鄂尔多斯盆地镇泾区块延长组长81储层成岩作用特征及其对储集物性的影响

鄂尔多斯盆地镇泾区块上三叠统延长组长81储层具有成分成熟度低,结构成熟度中等到好的特点,岩性以岩屑长石砂岩和长石岩屑砂岩为主.长81砂岩属于特低孔特低渗储层,以次生溶蚀粒间孔为主要储集空间.目前储层正处于中成岩阶段A期,主要发育压实、胶结、溶解和交代4种成岩作用类型.机械压实作用是造成长81储层埋深小于2000 m砂岩...     

Diagenesis and hydrocarbon emplacement in the Upper Triassic Yanchang Formation tight sandstones in the southern Ordos Basin,China

The Upper Triassic Yanchang Formation in the southern Ordos Basin is a tight sandstone but productive hydrocarbon exploration target. A variety of analyses, including thin-sections, cathodoluminescence (CL), scanning electron microscope (SEM), X-ray diffraction (XRD), fluorescence, isotope and fluid inclusion, have been used to analyse the reservoir petrology and diagenesis to understand the evolution of reservoir porosity. The sandstones are mostly feldspathic litharenites and lithic arkoses with low porosities (7.6% on average) and extremely low permeabilities (0.49 mD on average). A complex diagenetic alteration history of the reservoir caused different kinds of reduction in reservoir porosity. The carbonate cements are sourced from adjacent organic-rich mudstone and precipitated with a higher content near the edges of sandstone units and a lower content at the centres of sandstone units, resulting in two different diagenetic evolution paths. The analysis of porosity evolution history and hydrocarbon emplacement produced the following conclusions: (1) during the eodiagenesis period, mechanical compaction reduced the primary porosity dramatically from 40 to 19%; and (2) during the mesodiagenesis period, the first-phase oil charged the reservoir (porosity ranging from 14 to 19%) as bitumen along detrital grains forming a preferential pathway for subsequent oil emplacements. In the second-phase of oil charge, the reservoir porosity ranged from 9 to 14% limiting water–rock reactions. During the third-phase of oil charge, porosity of the reservoir remained at 9%. In general, the densification period (with a porosity close to 10%) of the reservoir occurs at the same time as the major oil emplacement and may be an explanation for the low oil saturation of the Chang 8 tight sandstone.     

志丹探区西南部长4+5段成岩作用及其对储层的影响

为了认清鄂尔多斯盆地志丹探区西南部三叠系延长组长4+5段有利储层的分布规律,通过大量岩石薄片和铸体薄片的镜下鉴定,并结合扫描电镜、阴极发光和X-衍射等测试结果,研究了该区长4+5段主要成岩作用及其对储层物性的影响.研究结果显示,长4+5段主要成岩作用类型有压实作用、胶结作用和溶蚀作用:强烈的压实作用及胶结作用导致其原生孔隙丧失殆尽,成为形成特低孔渗储层的主要原因,而晚期较弱的溶蚀作用,对储层的改善发挥了较大作用,从而使鄂尔多斯盆地延长组发育部分有利储层,形成较好的储集-成岩相带.     

鄂尔多斯盆地冯地坑-洪德地区延长组长4+5段储层砂岩成岩作用研究

鄂尔多斯盆地冯地坑-洪德地区长4+5段储层以长石砂岩和岩屑长石砂岩为主。本文通过岩心观察、薄片鉴定、扫描电镜、粘土矿物分析等,研究延长组长4+5段的成岩作用特征,分析其对储层物性的影响,确定储层砂岩的成岩作用阶段。早成岩期的压实作用和多种矿物的胶结作用是造成本区岩石孔隙体积减少的重要因素,而溶解作用尤其是长石的溶蚀作用是储层变好的重要控制因素。     

