Sedimentological aspects of the Humber Group (Upper Jurassic) of the South Central Graben, UK North Sea

Complex structure, poorly understood sedimentology and poor biostratigraphic control make the Upper Jurassic Humber Group of the South Central Graben one of the least understood and most complex hydrocarbon reservoirs of the North Sea. Detailed logging of available core from 19 exploration wells has been combined with an improved understanding of the relevance of trace fossils and a recognition of important base-level variations to provide a greatly enhanced understanding of the depositional system active within the area at that time. A new sedimentological model, based upon the distribution of facies and facies associations, illustrates that Upper Jurassic structure and consequent basin geometry were the principal controls upon the distribution of depositional environments. Rifting and second-order transgression controlled the back-stepping onlap patterns observed and higher frequency base-level fluctuations controlled the internal architecture of individual sandbodies. The model presented accounts for features of these deposits that were previously considered anomalous, such as the thickness of bioturbated sandstones, paucity of foreshore deposits and complex age relationships of sands.     

Plagioclase dissolution related to biodegradation of oil in Brent Group sandstones (Middle Jurassic) of Gullfaks Field, northern North Sea

Brent Group sandstones from the north side of the Gullfaks Oilfield contain mostly 5–8% albitic plagioclase, whereas plagioclase is almost absent in the same strata in the southern part of the field. Absence of plagioclase throughout the entire vertical extent of the Brent Group in the southern wells seems to rule out provenance as the principal explanation for differing plagioclase content, which is therefore interpreted as the result of diagenesis. Hypotheses for the nature of the inferred leaching event include epigenetic meteoric diagenesis and mesogenetic release of acid components from clay minerals or kerogen, but these explanations are unable to account for the observed spatial distribution of the plagioclase-bearing and plagioclase-free sandstone intervals. However, overall correspondence between the area lacking plagioclase and oil compositions having both anomalously high CO₂ and organic geochemical indications of advanced biodegradation suggest a link between plagioclase dissolution and biodegradation of the present oil column. It is, therefore, proposed that acid components from biodegradation selectively reacted with albitic plagioclase to form kaolin, releasing sodium bicarbonate into the residual water. The plagioclase-free sandstones contain more kaolin than the plagioclase-bearing sandstones, as would be expected due to aluminium conservation. However, the wide and overlapping ranges of kaolin content in both groups suggest that most of the kaolin originated from processes other than biodegradation-driven plagioclase alteration, potentially including both epigenetic and mesogenetic dissolution, as well as deposition of detrital kaolin and kaolin precursors.     

Microtextures, geochemistry and geochronology of authigenic xenotime: constraining the cementation history of a Palaeoproterozoic metasedimentary sequence

An ≈ 26 m thick unit of phosphatic sandstone and black shale (the Phosphatic Unit) in the Palaeoproterozoic Mount Barren Group of south-western Australia contains abundant authigenic xenotime crystals showing well-preserved diagenetic textures. Despite extensive regional deformation and thermal metamorphism, the peak of which occurred at ≈ 1205 Ma, the Phosphatic Unit was preserved as a low-strain envelope because of its pre-compaction carbonate and phosphate cementation. In situ U–Pb geochronology of xenotime reveals four discrete age populations at 1693 ± 4, 1645 ± 3, 1578 ± 10 and 1481 ± 21 Ma. When integrated with petrography, the age data place a timeframe on: (i) sediment deposition; (ii) phosphogenesis; (iii) diagenetic cement infilling; (iv) diagenetic pyrite formation; (v) secondary porosity generation; (vi) hydrocarbon migration; (vii) burial compaction; and (viii) hydrothermal alteration, up until peak thermal metamorphism. Xenotime growth at ≈ 1693 Ma occurred prior to compaction, whereas xenotime growth at ≈ 1645 Ma occurred during burial. Xenotime growth at ≈ 1580 Ma and at ≈ 1480 Ma appears to be the far-field record of thermotectonic events associated with intracontinental extension and magmatism recorded elsewhere in Australia. Geochemical analysis, integrated with geochronology, shows a systematic increase in MREE/HREE in xenotime crystals with decreasing age and with increasing stratigraphic depth. Coupled with a decrease in xenotime abundance and age with depth, it suggests that: (i) the main focus of porosity infilling was at the top of the Phosphatic Unit and progressed downwards over the > 200 Myr period of porosity infilling, and (ii) the changes in xenotime REE chemistry may be due to an influx of MREE from increasing amounts of dissolved apatite or changes, with respect to REE solubility, in the physiochemical nature of the fluids with burial depth.     

