mass movements along a rapidly uplifting river valley an example from the upper jinsha river southeast margin of the tibetan plateau because of the rapid uplift of the tibetan plateau since the middle and late pleistocene the surrounding mountainous areas of the tibetan plateau also show a rapid uplift trend the study area is located in the upper reaches of the jinsha river southeastern tibetan plateau tectonically the study area is located in the eastern segment of the jinshajiang suture zone which is the weak belt of the lithosphere affected by the rapid uplift steep slopes are formed by the rapid vertical down cutting of rivers in the studied reach under the sustained action of the gravitational field the deformation and failure of slopes readily occur to better formulate development and risk mitigation plans a mass movement inventory map at the scale of 1 50 000 was prepared for the 100 km long reach a total of 247 mass movements of different types sizes and relative ages were mapped via image interpretation and detailed field survey the statistical analysis of their spatial distributions was performed using two relative concentration indexes the correlations between the spatial distribution and six common factors were analyzed elevation slope angle slope aspect lithology active fault and river system the geomorphological survey shows that the mass movements locally play an important role in the present day landscape evolution in addition the geomorphic features indicate that the mapped giant landslides can be classified as very old landslides and they very likely occurred under a different morphoclimatic environment than that of the present

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

kaolin cements in limestones potential indicators of organic rich pore waters during diagenesis abstract kaolin dickite and or kaolinite cement occurs locally in the chalk group upper cretaceous lower tertiary in the north sea and the derbyhaven beds lower carboniferous on the isle of man united kingdom stable isotope and petrographic data indicate that the kaolin cements in both the chalk group and derbyhaven beds precipitated during late diagenesis in either marine or modified marine pore waters kaolin cements in both formations are associated with hydrocarbons which suggests that organic complexing of aluminum may be responsible for precipitation of kaolin cement published experimental studies have demonstrated that the mobility of aluminum is greatly increased by complexing with organic acids in such systems aluminum would be released and made available for clay mineral precipitation when the organo aluminum complexes are destabilized increased whole rock 13c values in chalk group limestones from the eldfisk field where kaolin cement was widespread provides corroborating evidence for the late diagenetic decarboxylation of organic acids which would have released complexed aluminum the presence of kaolin cement may thus be an indicator of the former presence of organic rich pore fluids in limestones and provide an additional tool for determination of the pore water history of limestones and sedimentary basins

burial dissolution of micrite in middle east carbonate reservoirs jurassic cretaceous keys for recognition and timing abstract this paper discusses burial diagenesis and especially a burial dissolution phase and its effects on micrite microtexture in reservoirs in the middle east three microporous reservoirs were selected the cenomanian mishrif formation in iraq the kimmeridgian tithonian arab d and the barremian kharaib formations both in abu dhabi staining cathodoluminescence and scanning electron microscopy were used for petrographic observations in the selected reservoirs three typical micrite microtextures are distinguished micro rhombic compact anhedral and rounded the chronology of diagenesis based on petrographic observations shows that mineralogical stabilisation through high magnesian calcite replacement by low magnesian calcite led to the formation of micro rhombic micrites non reservoir facies such as seals present compact anhedral micrites correlation between these micrites and high aluminium content suggests alumino silicate minerals clays as a catalyst for pressure dissolution and thus for reservoir degradation through cementation micrites from the upper part of oil reservoirs are dissolved during burial diagenesis dissolution is responsible for rounded shape and for reduction of crystal size it also implies pore diameter and pore throat size enlargement comparison of porosity and permeability with scanning electron microscope data confirms that rounded micrites are better reservoirs than micro rhombic micrites

petrological and geochemical constraints on diagenesis and deep burial dissolution of the ordovician carbonate reservoirs in the tazhong area tarim basin nw china abstract deeply buried 4500 7000 m ordovician carbonate reservoirs in the tazhong area tarim basin nw china show obvious heterogeneity with porosity from null in limestones and sweet dolostones to 27 8 in sour dolostones from which economically important oils sour gas and condensates are currently being produced petrographic features c o sr isotopes were determined and fluid inclusions were analyzed on diagenetic calcite dolomite and barite from ordovician reservoirs to understand controls on the porosity distribution ordovician carbonate reservoirs in the tazhong area are controlled mainly by initial sedimentary environments and eo genetic and near surface diagenetic processes however vugs and pores generated from eogenetic and telogenetic meteoric dissolution were observed to have partially been destroyed due to subsequent compaction filling and cementation in some locations or wells especially zg5 zg7 oilfield nearby zg5 fault burial diagenesis e g thermochemical sulfate reduction tsr probably played an important role in quality improvement towards high quality reservoirs c2 calcite and dolomite cements and barite have fluid inclusions homogenization temperatures ths from 86 to 113 c from 96 to 128 c and from 128 to 151 c respectively we observed petrographically corroded edges of these high temperature minerals with oil inclusions indicating the dissolution must have occurred under deep burial conditions the occurrence of tsr within ordovician carbonate reservoirs is supported by c3 calcite replacement of barite and the association of sulfur species including pyrite anhydrite or barite and elemental sulfur with hydrocarbon and 12c rich as low as 7 2 v pdb c3 calcite with elevated ths 135 153 c the tsr may have induced burial dissolution of dolomite and thus probably improved porosity of the sour dolostones reservoirs at least in some locations in contrast no significant burial dissolution occurred in limestone reservoirs and non tsr dolostone reservoirs the deeply buried sour dolostone reservoirs may therefore be potential exploration targets in tarim basin or elsewhere in the world

