relation between depositional environment and the elemental composition of early diagenetic siderite early diagenetic siderites from marine and fresh water depositional environments are characterized by distinctive compositional trends siderite from fresh water environments is often relatively pure i e greater than 90 mol feco sub 3 and commonly attains end member composition siderite from marine environments however is always extremely impure and has extensive substitution of mg up to 41 mol and to a lesser extent ca up to 15 mol for fe in the siderite lattice in addition marine siderite generally contains less mn and has a higher mg ca ratio than fresh water siderite this compositional variation appears to result from differences in the chemistry of early marine and meteoric pore waters inasmuch as early marine pore waters generally have a higher mg sup 2 ca sup 2 ratio and contain less mn sup 2 and fe sup 2 and more ca sup 2 and mg sup 2 than meteoric waters

reservoir characteristics and their effects on hydrocarbon accumulation in lacustrine turbidites in the jiyang super depression bohai bay basin china abstract a great difference exists between the hydrocarbon charging characteristics of different tertiary lacustrine turbidites in the jiyang super depression of the bohai bay basin east china based on wireline log data core observation and thin section analyses this study presents detailed reservoir property data and their controlling effects from several case studies and discusses the geological factors that govern the hydrocarbon accumulation in turbidite reservoirs the lacustrine fluxoturbidite bodies investigated are typically distributed in an area of 0 5 10 km2 with a thickness of 5 20 m the sandstones of the tertiary turbidites in the jiyang super depression have been strongly altered diagenetically by mechanical compaction cementation and mineral dissolution the effect of compaction caused the porosity to decrease drastically with the burial depths especially during the early diagenesis when the porosity was reduced by over 15 the effect of cementation and mineral dissolution during the late stage diagenesis is dominated by carbonate cementation in sandstones high carbonate cement content is usually associated with low porosity and permeability carbonate dissolution secondary porosity zone and primary calcite dissolution is believed to be related to thermal maturation of organic matter and clay mineral reactions in the surrounding shales and mudstone two stages of carbonate cementation were identified the precipitation from pore water during sedimentation and secondary precipitation in sandstones from the organic acid dissolved carbonate minerals from source rocks petrophysical properties have controlled hydrocarbon accumulation in turbidite sandstones high porosity and permeability sandstones have high oil saturation and are excellent producing reservoirs it is also noticed that interstitial matter content affects the oil bearing property to some degree there are three essential elements for high oil bearing turbidite reservoirs excellent pore types low carbonate cement 15 and average permeability 10 md

evolution characteristics of transtensional faults and their impacts on hydrocarbon migration and accumulation a case study from the huimin depression bohai bay basin eastern china abstract the study of transtensional faults is significantly important for analysing migration and accumulation of hydrocarbons in petroliferous basins the purpose of this study is to present disparities in the temporal and spatial evolution of two boundary transtensional faults northern linshang fault and southern xiakou fault in the huimin depression by 2d structural restoration and quantitative calculations of fault activity this paper also reveals the controls of transtensional faults on the migration and accumulation of hydrocarbons by considering the stress normal to the fault plane p the shale gouge ratio sgr of the fault zone and models of migration pathways the results show that the transtensional systems within the huimin depression were generated from the reactivation of basement faults under an oblique extensional stress field the linshang fault linked two initially separated segments and then behaved as a single fault since the ek period whereas the xiakou fault retained significant three segment growth properties during the paleogene during the hydrocarbon charging period the weak activity and strong vertical sealing restricted the occurrence of vertical migration around the xiakou fault while the relatively strong movement and weak vertical sealing resulted in multilayer hydrocarbon accumulations around the linshang fault meanwhile the vertically closed fault with high sgr 0 6 acted as barriers and trapped hydrocarbons on the lateral migration pathways whereas the vertically closed fault with low sgr

role of organic and inorganic reactions in development of secondary porosity in sandstones abstract the development of secondary porosity in many sandstones is the result of aluminosilicate and or carbonate dissolution the dissolution of aluminosilicates and creation of secondary porosity is a problem of aluminum mobility our experimental data demonstrate that it is possible to increase significantly the mobility of aluminum and to transport it as an organic complex in aliphatic acid solutions the same aliphatic acid solutions have the capability of destroying carbonate cements and debris carothers and kaharka have shown that concentrations of aliphatic acid anions range up to 5 000 ppm over the temperature range 80 to 200 c in some oil field formation waters our experiments show that acetic acid solutions at the same concentrations and over the same temperature range can increase the solubility of aluminum by a half an order of magnitude whereas oxalic acid solutions increase the solubility of aluminum by an order of magnitude the textural relations observed in the experiments are identical to those observed in sandstones containing secondary porosity as a result of aluminosilicate dissolution a natural consequence of the burial of sedimentary prisms is the maturation of organic material these maturation reactions result in the evolution of significant amounts of organic acids and carbon dioxide the experiments suggest that the development of secondary porosity in a sandstone as a result of aluminosilicate or carbonate dissolution is the natural consequence of the interaction of organic and inorganic reactions during progressive diagenesis the degree to which secondary porosity develops depends on the ratio of organic to inorganic matter the sequence rates and magnitude of diagenetic reactions fluid flux and sand shale geometry end of article last page 635

diagenetic alterations and reservoir heterogeneity within the depositional facies a case study from distributary channel belt sandstone of upper triassic yanchang formation reservoirs ordos basin china abstract diagenesis is of decisive significance for the reservoir heterogeneity of most clastic reservoirs linking the distribution of diagenetic processes to the depositional facies and sequence stratigraphy has in recent years been discipline for predicting the distribution of diagenetic alterations and reservoir heterogeneity of clastic reservoirs this study constructs a model of distribution of diagenetic alterations and reservoir heterogeneity within the depositional facies by linking diagenesis to lithofacies sandstone architecture and porewater chemistry during burial this would help to promote better understanding of the distribution of reservoir quality evolution and the intense heterogeneity of reservoirs based on an analogue of deltaic distributary channel belt sandstone in upper triassic yanchang formation 83 sandstone plug samples were taken from 13 wells located along this channel belt an integration of scanning electron microscopy thin sections electron microprobe analyses rate controlled porosimetry rcp gas flow measurements of porosity and permeability and nuclear magnetic resonance nmr experiments together with published data were analysed for the distribution mineralogical and geochemical characteristics of detrital and diagenetic components and the distribution of reservoir quality within the distributary channel belt distribution of diagenetic alterations and reservoir heterogeneity within the distributary channel belt sandstones include i formation of high quality chlorite rims in the middle part of thick sandstones with coarser grain sizes and a lower content of ductile components resulted from the greater compaction resistance of these sandstones providing larger pore spaces for chlorite growth leading to formation of the intergranular pore wide sheet like throat and intergranular pore intragranular pore wide sheet like throat 15 k 1md in the middle part of thick sandstones ii formation of thinner chlorite rims in the middle part of thinner sandstones is associated with the intergranular pore intragranular pore narrow sheet like throat 9 18 o vpdb 23 4 to 16 6 13 c vpdb 4 0 to 2 3 favoured in the top and bottom of the channel sandstone which near the sandstone mudstone bouding surface destroying pore space

evolution of the lian river coastal basin in response to quaternary marine transgressions in southeast china abstract the coastal basin deposit in the lian river plain is among the thickest quaternary sequences along the southeastern coast of china the clastic sediment accumulated in a variety of environmental settings including fluvial channel estuary coastal and marine conditions detailed investigation of lithofacies grain size distributions magnetic susceptibility microfossils and chronology of marine core cn01 compared with regional cores and combined with offshore seismic reflection profiles has allowed us to correlate the spatial stratigraphy in the inner and outer plain and the seismic units grain size distribution analysis of core cn 01 through compositional data analysis and multivariate statistics were applied to clastic sedimentary facies and sedimentary cycles results show that these methods are able to derive a robust proxy information for the depositional environment of the lian river plain we have also been able to reconstruct deltaic evolution in response to marine transgressions on the basis of dating results and chronostratigraphy the estimated age of the onset of deposition in the lian river coastal plain was more than 260 kyr bp three transgressive sedimentary cycles revealed in many regional cores support this age model detailed lithological and microfossil studies confirm that three marine m3 m2 and m1 and three terrestrial t3 t2 and t1 units can be distinguished spatial correlation between the inner plain outer plain typical cores characterized by marine transgression cycles and offshore seismic reflectors reveals coherent sedimentary sequences two major boundaries unconformity and erosion surfaces can be recognized in the seismic profiles and these correspond to weathered reddish and or variegated clay in the study core suggesting that quaternary sediment changes on the lian river plain were largely controlled by sea level variations and coastline shift during glacial interglacial cycles

distribution chemistry isotopic composition and origin of diagenetic carbonates magnus sandstone north sea abstract diagenetic ferroan carbonates grew in the upper jurassic reservoir sandstones of the magnus oilfield in porewaters which differed in composition across the field these porewaters remained compositionally different and stratified for at least 35 m y variations in carbonate chemistry across the field are attributable to these porewater variations which resulted from displacement of marine depositional water from the crest of the field by meteoric water during late cimmerian subaerial exposure original depositional facies and detrital mineralogy strongly influenced diagenetic carbonate distribution rare diagenetic calcite occurs as discrete rhombic crystals diagenetically late magnesian siderites have developed throughout the reservoir sandstone and are commonly intimately associated with altered detrital biotite grains poikilotopic ankerite cement postdates calcite and siderite and occurs only adjacent to mudstones and in thin sandstones within mudstones three compositional growth zones in siderite crystals are observed across the field from crest to flank in all three wells studied a similar trend of compositional evolution through time is observed in both two and three zoned rhombs first formed siderite is relatively magnesian intermediate zones are more ferroan and outer zones are at least as magnesian as the first stage these individual grain variations overprint a fieldwide variation where siderite is more ferroan in the crestal samples up to 87 mol fe mn and more magnesian downdip up to 58 mol mg this reflects the greater influence of relatively fe rich meteoric derived water in the crest and the greater influence of marine derived mg rich porewater downdip ankerite shows a similar variation in fe and mg abunda ces across the field crest and flank maximum 25 and 17 mol fe mn respectively 27 and 43 tool mg and developed due to release of mg fe ca and hco3 ions from mudstones into adjacent sandstones following dissolution of detrital minerals and organic decarboxylation reactions both siderite and ankerite have lower 18o at the crest of the oilfield than downdip respectively siderite 16 0 and 17 6 smow ankerite 17 7 and 21 4 these differences in 18o reflect the retention during burial diagenesis of a larger component of meteoric water in the crest of the field below the unconformity whereas downdip porefluid contained a larger marine derived component strong organic influence on 13c 8 0 to 14 6 pdb for magnesian siderite 7 7 to 13 6 for ankerite closed system 34s values up to 15 7 for late cubic pyrite and stratified 18o from crest to flank of the field argue against large scale porewater movements diagenetic porewater stratification is strongly supported by the parallel but distinct geochemical fingerprints of siderite and ankerite cements from the crest to the flank of the field

