in salinity from a pre tsr value of 25 wt down to 5 wt as a result of water created as a byproduct of progressive tsr we have studied the isotopic character of the water that resulted from tsr in the feixianguan formation by analyzing the oxygen isotopes of tsr calcite and determining the oxygen isotopes of the water in equilibrium with the tsr calcite at the temperatures determined by aqueous fluid inclusion analysis we have compared these tsr waters to water that would have been in equilibrium with the bulk rock also at the temperatures determined by aqueous fluid inclusion analysis we have found that the tsr waters are relatively depleted in oxygen isotopes by up to 8 compared to what would be expected at equilibrium between the bulk rock and water since this type of water was specifically derived from anhydrite the generation of relatively large volumes of low salinity low 18o water associated with advanced tsr in the feixianguan formation has also been reported in the permian khuff formation in abu dhabi and from sour devonian fields in the western canada basin this suggests that tsr derived water may be a common phenomenon the effects of which on mesogenetic secondary porosity and reservoir quality have previously been underappreciated

origin and reservoir characteristics of upper ordovician trenton black river hydrothermal dolomite reservoirs in new york in the past decade more than 20 new natural gas fields have been discovered in laterally discontinuous dolomites of the upper ordovician black river group in south central new york the dolomites form around basement rooted wrench faults that are detectable on seismic data most fields occur in and around elongate fault bounded structural lows interpreted to be negative flower structures away from these faults the formation is composed of impermeable limestone and forms the lateral seal for the reservoirs in most cases the faults die out within the overlying trenton limestone and utica shale most porosity occurs in saddle dolomite coated vugs breccias and fractured zones matrix porosity is uncommon in the black river cores described for this study the patchy distribution around basement rooted faults and geochemical and fluid inclusion analyses supports a fault related hydrothermal origin for the saddle and matrix dolomites this play went for many years without detection because of its unconventional structural setting i e structural lows versus highs using the appropriate integrated structural stratigraphic diagenetic model more hydrothermal dolomite natural gas reservoirs are likely to be discovered in the black river of new york and in carbonates around the world

milankovitch climatic signals in lower triassic olenekian peritidal carbonate successions nanpanjiang basin south china abstract meter scale peritidal carbonate strata from an isolated platform in the nanpanjiang basin of south china were tested for the presence of periodic or quasi periodic climatic signals these signals provide information on factors controlling cyclic sedimentation in the equatorial eastern tethys during the early triassic greenhouse period parasequences are composed of shallowing upward successions of subtidal facies including skeletal packstone and grainstone calcimicrobial boundstone and oolitic packstone and grainstone intertidal laminated lutite and ribbon rock and rare supratidal microbial laminite parasequence stacking patterns display three orders of cyclicity suggesting hierarchical stratigraphic relationships gamma analysis of kominz and bond earth planet sci lett 48 1990 233 244 was used to estimate facies dependent thickness time conversion factors i e s and to construct tuned and untuned time series for two stratigraphic sections spectra of these time series indicate the presence of quasi periodic signals with prominent short eccentricity 94 9 131 2 kyr short obliquity 35 8 kyr and long precessional index 21 2 kyr peaks and minor long eccentricity 412 9 kyr long obliquity 45 3 kyr short precessional index 17 7 kyr and constructional tone 9 7 kyr peaks when calibrated on the tuned spectra thus milankovitch climatic forcing probably greatly influenced sedimentation the calibration indicates that the subtidal facies has a sedimentation rate of 24 6 30 7 cm kyr and the intertidal supratidal facies has a rate of 2 7 6 0 cm kyr the estimated duration of deposition of the two sections is 1139 1423 kyr and corresponds to a stratigraphic completeness of 32 92 the completeness is much greater than that of icehouse stratigraphic records we speculate that variations in carbonate productivity and environmental conditions driven by milankovitch climatic forcing combined with low amplitude sea level fluctuations were likely major controls on cyclic sedimentation furthermore evolutional spectra of the two sections show that dominant milankovitch climatic forcings varied from short eccentricity obliquity to long precessional index during the course of sedimentation some spectral differences between the two sections suggest variations in depositional recording mechanisms of milankovitch climatic signals between platform interior and windward platform margin