late leaching under deep burial conditions a case study from the miocene zhujiang carbonate reservoir south china sea abstract the zhujiang carbonates are the hydrocarbon reservoir rock of the liuhua 11 1 field they comprise three porous zones of around 20 m intercalated with tight zones parallel to bedding thin section cathodoluminescence and stable isotope data were combined with well log information in order to investigate the origin and spatial distribution of the abundant porosity the vast majority of pores in the porous zones was generated or overprinted in deep burial settings by late leaching two tight zones however lack this porosity because of meteoric cementation and increased compaction and related calcite precipitation prior to late leaching we propose that migrating corrosive fluids were diverted along bedding parallel flow barriers of the tight zones and leached the intervals in between the porous zones thereby emphasizing the depositional pattern

diagenetic history and porosity evolution of an early miocene carbonate buildup upper burman limestone yadana gas field offshore myanmar abstract the miocene yadana carbonate platform is a major offshore gas reservoir located in the andaman sea offshore myanmar the integration of petrographic analysis of carbonate rocks stable isotope measurements and seismic interpretation provided new insights into the diagenetic history of the upper burman limestone reservoir the high 28 average porosity of the reservoir is dominantly related to strong development of microporosity moldic and vuggy porosity mud supported coral rich sediments record significant early marine dissolution of aragonite and early lithification of matrix that has led to the development of dense layers stable isotope and petrographic data from the top of the reservoir suggest that the long term depositional hiatus middle to late miocene was related to platform drowning and non deposition in a deep marine setting moderate dolomitization has affected the top and flanks of the platform during the depositional hiatus microporosity development and sparry calcite cementation mainly occurred in marine to marine shallow burial environments a major decrease in porosity averaging 10 below the gas water contact suggests a major phase of porosity evolution during and or after the hydrocarbon emplacement which is mainly related to 1 increased moldic and vuggy porosity development within the gas zone 2 microporosity preservation in loosely packed subrounded micrites within the gas zone and 3 microporosity reduction by calcite precipitation within the water zone leading to the development of densely packed euhedral micrites

equatorial carbonates an earth systems approach the hypothesis here is that an earth systems processes to products approach can be used to better develop predictive models for the recognition and assessment of under evaluated equatorial carbonate systems warm temperatures together with common clastic fresh water and nutrient influx as well as basinal settings in the equatorial tropics all have a major impact on carbonate deposition and diagenesis specific features of equatorial carbonate systems resulting from the combination of processes acting in the region include common occurrence of photoautotrophs and heterotrophs aragonitic and or calcitic dominant mineralogies lack of coated grains or aggregates common associations with clastics lack of associations with evaporites and diversity of platform types including oligophotic ones additional diagenetic characteristics include common micritization and bioerosion paucity of marine cements extensive vadose dissolution and concomitant phreatic cementation there is also significant replacement of aragonite by calcite in regions of meteoric groundwater flow common burial compaction and leaching as well as localized massive dolomitization via sea water or continental derived groundwater flow although equatorial carbonates fall into the warm water photozoan association many of the features described above are at odds with models derived from their warm water arid zone counterparts instead a range of the equatorial carbonate features show some similarities with those formed in cool waters and there have been difficulties separating carbonates from these two very different climatic regimes recommendations for the recognition of phanerozoic regional equatorial carbonate development are i a diversity of calcitic and or aragonitic photoautotrophs plus ii common elements of the heterozoan association plus iii independent for example isotopic evidence for warm temperatures 22 c additional indicators towards a humid equatorial setting are iv situation in appropriate palaeolatitudes v lack of association with sedimentary evaporites coated grain or aggregates and vi geochemical evidence for reduced marine salinity and or nutrient upwelling the aim is that this work will lead to greater awareness and understanding of equatorial carbonate systems and contribute to the development of globally predictive models to better understand past and likely future environmental change