致密砂岩储层成岩作用与孔隙演化:以川西南上三叠统为例

致密砂岩储层成岩作用控制孔隙演化,并进一步决定相对优质储层分布。川西南上三叠统砂岩储层整体孔隙度小于6%,基质渗透率小于0.1×10-3μm2,属于典型致密砂岩储层。铸体薄片和储层流体包裹体综合分析表明,砂岩储层经历了压实作用、胶结作用、溶蚀作用和交代作用,储集空间以粒内溶孔、铸模孔和粒间溶孔为主。综合各成岩矿物共生组合关系,结合储层流体包裹体均一化温度,确定各成岩事件和成岩矿物形成相对顺序:压实作用→第Ⅰ期裂缝→第Ⅰ期伊利石胶结→第Ⅰ期石英加大→第Ⅰ期方解石胶结→第Ⅱ期石英加大、硅质充填→长石、岩屑溶蚀→绿泥石胶结或者伊利石胶结→第Ⅱ期方解石胶结→长石溶蚀作用→第Ⅱ期破裂作用→第Ⅲ期石英加大、硅质充填→第Ⅲ期碳酸盐胶结→碳酸盐交代长石和石英→第Ⅲ期构造破碎→晚期方解石和石英胶结成岩。镜质体反射率(Ro)及砂岩储层流体包裹体均一温度表明砂岩储层目前已达到中成岩A2期至B期,压实作用是储层致密化的最主要原因,造成孔隙度损失约27.5%,硅质和碳酸盐胶结作用仅损失孔隙度约5.3%。     

Diagenesis in Mesozoic sandstones from Spitsbergen and the North Sea — A comparison

Mesozoic and Tertiary sandstones on Spitsbergen are generally tight and well cemented while Mesozoic and Tertiary sandstones from the North Sea include highly porous reservoir rocks.It is concluded that one of the most important controlling mechanisms for preservation of porosity is the build up of high pore pressures which effectively reduce pressure solution at grain contacts and the incidence of quartz overgrowth.High pore pressures are most commonly developed in onlapping sedimentary sequences with an effective shale seal like in the North Sea.Extensive diagenetic growth of kaolinite at the expence of clastic feldspar, which is commonly observed, requires a large flux of low salinity water. The most probable mechanism is fresh ground water drive through sandstones which have served as aquifers. During the late stages of burial (1–3 kms) the diagenetic reactions must have taken place within an isochemical semi-closed system. Calculations show that porewater expelled from compacting shales is insufficient to cause significant changes in the bulk chemistry of thick sandstones. Microprobe analyses carried out on Jurassic sandstones from the Statfjord field show that there are a continuous series of transitions from clastic mica to illite and kaolinite as indicated by a gradual loss of potassium. Evidence is presented to show that silica cementation is retarded in the presence of clastic mica while carbonate and feldspar cementation is favoured in this local high pH environment between sheets of mica. Electron probe analyses show that diagenetic feldspar overgrowths are pure potassium feldspar with a composition significantly different from that of clastic feldspar. During late diagenesis the porewater was most probably near equilibrium with the reactive phases.