the fate of co2 derived from thermochemical sulfate reduction tsr and effect of tsr on carbonate porosity and permeability sichuan basin china abstract this article discusses the role of methane in thermochemical sulfate reduction tsr the fate of tsr derived co2 and the effect of tsr on reservoir porosity and permeability and the causes of the anomalously high porosity and permeability in the lower triassic soured carbonate gas reservoirs in the northeast sichuan basin southwest china the lower triassic carbonate reservoirs were buried to a depth of about 7000 m and experienced maximum temperatures up to 220 c before having been uplifted to the present day depths of 4800 to 5500 m but they still possess porosities up to 28 9 and permeabilities up to 3360 md the present day dry gas reservoirs evolved from a paleo oil accumulation and experienced varying degrees of tsr alteration as evidenced from the abundant sulfur rich solid bitumens and varying h2s and co2 concentrations tsr occurred mainly within the oil and condensate wet gas windows with liquid hydrocarbons and wet hydrocarbon gases acting as the dominant reducing agents responsible for sulfate reduction sulfur rich solid bitumen and h2s generation and calcite precipitation methane dominated tsr was a rather late event and had played a less significant role in altering the reservoirs intensive h2s and co2 generation during tsr resulted in calcite cementation rather than carbonate dissolution which implies that the amount of water generated during tsr was volumetrically insignificant 13c depleted co2 derived from hydrocarbon oxidation preferentially reacted with ca2 to form isotopically light calcite cements and the remaining co2 re equilibrated with the 13c enriched water rock systems with its 13c rapidly approaching the values for the host rocks which accounted for the observed heavy and relatively constant co2 13c values the carbonate reservoirs suffered from differential porosity loss by tsr involved solid bitumen generation and tsr induced calcite and pyrite precipitation intensive tsr significantly reduced the porosity and permeability of the intervals expected to have relatively high sulfate contents the evaporative platform dolostones and the platform margin shoal dolostones immediately underlying the evaporative facies early oil charge and limited intensity of tsr alteration together with very low phyllosilicate content and early dolomitization accounted for the preservation of anomalously high porosities in the reservoirs above the paleo oil water contact a closed system seems to have played a special role in preserving the high porosity in the gas zone reservoirs below the paleo oil water contact the closed system which is unfavorable for deep burial carbonate dissolution and secondary porosity generation was favorable for the preservation of early formed porosity in deeply buried carbonates especially sucrosic and vuggy dolostones have a high potential to preserve such porosity

thermochemical sulfate reduction and fluid evolution of the lower triassic feixianguan formation sour gas reservoirs northeast sichuan basin china the dolomite hosted lower triassic feixianguan formation from the northeast sichuan basin china is an economically important reservoir that contains sour natural gas these reservoirs were initially filled with oil later replaced by gas during burial to 7000 m 22 965 ft followed by uplift to about 4000 m 13 123 ft we have studied the souring process thermochemical sulfate reduction tsr and diagenetic evolution of the feixianguan formation using detailed petrology fluid inclusion studies and stable isotope data from carbonate minerals pre tsr diagenesis included in time order the eodiagenetic main stage of dolomitization by a reflux mechanism fracture related calcite cementation barite quartz celestite and fluorite mineralization and a dolomite recrystallization stage thermochemical sulfate reduction resulted in anhydrite replacement by calcite petroleum destruction formation of sulfur rich pyrobitumen and elemental sulfur and generation of large volumes of and diagenesis during tsr can be subdivided into oil stage tsr and gas stage tsr with oil stage tsr defined by the presence of primary oil and bitumen inclusions in the tsr calcite based on aqueous inclusion homogenization temperatures oil stage tsr commenced at a temperature of 116 c with a mode between 130 c and 140 c gas stage tsr started at a temperature of 135 c and continued to maximum burial temperatures of about 220 c trace amounts of pyrite barite quartz and celestite grew during tsr post tsr diagenesis was dominated by fracture related calcite precipitation as well as celestite and anhydrite crystallization formation water salinity increased from depositional values 3 5 wt up to 24 wt during pre tsr dolomite recrystallization probably because of an influx of evaporite associated water from the overlying jialingjiang formation although pre tsr barite quartz celestite and fluorite mineralization was associated with a transient decrease in water salinity during tsr formation water salinity decreased from 26 wt to as low as 4 wt as a result of water being produced during tsr reactions