the fate of mantle derived carbon in a continental sedimentary basin integration of c he relationships and stable isotope signatures abstract the isotopic composition and abundances of the rare gases he ne and ar and active gases co2 ch4 have been determined in a series of commercial gas reservoirs in the pannonian and vienna basins of hungary and austria respectively in these zones of continental extension significant components of mantle derived 4he up to 39 8 and 21ne up to 58 are identified the results of this study indicate a major component of mantle derived carbon in these systems as well ranging in composition from close to 100 co2 to ch4 dominated reservoirs with trace concentrations ppm of co2 these gas reservoirs provide a unique opportunity to examine the relationship of the conservative rare gases to the active gas components and to examine the sources and sinks for mantle and crustal derived carbon phases with the exception of kismarja gas field which shows evidence of addition of 3he depleted crustal co2 all gas fields exhibit a trend whereby the most co2 rich samples approach co 2 3 he mntl values similar to morb 2 109 to 7 109 while co2 depleted samples extend to co 2 3 he mntl values as low as 105 the lack of any correlation of co 2 3 he mntl ratios with r ra values indicates the observed trends are not a function of mixing between crustal and mantle derived endmembers but instead reflect progressive loss of the mantle derived co2 carrier phase during volatile transport and emplacement in the continental crust based on stable isotopic signatures from the kismarja field a crustal co2 endmember with an isotopic composition of 6 8 and a mantle co2 endmember with an isotopic composition no more depleted in 13c than 5 0 can be identified one of the few instances where the isotopic signatures of mantle and crustal derived co2 can be reliably distinguished in a continental setting covariation in 13cco2 and 13cch4 values in the gas fields is used to place constraints on two alternative models whereby the trends in percent co2 and co 2 3 he mntl ratios can be accounted for by 1 loss of the mantle derived co2 carrier phase 2 addition of crustal derived co2 3 addition of crustal derived thermogenic ch4 and potentially 4 conversion of the co2mntl carrier phase to mantle derived ch4 based on an estimated total mantle 4he flux for the pannonian basin of 4 2 108 atoms m 2 s 1 mantle carbon flux estimates for this basin alone range from 3 108 g c yr to 1 109 g c yr which over the lifetime of the basin is only 4 5 orders of magnitude less than flux estimates based on the total area of the spreading ridges clearly the addition of mantle derived carbon to the crust in areas of continental extension is significant and may have been previously underestimated we demonstrate here that integration of 13c data with rare gas isotopic tracers provides an important tool in constraining models of mantle carbon sources and sinks in continental settings

biodegradation of oil in uplifted basins prevented by deep burial sterilization biodegradation of crude oil by bacterial activity which has occurred in the majority of the earth s oil reserves is known to reduce greatly the quality of petroleum in reservoirs for economically successful prospecting for oil it is therefore important to understand the processes and conditions in geological formations that lead to oil biodegradation although recent studies speculate that bacterial activity can potentially occur up to temperatures as high as 150 degrees c refs 3 4 it is generally accepted that effective petroleum biodegradation over geological timescales generally occurs in reservoirs with temperatures below 80 degrees c ref 2 this appears however to be at odds with the observation that non degraded oils can still be found in reservoirs below this temperature here we compile data regarding the extent of oil biodegradation in several oil reservoirs and find that the extensive occurrence of non biodegraded oil in shallow cool basins is restricted to those that have been uplifted from deeper hotter regions of the earth we suggest that these petroleum reservoirs were sterilized by heating to a temperature around 80 90 degrees c during deep burial inactivating hydrocarbon degrading organisms that occur in the deep biosphere even when such reservoirs are subsequently uplifted to much cooler regions and filled with oil degradation does not occur implying that the sterilized sediments are not recolonized by hydrocarbon degrading bacteria

prevention of carbonate cementation in petroleum reservoirs by the time a carbonate unit has been buried to the depths of most petroleum reservoirs the significant question is often not how did the pores originate but rather why are they still there preservation of porosity regardless of its origin is a consequence of one or more of the following mechanisms 1 minimal burial 2 reduced burial stress generally due to overpressured pore fluids 3 increased framework rigidity which prevents compaction 4 exclusion of pore waters by petroleum entry 5 stable mineralogy 6 permeability barriers isolating porous intervals from diagenetic fluids and 7 pore resurrection a consequence of the temporary filling of pores with cement that is subsequently removed examples from the stratigraphic record demonstrate that each of these pore preserving mechanisms may control reservoir quality

