贵州紫云上石炭统叶状藻礁灰岩的成岩作用

贵州紫云县猴场镇扁平村的上石炭统中的叶状藻礁及其周边灰岩中发育强烈的成岩作用和胶结物,这些胶结物在猴场研究区内是显著的和有代表性的。通过观察、分析野外露头、光片、薄片、薄片的阴极发光和染色,来研究礁体岩石的成岩作用,确定了成岩作用序列、成岩环境、成岩阶段。成岩作用类型主要有泥晶化、溶蚀、胶结、新生变形、机械压实、剪切或...     

贵州紫云上石炭统叶状藻礁灰岩胶结物特征

贵州紫云县猴场镇扁平村的上石炭统叶状藻礁及其周边灰岩中广泛发育大量的各类胶结物。通过对胶结物的形态、结构和阴极发光特征以及胶结物间的接触关系的研究,可以确定成岩作用的先后并识别成岩环境。浅海海底同生成岩阶段大的孔隙中形成等厚环边针状胶结物、葡萄状胶结物,小的孔隙里形成微晶胶结物。早成岩阶段形成微亮晶和斑块状亮晶方解石胶结物和放射纤维扇状胶结物,表生成岩阶段的溶蚀作用和胶结作用强烈,胶结物类型有斑块状或等粒的方解石胶结物和等厚环壁柱状胶结物,等厚环壁柱状胶结物在所有胶结物中体积是较大的。早期胶结作用使叶状藻礁灰岩孔隙度大为降低。中、晚成岩阶段,孔隙被等厚环壁刃状胶结物和晶簇或斑块状亮晶方解石所充填,有些先成的胶结物被热液改造。后生作用阶段发生的主要是构造破裂作用,其中少数裂隙被红褐色含Fe2O3微晶层和晶体粉砂及渗流豆粒充填。叶状藻礁灰岩的孔隙在晚成岩阶段前或中被胶结而之后没有创造出大且连通的孔隙,是它没能成为油气储集层的原因之一。     

Early lithification and hardgrounds in upper Albian and Cenomanian calcarenites,southwest England

Early diagenetic lithification of calcarenites on the sea floor led to the development of a variety of hardgrounds, intraformational conglomerates, breccias and boulder beds in southwest England during late Albian and Cenomanian time. Cemented nodules commonly developed below the sea floor, and these were avoided by burrowing infauna. In some instances the nodules were exhumed and reworked on the sea floor to form distinctive intraformational conglomerates; exposed nodules frequently became bored and encrusted by organisms and mineralised by glauconite and phosphate. In other cases, sea floor cementation produced true hardgrounds whose upper surfaces were affected by the same processes, but the hardened layers were only a few tens of centimetres thick and were underlain by soft sediment. Fracturing and brecciation of some hardground layers occurred through differential compaction and through undermining and collapse as the result of burrowing and erosion beneath the hardened layer. Reworking of clasts produced in this manner yielded intraformational breccias. Petrographic analysis reveals multiple generations of carbonate cement, commonly beginning with syntaxial overgrowths on echinoderm fragments and “dog tooth” spar on polycrystalline carbonate grains. A progression exists in the Albian-Cenomanian succession of southwest England from relatively simple hardgrounds and intraformational conglomerates low in the sequence up into complex hardgrounds that may record many stages of sediment accretion, cementation, mineralisation and erosion. This progression appears to record increasing water depths and increased sea floor diagenesis.     