meteoric water diagenesis in late cretaceous canyon fill turbidite reservoirs from the espirito santo basin eastern brazil abstract parameters controlling the diagenetic evolution of passive margin marine turbidites which are important targets of hydrocarbon exploration are poorly constrained in the literature this study aims to unravel the conditions of diagenesis and its impact on the reservoir quality evolution from late cretaceous canyon filling turbiditic sandstones of the onshore portion of espirito santo basin eastern brazil kaolinization 18 o 13 3 to 15 2 d 96 6 to 79 6 and dissolution of framework silicate grains is attributed to meteoric water incursion during eodiagenesis in response to a considerable fall in relative sea level eogenetic alterations also include cementation by siderite average 18 o 7 2 13 c 9 3 and pyrite progressive sediment burial present depths 1530 2027 m resulted in the formation of poikilotopic calcite ferroan dolomite ankerite average 18 o 7 9 13 c 2 9 minor amounts of quartz overgrowths and in partial dickitization of kaolinite isotopic values of calcite and dolomite ankerite follow two trends of co variance of 13 c with decreasing 18 o and increasing temperature from a composition closer to marine 0 one trend goes towards positive 13 c values up to 22 4 for calcite 18 6 for dolomite ankerite indicating increasing input of carbonate from methanogenic fermentation the other trend develops towards negative 13 c values down to 17 2 for calcite 15 for dolomite ankerite suggesting increasing contribution from thermal decarboxylation with increasing temperature and depth despite the presence of various cement types mechanical compaction was more important than cementation in reducing depositional porosity in the onshore urucutuca sandstones

extension of berner s new geochemical classification of sedimentary environments to ancient sediments using stable isotopes of carbon and oxygen in siderite nodules and lithology of host shales one can recognize geochemical environments proposed by berner in devonian carboniferous sediments of the appalachian basin his oxic post oxic and sulfidic diagenetic environments are well represented sediments of the methanic environment may be present but the evidence is equivocal the sulfidic sediments can be further subdivided into well laminated and bioturbated corresponding to the presence of h 2 s in the overlying water in the first case but confined to the sediment pore waters in the second journal abstract

late cretaceous cenozoic tectonic sedimentary evolution and u enrichment in the southern songliao basin abstract late cretaceous cenozoic tectonic sedimentary evolution is fundamental to u uranium migration and u enrichment in the southern songliao basin northeast china to establish the genetic connection between tectonic sedimentary evolution and u enrichment this paper presents a mineralogical study apatite fission track aft analysis and sem cl analysis of quartz grains two stages of mineralization were proposed including the u pre enrichment stage during the late cretaceous and the ore forming stage during the cenozoic based on the density plots of the aft ages paleoclimate studies and sem cl analysis of quartz grains it is inferred that the u pre enrichment was closely related to the rapid uplift of the great xing an range and the zhangguangcai range and the o2 rich atmosphere during 97 90 ma in the southern songliao basin the o2 rich atmosphere increased u mobility and led to the formation of red beds during the late cretaceous which widely developed u pre enrichment in the basins of northeast china the mineralogical study reveals that the qianjiadian uranium deposit developed in two stages the diagenetic process experienced the transition from early alkaline to late acidic conditions to promote the stage i mineralization u pre enrichment the dominant occurrence spaces for u were the dissolution pores of quartz tightly packed with calcite cement u enrichment during alkaline ambient conditions was mainly via adsorption with the increase of the buried depth the early alkaline diagenesis returned to acidic conditions due to the abundant organic material u vi was reduced to pitchblende due to the reducing capacity of the organic matter om stage ii ore forming stage mineralization was strictly related to the hydrocarbon re concentration according to the thermal history study of the cc1 and q2 boreholes the southern songliao basin experienced four episodes of rapid uplift during the cenozoic at 83 80 ma 68 52 ma 33 23 ma and 8 ma respectively the cenozoic uplifts led to hydrocarbon migration at 65 ma 23 ma and present the re concentration of hydrocarbon prompted the u mineralization in the qianjiadian uranium deposit in the southern songliao basin during the stable period of tectonic movements the reducing conditions caused by hydrocarbons promoted u mineralization the dominant spaces of coffinite were intergranular dissolved pores formed during the transformation from the feldspar to the kaolinite under acidic conditions the post rift phase and structural inversion phase directly controlled the u pre enrichment stage and the ore forming stage respectively overall the tectonic sedimentary evolution played an important role in controlling the u leaching from granites the dominant occurrence spaces and u recycling

origin of calcite cements and their impact on reservoir heterogeneity in the triassic yanchang formation ordos basin china a combined petrological and geochemical study abstract calcite cement is a typical product of fluid rock interaction in petroleum reservoirs and its migration has a major impact on reservoir heterogeneity a case study of a closed diagenetic system was carried out on the upper triassic yanchang formation of ordos basin china the most successful exploration region for tight sandstone oil in china the investigated sandstones are mainly lithic arkoses q31f42l27 three occurrences of calcite cement are the most important diagenetic minerals affecting reservoir quality early calcite completely filling intergranular volumes late calcite filling pores after chlorite coating distributed in oil layers or locally replacing feldspar and crystal stock calcite typically distributed in dry layers 13c values of the sandstones are in the range 9 69 to 2 05 indicating that calcite is derived mainly from thermodynamic decarboxylation of organic matter dark mudstone interbedded with sandstone and hydrocarbon emplacement are the two crucial sources of organic fluid providing sufficient ca2 mn2 fe2 and mg2 for calcite precipitation through interaction with feldspar and other silicates such as dissolution and replacement together with transformation of smectite to illite manganese was continuously enriched in calcite during burial diagenesis driven by compaction of interbedded mudstone and fluid overpressure calcite migrated widely into calcite cemented zones cczs where fluid rock interaction was strong such as in the margins of sandstone bodies intrastratal sandstones with original high permeability zones and thin sandstones thereby forming an important organic co2 sink potential high quality reservoirs are distributed mainly in the central part of the closed system restricted by calcite cemented zones and are an exploration target our data suggest that frequently alternating sandstone and mudstone sequences comprise a complete organic co2 source sink system controlling the migration of calcite cement in a closed diagenetic system and reservoir heterogeneity by fluid rock interaction

strontium isotopes in brines and associated rocks from cretaceous strata in the mississippi salt dome basin southeastern mississippi u s a abstract sr concentrations and 87 sr 86 sr ratios have been determined for brines from formations spanning most of the cretaceous in two oilfields in the mississippi salt dome basin and from local salt domes chalk sands and shales the selma group is predominantly a fossiliferous chalk whereas the remainder of the stratigraphic sequence consists of sands and shales with only minor carbonate phases the sampled interval ranges from 2000 to just over 4000 m below land surface sr concentrations range from 443 to 1718 mg kg 1 for brines from baxterville field and from 528 to 1762 mg kg 1 for brines from soso field brine 87 sr 86 sr ratios range from 0 7071 to 0 7116 at baxterville field and from 0 7083 to 0 7119 at soso field with the exception of brines from the selma chalk all 87 sr 86 sr ratios are more radiogenic than cretaceous seawater on plots of 87 sr 86 sr vs sr concentration brines from individual formations within a producing field occupy distinct non overlapping areas with the exception of a single sample in some cases individual reservoirs within formations can be distinguished brines from formations that are present in both fields which are 85 km apart do not have the same 87 sr 86 sr ratio in each field brine sr concentration increases with depth 87 sr 86 sr ratios increase with depth to 3000 m and then appear to level off these trends are interpreted to be caused by dissolution or recrystallization of carbonate minerals which contain significant amounts of non radiogenic sr and decomposition of rb rich detrital micas and k feldspar which are sources of radiogenic 87 sr salt and cap rock from domes in southeastern mississippi have an average 87 sr 86 sr ratio of 0 70707 consistent with a middle jurassic age for the louann salt in baxterville field chalk from the upper cretaceous selma group has a cretaceous seawater 87 sr 86 sr ratio of 0 7076 87 sr 86 sr ratios range from 0 7091 to 0 7111 for sr leached from sands and shales from both baxterville and soso fields if these values reflect the 87 sr 86 sr values of the easily dissolved interstitial carbonate phases they indicate that the calcite cements precipitated from sedimentary fluids which were more radiogenic than the seawater of that time but which were not as evolved as the brines which are now present alternatively they may reflect leaching of radiogenic sources

carbonate cementation patterns and diagenetic reservoir facies in the campos basin cretaceous turbidites offshore eastern brazil massive turbidite arkoses contain more than 80 of the oil reserves of the campos basin the main petroleum province of brazil the porosity and permeability distribution in the namorado albian cenomanian and carapebus turonian santonian sandstones is