mesogenetic dissolution its role in porosity development in carbonate reservoirs models of porosity formation in carbonate rocks have stressed subaerial exposure and attendant shallow meteoric diagenesis porosity formation also occurs in deep burial or mesogenetic settings as a result of dissolution enlargement of preexisting pores porosity enhancement and creation of new pore systems brines charged with organic acids carbon dioxide and or hydrogen sulfide derived from organic matter diagenesis and thermochemical sulfate reduction are the likely fluids causing significant mesogenetic dissolution enhanced and newly created mesogenetic pore types can mimic pore types formed in shallow meteoric environments and therefore the mesogenetic origin of some porosity may go unrecognized

diagenesis of the devonian southesk cairn carbonate complex alberta canada marine cementation burial dolomitization thermochemical sulfate reduction anhydritization and squeegee fluid flow abstract one of the largest geological features in the deep part of the alberta basin in western canada is the middle to upper devonian southesk cairn carbonate complex sccc the sccc is located at depths of about 5000 m adjacent to the disturbed belt of the rocky mountains rising to depths of 3000 m in a northeastward direction over a distance of 150 km the sccc produces oil and gas with up to about 30 h2s from several stratigraphic levels the diagenesis of the sccc was investigated by means of petrography and geochemistry stable isotope compositions were determined for fibrous calcite cements matrix dolomite saddle dolomite and sparry calcite fibrous calcites have stable isotope values of 18o 7 5 to 4 5 pdb and 13c 2 0 1 0 pdb matrix dolomite samples have stable isotope values of 18o 8 1 to 4 9 pdb and 13c 0 to 1 0 pdb saddle dolomite samples range from 18o 9 8 to 7 1 pdb and 13c 1 0 to 0 pdb sparry calcite samples have 18o 13 7 to 7 2 pdb and 13c 24 0 to 0 pdb the sr isotope values of all analyzed samples span a broad range from 0 7080 to 0 7320 showing considerable scatter but also one distinctive pattern i e values higher than 0 7220 occur only at depths in excess of about 4000 m whereas lower values scatter throughout the entire depth range furthermore the sr isotope values of sparry calcites decrease northeastward similarly the fluid inclusion homogenization temperatures of these calcites display a general decrease updip whereas the salinities show no general spatial trends the uncorrected homogenization temperatures range from about 85 to 180 c and the nacl equivalent salinities range from about 17 to 24 weight the d values of brines obtained from seven wells range from 79 to 20 smow and their 18o values range from 1 5 to 8 7 smow the sulfur isotope values of the dissolved sulfate ranges from 34s 10 1 to 24 5 cdt the sr isotope values of the brine samples range from 0 7109 to 0 7255 elemental sulfur isotope values cluster tightly around 34s 18 0 to 18 4 cdt the values of secondary anhydrites range from 34s 18 4 to 26 7 cdt values for solid bitumen vary from 34s 19 5 to 20 2 cdt integration of all data reveals a complex diagenetic evolution from the middle devonian to the recent the paragenetic sequence consists of 24 distinct diagenetic phases which have been grouped into 5 diagenetic stages stage i density driven convection in near surface settings likely superimposed on and or aided by tidal pumping facilitating marine cementation stage ii cellular convection facilitating pervasive matrix dolomitization aided by compaction stage iii hydrocarbon migration stage iv thermochemical sulfate reduction forming sour gas and regional squeegee flow forming sparry calcite cements and localized flow through faults stage v fluid flow driven by erosional rebound in the overlying mesozoic strata and remnant squeegee flow this study yielded several important results 1 the prolific repertoire of diagenetic textures found in the southesk cairn carbonate complex serves as a means of comparison for petroliferous carbonate complexes elsewhere especially in sour gas provinces close to orogenic fronts 2 the diagenetic evolution contains long periods of relative tranquility that were punctuated by several episodes of intense diagenetic activity 3 both thermochemical sulfate reduction and squeegee type fluid flow are recorded in late diagenetic sparry calcite cements 4 the effect of the various burial diagenetic episodes on the hydrocarbon reservoir properties throughout the entire reef complex was highly variable specifically matrix dolomitization thermochemical sulfate reduction and the emplacement of secondary and tertiary anhydrites affected the hydrocarbon reservoir properties throughout the entire reef complex on the other hand squeegee fluid flow was insignificant in terms of affecting the reservoir properties