A review of the origin and setting of tepees and their associated fabrics

Carbonate hardgrounds often occur at the surface of shallow subtidal to supratidal, lacustrine, and subaerial carbonate shelf sediments. These are commonly disrupted and brecciated when the surface area of these crusts increases. In the subtidal environment, megapolygons form when cementation of the matrix causes the surface area of the hardgrounds to expand. Similar megapolygons form in the supratidal, lacustrine and subaerial settings when repeated incremental fracturing and fracture fill by sediment and/or cement also causes the area of the hardgrounds to expand. The arched up antiform margins of expansion megapolygons are known as tepees. The types of tepees found in the geological record include: (1) Submarine tepees which form in shallow carbonate-saturated waters where fractured and bedded marine grainstones are bound by isopachous marine-phreatic acicular and micritic cements. The surfaces of these brecciated crusts have undergone diagenesis and are bored. Unlike tepees listed below they contain no vadose pisolites or gravity cements; (2) Peritidal and lacustrine tepees are formed of crusts characterized by fenestral. pisolitic and laminar algal fabrics. This similarity in fabric makes these tepees of different origins difficult to separate. Peritidal tepees occur where the marine phreatic lens is close to the sediment surface and the climate is tropical. They are associated with fractured and bedded tidal flat carbonates. Their fracture fills contain geopetal asymmetric travertines of marine-vadose origin and/or marine phreatic travertines and/or Terra rossa sediments. The senile form of these peritidal tepees are cut by labyrinthic dissolution cavities filled by the same material. Lacustrine tepees form in the margins of shallow salinas where periodic groundwater resurgence is common. They include groundwater tepees which form over evaporitic ‘boxwork’ carbonates, and extrusion tepees which also form where periodic groundwater resurgence occurs at the margins of shallow salinas, but the dominant sediment type is carbonate mud. These latter tepee crusts are coated and crosscut by laminated micrite; the laminae extend from the fractures downward into the underlying dolomitic micrite below the crust. Both peritidal and lacustrine tepees form where crusts experience alternating phreatic and vadose conditions, in time intervals of days to years. Cement morphologies reflect this and the crusts often contain gravitational, meniscus vadose cements as well as phreatic isopachous cement rinds. (3) Caliche tepees which are developed within soil profiles in a continental setting. They are formed by laminar crusts which contain pisolites, and fractures filled by micritic laminae, microspar, spar and Terra rossa. Most of the cements are gravitational and/or meniscoid. In ancient carbonates, when their cementation and diagenetic fabric can be interpreted, tepee structures can be used as environmental indicators. They can also be used to trace the evolution of the depositional and hydrological setting.     

磷块岩的胶结作用

磷块岩胶结物有泥质、硅质、磷质和碳酸盐质4种,共形成19种胶结结构,其中尤以磷泥晶环边结构、等厚纤状环边结构、似重力式结构、磷质纤状环边叠加云质亮晶充填结构特征突出,具有指示沉积成岩环境的意义。4种类型的胶结物在剖面和平面上的演变与水体深度和沉积成岩环境有关,而胶结作用的地球化学特征,既是磷块岩的环境指示,又反映微生物的影响状况。     

Late Holocene to Recent aragonite-cemented transgressive lag deposits in the Abu Dhabi lagoon and intertidal sabkha

Modern cemented intervals (beachrock, firmgrounds to hardgrounds and concretionary layers) form in the lagoon and intertidal sabkha of Abu Dhabi. Seafloor lithification actively occurs in open, current-swept channels in low-lying areas between ooid shoals, in the intertidal zone of the middle lagoon, some centimetres beneath the inner lagoonal seafloor (i.e. within the sediment column) and at the sediment surface the intertidal sabkha. The concept of ‘concretionary sub-hardgrounds’, i.e. laminar cementation of sediments formed within the sediment column beneath the shallow redox boundary, is introduced and discussed. Based on calibrated radiocarbon ages, seafloor lithification commenced during the Middle to Late Holocene (ca 9000 cal yr bp ), and proceeds to the present-day. Lithification occurs in the context of the actualistic relative sea-level rise shifting the coastline landward across the extremely low-angle carbonate ramp. The cemented intervals are interpreted as parasequence boundaries in the sense of ‘marine flooding surfaces’, but in most cases the sedimentary cover overlying the transgressive surface has not yet been deposited. Aragonite, (micritic) calcite and, less commonly, gypsum cements lithify the firmground/hardground intervals. Cements are described and placed into context with their depositional and marine diagenetic environments and characterized by means of scanning electron microscope petrography, cathodoluminescence microscopy and Raman spectroscopy. The morphology of aragonitic cements changes from needle-shaped forms in lithified decapod burrows of the outer lagoon ooidal shoals to complex columnar, lath and platy crystals in the inner lagoon. Precipitation experiments provide first tentative evidence for the parameters that induce changes in aragonite cement morphology. Data shown here shed light on ancient, formerly aragonite-cemented seafloors, now altered to diagenetic calcites, but also document the complexity of highly dynamic near coastal depositional environments.     