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

factors controlling reservoir properties and hydrocarbon accumulation of the eocene lacustrine beach bar sandstones in the dongying depression bohai bay basin china abstract lacustrine beach bar sandstone reservoirs are economical important exploration targets in the dongying depression bohai bay basin the factors that controls reservoir quality and hydrocarbon accumulation in lacustrine beach bar sandstones were systematically investigated through an integrated petrographic petrophysical and diagenesis analysis the reservoir properties of the beach bar sandstones are mainly affected by compaction carbonate cementation and dissolution significant negative relationships exist between both the porosity and permeability and the volume of carbonate cement carbonate cement is mainly distributed along the sandstone mudstone contacts sandstones with thicknesses less than 1 m are generally tightly cemented to form zones with low petrophysical properties the material source of the carbonate cement in the tightly cemented zone was highly likely from the interbedded mudstone content of carbonate cements in the center of the thick sandstones is low whereas compaction is the dominant diagenetic process with some dissolution there is a positive correlation between the petrophysical properties of the beach bar sandstones and the content of kaolinite the petrophysical properties have an inverse correlation with the contents of illite and illite smectite mixed layer clays reservoir petrophysical property largely controls hydrocarbon accumulation in the beach bar sandstones compared with non oil bearing sandstones oil bearing sandstones have higher porosity and a larger pore throat size sandstones with a porosity of less than 8 5 and or low permeability of less than 1 02 10 3 m2 are mostly incapable of accumulating oil

sources of ca2 in the major carbonate cements in eocene sandstones and conglomerates evidence from sr isotopes sr ca ratios and rare earth elements abstract different sources of calcium have important effects on the formation and distribution of carbonate cements on the basis of identifying all possible sources of calcium comprehensive quantitative analyses of sr c and o isotopic values and trace elements including rare earth elements were conducted to determine the ca2 sources and their changes in the same type of carbonate cements at different vertical positions in the same reservoirs ferroan calcite and ankerite were the major types of carbonate cements ca2 in ferroan carbonate cements in the sandstones near the mudstone sandstone m s interfaces was derived primarily from diagenetic fluids mixed with ca2 originating from the transformation of smectite to illite and from original sedimentary water in the adjacent mudstones however ca2 in the ferroan carbonate cements in the sandstones away from the m s interfaces originated from the transformation of smectite to illite the dissolution of carbonate minerals and original sedimentary water in source rocks the gradual change of ca source of the same type of ferroan carbonate cements indicates the diversity of migration direction of diagenetic paleo fluid in the same period during the transformation of smectite to illite concentration gradients were created from mudstones to sandstones ca2 mg2 fe2 and other cations in the mudstones diffused to the adjacent sandstones and provided sources of ferroan carbonate cements at the edge of the sandstones pore water with ca2 mg2 fe2 co32 and other cations in the source rocks was collectively charged into sandstones during oil emplacement and promoted the precipitation of ferroan carbonate cements in residual pore water due to oil emplacement and material supplied by the adjacent mudstones the content of ferroan carbonate cements in the sandstones was lower and the reservoir porosity was higher at a greater distance from the m s interfaces however in thick bedded sandstones due to lack of material supply from the adjacent mudstones ca2 mg2 fe2 and co32 in the ferroan carbonate cements were mainly derived from source rocks as a result no significant changes were observed in the content of ferroan carbonate cements and reservoir porosity in the vertical direction

geochemical constraints on the origins of calcite cements and their impacts on reservoir heterogeneities a case study on tight oil sandstones of the upper triassic yanchang formation southwestern ordos basin china calcite cementation has been identified as an active process in the upper triassic yanchang formation throughout its burial history and as a major diagenetic factor causing strong reservoir heterogeneities the origins of calcite cements and their relevance to reservoir heterogeneities were investigated using a suite of petrographic and geochemical methods including optical microscopy with fluorescence and cathodoluminescence scanning and backscattered electron microscopy with energy dispersive spectrometry x ray diffraction x ray fluorescence electron probe microanalysis quantitative evaluation of minerals by scanning electron microscopy fluid inclusion analysis and carbon and oxygen stable isotope analyses the sandstones are compositionally immature with relatively high amounts of volcanic rock fragments the two generations of calcite cements are ca i and ca ii the ca i calcites are distributed along the interface of sandstone and mudstone units and were formed during the late triassic to early jurassic at formation temperatures of approximately 90 c the ca ii calcite mainly developed in the lower part of the fining upward sandstone units and was formed in the late jurassic at higher temperatures of approximately 110 c the origins of calcite cements were constrained by geochemical and isotope measurements fluid inclusion homogenization temperature and in situ element analysis the ca i calcite cement originated from dissolution of the lacustrine depositional carbonates in the interbedded mudstones and reprecipitation in the adjacent sandstones the ca ii calcite was mainly related to organic matter decarboxylation with ca2 having been provided internally by volcanic fragment alteration and plagioclase dissolution calcite cementation had caused strong reservoir heterogeneities in the yanchang formation tight sandstones the ca i calcite cementation destroyed reservoir properties along the interface of sandstones and mudstones the lower parts of the fining upward sandstone units were tightly cemented by ca ii calcite although they originally had high porosity and permeability the middle upper parts of the fining upward sandstone units contain less calcite cements and thus have better preserved reservoir pores because of oil emplacement inhibiting the calcite cementation processes

formation of zoned ankerite in gravity flow sandstones in the linnan sag bohai bay basin eastern china evidence of episodic fluid flow revealed from in situ trace elemental analysis abstract it has been shown that ankerite in clastic reservoirs can preserve an extended record of diagenesis and thus the fluid flow history in sedimentary basins ankerite cements with distinct zoning textures are well developed in gravity flow sandstones associated with magmatic intrusions in the linnan sag eastern china this example offering a rare opportunity to investigate the history of complex diagenetic fluid flow events in a lacustrine rift basin an integrated study including thin section petrography fluid inclusion micro thermometry carbon and oxygen isotope sem eds xrd cl bse epma la icp ms analyses was undertaken the aim is to resolve the mineralogical geochemical and isotopic composition of the sandstone and constitutional minerals the sandstones are mainly of lithic arkoses q43f34l23 with an average carbonate cements of approximately 8 2 wt the carbonate cements are dominated by zoned ankerite appearing as rhombohedral and poikilotopic blocky crystals calcite ferroan calcite dolomite and siderite cements are also present non carbonate cements including barite authigenic clay minerals and quartz overgrowth are common in the sandstone the 13 c values of ferroan calcite range from 4 2 to 1 2 pdb ferroan calcite was re precipitated at a temperature of 93 1 c at the expense of the dissolution of eodiagenesis carbonate cement in the sandstone ankerite was precipitated at temperature ranges from 107 8 to 140 9 c accompanied with the formation of pore filling barite the 13c values of ankerite range from 1 8 to 1 1 and 18o values range from 14 8 to 10 5 indicating an involvement of hydrothermal fluids in the formation of the zoned ankerite the hydrothermal fluids were probably related to the mafic magmatic activities and migrated into the sandstones along faults these fluids are rich in iron and magnesium and also contains abundant inorganic carbon dioxide and sulphate episodic charging into the reservoir the abundances of most of the elements including magnesium iron rees and trace elements in the zoned ankerite consistently decrease away from the magma intrusion centres we therefore conclude that multiple magmatic activities were primarily responsible for the zoned ankerite precipitation in the research area the intimate relationship between magmatic activity and carbonate cementation in sandstone reservoirs documented in this paper may have significant implications for studying fluid flow in lacustrine rift basins

diagenetic evolution and porosity destruction of turbiditic hybrid arenites and siliciclastic sandstones of foreland basins evidence from the eocene hecho group pyrenees spain abstract this study aims to unravel the impact of diagenetic alterations on porosity loss of foreland basin turbiditic hybrid arenites and associated siliciclastic sandstones of the eocene hecho group south central pyrenees spain in this succession hybrid arenites and calclithites are extensively cemented by mesogenetic calcite cement 18ovpdb 10 0 to 5 8 th mode 80 c salinity mode 18 8 wt eq nacl fe dolomite 18ovpdb 8 5 to 6 3 and trace amounts of siderite the extent of carbonate cementation is interpreted to be related to the amounts of extrabasinal and intrabasinal carbonate grains which provided nuclei and sources for the precipitation and growth of carbonate cements other diagenetic alterations such as pyrite and albitization had no impact on reservoir quality scarce early diagenetic cements coupled with abundant ductile carbonate and siliciclastic framework grains have led to rapid porosity loss owing to compaction conversely abundant quartz in the sandstones prevented rapid loss of porosity by mechanical compaction reservoir quality was affected by mesogenetic cementation by quartz overgrowths calcite and dolomite intergranular pressure dissolution of quartz grains and formation of fracture filling calcite cement 18ov pdb values from 10 4 to 7 8 th temperatures of 150 c which are attributed to deep circulation of hot meteoric waters during extensional stages of tectonism the results of this study illustrate that diagenetic evolution pathways of the arenites and sandstones are closely linked to the variation in detrital composition particularly the proportion and types of extrabasinal noncarbonates extrabasinal carbonates and intrabasinal carbonate grains these insights suggest that marine turbiditic hybrid arenites and calclithites of foreland basins are subjected to more rapid and extensive porosity loss owing to compaction and cementation than associated siliciclastic sandstones degradation of reservoir quality makes these hybrid arenites calclithites and sandstones suitable as tight gas reservoirs but only if fracture porosity and permeability develop during tectonic deformation