reservoir quality variation on an eocene carbonate ramp el garia formation offshore tunisia structural control of burial corrosion and dolomitisation abstract study of four closely spaced el garia formation reservoirs offshore tunisia has identified diagenetic controls on variation in reservoir quality the ashtart didon and zarat fields have been affected by significant dissolution porosity which occurred during burial diagenesis with vuggy pores preferentially forming in micritic sediment however inter crystalline porosity due to dolomitisation is the principal control on reservoir quality in the hasdrubal field this spatial variation in porosity type reflects to some extent the structural and sedimentological isolation of hasdrubal within the facies trend surrounded by deep water embayments and unlike the other studied fields not directly underlain by salt diapirs burial corrosion appears to have formed where mid to outer ramp mud rich el garia formation facies directly overlie salt domes a consequence of the circulation of aggressive fluids related to increased heat flow above the salt inorganic co2 focused into the reservoir prior to hydrocarbon emplacement by faults related to salt movement is the most likely candidate responsible for the burial dissolution porosity observed in the ashtart didon and zarat fields a salt diapir does underlie the most basinward north east margin of hasdrubal and minor dissolution porosity is present but we conclude that the absence of significant dissolution in hasdrubal is due to its offset position away from the highly faulted roof zone immediately above the salt although we favour this model we cannot rule out that the absence of dissolution porosity in hasdrubal may reflect the early emplacement of hydrocarbons from an older non bou dabbous formation source or differences in the timing of salt movement and consequent fault creation below hasdrubal compared to the other fields local temperature and or acid concentration effects or a different source of co2 differences in the distribution of dissolution porosity in the ashtart didon and zarat fields may be related to the presence of intra reservoir permeability barriers due to increased nummulithoclastic fragmented nummulites content this may also explain the strata bound nature of the dolomite in hasdrubal which like the dissolution porosity in ashtart is overlain by sediments with an increased nummulithoclastic content although dolomite is present in variable quantities in all of the studied fields the significant volume of dolomite in hasdrubal may reflect a local plumbing effect with dolomitising fluids entering the reservoir via a fault network that does not affect the other fields

coupled alteration of hydrothermal fluids and thermal sulfate reduction tsr in ancient dolomite reservoirs an example from sinian dengying formation in sichuan basin southern china abstract to better understand the fluid activities and their roles in the formation of petroleum reservoirs dolomite was thoroughly studied in the ancient sinian dengying formation in sichuan basin southern china the high homogenization temperatures above 190 c of fluid inclusions positive eu anomaly and o and sr isotopic values of coarse crystalline pore filling dolomite fd in dissolution pore spaces have suggested that the fds are of hydrothermal origin there are also other minerals pyrite organic compounds pyrobitumen and gas components h 2 s co 2 and ch 4 in pore spaces the sulfur isotope compositions of pyrite and h 2 s along with the carbon isotopes of co 2 have shown the involvement of thermochemical sulfate reduction tsr hydrothermal activities during burial diagenesis of the dengying formation provided high temperature environment necessary for tsr to proceed the presence of co 2 and h 2 s in both processes enhanced fluid acidities and consequently porosity and permeability of the dengying formation emplacement of petroleum and acidic fluids helped to maintain pre existing reservoir spaces during deep burial stage overall the coupled hydrothermal activities and tsr provided favorable environments to allow for maintaining high quality reservoirs

possible effects of thermal degradation of organic matter on carbonate paragenesis and fluorite precipitation in mississippi valley type deposits heating of organic matter to progressively higher temperatures causes a sequence of chemical changes in which carbonate minerals are first dissolved then precipitated and then again dissolved the initial carbonate dissolution is related to the production of organic acids as heating continues organic acids begin to decompose to carbon dioxide and methane this addition of carbon dioxide to a solution with an organic acid ph buffer causes carbonates to precipitate eventually a temperature is reached at which any remaining organic acids quickly decompose addition of carbon dioxide to a solution in which carbonate species rather than organic acids now control the ph lowers the ph and causes renewed carbonate dissolution the similarity of this sequence to the carbonate paragenesis of mississippi valley type deposits suggests that the carbonate paragenesis in these deposits may be related to the thermal alteration of organic matter by competing with fluoride for magnesium bicarbonate ions from decarboxylation off organic matter and dissolution of carbonates may break the mgf complex form the mghco3 complex and thereby liberate fluoride to trigger the precipitation of fluorite

deep burial dissolution in an oligocene miocene giant carbonate reservoir perla limestone gulf of venezuela basin implications on microporosity development abstract sedimentary rocks tend to progressively lose porosity with depth due to mechanical and chemical compaction in carbonates this trend is hard to predict since many factors can create porosity in the burial setting the oligocene miocene perla limestone a giant gas reservoir located in the gulf of venezuela basin shows a complex porosity system marked by a strong diagenetic control despite exhaustive depositional facies modelling carried out in the carbonate reservoir inconsistencies remained when distributing petrophysical properties with depositional facies these inconsistencies become more important in areas strongly affected by intense diagenetic processes after the initial investigation burial dissolution was identified as the main diagenetic process affecting porosity in this succession to understand the origin of the burial dissolution process and its distribution along the reservoir detailed petrographic mineralogical and isotopic studies were carried out on recovered cores from different exploration wells results show a series of diagenetic features which were associated with the evolution of the strata including burial dissolution dolomitization and compaction as the main processes after testing several hypotheses the results support an inorganic co2 model as responsible for the burial dissolution process in the perla limestone according to this model deep burial dissolution created pervasive microporosity with subordinated moldic and vuggy pores that enhanced the reservoir quality in localized areas of the carbonate succession these features are located where the ascent of hydrothermal co2 rich fluids funneled by discontinuity surfaces faults fractures stylolites etc reached the perla limestone