depositional environments organic carbon accumulation and solar forcing cyclicity in smackover formation lime mudstones northern gulf coast upper jurassic smackover lime mudstones a major hydrocarbon source rock in the u s gulf coast show three distinct lithofacies the basal laminated lithofacies consists of organic and siliciclastic rich lamina sets that alternate with organic poor clean carbonate lamina sets both lamina sets are composed of lamina couplets interpreted to be varves this lithofacies lacks current induced structures allochems and rip up clasts benthic and planktonic fossils are absent one organic rich lamina set and one organic poor lamina set comprise one cycle 0 5 3 cm thick the middle thin bedded lithofacies is composed of organic and siliciclastic rich carbonate beds that alternate with organic poor clean carbonate beds both bed types are internally laminated one organic rich bed and one organic poor bed comprise one cycle 2 60 cm thick the upper burrowed lithofacies is extensively bioturbated massive shows no evidence of cyclicity and contains little organic carbon sedimentologic characteristics suggest that the basal laminated lithofacies was deposited under anoxic conditions below storm wave base in a basinal environment the middle thin bedded lithofacies is copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received march 5 1996 revised manuscript received august 2 1996 final acceptance december 10 1996 2basin research institute louisiana state university baton rouge louisiana 70803 3lss international 1440 lake front circle suite 180 the woodlands texas 77380 formerly with department of geology and geophysics louisiana state university baton rouge louisiana 70803 4department of geology and geophysics louisiana state university baton rouge louisiana 70803 the study was supported by grants from the center for energy studies and the basin research institute of louisiana state university we thank h d klemme and g t moore for providing unpublished data p meyers and l m pratt reviewed an earlier version of the manuscript and offered valuable comments j wrenn kindly provided characteristics of organic matter we thank j kropog w s leblanc k lyle p o neill and b simpson for technical assistance we appreciate the helpful comments of r l folk b kirkland and c h moore critical comments by k t biddle n b harris b a prather and p d wagner significantly improved the quality of concepts presented

a method to decipher the time distribution in astronomically forced sedimentary couplets abstract sedimentary couplets that are generated by astronomical forcing are common in pelagic and hemipelagic depositional settings this study disentangles the time scales sedimentation rates of the two lithofacies that contribute to such astronomically forced couplets by introducing the alpha method this new method can be applied to precession or obliquity forced sedimentary records and compares the frequency modulation of an astronomical cycle model with the thicknesses of the couplets the method is demonstrated on a synthetic model of a precession index forced succession of limestone marlstone couplets finally the methods is applied to two case studies for the middle cenomanian eagle ford formation iona 1 core texas sedimentation rates are estimated as 0 85 1 02 cm kyr for the marlstone and 4 70 5 65 cm kyr for the limestone for the middle eocene iodp expedition 342 site u1408 sedimentation rates are 1 70 1 84 cm kyr for the white nannofossil ooze and 2 54 2 75 cm kyr for the greenish nannofossil rich clay more generally studies of paleoclimate and geochemical evolution at the sub milankovitch scale can benefit from this method