pore fluid evolution distribution and water rock interactions of carbonate cements in red bed sandstone reservoirs in the dongying depression china the compositions distribution and its interaction with rocks of the evolving pore fluids controls the distribution of carbonate cements and reservoir storage spaces the reservoir quality of the red bed sandstone reservoirs in the dongying depression was investigated by an integrated and systematic analysis including carbonate cement petrology mineralogy carbon and oxygen isotope ratios and fluid inclusions the investigation was also facilitated by probing the mineral origins precipitation mechanisms pore fluid evolution and distribution and water rock interaction of carbonate cements and their influences on reservoir quality diagenetic evolving fluids in the interbedded mudstones are the main source for the precipitation of calcite cements that completely fill the intergranular volume cfiv calcite with heavier oxygen and carbon isotopes the ferro carbonate cements in the reservoir sandstone are enriched in lighter carbon and oxygen isotopes in addition to the cations released by the conversion of clay minerals in reservoirs products of organic acid decarboxylation and the associated feldspar dissolution process provide important sources for such carbonate cementation the carbon isotopes of co2 and the oxygen isotopic composition of fluids equilibrated with the cfiv calcite ferro calcite dolomite and ankerite cements indicate that the pore in the red bed reservoirs experienced high salinity fluids which evolved from the early formed interbedded mudstones through organic acid input and to organic acid decarboxylation pore fluids from nearby mudstones migrated from the edge to the centre of sandbodies causing strong calcite cementation along the sandbody boundaries and forming tight cementation zones pore fluids associated with organic co2 and acids and organic acid decarboxylation are mainly distributed in the internal portion of sandbodies causing feldspar dissolution and precipitation of ferro carbonate cements the distribution of pore fluids caused the zonal distribution of carbonate cements in sandbodies during different periods this may be advantageous to preserve the porosity of reservoirs as exemplified by the distribution of high quality reservoirs in the red bed sandbodies

diagenesis and its impact on the reservoir quality of the fourth member of xujiahe formation western sichuan depression china abstract the fourth member of the xujiahe formation t3x4 in the western sichuan depression is typical of the tight gas sandstones of china the member is composed of three types of sandstone litharenite sublitharenite and feldspathic litharenite some of these sandstones contain a large proportion of carbonate rock fragments these calcarenaceous sandstones ccss can be differentiated from ordinary sandstones oss the reservoir properties of the fourth member are quite poor average porosity 5 9 average permeability 1 41 md these properties were influenced by several diagenetic processes including compaction feldspar dissolution precipitation of carbonate cements and clay cementation mineral distribution patterns and quantitative data show that the authigenic quartz in t3x4 sandstones is mainly sourced from sei reactions feldspar dissolution and kaolinite illitization quartz cements occupy only a small fraction of the reservoir space carbonate cementation primarily calcite is dominant in the research area mineral distribution patterns and isotopic composition trends suggest that the carbonate cements in the sandstones originated both inside and outside of the sandstones dissolution of inorganic carbonate rock fragments is the main internal carbon source of carbonate cements in ccss in oss the contribution is much smaller organic carbon derived from the thermal evolution of kerogen in the mudstones that migrated into sandstones via diffusion is the main carbon source in oss due to a combination of compaction of ductile fragments and carbonate cementation t3x4 reservoir sandstones in western sichuan depression exhibit low porosities in t3x4 sandstones porosity is highest for samples where both the detrital ductile grain content and carbonate cement content are low

diagenesis and reservoir quality of the lower cretaceous quantou formation tight sandstones in the southern songliao basin china abstract the lower cretaceous quantou formation in the southern songliao basin is the typical tight oil sandstone in china for effective exploration appraisal and production from such a tight oil sandstone the diagenesis and reservoir quality must be thoroughly studied first the tight oil sandstone has been examined by a variety of methods including core and thin section observation xrd sem cl fluorescence electron probing analysis fluid inclusion and isotope testing and quantitative determination of reservoir properties the sandstones are mostly lithic arkoses and feldspathic litharenites with fine to medium grain size and moderate to good sorting the sandstones are dominated by feldspar quartz and volcanic rock fragments showing various stages of disintegration the reservoir properties are quite poor with low porosity average 8 54 and permeability average 0 493 md small pore throat radius average 0 206 m and high displacement pressure mostly higher than 1 mpa the tight sandstone reservoirs have undergone significant diagenetic alterations such as compaction feldspar dissolution quartz cementation carbonate cementation mainly ferrocalcite and ankerite and clay mineral alteration as to the onset time the oil emplacement was prior to the carbonate cementation but posterior to the quartz cementation and feldspar dissolution the smectite to illite reaction and pressure solution at stylolites provide a most important silica sources for quartz cementation carbonate cements increase towards interbedded mudstones mechanical compaction has played a more important role than cementation in destroying the reservoir quality of the k 1 q 4 sandstone reservoirs mixed layer illite smectite and illite reduced the porosity and permeability significantly while chlorite preserved the porosity and permeability since it tends to be oil wet so that later carbonate cementation can be inhibited to some extent it is likely that the oil emplacement occurred later than the tight rock formation with the porosity close to 10 however thicker sandstone bodies more than 2 m constitute potential hydrocarbon reservoirs

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

compaction of lithic sands experimental results and applications compaction experiments showed that the degree of physical compaction of a lithic sand is related to the amount and type of lithic material present these experiments produced a product that is texturally indistinguishable from natural lithic sandstones generally sedimentary lithic fragments are more ductile than metamorphic lithic fragments volcanic lithic fragments when altered to phyllosilicate minerals by weathering or diagenesis are extremely ductile mineralogy and intragranular microporosity influence ductility ratios based on weight percent of soft ductile minerals clay and micas to hard brittle minerals quartz and feldspar within the lithic fragments provides a good indication of ductility a ratio greater than 1 indicates the lithic material will be h ghly ductile microporous lithic fragments are more ductile than dense lithic fragments physical compaction models were developed based on experimental data from well sorted lithic sandstones for which the percentage of preserved porosities were functions of the effective stress the type of lithic material and the quantity of lithic material these models are derived from laboratory sandstones with at least 25 lithic grains in the following categories moderately ductile metamorphic lithic fragments highly ductile shale lithic fragments and extremely ductile altered volcanic lithic fragments application of the compaction models of the experimental compaction techniques to modern subsurface sand samples or outcrop equivalents can provide valuable estimates of the preserved porosities i e reservoir potential for lithic sandstones that may be exploratory objecti es in frontier basins examples using the models and experimental compaction techniques for exploration are given experimental data showed that precompaction cement and overpressure may be beneficial to porosity preservation an early formed partial cement can retard compaction and preserve porosity by stabilizing the sand pack a later cement does not have this beneficial effect overpressure when developed early retards compaction by reducing the effective stress of course a late developing overpressure is not effective at preserving porosity because compaction is an irreversible process

diagenesis and evolution of the lower eocene red bed sandstone reservoirs in the dongying depression china abstract the diagenetic evolution of the lower eocene red bed high quality reservoirs in the dongying depression was systematically investigated through an integrated petrographic petrophysical fluid evolution and thermal history analysis the reservoirs experienced several phases of diagenesis including compaction carbonate cementation gypsum and ankerite cementation feldspar and carbonate dissolution quartz overgrowth cementation and clay mineral cementation and transformation an early alkaline diagenetic environment caused significant cementation of calcite and gypsum in the reservoir sandstone units near the sandstone mudstone contacts acidic formation fluids caused widespread dissolution of the feldspar and carbonate cements leading to synchronous occurrences of primary intergranular pores and acidic dissolution pores from the middle parts of the reservoir sandstone units to the sandstone mudstone contacts dissolution wanes gradually with ferro carbonate progressively diminishing migration of exogenetic thermal fluid along faults may have contributed to the gradual decrease in dissolution and the gradual increase in cementation from the lower part to the upper part of fault blocks the alternating alkaline and acidic diagenetic environments and the distribution of diagenetic products caused the development of the high quality red bed reservoirs in the middle part of the thick layered sandstone units in the lower part of fault blocks because of the spatial and temporal relationships between the diagenetic stages and their products lithological traps of a diagenetic origin were developed in the red bed reservoirs

quantitative evaluation of the reservoir potential and controlling factors of semi deep lacustrine tempestites in the eocene lijin sag of the bohai bay basin east china abstract ancient lacustrine storm deposits that act as petroleum reservoirs are seldom reported the lijin sag which is located in the southeastern corner of the bohai bay basin in east china is a ne sw trending cenozoic half graben basin some of its eocene deposits bindong deposits were interpreted as lacustrine tempestites the bindong tempestites which developed in the lower part of the upper fourth member of the shahejie formation es41 constitute a new kind of petroleum reservoir and are novel petroleum exploration targets in the bindong area however the characteristics of the es41 bindong tempestite reservoirs and their controlling factors are poorly understood point count analyses of thin sections scanning electron microscope image analyses x ray diffractometry data and the petrophysical parameters of the bindong tempestite reservoirs were utilized to estimate the reservoir quality the reservoirs have undergone significant diagenetic alteration which can be divided into negative and positive aspects the negative alteration includes compaction authigenic minerals and cementation such as carbonates clay minerals and overgrowths of quartz and feldspar the uneven distribution of carbonate cement increased the reservoir s heterogeneity with carbonate cement commonly precipitating along the sandstone mudstone contacts the primary porosity was severely reduced because of compaction and extensive carbonate cementation positive alteration includes dissolution carbonate cementation undercompaction and fractures carbonate cementation exhibited both positive and negative effects on the reservoir quality overall the objective reservoir quality is quite poor a quantitative evaluation of the reservoirs potential was conducted the cutoff values of several of the reservoir s parameters were calculated the lower limits of the porosity and permeability are 8 35 5 85 and 1 2587 0 2753 10 3 m2 respectively depending on the depth the upper limits of the carbonate and mud content are approximately 18 5 and 9 10 respectively a fundamental understanding of these characteristics will provide necessary information for extracting hydrocarbons from analogous subsurface reservoirs