crustal structure of the west bengal basin india from deep seismic sounding investigations summary basement and sub basement crustal configuration of parts of the west bengal basin has been delineated from inverse and forward modelling of refraction and wide angle reflection data obtained along two east west profiles a beliatorburdwan bangaon profile 1 about 180 km long and b gopali tamluk port canning profile 2 about 140 km long the study has revealed the existence of four groups of prominent refractors velocities 1 7 2 1 km s 2 7 3 2 km s 3 74 2 km s and 4 6 5 3 km s overlying the crystalline basement with a velocity of 5 8 6 2 km s rajmahal traps velocity 4 6 5 3 km s averaging 4 8 km s l occur beneath the base of the tertiary sediments in the middle parts of the profiles the presence of sub trappean gondwanas is also indicated from the wide angle reflection modelling supported by deep well lithological information in general depth to the basement is found to increase from west to east with a maximum depth of around 10 to 11 km at the extreme eastern parts of the profiles a basement upwarp with a sharp flexure west of bishnupur on profile 2 in combination with the fault west of shantipur on profile 1 possibly brings into focus the regionally extending nne ssw hinge zone a low velocity layer lvl of about 2 to 3 km thickness velocity 5 6 5 8 km s has been inferred in the upper crust in this area in general the thickness of the upper including the sedimentary column and the middle 6 4 6 6 km s velocity parts of the crust is found to be larger thickness varying between 22 and 27 km than the lower crustal column interval velocity 6 8 7 0 km s l with a thickness of 5 to 10 km the moho lies at varying depths of 36 to 26 km along profile 1 with a prominent domal feature of about 40 km width between satgachia and west of shantipur in the middle part of profile 2 the moho occurs at depths of 32 to 34 km with a mild flexure east of tamluk configuration of the moho indicates that the crust is more akin to continental in nature in the western and middle parts of the basin whereas significantly thinned continental crust is indicated in the eastern and southeastern parts

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

sediment production and dispersal on foraminifera dominated early tertiary ramps the eocene el garia formation tunisia larger benthic foraminifera nummulites are common within eocene circumtethyan limestones despite their importance as sediment producers contradictions in the literature constrain current understanding about the location of the nummulites factory the el garia fm was deposited on a ramp with localized palaeohighs and whilst some authors suggested that the locus of nummulites production was in shallow water across the palaeohighs others concluded that production was significantly reduced over these palaeohighs and concentrated in the surrounding deeper 30 60 m water there are also marked dissimilarities between recent models in terms of the continuity correlation and resolution of depositional sequences to assess these models we integrate studies of the architecture and geometry of the el garia fm with detailed taphonomic biometric biofabric and palaeoecological characterization of nummulites tests we conclude that the highest rates of sediment production occurred in euphotic water over the palaeohighs and in nearshore environments nummulites on the palaeohighs were transported into the surrounding deeper water by oceanic and storm currents that swept the platform top producing a nummulitic sediment package that thickened and became increasingly fine grained and fragmented into outer ramp environments this transport exerted a major control on development of the ramp like geometries often seen at outcrop our findings question the validity of a recent sequence stratigraphic model that identifies decimetre scale milankovitch cycles even in largely allochthonous bio retextured mid outer ramp sediments our findings also suggest that the thin packages of el garia fm on the palaeohighs which have previously been interpreted as condensed sections that can be correlated with thicker more distal accumulations actually represent remnants of the sediment that was produced on the highs and exported into the basin