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Zusammenfassung Die mesozoischen und tertiären Sandsteine von Spitzbergen sind im allgemeinen gut zementiert, während die gleichalten Sandsteine aus der Nordsee hochporöse Speichergesteine umfassen.Man erkennt, daß die Erhaltung der Porosität auf einen hohen Porenwasserdruck zurückzuführen ist. Dadurch wird Drucklösung an Kornkontakten sowie Weiterwachsen von Quarz verhindert.Hoher Porendruck entsteht gewöhnlich da, wo überlappende Sedimentserien mit einer Abdichtung durch Schiefer vorliegen—wie in der Nordsee.Ausgedehntes diagenetisches Wachstum von Kaolinit auf Kosten klastischer Feldspäte erfordert einen hohen Durchfluß von niedrigsalinem Wasser. Der wahrscheinlichste Mechanismus ist in einem Grundwasserstrom zu suchen, der durch die Sandsteine fließt, die als Wasserleiter dienen. Während der späten Stadien der Absenkung (1–3 km) müssen diese diagenetischen Reaktionen in einem halbgeschlossenem isochemischen System abgelaufen sein. Berechnungen aus Porenwasser, das von Schiefern gewonnen wurde, zeigen, daß dieses nicht ausreicht, bedeutende änderungen in der Gesamtchemie mächtiger Sandsteine zu bewirken.Mikrosondenanalysen, die an jurassischen Sandsteinen aus dem Statfjord-Feld vorgenommen wurden, zeigen, daß hier eine kontinuierliche Serie von übergängen aus klastischem Glimmer nach Illit und Kaolinit vorliegt. Eine stufenweise Abnahme von Kalium deutet dies an. Quarz-Zementation tritt zurück bei Anwesenheit von klastischem Glimmer, während Karbonat- und Feldspat-Zementation unter den Bedingungen eines hohen pH-Wertes zwischen den Glimmer-Blättern bevorzugt erscheint. Mikrosondenanalysen zeigen, daß der diagenetisch aufgewachsene Feldspat ein reiner Kali-Feldspat ist, der sich beträchtlich von den klastischen Feldspäten unterscheidet. Während der späten Diagenese stand das Porenwasser nahezu im Gleichgewicht mit den reagierenden Phasen.

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Résumé Les grès mésozoÏques et tertiaires du Spitzberg sont en général bien cimentés, tandis que les grès de mÊme âge de la Mer du Nord comprennent des roches-réservoirs très poreuses. On en conclut que l'un des plus importants mécanismes régissant le maintien de la porosité est la mise en jeu de fortes pressions dans les pores de la roche, ce qui effectievement réduit la dissolution par la voie de la pression s'exerÇant au contact des grains et l'intervention de la croissance des grains de quartz.De fortes pressions dans les pores sont le plus couramment développées dans les séquences sédimentaires isolées par un recouvrement argileux, comme dans la Mer du Nord.La croissance diagénétique à grande échélle de la kaolinite aux dépens du feldspath détritique, communément observée, requiert une grande circulation d'eau à faible salinité. Le mécanisme le plus probable est celui de la circulation d'eau douce dans des grès aquifères. Durant les derniers stades de l'enfouissement (1–3 km) les réactions diagénétiques se sont produites dans un système isochimique à moitié fermé. Des calculs montrent que l'eau poreuse chassée des argilites à la suite de leur compaction est insuffisante pour causer des changements de quelqu'importance dans la composition chimique des grès en masses épaisses. Des analyses à la microsonde sur des grès jurassiques de la région du Statfjord montrent qu'il existe une gamme continue de transitions depuis le mica détritique jusqu'à l'illite et la kaolinite, ainsi qu'il ressort de la perte graduelle du potassium. La preuve est donnée que la cimentation par la silice est retardée par la présence de mica détritique, tandis que la cimentation par le carbonate et par le feldspath est favorisée dans ce milieu restreint à pH élevé entre lamelles de mica. Des analyses électroniques montrent que les accroissements diagénétiques de feldspath sont faits de feldspath potassique pur d'une composition notablement différente de celle del feldspath détritique. A la fin de la diagénèse l'eau intersticielle était très probablement proche de l'équilibre avec les phases réagissantes.

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Triassic breakup of Pangea in southern Mexico: Thermochronological evidence from the Tianguistengo formation

Southern Mexico is a key area for unraveling the tectonic evolution of North America because it contains the stratigraphic and structural record of the major tectonic events that shaped this continental mass, such as the breakup of Pangea and the growth of the North America Cordilleran Orogeny. However, multiple reactivations of faults and erosion of the stratigraphic record do not permit to adequately assess the timing of these tectonic events. Although most authors suggested that lithospheric extension and exhumation of continental blocks during Pangea breakup started in Mexico by Early Jurassic time, works published in the last decade provide an increasing number of thermo-tectonic evidence of an earlier phase of continental thinning. In this work, we present detrital apatite thermochronological and geochemical data (trace elements including rare earth elements) from fluvial deposits of the Tianguistengo formation, which is the oldest stratigraphic unit of the Otlaltepec Basin, a major basin in southern Mexico that has been linked to Pangea breakup. Our data show that at least a part of the Tianguistengo formation was derived from the adjacent Pennsylvanian–Cisuralian Totoltepec pluton. Apatite fission-track-based time–temperature modelling for unreset apatite populations suggests that the main exhumation of the Totoltepec pluton, which prompted the deposition of a part of the Tianguistengo formation, took place during Late Triassic time. Thus, our results suggest that Pangea breakup in southern Mexico started at least by Middle–Late Triassic time (240–230 Ma), as it is recorded in the Otlaltepec Basin.     