petrography and composition of authigenic feldspars oligocene frio formation south texas abstract detrital feldspar assemblages 1 to 4 km burial depth in oligocene sandstones from south texas are extensively altered by dissolution in some samples cementation in secondary pores principally by albite but also by authigenic k feldspar and other minerals occurs subsequent to the dissolution and can lead to wholesale replacement textures solid state diffusion of cations has not been important in the replacement process information from conventional light microscopy sem examination and back scattered electron imaging suggests that relative rates of dissolution versus replacement varied widely among different detrital grains through time even on the scale of a thin section dissolution of detrital feldspars and precipitation of relatively pure albite and k feldspar take place ver a large depth interval and thus can be markedly separated in time vacuolized albite occurring as irregular veins within detrital grains is associated with little visible secondary porosity and forms when precipitation is rapid relative to dissolution of the detrital grain more rapid dissolution results in formation of larger scale intragranular secondary pores commonly filled by subsequent precipitation of clear albite within the former grain volume electron microprobe analyses show that the composition of vacuolized albite and authigenic k feldspar deviate somewhat from ideal compositions presumed typical for authigenic feldspars regardless of their cause these subtle variations in composition have important ramifications for basin scale mass balance calculations requiring accurate estimates of the volume of authigenic feldspar

a type of continuous petroleum accumulation system in the shulu sag bohai bay basin eastern china abstract this study analyzed crude oils from the lower part of the third member of the eocene shahejie formation es 3 l and three prospective source rocks from the shulu sag bohai bay basin eastern china using a variety of organic geochemical methods biomarker characteristics were used to interpret source rock organic matter input and depositional environment and oil source rock correlation the biomarker data indicate that the crude oils originated from the es 3 l source rock which contains a mixture of plankton and land plant organic matters deposited in brackish fresh water under reducing conditions the oil in the es 3 l is self sourced instead of migrated from the overlying source rocks the petroleum generation potential of the es 3 l source rock was evaluated using organic geochemistry total organic carbon toc values for approximately 100 samples are between 1 02 and 4 92 wt and hydrogen indices range from 285 to 810 mg hydrocarbons g toc the es 3 l source rock contains mainly type ii and iii kerogen and most of the samples are thermally mature the data show that the es 3 l source rock has good potential for liquid hydrocarbon generation the es 3 l rock also acts as the oil reservoir having very low bulk porosity and permeability various types of storage space in the marlstone and carbonate rudstone in the es 3 l of the shulu sag include 1 fractures 2 intergranular pores 3 dissolution pores 4 organic matter pores 5 intragranular pores and 6 seams around gravels pore size ranges from nanometers to millimeters because the oil was generated and stored in es 3 l strata which lack any obvious trap and seal and have low permeability the unit represents a continuous petroleum accumulation

diagenesis and reservoir quality evolution of palaeocene deep water marine sandstones the shetland faroes basin british continental shelf abstract the palaeocene deep water marine sandstones recovered from six wells in the shetland faroes basin represent lowstand transgressive and highstand systems tract turbiditic sediments mineralogic petrographic and geochemical analyses of these siliciclastics are used to decipher and discuss the diagenetic alterations and subsequent reservoir quality evolution the middle upper palaeocene sandstones subarkoses to arkoses from the shetland faroes basin british continental shelf are submarine turbiditic deposits that are cemented predominantly by carbonates quartz and clay minerals carbonate cements intergranular and grain replacive calcite siderite ferroan dolomite and ankerite are of eogenetic and mesogenetic origins the eogenetic alterations have been mediated by marine meteoric and mixed marine meteoric porewaters and resulted mainly in the precipitation of calcite 18 o v pdb 10 9 and 3 8 trace amounts of non ferroan dolomite siderite 18 o v pdb 14 4 to 0 6 as well as smectite and kaolinite in the lowstand systems tract lst and highstand systems tract hst turbiditic sandstone below the sequence boundary minor eogenetic siderite has precipitated between expanded and kaolinitized micas primarily biotite the mesogenetic alterations are interpreted to have been mediated by evolved marine porewaters and resulted in the precipitation of calcite 18 o v pdb 12 9 to 7 8 and fe dolomite ankerite 18 o v pdb 12 1 to 6 3 at temperatures of 50 140 and 60 140 c respectively quartz overgrowths and outgrowth which post and pre date the mesogenetic carbonate cements is more common in the lst and tst of distal turbiditic sandstone discrete quartz cement which is closely associated with illite and chlorite is the final diagenetic phase the clay minerals include intergranular and grain replacive eogenetic kaolinite smectite and mesogenetic illite and chlorite kaolinite has been subjected to mesogenetic replacement by dickite the k feldspar and plagioclase grains have been albitized dissolution of calcite cement and of framework grain feldspar volcanic fragments and mud intraclasts has resulted in a considerable enhancement of reservoir quality

porosity permeability and textural heterogeneity of reservoir sandstones from the lower cretaceous putaohua member of yaojia formation weixing oilfield songliao basin northeast china abstract with the granulometric analyses mineral composition x ray diffraction xrd analyses porosity and permeability measurements scanning electron microscopy sem observations and mercury porosimetry measurements of the reservoir sandstones from four wells in the weixing oilfield songliao basin ne china the porosity permeability and pore structural heterogeneity features of the putaohua member sandstone of the lower cretaceous yaojia formation have been investigated in this paper the sandstones are mostly characterized by a similar deposition in continental detrital environments with quartz and feldspar taking the majority 97 and argillaceous minerals illite kaolinite and chlorite the minority 3 however their granule size sorting index porosity and permeability vary significantly in contrast to the generality that high porosity permeability often corresponds to large grain sizes and sorting indexes the porosity and permeability here show no good correlations to the grain size and sorting index suggesting the presence of a remarkable textural heterogeneity in terms of sequential change in permeability along the well depth cyclic sedimentation modes have been identified in some sub layers despite the porosity data showing little response to those scenarios this may be ascribed to the sandstone s peculiar intergranular pore structures in which the cementation occurred in pore lining and filling manners to different extents depending on their depositional environments the distributary channel facies tends to have lowest detrital clay content and rather open packed texture in pore lining cementation manner whereas sand sheet facies has pore filling manner the hg porosimetry analytical data are in fairly good agreement with the heterogeneous pore structures observed by sem large sized pores and channels connecting them though only accounting for a low percentage of the total porosity are believed to contribute mostly to the effective permeability in the sandstone reservoir such textural heterogeneities should be closely associated with the dynamic changing sedimentary facies of sandstones in the continental environment

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

holocene hydrological and sea surface temperature changes in the northern coast of the south china sea abstract in order to reconstruct the holocene environmental history of a coastal site in the northern south china sea this study analysed the organic carbon isotope ratios 13corg and alkenone unsaturation ratios uk 37 from a 36 5 m long sediment core drilled at seabed in the mouth region of the pearl river estuary and generated a coupled hydrological and temperature record this record reveals changes of monsoon induced sediment discharge and sea surface temperature of the holocene in four stages in stage i the site was under fluvial conditions prior to postglacial marine transgression stage ii saw an increase of sea surface temperature from c 23 0 c to 27 0 c associated with a strengthened summer monsoon from c 10 350 to 8900 cal years bp this was also a period of rapid sea level rise and marine transgression during which the sea inundated the palaeo incised channel i e the lower part of the t shape accommodation space created by the rising sea in these 1500 years fluvial discharge was strong and concentrated within the channel and the high sedimentation rate 11 8 mm year was very close to the rate of sea level rise in the subsequent 2000 years stage iii sea level continued to rise and the sea flooded the broad seabed above the palaeo incised channel resulted in fluvial discharge spreading thinly across the wide accommodation space and a much reduced sedimentation rate 1 8 mm year sea surface temperature in this stage reached 27 3 c initially but dropped sharply to 26 1 c towards c 8200 cal years bp the final stage covers the last 7000 years and the site was under a stable sea level sedimentation in this stage varied a little but averaged at 1 8 mm year whilst fluvial discharge and sea surface temperature didn t change much two short periods of hydrological and temperature change were observed which are related to the climatic cooling events of c 4200 cal years ago and the little ice age

geomorphologic changes in the lower pearl river delta 1850 2015 largely due to human activity abstract we use 165 years of navigational and bathymetric data and 60 years of sediment discharge data to document and explain geomorphic changes in the pearl river delta prd and its estuary which in recent years has experienced rapid urbanization and explosive economic growth a series of british and chinese navigation charts dating back to 1850 show that since then the prd has gained 1160 km2 of new land most of it reclaimed land derived from disposal of dredge spoils accordingly the prd estuary has decreased by 35 in ocean area and by 39 in water volume changes were most dramatic in the western part of the delta where the modaomen and huangmao seas decreased in ocean area by 62 and 39 respectively to the east upper lingding bay ldb decreased in ocean area by 36 in contrast the lower part of ldb decreased by only 3 in area although shoaling more by than 2 m some of which may come via influx of sediment from the south china sea since 1995 the rate of land accretion in the prd has declined to 40 of what it was over the previous 145 years and to

modern fine grained sedimentation spatial variability and environmental controls on an inner pericontinental shelf hong kong abstract fine grained sediments in hong kong form part of an extensive cover that extends from nearshore locations to the mid continental shelf this featureless mud blanket is associated with sediment inputs from the pearl river and has generally been considered to be homogeneous in its characteristics however detailed sedimentological and geochemical analyses have revealed that the superficial deposits show systematic spatial variations that reflect contrasts in deposition between the pearl river estuary to the west and fully marine conditions to the east results indicate significant differences in particle size accumulation rates biogenic silica organic carbon content c n ratios and stable carbon and nitrogen isotopes that reflect location with respect to the pearl river delta calcium carbonate content varies widely tending to be higher close to land major controls over spatial variability include rainfall patterns and the supply of estuarine suspended particulate matter the effects of palaeotopography and modern water depth resuspension induced by monsoon winds and typhoons and human impacts on the sea floor

note on the importance of hydrocarbon fill for reservoir quality prediction in sandstones oil emplacement retarded the rate of quartz cementation in the brae formation deep water sandstone reservoirs of the miller and kingfisher fields united kingdom north sea thus preserving porosity despite the rocks being buried to depths of 4 km and 120 degreesc quartz precipitation rates were reduced by at least two orders of magnitude in the oil legs relative to the water legs important contrasts in quartz cement abundances and porosities have emerged between the oil and water legs where reservoirs have filled with hydrocarbons gradually over a prolonged period of time 15 m y the earlier the hydrocarbon fill the greater is the degree of porosity preservation failure to consider this phenomenon during field development could lead to overestimation of porosity and permeability in the water leg potentially leading in turn to poor decisions about the need for and placement of downflank water injectors during exploration the retarding effect of oil on quartz cementation could lead to the presence of viable reservoirs situated deeper than the perceived regional economic basement