海南岛全新世海滩岩的胶结作用与成岩环境的关系

我国海南岛及南海诸岛沿岸,广泛发育海滩岩。1980年我队赴海南岛考察现代沉积时,对海滩岩的分布、岩性特征及其与周围环境的关系进行了观察和采样。样品采自崖县鹿回头三亚湾水尾岭海蚀崖、西洲岛、小东海、东瑁岛、西瑁岛、天涯海角,乐东县莺歌海,文昌县渔业等地(图1)。有关的地质、地貌及岩性特征等,已有许多描述,对海滩岩的岩石学及成岩作用也有许多研究。本文侧重探讨海滩岩的胶结作用及与成岩环境的关系。     

Genesis and diagenetic evolution of Habo formation,Kachchh, Gujarat

The Kachchh Basin is a pericratonic rift basin situated at the western margin of the Indian plate. The Habo Dome embodies an important exposure of Bathonian to Kimmergian sediments among the Kachchh Mainland exposures. Based on vertical facies transitions, facies associations were documented: mixed shallow marine (Facies association 1), shoreface and lagoon deposits (Facies association II) and subtidal innershelf below fair weather wave base (Facies association III). The documented facies associations reflect that Habo Dome sediments deposited in a variety of environments from shallow marine to fluvio-deltaic and were strongly influenced by fluctuation of relative sea level. The dominance of floating grains and point contacts in the sandstone indicate that detrital grains do not show much pressure effects as a result of either shallow burial or early cementation. The sandstones were cemented by iron oxide, carbonate and silica in order of abundance. Three types of cements, blocky, rim and fibrous cement occur in the studied limestone representing phreatic, fresh water phreatic and deep burial diagenetic stages. Neomorphism and micritization are common. Both primary and secondary porosity exists in these sediments. Different graphs of porosity versus depth suggest a depth of burial in the range of 615–769 m.     

Evolution of pore space in the Poza Rica trend (Mid-Cretaceous), Mexico

ABSTRACT The Poza Rica trend of the Tampico embayment, Mexico, will ultimately produce more than 2.3 × 10⁹ barrels of oil from Mid-Cretaceous (Albian-Cenomanian) basin-margin deposits. Bioclastic grainstone, packstone, and wackestone are interbedded with polymictic lime breccia and dolomitized debris; all were deposited by sediment gravity flow. Indigenous sediment was pelagic lime mud. Typical reservoir porosities are about 10%; permeabilities average 2 md and rarely exceed 100 md. Porosity is largely the result of selective dissolution of rudist fragments, which were originally aragonite. Detailed petrographic study, with emphasis on the diagenetic products, allows quantitative assessment of porosity at various diagenetic stages from original sediment to reservoir rock. A relatively simple diagenetic history is reflected by about 90% of the samples studied: primary porosity was reduced through lithification of matrix mud and initial cementation by clear, equant to bladed, non-ferroan calcite. Later dissolution produced extensive skelmoldic and minor vuggy porosity. Subsequently, non-ferroan calcite cement reduced porosity before the emplacement of hydrocarbons. Reconstructed sediment porosities are comparable to, but lower than, modern counterparts. The initial phase of cementation and presumed lithification of mud greatly reduced porosity in all lithologies, but appreciably more porosity persisted in grainstone and packstone than in wackestone or mudstone. Dissolution produced a porosity resurrection, which exceeded that of the initial sediment in some grainstones. Calcite cementation and local multiphase quartz cementation and dolomitization reduced porosity to present average values of 8–12% in grain-supported rocks and 3% in mud-supported rocks. The greater persistence of primary porosity and, therefore, permeability in grain-supported rocks probably accounts for their greater secondary porosity development and ultimate reservoir quality. Geometrically averaged permeabilities range from only 0.17 md in wackestone to 3.85 md in dolomite, but differ significantly with rock type and grain size. Permeability increases with porosity in all lithologies; the rate of increase is greater at higher porosities and with coarser grain sizes. The agent for both early cementation and development of secondary porosity appears to have been meteoric water. Subaerial exposure appears to be ruled out, however, by a basin-margin depositional environment and continued burial beneath Upper Cretaceous pelagic sediments. Early exposure to meteoric water can be explained by a hydrologic head developed during penecontemporaenous exposure that produced cavernous porosity in the adjacent Golden Lane trend. Descending meteoric water likely emerged as submarine springs along the Tamabra trend. Deposition of pelagic limestone during the Turonian blanketed part of the Golden Lane escarpment to enhance development of a large freshwater lens; gaps in the blanket localized springs and influenced flow patterns within the Poza Rica field. Analogous freshwater circulation exists today in northern Florida.     