burial diagenesis and porosity evolution upper jurassic smackover arkansas and louisiana the jurassic upper smackover ooid lime grainstone of the central gulf coast has been a major hydrocarbon exploration target for nearly half a century study of the geologic and sedimentologic framework petrography two phase fluid inclusions and trace element and stable isotope chemistry indicates the following diagenetic history the upper smackover is a blanket ooid grainstone ranging in thickness from 300 to 400 ft 91 to 122 m and covering at least 4 000 sq mi 10 400 sq km in the northern part of the arkansas shelf a regional meteoric water system was established in the grainstone shortly after deposition and prior to significant burial this meteoric water system gave way to conditions which were increasingly more marine toward the south near the louisiana arkansas state line the fresh to marine hydrologic gradient is responsible for the strong precompaction regional diagenetic overprint that eventually controlled available diagenetic pathways for the unit during burial three diagenetic zones are present 1 a northern zone dominated by oomoldic porosity and preburial spar cementation 2 a sou hern zone characterized by compaction effects late subsurface cement primary porosity preservation and late secondary subsurface porosity generation and 3 a transitional zone having characteristics common to both the northern and southern zones during the shallow burial phase compaction became increasingly more important to the south in zones dominated by marine connate fluid whereas rock integrity and a secondary oomoldic porosity system had already been established in the northern zone as burial increased brine originating in the underlying louann salt reached the shelf grainstone and began to displace the original connate fluid the brine triggered the inversion to calcite of remaining grains and cement composed of unstable mineralogy when the louann brine had displaced the original connate fluid caso4 replacement as gypsum and anhydrite was common across all three diagenetic zones also precompaction fine cement associated with oomoldic porosity was recrystallized and reequilibrated with the concentrate brine localized replacement dolomitization may well have developed throughout the burial history of the sequence but was most commonly postcompactional postcompaction poikilitic calcite cement and baroque dolomite were precipitated from the louann brine under conditions of elevated temperature and pressure as subsidence continued organic material from the basinward equivalents of the upper smackover lime grainstone arrived on the shelf and catalyzed so4 reduction resulting in pyrite and galena replacement in the southern and transitional zones as hydrocarbon maturation continued in the fine grained equivalents of the upper smackover h2s and co2 by products of these processes began to increase in the brine of the southern and transitional zones ultimately causing regional dissolution of existing carbonate and significantly enhancing preserved porosity porosity in the northern zone was established early and preserved through moderate burial although porosity is well developed permeability has been largely destroyed by recrystallization dolomitization is a prerequisite for economic reservoir development in the northern zone porosity in the southern zone is solution enhanced preserved primary intergranular porosity compaction is the most important porosity destruction process compaction in the absence of an early diagenetic overprint is affected primarily by grain type and secondarily by size and sorting pellets and oncolites compact easily yet destroy porosity during burial ooids and rhodolites resist compaction during burial therefore are related to porosity preservation grain type distribution as well as size and sorting are controlled by depositional environment processes with the most favorable areas for porosity development concentrations of ooids and rhodolites associated with early salt related structural movement porosity distribution in the southern zone therefore can be outli ed with the aid of geophysical end page 597 techniques porosity in the transitional zone is early secondary or preserved primary intergranular or a combination porosity in this zone therefore is highly unpredictable and a strong potential exists for diagenetic traps unrelated to structure or original depositional environment documentation of significant deep subsurface secondary porosity possibly associated with hydrocarbon maturation opens the potential for hydrocarbon exploration in deeply buried carbonate dominated basins this would include carbonate rocks heretofore thought to be well below the depth of carbonate porosity maintenance because of lithostatic load

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

a triassic homoclinal ramp from the western tethyan realm western balkanides bulgaria integrated insight with special emphasis on the anisian outer to inner ramp facies transition abstract the triassic carbonate succession exposed in the western balkanides is analyzed in terms of depositional evolution sequence stratigraphy sedimentation rate environmental constraints and biota distribution deposition took place on a storm dominated ramp which existed at the nw end of the tethys ocean from late olenekian to early carnian time subtidal intertidal and supratidal sediments were accumulated in three zones of the ramp profile inner ramp mid ramp and outer ramp the calculated average sedimentation rate is typical for other ancient marine shallow water carbonates including those deposited on ramps newly accomplished sedimentological study on part of the anisian interval reveals a homoclinal ramp model with episodic initial distal steepening two main phases of the ramp evolution are distinguished on the basis of changing relative paleobathymetry transgressive e g retrogradational depositional trend and regressive e g progradational depositional trend the maximum flooding zone is marked by the terebratula beds having late pelsonian age the inferred environmental constraints of the ramp system comprise normal marine to hypersalinity warm well illuminated water aerobic and rarely dysaerobic bottom conditions dominantly soft lime mud substrate intensive storm activity temporal seismic influence presumably related to early triassic rifting the vertical distribution of marine fauna indicates shifting from germanic peri tethyan affinity to alpine tethyan affinity in anisian time the sedimentation was not characterized by formation of large buildups because of the paucity or lack of metazoan and other reef builders the studied rocks are well correlated with the triassic ramp succession outcropping in the tizsa meagunit of southern hungary similarities to the german polish and spanish muschelkalk are also clearly outlined and some parallel can be drawn with other platform carbonates from the nw tethys shelf area and peri tethys basins

thermal impact of salt simulation of thermal anomalies in the gulf of mexico salt and related structures have played important roles in controlling hydrocarbon accumulations in the gulf of mexico using a two dimensional fluid flow compaction model which allows for both conduction and convection of heat an examination is given of the effects on thermal patterns of the combined influence of multiple salt features including diapirs pillows sheets and wedges the focusing and defocusing of heat due to the higher thermal conductivity of salt are accounted for in the modeling the results show that there could be as much as a 30 c anomaly above multi salt bodies due to the focusing of heat by salt and as much as 50 c temperature contrast between internal salt positions and sediments external to the salt in the deep part of a section the magnitude of the thermal anomaly depends on the size or width of the salt and on the depth of the rooted salt the modeled results provide estimates of the influence of salt in expanding the oil generation window by approximately half of the salt thickness