late quaternary environmental changes in hong kong abstract the offshore quaternary deposits of hong kong include a wide range of clastic sediments that accumulated in a variety of settings including lacustrine channel overbank coastal and marine environments detailed studies of the sedimentology and diatoms of several long continuously sampled cores combined with seismic reflection profiles has enabled the reconstruction of late pleistocene and holocene palaeoenvironments in hong kong the quaternary sediments comprise several informal seismic units that are separated by marine planation tidal scour and fluvial erosion surfaces seismic unit 1 represents pre quaternary bedrock and weathered bedrock which is separated from a complex assemblage of pleistocene channel floodplain backswamp and lacustrine deposits seismic unit 2 by a major erosion surface u1 sediments that represent the basal part of unit 2 have been dated at about 248 000 yr b p well preserved laminations at several horizons suggest periodic deposition and probably seasonal flooding the presence of kaolin indicates strong chemical weathering and warm humid conditions at the time of deposition seismic unit 3 consists of marine mud that unconformably overlies a fluvial erosion surface fe1 cut into deposits represented by seismic unit 2 the sediments comprising unit 3 are tentatively correlated with oxygen isotope stage 5e and high sea levels at about 128 000 105 000 yr b p a marine planation surface r1 terminates unit 3 and is overlain by coastal and nearshore deposits comprising seismic unit 4 at about 95 000 to 80 000 yr b p this phase of marine deposition was terminated by a major fall in sea level that caused widespread fluvial incision across hong kong and produced surface fe2 a subsequent rise in sea level after about 18 000 yr b p led to deposition of a complex succession of marine sediments and erosion surfaces initially homogeneous mud seismic unit 5a was laid down tidal currents perhaps related to base level changes then caused incision and the formation of surface ts1 at several localities further shallow marine mud comprising unit 5b was laid down between about 9500 and 8000 yr b p a planar erosion surface r2 with local palaeosols and minor incision cuts across earlier deposits the overlying seismic unit 5c represents diatomaceous marine mud and a basal sand at one coring site these sediments have been divided into five major diatom zones that reflect changes in salinity and sea level modern sea level was attained by about 6000 yr b p in broad terms the late quaternary record is one of two periods of marine flooding separated by a major regressive episode with no evidence of older quaternary marine sediments being present

the strontium isotopic composition and origin of burial cements in the lincolnshire limestone bajocian of central lincolnshire england strontium isotopic composition 87sr 86sr of two petrographically chemically and isotopically 18o and 13c distinct phases of burial calcites from the lincolnshire limestone are indistinguishable 0 70820 26 the mean 87sr 86sr ratio of these phases is considerably more radiogenic than 87sr 86sr ratios of bajocian marine waters 0 70725 neither bajocian marine waters nor meteoric waters buffered by host marine carbonate in the limestone could have precipitated the burial spars radiogenic strontium may have been contributed from k feldspar dissolution and or clay recrystallization either within clastic portions of the limestone itself or from major clastic units adjacent to the limestone alternatively palaeozoic marine waters or remobilized palaeozoic marine carbonate and or sulphate could have supplied the necessary radiogenic strontium

erosion and sedimentation associated with the last sea level rise offshore hong kong south china sea abstract high resolution seismic data reveals an extensive planar seaward dipping reflector offshore south of hong kong this is interpreted as a ravinement surface created during the last transgression and is associated with ca 2 5 m of shoreface sand which is overlain by a 6 15 m thick sheet of prodelta marine shelf sediments on seismic profiles the shoreface sand is characterised by a chaotic reflector pattern and the sheet by low amplitude parallel flat or low angle continuous reflectors particle size data reveal a westward coarsening and decrease in sorting which reflects increasing proximity to the pearl river foraminiferal and diatom assemblages demonstrate the same overall westward shallowing trend with increase in inner shelf foraminifera and brackish planktonic diatom species a phase of early holocene shoreface sand and intertidal silt deposition in eastern and central areas culminated in a brief period of subaerial exposure this was succeeded by an extensive period of shelf and inner shelf deposition brought about by variation in balance between the rate of sea level rise and accommodation and the discharge and sediment supply from the pearl river

cementation and burial history of a low permeability quartzarenite lower cretaceous travis peak formation east texas fine grained quartzarenites and subar koses in the lower cretaceous travis peak formation were extensively modified during burial diagenesis timing of diagenetic events can be constrained by combining petrographic and geochemical data with data on subsidence and thermal history the geothermal gradient in the east texas study area is 38 c km now but it may have been as high as 44 c km when the travis peak was deposited because of elevated heat flow caused by crustal stretching associated with rifting of the gulf of mexico illite rims and dolomite were the first authigenic minerals to precipitate in travis peak sandstones dolomite probably formed soon after deposition at about 25 c from water with a 18 o composition near smow next extensive quartz cement averaging 17 of the rock volume in well sorted sandstones occluded much of the primary porosity the average 18 o composition of the quartz overgrowths indicates that they precipitated from meteoric fluids at temperatures of between 55 and 75 c at depths of 1 to 1 5 km dissolution of orthoclase and albitization of plagioclase followed quartz cementation and occurred prior to mid cretaceous movement of the sabine uplift illite chlorite and ankerite precipitated after feldspar diagenesis the ankerite may have precipitated over a range of temperatures from about 80 to 125 c from fluids with 18 o composition of about 2 smow 2 is the average present composition of travis peak water most diagenesis ended when oil migrated into the travis peak later de asphalting of the oil by solution of gas filled much of the remaining porosity with reservoir bitumen in some zones near the top of the formation

intergranular pressure solution in four quartzose sandstones abstract cathodoluminescence petrography and scanning electron microscopy reveal that several geologic variables significantly influenced intergranular pressure solution and quartz cementation in the bromide ordovician oklahoma upper minnelusa permian wyoming nugget jurassic utah and tuscarora silurian pennsylvania sandstones a linear relationship exists between mean grain size and intergranular pressure solution with finer grained samples having experienced more intergranular pressure solution than coarser grained ones samples that display pervasive illite grain coatings have also experienced more intergranular pressure solution than those with no or poorly developed grain coatings sorting and limited framework grain compositional variability have exerted little influence o intergranular pressure solution silica budgets indicate that in the bromide upper minnelusa and nugget sandstones more quartz has been dissolved by intergranular pressure solution than is present as cement these sandstones have exported silica during burial diagenesis in the tuscarora the amount of quartz dissolved by intergranular pressure solution is about equal to the amount of quartz cement present implying approximate mass balance however petrographic evidence suggests that most of the quartz cement in at least three of these sandstones predates most intergranular pressure solution implying that they acted as silica importers during shallow diagenesis and silica exporters during deeper diagenesis intergranular pressure solution has played varying roles in determining the ultimate porosity of these four sandstones in the nugget intergranular pressure solution was the single most important diagenetic process that influenced porosity evolution in the bromide intergranular pressure solution was slightly more important than mechanical compaction and considerably more important than cementation in the tuscarora intergranular pressure solution was slightly more important than mechanical compaction and of about equal importance to quartz cementation in the upper minnelusa intergranular pressure solution was less important than both mechanical compaction and gypsum anhydrite cementation

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

reconstruction of holocene monsoon history from the pearl river estuary southern china using diatoms and carbon isotope ratios this study employs a multiproxy approach in the reconstruction of holocene monsoon history from sedimentary sequences obtained from the pearl river estuary southern china a set of modern surface sediment samples were collected within and outside the estuary and a sediment core was obtained from the mouth of the estuary a range of modern environmental variables were recorded including water salinity summer winter and annual average and water depth the diatom results show a good relationship with water salinity with marine diatoms dominating the more distal samples from outside the main estuary and freshwater diatoms dominating the proximal samples influenced by higher freshwater flux this pattern is matched by the bulk organic carbon isotopes the isotope values vary between 21 1 0 3 0 for samples from the fully marine environment between 23 2 0 8 and 23 7 0 8 for samples from the mid to outer estuary brackish water environment and less than 24 7 1 3 for samples from the inner estuary close to the freshwater sources similarly the c n ratios vary from below 7 0 6 in the marine end members to over 14 8 3 0 in the freshwater end members both the diatom and carbon data from the sediment core reveal a significant increase in freshwater flux from 8500 cal yr bp resulting from the enhanced summer monsoon regime in early holocene the strength of freshwater flux reached its highest between 7500 and 6000 cal yr bp in the last 6000 years freshwater flux decreased towards present reflecting a gradual weakening of the summer monsoon

geochemistry of carbonate cements in the sag river and shublik formations triassic jurassic north slope alaska implications for the geochemical evolution of formation waters carbonate cements calcite siderite dolomite and ankerite formed throughout the diagenetic history of the sag river and shublik formations the trace element and isotopic geochemistry of these cements varies as a function of the timing of precipitation earliest calcites formed prior to significant compaction of the sediment are relatively enriched in mg up to 4 4 mol and have 87sr 86sr values mean 0 707898 compatible with the original marine pore waters later calcites are relatively fe rich up to 5 0 mol and are characterized by increasing 87sr 86sr values up to 0 712823 and sr content with decreasing age the fe content of zoned siderite and dolomite ankerite rhombs increases towards the outside of the rhombs i e increasing fe content with decreasing age these geochemical variations appear principally to result from changes in pore water chemistry during diagenesis the increase in 87sr 86 sr and sr content of the cements is most likely due to interaction between pore waters and 87 sr rich clay and possibly feldspar in ellesmerian mudrocks whole rock 87sr 86 sr signatures for the mudrocks are 0 716 pore water fe2 concentration was probably controlled by diagenetic alterations involving fe bearing minerals e g pyrite precipitation a reconnaissance examination of carbonate cements in the overlying kingak shale indicates that similar alterations occurred in the kingak the low 18 o value of some calcite cements 11 96 pdb suggests that an influx of meteoric water may have occurred in the mid neocomian though the low value could also result from an abnormally high geothermal gradient associated with mid neocomian rifting