Alterations in guano phosphates and mio-pliocene carbonates of table mountain Santa Barbara,Curacao

The Table Mountain Santa Barbara, consisting of the Mio-Pliocene Seroe Domi Limestone Formation, was probably covered with a guano layer in the Pleistocene (?). Solutions, carrying guano-derived phosphate, percolated downwards, resulting in partial phosphatization of the underlying limestones. Only two phosphate minerals are present in the Table Mountain: apatite, and whitlockite.Phosphatization includes both replacement of the original carbonate or (pre-phosphate) dolomite, and cementation in primary- as well as secondary porosity (cryptocrystalline, isotropic phosphate cement, (micro)crystalline apatite, isopachous apatite fringe cement, multiple-zoned apatite crusts, rhombic whitlockite cements). Two factors controlled the final distribution of the phosphate within the Table Mountain. Firstly, changing positions of the sea level (and, therefore, of the diagenetic environments) determined the overall distribution of the phosphatized interval, a horizontal layer (98–128 m above sea level) sandwiched between two non-phosphatized limestone units.Original carbonate facies and -mineralogy was the second controlling factor causing the final variations in degree of phosphatization. The limestones comprise two lithofacies: (a) coralliferous limestone lenses erratically distributed amidst; and (b) micritic limestones. These coralliferous limestones are phosphatized preferentially forming so-called “phosphate pockets”, sharply outlined within a non-phosphatized micritic limestone “host”. These pockets are characteristically organized into five zones which are described in detail. Higher original porosity/permeability of the coralliferous limestone lenses as compared to the micrites, additionally enhanced by a phase of pre-phosphate dolomitization, determined preferential phosphatization of this facies type.A second controlling factor was the original carbonate mineralogy and resulting dia-     

Marine cements in mid-Tertiary cool-water shelf limestones of New Zealand and southern Australia

Large areas of southern Australia and New Zealand are covered by mid‐Tertiary limestones formed in cool‐water, shelf environments. The generally destructive character of sea‐floor diagenesis in such settings precludes ubiquitous inorganic precipitation of carbonates, yet these limestones include occasional units with marine cements: (1) within rare in situ biomounds; (2) within some stacked, cross‐bedded sand bodies; (3) at the top of metre‐scale, subtidal, carbonate cycles; and (4) most commonly, associated with certain unconformities. The marine cements are dominated by isopachous rinds of fibrous to bladed spar, interstitial homogeneous micrite and interstitial micropeloidal micrite, often precipitated sequentially in that order. Internal sedimentation of microbioclastic micrite may occur at any stage. The paradox of marine‐cemented limestone units in an overall destructive cool‐water diagenetic regime may be explained by the precipitation of cement as intermediate Mg‐calcite from marine waters undersaturated with respect to aragonite. In some of the marine‐cemented limestones, aragonite biomoulds may include marine cement/sediment internally, suggesting that dissolution of aragonite can at times be wholly marine and not always involve meteoric influences. We suggest that marine cementation occurred preferentially, but not exclusively, during periods of relatively lowered sea level, probably glacio‐eustatically driven in the mid‐Tertiary. At times of reduced sea level, there was a relative increase in both the temperature and the carbonate saturation state of the shelf waters, and the locus of carbonate sedimentation shifted towards formerly deeper shelf sites, which now experienced increased swell wave and/or tidal energy levels, fostering sediment abrasion and reworking, reduced sedimentation rates and freer exchange of sediment pore‐waters. Energy levels were probably also enhanced by increased upwelling of cold, deep waters onto the Southern Ocean margins of the Australasian carbonate platforms, where water‐mass mixing, warming and loss of CO₂ locally maintained critical levels of carbonate saturation for sea‐floor cement precipitation and promoted the phosphate‐glauconite mineralization associated with some of the marine‐cemented limestone units.     