influence of a major exposure surface on the development of microporous micritic limestones example of the upper mishrif formation cenomanian of the middle east microporous platform top limestones of the cenomanian mishrif formation offshore qatar were studied in order to investigate the diagenetic processes associated with the top mishrif subaerial unconformity and its influence on the development of microporosity in underlying carbonates petrographical and stable isotope results indicate that complex diagenetic changes occurred during subaerial exposure of the mishrif formation including pervasive dissolution and meteoric cementation as well as neomorphism of the micritic matrix micrites at the top of the mishrif formation are coarse i e 2 m sub rounded and very dull luminescent under cathodoluminescence in this uppermost part of the studied interval the limestone matrix first underwent dissolution of unstable grains in the vadose zone with subsequent precipitation of low magnesium calcite lmc overgrowths within an oxidising phreatic setting this process explains the poor luminescence of the micrite crystals and their relatively coarse crystallometry which results in the present day in relatively good reservoir properties 13c ratios within the microporous limestones are negative up to 4 v pdb due to the incorporation of isotopically light carbon derived from palaeosols which developed during exposure by contrast fine i e 2 m polyhedral and bright luminescent micritic matrix with higher 13c values up to 4 v pdb and lower permeability are dominant within the underlying carbonate interval these fine micrites are likely to be the product of early neomorphic recrystallisation in the lowermost part of the palaeo aquifer associated with the top mishrif unconformity and were precipitated from meteoric or mixed dysoxic waters which were slightly supersaturated with respect to calcite

carbonate porosity creation by mesogenetic dissolution reality or illusion discussion in their recent discussion of the recognition of late burial dissolution of carbonates ehrenberg et al 2012 use our article beavington penney et al 2008 as an example of a study that mistakes late burial mesogenetic porosity for pores that formed much earlier in the diagenetic sequence commenting on our study of reservoir quality variation in eocene nummulitic limestones ehrenberg et al 2012 p 229 note that

dolomitization and chertification of the early eocene rus formation in abu dhabi united arab emirates abstract the early eocene carbonates of the rus formation in the jabal hafit area near al ain united arab emirates were sampled to study their diagenetic history these carbonates were cyclically deposited in a shallow water environment alternating with marls or marly limestones and are characterized by extensive dolomitization and chertification with associated glauconitization the cyclic nature of sedimentation suggests that deposition was eustatically driven the diagenetic history of the limestones proceeded through the following path 1 marine cementation of allochems concomitant with formation of micrite envelopes 2 micritization of peloids ooids and skeletal grains 3 neomorphism of formerly high mg calcite and aragonite grains to low mg calcite 4 partial to complete dolomitization 5 partial chertification of both calcite and dolomite 6 glauconitization of chert 7 fracturing during shallow burial and 8 precipitation of late coarse calcite spar or mosaic and saddle baroque dolomite spar filling voids and fractures porosity is virtually nil in these rocks except for infrequent unfilled fractures vugs and oomolds due primarily to cementation and chertification biogenic grains are abundant in the limestones with nummulites sp alveolinid and miliolid foraminifera and crinoid ossicles and other echinoderm fragments being most common the tests of nummulites sp are commonly replaced by length fast chalcedony fibers which are occasionally later replaced by glauconite spherulitic and zebraic chalcedony and more rarely megaquartz occur as void filling cements and fringing chalcedonic fibers frequently rim the tests of skeletal grains all except the fringing chalcedonic fibers were subject to glauconitization prior to the chertification process is an extensive dolomitization period rhombic and sucrosic dolomite partially to completely replaced calcite at a relatively early diagenetic stage silicification caused the replacement of calcite by chert at a much more rapid rate than that of dolomite which was found floating within a chertified matrix fractures occurred both early and late in the rus and are usually filled by a late coarse mosaic calcite spar or coarse mosaic and saddle dolomite spar in completely dolomitized samples