the origin and fate of silt sized carbonate in subsurface miocene oligocene mudstones south texas gulf coast combined petrographic and geochemical data document several kinds of detrital carbonate in subsurface miocene oligocene mudstones of the texas gulf coast in the extreme south of texas in muddy sediments deposited by ancient precursors to the modern rio grande mudstone carbonate is dominated by extrabasinal detritus derived from cretaceous limestones of the edwards plateau further north oligocene mudstones contain carbonate that is mostly syndepositional skeletal material minor amounts of authigenic carbonate are found as replacements of silt sized feldspars in all the mudstones depth related shifts in 18ocalcite and 13ocalcite suggest that small amounts of authigenic carbonate also form by replacement of detrital carbonate recrystallization although this cannot be demonstrated petrographically pervasive carbonate cementation in primary pores in mudstones is not generally observed textural evidence of carbonate loss through pressure dissolution is widespread providing a mechanism for the depth related decline in carbonate content observed in many gulf coast wells carbonate dissolution in mudstones and export of the caco3 implies a massive acid source probably resulting from silicate reactions within the mudstones during burial diagenesis

process controls on the development of stratigraphic trap potential on the margins of confined turbidite systems and aids to reservoir evaluation stratigraphic trapping at pinch out margins is a key feature of many turbidite hosted hydrocarbon reservoirs in systems confined by lateral or oblique frontal slopes outcrop studies show that there is a continuum between two geometries of pinch out configuration in type a turbidites thin onto the confining surface although the final sandstone pinch out is commonly abrupt and individual beds tend not to erode into earlier deposits in type b turbidite sandstones commonly thicken toward the confining slope and beds may incise into earlier deposits these two types may occur in combination to give a wide spectrum of pinch out characteristics our analysis suggests the principal control in determining pinch out character is flow magnitude with smaller flows producing type a and larger flows producing type b in areas of poor seismic control it can be difficult to assess either pinch out character or the proximity of wells to confining slopes because estimates of paleoflow magnitude can be made from core or high quality log image data however it is possible to make reasonable estimates of pinch out character even from wells such as exploration wells which may be placed conservatively away from the field margins furthermore systematic paleoflow variations and thickness trends are commonly seen in individual turbidite sandstones as they approach confining slopes for example dispersal directions indicate flow deflection parallel with the strike of confining topography beds thin toward type a onlaps and thicken toward type b onlaps these relationships can be exploited via analysis of vertical successions to constrain well position with respect to the slope similarly the presence location and frequency of locally derived debrites can provide information on the presence and proximity of confining slopes begin page 972

application of biomarkers to petroleum migration in the linyi fault area huimin depression bohai bay basin china a geochemical investigation was carried out on 32 crude oil samples to investigate the origin of the oil and to trace the migration direction in the linyi fault area huimin depression bohai bay basin china the oils which were characterized by low gammacerane content and a dominance of c27 over c29 regular steranes originated from the source rock of the third member of the shahejie formation es3 in the linnan sag a sequential charge of low and high maturity oils from the es3 source rocks resulted in a decrease in oil maturity in the migration direction the petroleum migration direction in the footwall of the linyi fault was northward away from the fault and the migration direction at the southwest end in the hanging wall of the linyi fault was westward along the fault as evidenced by lateral gradients of biomarker parameters of various maturity density and viscosity

summer monsoon induced upwelling dominated coastal sea surface temperature variations in the northern south china sea over the last two millennia the south china sea scs situated to the north of the indo pacific warm pool ipwp is under the strong influence of the asian monsoon system however coastal sea surface temperature sst reco

low mg calcite marine cement in cretaceous turbidites origin spatial distribution and relationship to seawater chemistry lower cretaceous hauterivian bioclastic sandstone turbidites in the scapa member north sea basin were extensively cemented by low mg calcite spars initially as rim cements and subsequently as concretions five petrographically distinct cement stages form a consistent paragenetic sequence across the scapa field the dominant and pervasive second cement stage accounts for the majority of concretions and is the focus of this study stable isotope characterization of the cement is hampered by the presence of calcitic bioclasts and of later cements in sponge spicule moulds throughout the concretions nevertheless trends from whole rock data augmented by cement separates from synlithification fractures indicate an early calcite 18o value of 0 5 to 1 5 pdb as such the calcite probably precipitated from marine pore fluids shortly after turbidite deposition carbon isotopes 13c 0 to 2 pdb and petrographic data indicate that calcite formed as a consequence of bioclastic aragonite dissolution textural integrity of calcitic nannoplankton in the sandstones demonstrates that pore fluids remained at or above calcite saturation as expected for a mineral controlled transformation electron probe microanalyses demonstrate that early calcite cement contains 2 mol mgco3 despite its marine parentage production of this cement is ascribed to a combination of an elevated aragonite saturation depth and a lowered marine mg2 ca2 ratio in early cretaceous calcite seas relative to modern oceans scapa cement compositions concur with published models in suggesting that hauterivian ocean water had a mg2 ca2 ratio of 1 this is also supported by consideration of the spatial distribution of early calcite cement in terms of concretion growth kinetics in contrast to the dominant early cement late stage ferroan 18o depleted calcites were sourced outwith the scapa member and precipitated after 1 2 km of burial our results emphasize that bioclast dissolution and low mg calcite cementation in sandstone reservoirs should not automatically be regarded as evidence for uplift and meteoric diagenesis

an evolutionary model for the holocene formation of the pearl river delta china this paper reconstructs the evolutionary history of the pearl river delta over the last 9000 years and investigates land sea interaction in a large deltaic complex which formed under the influence

calcite cement in permian deep water sandstones delaware basin west texas origin distribution and effect on reservoir properties calcite cement is the dominant control on reservoir quality in turbidite sandstones of the upper permian bell canyon formation delaware basin these well sorted very fine grained arkoses were deposited in a basin floor setting by channel levee systems terminating in broad lobes calcite cement distribution in the east ford and geraldine ford fields was mapped using core log and thin section data calcite is concentrated in tightly cemented zones that are mostly less than 1 ft 0 3 m thick areas that have high percentages of calcite cemented sandstone 20 occur along the margins of the sandstone bodies in overbank and lobe deposits where sandstone pinches out into siltstone areas that have the lowest percentage of calcite cemented sandstone 10 occur where the sandstone is thickest in the channel facies isotopic composition of the calcite 13c 1 8 to 3 0 relative to the peedee belemnite pdb 18o 4 6 to 6 3 pdb is consistent with the source of calcium carbonate being from dissolution of detrital carbonate rock fragments and marine skeletal debris because internal sources of calcite were apparently insufficient to account for the cement volume cement components are interpreted as having been transported into the sandstones from organic rich basinal siltstones and limestones feldspars buffered acidic formation waters near where they entered the sandstone resulting in calcite concentrated near the sandstone margins the calcite formed near maximum burial depths of 4800 ft 1 5 km and temperatures of 104f 40c from marine pore waters with 18o of approximately 0 relative to standard mean ocean water most of the calcite cemented zones are interpreted as being concretions that extend no more than a few meters laterally production data and geophysical log correlations suggest that some cemented zones are laterally continuous at least 1000 ft 300 m and cause vertical reservoir compartmentalization laterally extensive calcite layers may be associated with the base of turbidite deposits

the impact of diagenesis on the heterogeneity of sandstone reservoirs a review of the role of depositional facies and sequence stratigraphy diagenesis exerts a strong control on the quality and heterogeneity of most clastic reservoirs variations in the distribution of diagenetic alterations usually accentuate the variations in depositional porosity and permeability linking the types and distribution of diagenetic processes to the depositional facies and sequence stratigraphic framework of clastic successions provides a powerful tool to predict the distribution of diagenetic alterations controlling quality and heterogeneity the heterogeneity patterns of sandstone reservoirs which determine the volumes flow rates and recovery of hydrocarbons are controlled by geometry and internal structures of sand bodies grain size sorting degree of bioturbation provenance and by the types volumes and distribution of diagenetic alterations variations in the pathways of diagenetic evolution are linked to 1 depositional facies hence pore water chemistry depositional porosity and permeability types and amounts of intrabasinal grains and extent of bioturbation 2 detrital sand composition 3 rate of deposition controlling residence time of sediments at specific near surface geochemical conditions and 4 burial thermal history of the basin the amounts and types of intrabasinal grains are also controlled by changes in the relative sea level and therefore can be predicted in the context of sequence stratigraphy particularly in paralic and shallow marine environments changes in the relative sea level exert significant control on the types and extent of near surface shallow burial diagenetic alterations which in turn influence the pathways of burial diagenetic and reservoir quality evolution of clastic reservoirs carbonate cementation is more extensive in transgressive systems tract tst sandstones particularly below parasequence boundaries transgressive surface and maximum flooding surface because of the abundance of carbonate bioclasts and organic matter bioturbation and prolonged residence time of the sediments at and immediately below the sea floor caused by low sedimentation rates which also enhance the formation of glaucony eogenetic grain coating berthierine odinite and smectite formed mostly in tst and early highstand systems tract deltaic and estuarine sandstones are transformed into ferrous chlorite during mesodiagenesis helping preserve reservoir quality through the inhibition of quartz cementation the infiltration of grain coating smectitic clays is more extensive in braided than in meandering fluvial sandstones forming flow barriers in braided amalgamated reservoirs and may either help preserve porosity during burial because of quartz overgrowth inhibition or reduce it by enhancing intergranular pressure dissolution diagenetic modifications along sequence boundaries are characterized by considerable dissolution and kaolinization of feldspars micas and mud intraclasts under wet and warm climates whereas a semiarid climate may lead to the formation of calcrete dolocrete cemented layers turbidite sandstones are typically cemented by carbonate along the contacts with interbedded mudrocks or carbonate mudstones and marls as well as along layers of concentration of carbonate bioclasts and intraclasts commonly hybrid carbonate turbidite arenites are pervasively cemented proximal massive turbidites normally show only scattered spherical or ovoid carbonate concretions improved geologic models based on the connections among diagenesis depositional facies and sequence stratigraphic surfaces and intervals may not only contribute to optimized production through the design of appropriate simulation models for improved or enhanced oil recovery strategies as well as for co2 geologic sequestration but also support more effective hydrocarbon exploration through reservoir quality prediction