Order of diagenetic events controls evolution of porosity and permeability in carbonates

The properties of carbonate rocks are often the result of multiple, diagenetic events that involve phases of cementation (porosity occlusion) and dissolution (porosity enhancement). This study tests the hypothesis that the order of these events is a major control on final porosity and permeability. A three-dimensional synthetic model of grainstone is used to quantify trends that show the effect of early cementation, non-fabric selective dissolution, and then a second-generation of (post-dissolution) cement. Models are 3 mm³ with a resolution of 10 μm. Six simple paragenetic sequences are modelled from an identical starting sediment (without accounting for compaction) where the same diagenetic events are placed in different sequences, allowing for quantification of relative changes in the resultant porosity and permeability for each diagenetic event, the trajectory through time, as well as for each final rock. All modelled paragenetic sequences result in reductions in porosity and permeability, but the order of diagenetic events controls the trajectory and final rock properties. Differences in the order of early cement precipitation alone produce variable final values, but all follow the porosity–permeability relationship as expressed by the Kozeny-Carman equation. However, final values for the sequences which include a phase of dissolution fall on a new curve, which departs from that predicted by the Kozeny-Carman relationship. This allows an alternative form of porosity–permeability relationship to be proposed: κ = 2280ϕ–30,400, where ϕ is porosity (%) and κ is permeability (mD). Hence while the Kozeny-Carman relationship predicts porosity–permeability changes that occur with cementation, it is unable to capture accurately changes within the pore network as a result of dissolution. Although the results may be dependent on the properties of the initial carbonate sediment and simplified diagenetic scenarios, it is suggested that this new porosity–permeability relationship may capture some generalized behaviour, which can be tested by modelling further sediment types and diagenetic scenarios.     

鄂尔多斯盆地姬塬地区延长组长4+5低渗透储层成因

碎屑岩储层成岩作用复杂而强烈,对储层物性有着重要影响。应用岩相学研究方法,在对鄂尔多斯盆地姬塬地区延长组长4+5低渗砂岩储层成岩作用及其对物性影响定量研究基础上,探讨了低渗储层成因机理。成岩作用和孔隙演化研究表明,压实作用虽然造成12%～20%的孔隙度损失,但是压实后剩余孔隙度仍高达15%～23%,早期胶结作用使孔隙度损失很小,压实作用和早期胶结作用并没有使储层致密,不影响油气渗流。溶蚀作用进一步改善了储层,晚成岩阶段A期-B期形成的钠长石、亮晶方解石、白云石、自生高龄石和铁绿泥石等大量胶结物的晚期胶结作用使储层孔隙度仅有4%～6%,储层因此而致密,由此影响油气渗流。晚期裂隙作用对储层孔隙度贡献为6%～8%,改善了储层物性而使其成为有效储层。盆地演化及与之对应的成岩事件研究认为,印支运动前,姬塬地区长4+5储层处于浅埋藏阶段,经历压实作用和早期胶结作用,早-中侏罗世长 7烃源岩进入未成熟-低熟阶段,形成富含有机酸流体进入储层而发生溶蚀作用,早白垩世中-晚期进入深埋藏成岩阶段,晚期胶结物大量形成而使储层致密。生烃增压作用导致的裂隙以及晚白垩世以来的构造运动形成的裂缝对研究区长4+5有效储层形成具有非常重要意义,同时对低渗油气藏勘探也具有指导意义。     