鄂尔多斯盆地富县地区上三叠统长8砂岩储层物性的主要控制因素

研究认为富县地区上三叠统长8期三角洲前缘水下分流河道砂岩储层物性主要受沉积微相、砂岩成分和粒度以及成岩作用的控制。特别指出沉积微相对储层的控制,实际上是沉积(微)相带对砂体类型和物性特征的控制,不同沉积微相的砂岩在储集性能上存在明显的差异。表现为三角洲前缘水下分流河道砂体的孔隙度及渗透率均高于其它沉积微相内砂体的孔隙度和渗透率,而分流间湾的沉积物粒度相对较细,杂基含量高,压实作用强,不利于后期溶蚀流体的流动,储层物性较差。强烈的机械压实、压溶作用、碳酸盐自生矿物的胶结作用使孔隙结构变差,进一步降低砂岩的储集性能;由淡水淋滤及有机物质产生的溶蚀作用,对储层改善具有重要意义。     

鄂尔多斯盆地富县地区上三叠统长8砂岩储层物性的主要控制因素

研究认为富县地区上三叠统长8期三角洲前缘水下分流河道砂岩储层物性主要受沉积微相、砂岩成分和粒度以及成岩作用的控制。特别指出沉积微相对储层的控制,实际上是沉积（微）相带对砂体类型和物性特征的控制,不同沉积微相的砂岩在储集性能上存在明显的差异。表现为三角洲前缘水下分流河道砂体的孔隙度及渗透率均高于其它沉积微相内砂体的孔隙度和渗透率,而分流间湾的沉积物粒度相对较细,杂基含量高,压实作用强,不利于后期溶蚀流体的流动,储层物性较差。强烈的机械压实、压溶作用、碳酸盐自生矿物的胶结作用使孔隙结构变差,进一步降低砂岩的储集性能; 由淡水淋滤及有机物质产生的溶蚀作用,对储层改善具有重要意义。     

Permian–Triassic magmatism in response to Palaeotethys subduction and pre-Late Triassic arrival of northeast Gondwana-derived continental fragments at the southern Eurasian margin: Detrital zircon evidence from Triassic sandstones of Central Iran

The closure of Palaeotethys that led to the collision of the Cimmerian blocks with the southern Eurasian margin causing the Eo-Cimmerian orogeny during the Early Mesozoic is still controversially discussed. The Triassic Nakhlak Group in Central Iran is a key sedimentary succession for better understanding the closure of Palaeotethys and the Eo-Cimmerian orogeny in the Middle East. The Nakhlak Group is composed of the Alam (Olenekian to Middle Anisian), Baqoroq (?Upper Anisian to Middle Ladinian) and Ashin (Upper Ladinian to ?Carnian) formations, which consist mainly of volcaniclastic sandstones, mixed siliciclastic conglomerates, and marine carbonates. Here we present for the first time detrital zircon UPb ages from the Nakhlak Group to unravel its provenance and constrain its palaeotectonic position within the Palaeotethyan realm. Most detrital zircons from the Nakhlak Group are euhedral and subhedral with Permian–Triassic ages (ca. 280–240 Ma) suggesting sediment supply from Permian–Triassic magmatic rocks of the Silk Road Arc. Minor zircon populations show pre-Permian Palaeozoic ages, with age peaks at ca. 320 Ma and 480 Ma, which are probably derived from the basement on which the magmatic arc developed. Neoproterozoic–latest Mesoproterozoic (ca. 550–1100 Ma) and Palaeoproterozoic (ca. 1800–2200 Ma) zircon grains are anhedral (rounded). The latter are prominent in the upper Baqoroq Formation (Middle Ladinian) suggesting recycling of older sedimentary rocks. Sandstone petrography points toward an additional metamorphic provenance for this formation. This short-lived provenance change can be explained by tectonic uplift in the source area that led to erosion of metamorphosed rocks with a northeast Gondwanan affinity. It clearly indicates that northeast Gondwana-derived continental fragments likely belonging to the Cimmerian blocks already arrived at the southern Eurasian margin in pre-Late Triassic time. Current palaeotectonic models of the closure of Palaeotethys and the Eo-Cimmerian orogeny in the Middle East during the Triassic may need to be revised.     