strontium isotopic evolution of oil field waters from carbonate reservoir rocks in bindley field central kansas u s a abstract oil field waters produced from mississippian carbonate reservoir rocks in bindley field kansas had an average salinity of about 42 8 mg l they were enriched in ca sr na k rb and li and depleted in mg relative to sea water at the same level of either cl or br concentration the average abundances of different elements are as follows na 13 460 mg l k 325 mg 1 rb 0 9 mg l li 12 mg l ca 1 515 mg l sr 42 mg l mg 430 mg l cl 23 000 mg l so4 2 630 mg l br 32 mg l the 87 sr 86 sr values of the waters ranged between 0 7221 and 0 7230 whereas the values of the host carbonate rocks were between 0 7090 and 0 7093 the very high 87 sr 86 sr values of the waters probably occurred as a result of their reaction with alkali feldspar minerals in buried precambrian crystalline rocks to the east of the bindley field the study demonstrates that despite their residence in carbonate reservoir rocks oil field waters can retain sufficiently distinct isotopic memory that may provide important dues about mineral water interactions in the chemical evolutionary history of the waters the study further indicates that very late diagenetic reactions in many carbonate rocks are of minor extent difference in the sr isotopic data among the oil field waters suggests that the sr isotopic data can be used as a reliable guide in recognising existence of separate pools in an oil field

modal distribution of pore throat size in sandy conglomerates from an alluvial fan environment lower karamay formation junggar basin west china abstract alluvial fan exhibits various modal distribution of pore throat size which effectively control the oil displacement efficiency and fluid movement however the genetic mechanism of the modal distribution of the pore throat size and its macroscopic distribution within alluvial fan is poorly understood based on core analysis this paper which take the sandy conglomerate reservoir within the alluvial fan in junggar basin as an example aims to study the variations in modal distribution of pore throat size and controlling mechanisms the results reveal that 1 the modal distribution types of the pore throat size are complicated and can be classified into six subtypes including unimodal type uni unimodal type uni unimodal type uni bimodal type bi bimodal type bi and multimodal type multi uni uni and uni is dominated by intergranular pores intragranular dissolved pores and micropores respectively bi bi and multi are characterized by the combination of several pore types 2 diagenesis including compaction cementation and dissolution rock composition the mudstone fragment content and rock texture the clay content control the volume of each pore throat type which significantly influences the modal distribution of the pore throat size lithofacies with low mudstone fragment and clay content and great grain sorting tend to have experienced weak compaction thereby retaining a large volume of intergranular pores and have been subjected to strong dissolution when adequate acidic ions occur in the porewater resulting in a large volume of intragranular dissolved pores strong dissolution reduced the rock compressive strength and further enhanced the degree of mechanical compaction during the burial process cementation occurred later than compaction and dissolution and its intensity is controlled by the supply of co2 and ca2 and compaction intensity thus reducing the volume of intergranular pores the content of mudstone fragment and clay varies among different lithofacies of different origins for traction current deposits lithofacies with smaller median grain size contain fewer mudstone fragments resulting in fewer micropores the clay content decreased first and then increased with decreasing median grain size and lithofacies with lower clay content cause less volume of micropores 3 lithofacies types combined with porewater condition for dissolution and lithofacies location affect the diagenetic intensity rock composition and rock texture variations thus resulting in various modal distribution of pore throat size within the alluvial fan moreover the rock texture especially grain sorting is very important for compaction and facilitates percolation of meteoric water and dissolution

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

calcite cement distribution and its effect on fluid flow in a deltaic sandstone frontier formation wyoming precipitation of extensive calcite cement during burial diagenesis can strongly modify the depositional permeability of a sandstone reservoir and affect fluid flow during production to predict subsurface flow through cemented reservoirs permeability distributions used in fluid flow models must reflect this diagenetic overprint calcite cements in sandstones commonly occur as irregularly distributed concretions which makes it difficult to predict diagenetic permeability modifications in the subsurface from typically spaced wells outcrops can provide a continuous image of heterogeneity produced by concretionary calcite cements the size and distribution of calcite concretions were mapped in outcrops of the frewens sandstone frontier formation in central wyoming large tabular calcite concretions in this deltaic sandstone generally follow basinward inclined bedding median thickness of the concretions is 0 6 m length is 4 2 m and width is 5 3 m the highest cement fraction is in the high permeability facies at the top of the sandstone body concretion centers are approximately poisson distributed within the sandstone the upward increasing cement fraction is caused by upward increasing concretion size lateral variation in the fraction of the sandstone cemented by calcite has a normal distribution with a mean of 12 s 5 spatial distribution of calcite cement in the frewens sandstone was modeled using indicator geostatistics variograms were inferred from outcrop maps of cement indicator semivariograms of cement have a range of 30 m horizontally and 2 5 m vertically dimensions that correspond approximately to the size of the largest concretions stochastic images of cement were created using indicator simulation with vertically varying cement proportion flow models indicate that concretions make flow paths more tortuous and retard flow in the coarser facies near the top of the sandstone the fastest path through the sandstone is in the lightly cemented high net to gross center of the sandstone body because the cement mainly occurs within the highest permeability facies in the sandstone body a model based on depositional facies alone would overestimate upscaled permeability of the frewens sandstone

depositional and diagenetic controls on the reservoir quality of lower cretaceous pendencia sandstones potiguar rift basin brazil the quality of the reservoirs of the lower cretaceous pendencia formation of the potiguar basin northeastern brazil is directly controlled by depositional facies related carbonate cementation and compaction the study of the interplay of these processes in the reservoirs offers an opportunity to unravel the diagenetic patterns of clastic sequences in interior rifts and in particular the role of carbonate cementation in poorly understood continental systems the pendencia formation is a thick sequence of fan deltaic fluvial deltaic turbiditic and lacustrine sandstones conglomerates and shales deposited during the rift stage of the basin the sandstones are predominantly arkoses average q49f40l11 with subordinate plutonic and volcaniclastic feldspathic litharenites compaction and cementation had similar importance in the destruction of porosity with a dominance of cementation in the turbidites and of compaction in the fluvial deposits carbonate cementation in pendencia reservoirs increases progressively with depth and is facies controlled eodiagenetic nonferroan calcite i d18opdb 10 7 to 4 0 d13cpdb 17 5 to 8 5 mesodiagenetic rhombohedral ferroan dolomite ankerite d18opdb 9 3 to 3 9 d13cpdb 1 7 to 1 1 and ferroan calcite ii d18opdb 17 2 to 6 8 d13cpdb 13 6 to 2 3 were precipitated at three distinct temperature intervals calculated from the d18o values 21 to 58 c 70 to 79 c and 85 to 150 c respectively according to the d13c values dissolved carbonate for calcite i was derived from oxidation and methanogenic fermentation of organic matter and from methane oxidation ferroan mesogenetic cements were derived from thermal decarboxylation of organic matter the shales were a major source of dissolved begin page 1720 carbonate as indicated by the isotopic similarity between their calcite d13cpdb 0 2 to 1 8 87sr 86sr 0 719 and most cements in the sandstones and by the peripheral cementation along the contacts of interbedded sandstones as a result of this cementation pattern thin turbiditic and deltaic sandstone beds are pervasively cemented the best reservoir quality potential is encountered in the partially cemented fluvial sandstones at moderate depths deltaic and turbiditic sandstones are more pervasively cemented by carbonate derived from the interbedded shales alluvial fan conglomerates and sandstones were flushed by telogenetic meteoric waters close to the borders of the basin and to the proximity to the postrift unconformity however porosity enhancement was very limited due to the precipitation of kaolinite and the intense compaction related to the compositionally immature detrital framework

identification of sedimentary diagenetic facies and reservoir porosity and permeability prediction an example from the eocene beach bar sandstone in the dongying depression china abstract accurate prediction of reservoir porosity and permeability is essential for prospecting hydrocarbon reserves and petroleum production capacity we propose an innovative reservoir porosity and permeability prediction method through identifying sedimentary diagenetic facies determining the porosity permeability trends using core measurement data extrapolating the spatial distribution of the sedimentary diagenetic facies using log data through the bayes discriminant analysis and predicting the reservoir porosity and permeability the essence of the method was illustrated and its effectiveness was demonstrated using the eocene beach bar sandstones in the dongying depression bohai bay basin eastern china the eocene beach bar sandstones are classified into fine sandstone siltstone and argillaceous siltstone based on grain sizes sorting and matrix contents the major diagenetic processes that influence the porosity and permeability of the beach bar sandstones are compaction carbonate cementation and feldspar dissolution seven sedimentary diagenetic facies were identified in the beach bar sandstones based on the lithological types and their corresponding diagenesis and influence extent on reservoir properties the variation ranges of porosity and permeability log k of these sedimentary diagenetic facies are typically less than 6 and 1 2 respectively both the porosity and permeability have well defined functional relationships with depth the sedimentary diagenetic facies can be identified effectively from logging data through the use of the bayes discriminant analysis and corresponding cross plots the porosity and permeability log k of the well evaluated in the study were predicted with errors in the range of 3 and 0 6 respectively based on the distribution and the fitting equation of the trend lines for the seven sedimentary diagenetic facies the predicted porosities and permeability of sedimentary diagenetic facies match the measured porosities and permeability well