The Effects of Diagenesis on the Reservoir Characters in Ridge sandstone of Jurassic Jumara dome,Kachchh, Western India

Ridge sandstone of Jurassic Jumara dome of Kachchh was studied in an attempt to quantify the effects of diagenetic process such as compaction, cementation and dissolution on reservoir properties. The average framework composition of Ridge sandstone is Q₈₀F₁₇L₃, medium-to coarse grained and subarkose to arkose. Syndepositional silty to clayey matrix (3% average) is also observed that occurs as pore filling. The diagenetic processes include compaction, cementation and precipitation of authigenic cements, dissolution of unstable grains and grain replacement and development of secondary porosity. The major cause of intense reduction in primary porosity of Ridge sandstone is early cementation which include silica, carbonate, iron, kaolinite, illite, smectite, mixed layer illite-smectite and chlorite, which prevents mechanical compaction. The plots of COPL versus CEPL and IGV versus total cement suggest the loss of primary porosity in Ridge sandstone is due to cementation. Cements mainly iron and carbonate occurs in intergranular pores of detrital grains and destroys porosity. The clay mineral occurs as pore filling and pore lining and deteriorates the porosity and permeability of the Ridge sandstone. The reservoir quality of the studied sandstone is reduced by clay minerals (kaolinite, illite, smectite, mixed layer illitesmectite, chlorite), carbonate, iron and silica cementation but on the other hand, it is increased by alteration and dissolution of the unstable grain, in addition to partial dissolution of carbonate cements. The potential of the studied sandstone to serve as a reservoir is strongly related to sandstone diagenesis.     

Diagenesis in the Middle Jurassic Khatatba Formation sandstones in the Shoushan Basin,northern Western Desert,Egypt

The Middle Jurassic Khatatba Formation acts as a hydrocarbon reservoir in the subsurface in the Western Desert, Egypt. This study, which is based on core samples from two exploration boreholes, describes the lithological and diagenetic characteristics of the Khatatba Formation sandstones. The sandstones are fine‐ to coarse‐grained, moderately to well‐sorted quartz arenites, deposited in fluvial channels and in a shallow‐marine setting. Diagenetic components include mechanical and chemical compaction, cementation (calcite, clay minerals, quartz overgrowths, and a minor amount of pyrite), and dissolution of calcite cements and feldspar grains. The widespread occurrence of an early calcite cement suggests that the Khatatba sandstones lost a significant amount of primary porosity at an early stage of its diagenetic history. In addition to calcite, several different cements including kaolinite and syntaxial quartz overgrowth occur as pore‐filling and pore‐lining cements. Kaolinite (largely vermicular) fills pore spaces and causes reduction in the permeability of the reservoir. Based on framework grain–cement relationships, precipitation of the early calcite cement was either accompanied by or followed the development of part of the pore‐lining and pore‐filling cements. Secondary porosity development occurred due to partial to complete dissolution of early calcite cements and feldspar. Late kaolinite clay cement occurs due to dissolved feldspar and has an impact on the reservoir quality of the Khatatba sandstones. Open hydraulic fractures also generated significant secondary porosity in sandstone reservoirs, where both fractures and dissolution took place in multiple phases during late diagenetic stages. The diagenesis and sedimentary facies help control the reservoir quality of the Khatatba sandstones. Fluvial channel sandstones have the highest porosities and permeabilities, in part because of calcite cementation, which inhibited authigenic clays or was later dissolved, creating intergranular secondary porosity. Fluvial crevasse‐splay and marine sandstones have the lowest reservoir quality because of an abundance of depositional kaolinite matrix and pervasive, shallow‐burial calcite and quartz overgrowth cements, respectively. Copyright © 2013 John Wiley & Sons, Ltd.     