Diagenesis and reservoir quality analysis in the Lower Cretaceous Qishn sandstones from Masila oilfields in the Sayun–Masila Basin,eastern Yemen

Lower Cretaceous sandstones of the Qishn Formation have been studied by integrating sedimentological, petrological and petrophysical analyses from wells in the Masila oilfields of eastern Yemen. These analyses were used to define the origin, type of diagenesis and its relation to reservoir quality. The sandstones of the Qishn Formation are predominately quartz arenite to subarkose arenite with sublitharenite and quartz wackes displaying a range of porosities, averaging 22.33%. Permeability is likewise variable with an average of 2844.2 mD. Cementation coupled with compaction had an important effect on porosity destruction after sedimentation and burial. The widespread occurrence of early calcite cement suggests that the sandstones of the Qishn Formation lost significant primary porosity at an early stage of its diagenetic history. In addition to poikilotopic calcite, several different cements including kaolinite, illite, chlorite and minor illite–smectite occur as pore‐filling and pore‐lining cements, which were either accompanied by or followed the development of the early calcite cement. Secondary porosity development occurred due to partial to complete dissolution of early calcite cements and feldspar grains. The new data presented in this paper suggest the reservoir quality of Qishn sandstones is strongly linked to their diagenetic history; hence, the reservoir quality is reduced by clay minerals, calcite and silica cements but is enhanced by the dissolution of the unstable grains, in addition to partial or complete dissolution of calcite cements and unstable grains. Copyright © 2015 John Wiley & Sons, Ltd.     

Structure, chemistry and origins of gypsum crusts in southern Tunisia and the central Namib Desert

Gypsum crusts are broadly defined as accumulations at or within about 10 m of the land surface from 0.10m to 5.0 m thick containing more than 15% by weight gypsum (CaSO₄·2H₂O) and at least 5.0% by weight more gypsum than the underlying bedrock. The deposits are often, but not invariably, consolidated owing to cementation by gypsum. The crusts are found in many of the world's deserts where mean monthly potential evaporation exceeds mean monthly precipitation throughout the year. Using structural, fabric and textural criteria, three main types of crust may be distinguished:(1) bedded crusts, found either at or beneath the land surface, which are made up of discrete horizontal strata up to 0.10 m thick, each showing a gradation in gypsum crystal size from less than 50 μm at the top to more than 0.50 mm at the base; (2) subsurface crusts, of which there are two forms, one made up of large, lenticular crystals (up to 0.50 m in diameter)—the desert rose crusts—and the other, a mesocrystalline form, with gypsum crystals up to about 1.0 mm in diameter; and (3) surface crusts, which are subdivided into columnar, powdery and cobble forms, all of which are made up of predominantly alabastrine gypsum (crystallites less than 50 μm in diameter). In southern Tunisia and the central Namib Desert, bedded crusts are found around ephemeral lakes and lagoons. They are characterized by size-graded beds, gypsum contents of 50–80% by weight and comparatively high concentrations of sodium, potassium, magnesium and iron. They are interpreted as shallow-water evaporites which accumulate when saline pools evaporate to dryness. Desert rose crusts or croûtes de nappe generally contain 50–70% by weight gypsum, and have higher sodium concentrations than the second subsurface form. Texturally they are characterized by poikilitic inclusion of clastic material within large lenticular crystals. They are interpreted as hydromorphic accretions, which precipitate in host sediments at near-surface water tables through the evaporation of groundwater. The second form of subsurface crust—the mesocrystalline—often occurs in close association with the various surface forms. Unlike the hydromorphic crusts, they are not restricted to low-lying terrain. They are characterized by gypsum contents reaching 90% by weight, and have a close chemical and textural similarity to columnar surface crusts. This mesocrystalline form represents an illuvial accumulation; the surface forms—excluding the bedded crusts—are exhumed examples at various stages of solutional degradation. Subsurface precipitation of gypsum from meteoric waters containing salts leached from the surface, results in displacive gypsum accumulation in the soil zone. In southern Tunisia, the gypsum is derived from sand and dust deflated from evaporitic basins; in the central Namib, salts dissolved in fog water are the most likely source. Where other salts are present, differential leaching may form two-tiered crusts, calcrete—gypsum or gypsum—halite, if rainfall is sufficient to mobilize the less soluble salt yet insufficient to flush the more soluble. Gypsum crust genesis is restricted to arid environments, and if their susceptibility to post-depositional alteration is acknowledged, they can provide valuable palaeoclimatic indicators.     