controls on sedimentation and cyclicity of the boquillas and equivalent eagle ford formation from detailed outcrop studies of western and central texas u s a

new perspectives on orbitally forced stratigraphy this survey of the current status of research into earth s orbitally forced paleoclimatic record summarizes recent developments in the theory of earth s orbital parameters and reviews how various techniques of data collection and analysis have fared in the search and recovery of orbital signals in ancient stratigraphy the emerging significance of the quasi periodicity of earth s orbital variations as a prin cipal tool in the analysis of orbitally forced stratigraphy is discussed in detail five case studies are presented that illustrate new directions in research a time series analysis of discontinuous strata b measurement of ultra high resolution stratigraphic signals c new perspectives on the 100 kyr pleistocene glaciation problem d strati graphic evidence for solar system resonance modes and e evaluating phanerozoic length of day from orbitally forced stratigraphy the study of how earth s orbital parameters came to be recorded in ancient stra tigraphy has recently moved into a new phase of scientific inquiry and discovery notable advances in celestial mechanics and in the analysis of stratigraphic data have given rise to new ways of solving old problems and have uncovered new problems for the first time new tools have facilitated the recovery of extended high resolution orbital signals from stratigraphy advanced analytical techniques have now been fully incorporated into the field an updated theory of earth s orbital parameters provides an accurate ephemeris for the past 16 million years for precision correlation between earth s orbital parameters and orbitally forced stratigraphy some orbital eccentricity modes can be used for orbital stratal cal ibrations to several hundred million years ago these breakthroughs have transformed the study of orbitally forced stratigra phy into an applied field that addresses long standing problems in disciplines as far ranging as geochronology geodynamics and astrodynamics orbital signals provide the framework for a high resolution orbital chronostratigraphy as far back

structurally controlled hydrothermal dolomite reservoir facies an overview structurally controlled hydrothermal dolomite htd reservoir facies and associated productive leached limestones are major hydrocarbon producers in north america and are receiving increased exploration attention globally they include multiple trends in the ordovician locally silurian and devonian of the michigan appalachian and other basins of eastern canada and the united states and in the devonian and mississippian of the western canada sedimentary basin they also occur in jurassic hosts along rifted atlantic margins in the jurassic cretaceous of the arabian gulf region and elsewhere hydrothermal dolomitization is defined as dolomitization occurring under burial conditions commonly at shallow depths by fluids typically very saline with temperature and pressure t and p higher than the ambient t and p of the host formation the latter commonly is limestone proof of a hydrothermal origin for htd reservoir facies requires integration of burial thermal history plots fluid inclusion temperature data and constraints on timing of emplacement hydrothermal dolomite reservoir facies are part of a spectrum of hydrothermal mineral deposits that include sedimentary exhalative lead zinc ore bodies and htd hosted mississippi valley type sulfide deposits all three hydrothermal deposits show a strong structural control by extensional and or strike slip wrench faults with fluid flow typically focused at transtensional and dilational structural sites and in the hanging wall transtensional sags above negative flower structures on wrench faults are favored drilling sites for htd reservoir facies saddle dolomite in both replacive and void filling modes is characteristic of htd facies for many reservoirs matrix replacive dolomite and saddle dolomite appear to have formed near contemporaneously and from the same fluid and temperature conditions the original host facies exerts a major influence on the lateral extent of dolomitization resultant textures pore type and pore volume breccias zebra fabrics shear microfractures and other rock characteristics record short term shear stress and pore fluid pressure transients particularly proximal to active faults high temperature hydrothermal pulses may alter kerogen in host limestones a process designated forced maturation basement highs underlying sandstone and or carbonate aquifers probably overpressured and overlying and internal shale seals and aquitards also may constrain or influence htd emplacement although many questions and uncertainties remain particularly in terms of mg and brine source and mass balance recognition and active exploration of the htd play continues to expand increasing use of three dimensional seismic imagery and seismic anomaly mapping combined with horizontal drilling oblique to linear trends defined by structural sags helps to reduce risk