成岩作用对河流-三角洲相砂岩储层物性演化的影响--以延长油区上三叠统延长组长2砂岩为例

延长油区上三叠统长 2地层河流相-三角洲相砂岩储层的物性明显受埋藏-成岩作用事件的影响。埋藏压实作用是导致砂岩孔隙丧失的主要原因,造成的平均孔隙度丧失为 17.8%。其中黑云母的早期成岩蚀变是造成原生孔隙丧失的一个重要原因。胶结作用造成的平均孔隙度丧失为 7.1%。其中碳酸盐胶结物和次生石英加大是造成砂岩物性降低的主要胶结物。碎屑颗粒周围绿泥石薄膜的存在阻止了一部分石英次生加大及碳酸盐胶结物的沉淀,使一部分原生粒间孔隙得以保存。晚期成岩阶段有机质分解形成的酸性流体及表生成岩作用阶段的大气降水是形成次生孔隙的主要原因,从而使长 2砂岩的物性得到改善     

Isotopic and trace element evidence for submarine lithification of hardgrounds in the Jurassic of eastern England

Geochemical and petrographic data suggest early submarine cementation of hardgrounds from the Lincolnshire Limestone Formation, Middle Jurassic, England. The three hardgrounds, from Cowthick, Castle Bytham and Leadenham quarries, developed in tidal-inlet, on-barrier and lagoonal sub-environments of a carbonate barrier-island complex. At Cowthick early composite (acicular-bladed) radial-fibrous cements, which pre-date aragonite dissolution, completely fill intergranular pore-space at the hardground surface; away from it isopachous fringing cements decrease in thickness. Microprobe analyses demonstrate zoning within the fringes with magnesium concentrations (> 2 wt % MgCO₃) higher than those in allochems or later, ferroan cement (?0.5 wt % MgCO₃, 1.7 wt % FeCO₃). At Castle Bytham early granular isopachous cements, which post-date aragonite dissolution, occur within 5 cm of the surface. At Leadenham early lithification is superficial and represented by ferruginous crusts and micritic internal sediment. Late blocky cement fills residual pore-space in all three examples. Carbon and oxygen isotopic composition of whole-rock samples taken at intervals away from each hardground surface demonstrate the increasing proportion of late ¹⁸O depleted cements (δ¹⁸O – 8 to – 10). Early cements must have a marine isotopic composition; different δ¹⁸O values from each hardground reflect the intensity of early lithification and exclusion of late cements at the hardened surface. There is no isotopic evidence for subaerial cement precipitation during possible emergence at Castle Bytham. Oyster samples (with δ¹⁸O, – 2.9 and δ¹³C, 2.4) give estimated palaeotemperatures of 22–25°C. Early cements from Cowthick are enriched in ¹⁸O and ¹³C (δ¹⁸O = 0 δ¹³C ? 3‰) compared to the oyster values. In conjunction with trace element data this is interpreted as evidence for high-magnesium calcite precursor cements which underwent replacement in a system with a low water: rock ratio. The intensity of early lithification is related to depositional environment: maximum circulation of sea-water producing the most lithified hardground (Cowthick). This is directly analogous to the formation of Recent hardgrounds.     

Calcite uranium–lead geochronology applied to hardground lithification and sequence boundary dating

Hardground discontinuities within carbonate platforms form important stratigraphic surfaces which can be used at basin scale to correlate sequence boundaries. Although these surfaces are commonly used in sequence strati‐graphy, the timing and duration of hardground lithification and the crystallization of early cements remain unexplored. Here, early calcite cements were dated by U‐Pb geochronology for five Jurassic hardgrounds, interpreted as third‐order sequence boundaries, situated within a well‐constrained petrographic, sedimentological and stratigraphic framework. The consistency or the slight deviation between the age of the cements and the stratigraphic age of deposition of the formations illustrate that cementation occurred early in the diagenetic history. The ages obtained on dogtooth cements, replacing aragonite in gastropod shells and pendant cements in intergranular spaces, match those of the standard Jurassic biostratigraphic ammonite Zones, making calcite U‐Pb geochronology a promising method for dating third‐order sequence boundaries of depositional sequences and refining the Jurassic time scale in the future.     