辫状河三角洲储层砂体建筑结构分析

本文应用建筑结构分析法研究塔河油田一区三叠系下油组砂体内部建筑结构。根据取心井岩心观察识别出12种岩相类型;通过连井剖面对比和测井曲线识别,采用层次分析的思路定义了7级界面;在岩相识别和界面划分的基础上划分出7种结构要素:河道滞留沉积(CHL)、河道充填(CH)、纵向砂坝(LB)、横向砂坝(TB)、落淤层(FS)、越岸细粒沉积(OF)和洪泛平原细粒(FF);建立了砂体建筑结构平面组合模式:平面以河道充填(CH)和砂坝(CB)为主。     

松潘-甘孜地块三叠系砂岩的地球化学特征及其意义

松潘-甘孜地块位居中国西南部,北邻华北地块,西与青藏高原毗邻,东南缘与扬子地块相连.该区出露的巨厚层砂岩SiO2含量变化范围大为48.64%～71.77%,稳定元素（Al2O3,Fe2O3,MnO）与不稳定元素（MgO,K2O,Na2O）基本持平,CIW值较低;La/Co值集中于3.0～4.5,Th/Co集中于1.0～1.5,La/Th值集中于2.7～3.1,Th/U值均大于4.0,轻稀土元素含量大于重稀土元素含量,具Eu负异常,稀土元素分布形态与上地壳一致;锆石的U-Pb年龄集中分布于1500～1900Ma、700～900Ma、200～400Ma,与扬子地块、南秦岭的岩浆活动相一致.分析表明松潘-甘孜地块具有稳定的物源区,主要以扬子地块为主;其碎屑母岩应主要源自上地壳,以长英质成分为主.松潘-甘孜地块在三叠纪时期处于大陆岛弧环境,周边地区基本处于稳定状态,没有大规模构造运动和岩浆活动,扬子地块向华北地块的俯冲明显减弱或可能已经停止,秦岭造山运动基本已经完成.     

鄂尔多斯盆地陇东地区延长组砂岩成岩作用及孔隙演化

大量岩石薄片、孔隙铸体薄片、扫描电镜与X衍射以及常规物性等分析资料研究表明,鄂尔多斯盆地陇东地区三叠系延长组砂岩主要为成分成熟度及结构成熟度较低的长石质岩屑砂岩、岩屑砂岩和长石砂岩。优质储层主要发育于粗粒富含刚性颗粒的砂岩中,这类砂岩在埋藏过程中由于早期快速埋藏加之泥岩、板岩及千枚岩岩屑含量高,经历了强烈的压实作用,使其由原始孔隙度大约为39%降低至18.5%;此后发生石英颗粒次生加大及绿泥石胶结,使孔隙有一定程度的损失(大约3.5%);紧随其后的是方解石及白云石碳酸盐胶结,损失孔隙度5%;后来出现碳酸盐胶结物与长石及岩屑的溶蚀作用,新增加孔隙度4％～8%,最晚期的是含铁方解石和含铁白云石胶结充注溶蚀孔隙和部分原生孔隙。     