middle eocene nummulites and their offshore re deposition a case study from the middle eocene of the venetian area northeastern italy abstract the middle eocene calcari nummulitici formation from northeastern italy venetian area represents a shallow marine carbonate ramp developed on the northern tethyan margin in the monti berici area its main components are larger foraminifera and coralline red algal communities that constitute thick carbonate sedimentary successions middle ramp and proximal outer ramp environments are recognized using component relationships biofacies and sedimentary features the middle ramp is characterized by larger flattened lenticular nummulites on palaeohighs between which rhodoliths formed larger nummulites palaeohighs containing nummulites millecaput nummulites crassus nummulites discorbinus and nummulites cf gizehensis developed more basin wards the following relatively quiet environments of basin wards of the palaeohighs represent areas of maximum carbonate production the transition between the distal middle and the proximal outer ramp settings is marked in the study area by a large erosional surface which is interpreted to have been formed as a result of an erosive channel body filled in by deposits re sedimented from shallower depths these off shore re sedimented channelized deposits ascribed to the shallow benthic zone sbz 15 lying on hemipelagic marls planktonic foraminiferal zone e9 p11 allow for a biostratigraphic correlation to the late lutetian the studied deposits represented by packstone to rudstones were displaced whilst still unlithified the lutetian bartonian regression along with the local tectonic activity promoted the production of a high amount of biogenic shallow water carbonates mainly produced in the mossano middle ramp settings these prograded towards the basinal areas with high sedimentation rate of carbonate deposits characterized by the larger nummulites rudstones such high amounts of sediment led to sediment instability which potentially could be mobilized either by return currents due to occasional major storms or by earthquakes induced by tectonic activity these will have led to the offshore re deposition of the nummulites sediments into deeper water setting via the observed channels since potential migration pathways are short such distal re sedimented channel filled material surrounded by hemipelagic marls is optimally placed for the formation of potential subsurface oil reservoirs

petroleum geology of east central tunisia the tectonic features of tunisia are complex and include folds all types of faults evaporite diapirs and the saharan flexure which separates a stable paleozoic province on the south from a subsidence zone of mesozoic and cenozoic strata of the offshore ashtart and tripolitania basins the remainder of the offshore region is mostly stable shelf of the pelagian craton which also extends onshore the present study focused on this shelf where two carbonate intervals contain proven hydrocarbon reservoirs the metlaoui formation of earliest eocene ypresian age and the zebbag formation of late cretaceous turonian age regionally well defined belts of metlaoui carbonates trend northwest on the northeast are open marine deep water micrites and marls with abundant planktonic foraminifers thick bars of nummulitid packstone grainstone were deposited in shallow water at an angle to the paleoshelf the reservoir is confined largely to the bars and visible effective porosity is best developed in those areas among the foraminifers filled with sand size nummulitid debris where secondary solution enlargement has occurred this lithology tested oil in two recent wildcat wells and is a commercial reservoir at sidi el itayem and ashtart fields distribution of zebbag carbonates is more complex a northwest trending platform was bounded on three sides by deep water where shale and micrite with planktonic foraminifers were deposited predominately back reef and lagoonal foraminifer rudist wackestones and packstones occur in narrow belts apparently controlled at least locally by block faulting they tested oil in two recent discoveries the most significant porosity is interparticle generally enlarged by solution in foraminifer packstones but intraparticle voids in foraminifers and rudists commonly contribute to the porosity analyses of surface and subsurface samples identified the bahloul basal turonian and bou dabbous ypresian formations as source rocks fluorescence spectra of several oils were compared with extracts from these samples and indicate the bahloul to be the source of oils in recent metlaoui and zebbag discoveries

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

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 carrizo sandstone at butler salt dome east texas basin u s a evidence for fluid sediment interaction near halokinetic structures abstract the eocene carrizo sandstone adjacent to the butler salt dome in the east texas basin was cemented locally as the result of processes similar to those that have created salt dome calcite cap rocks throughout the gulf coast the geometry of this anomalous cemented zone was established using carrizo exposures in a large quarry and cores in the contiguous area the cemented zone is delimited by a steeply dipping normal fault that is radial to the dome the cements extend southeast of the fault for about 750 m the carrizo is a fine to medium grained quartzarenite to sublitharenite with 42 to 50 cement principally poikilotopic calcite with local pyrite pyrite concretions are found on both northwest and southeast sides of the fault as well as along the fault plane whereas the calcite cement is present only to the southeast 13c data for calcite 19 to 37 suggest that bicarbonate was supplied mainly by bacterial oxidation of methane whereas 18o data for calcite 6 to 9 indicate that meteoric water was the principal diagenetic fluid 87sr 86sr values for calcite 0 7073 are within the range for middle jurassic seawater suggesting that dissolution of anhydrite from the louann salt provided the ca pyrite is more 34s rich 34s range of 10 to 16 than typical sulfides in gulf coast salt dome cap rocks and are interpreted to represent h2s supply from deep sour gas reservoirs petrographic and isotope evidence suggests that early h2s supply from deep sour gas reservoirs was channeled upward along the flank of the dome and outward along the radial fault resulting in early pyrite precipitation along the fault zone and in adjacent sediments early pyrite cementation along the fault may have formed a barrier to subsequent fluid circulation blocking the northwestward movement of both ca rich waters from the dome and deep natural gas moving along the fault thus confining calcite precipitation to the southeast bacterial oxidation of this thermogenic gas resulted in calcite precipitation although mixing with shallow ground water bicarbonate may account for the range in 13c values in the calcite cement we propose that this type of cementation occurs in similar settings associated with hydrocarbon leakage to shallow levels including adjacent to many salt domes in the gulf of mexico basin and may represent a previously unrecognized self sealing type of hydrocarbon trap

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