查干凹陷早白垩世碎屑岩储层成岩作用及孔隙演化

在对大量薄片和岩心观察分析基础上,结合扫描电镜和全岩分析、 X衍射粘土分析、 包裹体测温等手段,对查干凹陷碎屑岩储层的成岩作用和孔隙演化进行了研究。结果表明: 研究区碎屑岩储层主要为低成分成熟度、 低结构成熟度的长石岩屑砂岩,成岩作用呈现“强压实、 强胶结、 弱交代、 弱溶蚀“的“两强两弱”特征;成岩阶段主要处于中成岩A期,局部达中成岩B期至晚成岩期。两期胶结主要发生在苏二早期和乌兰苏海时期,中浅层（<2 500 m）较低孔隙度主要由于强压实和早期致密胶结所致;较高孔隙度是由于原生孔隙保存较好,长石及碳酸盐胶结物的溶蚀作用导致孔隙度增大。深层碎屑岩储层（>2 500 m）较低孔隙度除压实作用外,碳酸盐致密胶结是主因;较高孔隙度则为早期酸性溶蚀叠加晚期碱性溶蚀产生次生孔隙为主。晚期碱性溶蚀发生在油气充注之后,为无效溶孔。     

Morphology and genesis of nodular chalks and hardgrounds in the Upper Cretaceous of southern England

The Upper Cretaceous chalks of southern England are a thick sequence of rhythmically bedded, bioturbated coccolith micrites, deposited in an outer shelf environment in water depths which varied between 50 and 200–300 m. The products of sea floor cementation are widely represented in the sequence, and a series of stages of progressive lithification can be recognized. These began with a pause in sedimentation and the formation of an omission surface, followed by (a) growth of discrete nodules below the sediment-water interface to form a nodular chalk, erosion of which produced intraformational conglomerates. (b) Further growth and fusion of nodules into continuous or semicontinuous layers: incipient hardgrounds. (c) Scour, which exposed the layer as a true hardground. At this stage, the exposed lithified chalk bottom was subject to boring and encrustation by a variety of organisms, whilst calcium carbonate was frequently replaced by glauconite and phosphate to produce superficial mineralized zones. In many cases, the processes of sedimentation, cementation, exposure and mineralization were repeated several times, producing composite hardgrounds built up of a series of layers of cemented and mineralized chalk, indicating a long and complex diagenetic history. Petrographic study of early cemented chalks indicates lithification was the result of the precipitation of small crystals on and between coccoliths and coccolith fragments. By analogy with known occurrences of early lithification in Recent deeper water carbonates, the cement is believed to have been either high magnesian calcite or aragonite, and more probably the former. The vast scale of operations involved in the cementation process precludes carbonate in expelled pore fluids as the source of cement, whilst quantities of aragonite incorporated in sediment are also inadequate. This, plus the observed association of horizons of early lithification with pauses in sedimentation associated with omission surfaces suggests seawater as a source of cementing materials. Stratigraphic studies indicate that processes of early lithification leading to hardground formation proceeded to completion in intervals to be measured in tens or hundreds of years. Regional studies suggest that early lithification characterized relatively shallow water phases associated with regional regression over the whole of the area, whilst in detail, the distribution of mature mineralized hardground complexes is strongly correlated with sedimentary thinning and condensation over small areas and the buried flanks of massifs. Early cementation in more basinal areas is typically in the form of nodular developments and incipient hardgrounds, whilst day contents in excess of a few percent appear to have inhibited early lithification. The striking rhythmicity of hardgrounds and nodular chalks is no more than a particular expression of the overall rhythmicity of chalk sequences. The stage of early lithification reached in any instance is dependent on sediment type, the time interval represented by the associated omission surface and the degree of associated scour and erosion (if any). Chalk hardgrounds differ from most others described in the geological literature in their widespread distribution (individual hardgrounds may cover up to 1500 km²), the presence of striking glauconite and phosphate replacements of lithified carbonate matrices, their frequently sparse epifaunas, and boring infaunas dominated by clionid sponges. These differences reflect the deeper water shelf setting of the chalk, and the more open marine, oceanic circulatory system, both strikingly different from the setting of other, shallower water hardgrounds. Litho- and biostratigraphic variation in the chalk sequences of the area studied are summarized in an appendix.