Diagenesis of the Lower Cretaceous Kanmon Group sandstones, SW Japan

The Kanmon Group (Lower Cretaceous) is a non-marine sequence in the Inner Zone of southwest Japan and is divided into the lower Wakino (lacustrine) and the upper Shimonoseki (fluvial) subgroups. Major diagenetic changes in this group are compaction, iron-oxide cementation, calcite cementation and grain replacement, quartz overgrowth and pore-fill cementation, illite authigenesis, chlorite pore-fill cementation and grain replacement, albitization of feldspar, and grain replacement by pyrite. Two subgroups of the Kanmon Group present no significant differences in general diagenetic features, paragenetic sequence, or the degree of diagenetic changes despite differences in depositional environments (lacustrine vs. fluvial) and stratigraphic positions. However, some differences are recognized in the content and chemistry of authigenic minerals caused by different sandstone framework compositions. The content of authigenic clay minerals is higher in sandstones of the Shimonoseki Subgroup containing abundant volcanic rock fragments. In addition, the composition of chlorite, the most abundant authigenic clay mineral in Kanmon sandstones, is Mg-rich in the volcanoclastic Shimonoseki sandstones, compared to an Fe-rich variety in Wakino sandstones. The original sandstone composition played a significant role in pore-water composition and diagenetic reactions.The Wakino sandstones lost most of its porosity by compaction, whereas Shimonoseki sandstones are only compacted in the vicinity of the basin-bounding fault. The weakly compacted Shimonoseki sandstones, instead, were largely cemented by pore-filling calcite during early diagenesis; cementation prevented compaction during further burial. The Kanmon Group sediments were heated to about 300 °C based on illite crystallinity values.     

Diagenesis of sandstones in the back-arc basins of the western Pacific Ocean

ABSTRACT Sandstones occur in back-arc basins of the western Pacific at DSDP sites 299 (Sea of Japan), 297 (northern Shikoku Basin), 445 and 446 (Daito-Ridge-and-Basin Province), 453 (Mariana Trough), 286 (New Hebrides Basin) and 285 (South Fiji Basin). These sandstones are dominantly volcaniclastic arenites derived from andesitic island arcs. The degree of sandstone diagenesis is dependent on original composition, burial rate, heat flow history of the basin, and timing of sandstone deposition with respect to rifting processes and associated high heat flow.
Sandstones containing a larger proportion of volcaniclastic components showed more diagenetic effects than sandstones containing a significant volume of other rock fragments and mineral components. Sandstones deposited during early stages of rifting (sites 445, 446) with a slow burial rate and high crustal heat flow showed the greatest degree of downhole diagenetic change. These diagenetic changes include early pore-space reduction and rim cementation by clay minerals followed later by calcite, and subsequent pore-fill cementation by clinoptilolite, heulandite, analcite and later calcite. Replacement of recognizable volcanic rock fragments by chert, calcite and zeolites was observed in the deepest part of the hole. Sandstones deposited after rifting under conditions of associated lower heat flow showed considerably less diagenetic changes, particularly if burial was rapid.
The high heat flow associated with earliest rifting, associated fluid circulation driven by thermal convection, and slow burial rate controlled the diagenetic history of these sandstones. Thus, timing of sandstone deposition with rifting stage and associated burial rates were key factors in controlling sandstone diagenesis in back-arc basins.     

川中--川南地区上三叠统须家河组气藏异常压力分布及成因

通过对川中--川南地区须家河组气藏异常压力的纵向和平面分布特征研究,结合沉积背景、构造演化和生烃作用等分析了异常高压的成因;并利用剥蚀量、流体包裹体均一温度及改进的Lee--Kesler状态方程识别了古今地温差与剥蚀厚度及其与压力系数之间的关系。川中--川南须家河组各储层段压力系数在平面上由西北向南逐渐降低,纵向上呈常压(压力系数0.90～1.1)、高压(压力系数1.1～1.4)及超高压(压力系数1.4)三段式分布。生烃增压和储层的致密化是本区产生异常高压的主要原因。各气藏古今地温差为35℃～70℃,古今地温差与剥蚀厚度呈正相关关系、与压力系数呈负相关关系,说明川中--川南须家河组现今压力分布状态是地层抬升剥蚀造成的。     

Marine-like potash evaporite formation on a continental playa: case study from Chott el Djerid, southern Tunisia

An exceptional flood in January 1990 led to the formation of a large ephemeral lake on the Chott el Djerid, a salt playa in southern Tunisia. Repeated observations made during 1990 show that the ephemeral lake underwent four evolutionary stages: (1) initial flooding, (2) evaporative concentration of lake waters, (3) the movement of concentrated brine pools over the playa surface as a result of wind action, and (4) total desiccation of the lake by September 1990. During all four stages the brine chemistry of the lake was monitored. Water inflow into the Chott el Djerid basin was found to have a consistent Ca-SO₄-Cl-rich and HCO₃-CO₃-poor chemistry, reflecting the recycling of homogeneous assemblages of Cretaceous, Mio—Pliocene and Quaternary evaporites within the catchment. As the ephemeral lake shrank, these waters produced an Na-Mg-K-Cl-SO₄ brine which was similar to modern sea water. Mineral saturation data show that, during the desiccation of the lake, saturation with respect to both gypsum and halite was achieved and that the most concentrated brines were ultimately saturated with respect to potash phases. After the desiccation of the lake the main mineral phases found on the Chott included gypsum and halite. In addition, ephemeral deposits of carnallite (observed as carnallitite,3KMgCl_3·6H₂0 + NaCl) were found. This assemblage is that which would be expected to form if the waters had undergone salt norm evaporation at 1 bar pressure at 25°C (SNORM) in the evaporation model proposed by Jones and Bodine (1987). The nature of both the brine chemistry and evaporite mineralogy provides a new and rare example of marine-like potash-bearing evaporites being formed in a contemporary continental playa.     

Removal of organic matter from reservoir water: mechanisms underpinning surface chemistry of natural adsorbents

One of the key challenges in water treatment industry is the removal of organic compounds by cost-effective methods. This study evaluated the adsorptive removal of dissolved organic carbon (DOC) from reservoir water using fuller’s earth (FE) in comparison with natural (SQ) and modified quartz (MSQ) sands. The removal capacities of FE at different contact times, pH levels, adsorbent dosages and initial DOC concentrations were compared with both the quartz sands. The optimum DOC removals by FE and SQs were achieved at contact time of 60 and 30 min, pH level of 6 and 4, and at adsorbent dose of 1.5 g/150 mL and 10 g/100 mL, respectively. The adsorption capacity of FE (1.05 mg/g) was much higher compared to the MSQ (0.04 mg/g) and SQ (0.01 mg/g). Adsorption equilibrium data better fitted to the Freundlich model than to the Langmuir model, suggesting that adsorption occurred primarily through multilayer formation onto the surfaces of FE and SQ. The pseudo-second-order model described the uptake kinetics more effectively than the pseudo-first-order and intra-particle diffusion models, indicating that the mechanism was primarily governed by chemisorption. These observations were well supported by the physiochemical characteristics and charge behaviour of the adsorbents. In mass-transfer study, the results of liquid film diffusion model showed that the adsorption of DOC on FE was not controlled by film diffusion, but other mechanisms also played an essential role. This study demonstrates that FE is an effective adsorbent for the removal of DOC in surface water treatment.