准噶尔盆地玛湖凹陷风城组页岩自生长英质矿物的成因机理及其储层改造意义

玛湖凹陷二叠系风城组是近几年准噶尔盆地页岩油接替的主攻领域,但其页岩成岩改造强烈,原始岩性归属尚不明确。X射线粉晶衍射测得风城组页岩黏土矿物含量普遍较低（<10%）,长英质矿物含量较高（60%~80%）。薄片分析发现风城组长英质页岩存在三种类型：① 粉砂级（4~63 μm）长石和石英碎屑含量丰富,为典型的粉砂岩;② 燧石条带和团块发育,为富硅页岩;③ 长英质基质发育,呈非碎屑状,且火山尘及火山玻璃少见,其矿物组成和形态与典型的黏土岩、粉砂岩及沉凝灰岩明显不同。背散射图像和高精度扫描电镜测得该长英质基质由碎屑和自生石英、钾长石和钠长石共同组成,可见黏土矿物、碎屑钾长石和钠长石向自生石英转化,碎屑钾长石向自生钠长石转化。通过调研全球中新生代碱湖沉积中自生硅酸盐矿物组合特征和成岩演化规律,探讨了二叠系风城组“贫”黏土矿物和“富”自生长英质矿物的原因,并提出风城组发育一类“改造长英质页岩”,主要由原始黏土和长石碎屑矿物和火山物质经过多期成岩改造而成。早期高碱（pH>9）沉积和成岩环境增加了SiO2溶解度和元素铝的活性,导致黏土矿物、泥级—细粉砂级长英质碎屑、火山灰等在入湖后发生“溶解、转化”,形成次稳定的硅酸盐矿物,如沸石和含镁蒙皂石,在经历漫长埋藏成岩后进一步转化为更为稳定的石英、钾长石和钠长石。碱湖页岩的成岩改造过程消耗了黏土矿物和火山灰,极大增加了页岩脆性,同时伴生了大量基质溶孔和晶间孔,是一类优质的页岩储层。     

Diagenetic signatures of stratal surfaces in the Upper Jurassic Fulmar Formation, Central North Sea, UKCS

Sedimentation rate and changes in relative sea level affect early diagenetic cementation along key stratal surfaces within the deeply buried Upper Jurassic Fulmar Formation, South Central Graben, UKCS. As a result of the bioturbated nature of the shallow marine strata, variations in ichnofabrics and cross-cutting relationships between trace fossils are important in identifying and correlating stratal surfaces. Two transgressive and two regressive surfaces/sequence boundaries were studied, using petrographical, mineralogical and geochemical techniques. Cements, including early diagenetic pyrite framboids, grain-rimming apatite, microcrystalline dolomite and ankerite, occur at, above and immediately below both transgressive and forced regressive surfaces/sequence boundaries. Breaks or subdued rates of sedimentation associated with both types of surfaces meant that the sediment package resided within early diagenetic zones for prolonged periods of time, enhancing diagenetic reactions at and below the surfaces. The distribution of ankerite, despite being a deep-burial cement, was primarily controlled by concentrations of bioclasts that are particularly abundant at transgressive surfaces. The diagenetic character of the forced regressive surfaces/sequence boundaries is more complex than that of the transgressive surfaces, both in terms of mineralogy and paragenesis. This is attributed to the superimposition of diagenetic assemblages as a result of erosion, re-sedimentation and sediment by-passing. Although the diagenetic signature of the surfaces has been modified by dissolution/replacement reactions during deep burial, early diagenetic signatures can still be distinguished.     

Quartz types in the Upper Pennsylvanian organic-rich Cline Shale (Wolfcamp D), Midland Basin,Texas: Implications for silica diagenesis,porosity evolution and rock mechanical properties

The origin and form of quartz in mudrocks has significant implications for interpretation of depositional environments, diagenetic pathways, mechanisms of porosity reduction and rock mechanical-property evolution. Quartz types in the Upper Pennsylvanian Cline Shale, Midland Basin, Texas, were examined using a combination of field-emission scanning electron microscopy-based energy-dispersive spectroscopy elemental mapping (to determine mineralogy) and scanning electron microscopy-based cathodoluminescence imaging (to determine quartz types) with the goal of elucidating a high-resolution imaging protocol at the micrometre scale for shale petrology. Also, the unconfined compressive rock strength of shale samples with contrasting proportions of different quartz types was measured using Equotip Bambino analyses. The results suggest that extrabasinal detrital quartz, which accounts for an average of 26 vol.% of the rock in all analyzed samples, is the dominant form of quartz in the Cline Shale. The intergranular clay-size microquartz, which accounts for an average of 10 vol.% of the rock in all analyzed samples, is the dominant form of authigenic quartz. Dissolved radiolarians and sponge spicules are likely sources of silica for clay-size microquartz and other authigenic quartz showing pale-mauve to dark greyish cathodoluminescence colour. Some authigenic quartz in the form of intragranular pore filling and mollusc skeletal replacement displays bright-reddish cathodoluminescence colour, which may be associated with silica released at a different time in the rock's diagenetic history, such as during smectite illitization. Porosity reduction in the Cline Shale predominantly resulted from compaction because of extremely low intergranular volume and the general lack of early cementation. Quartz form significantly impacts rock mechanical properties in the Cline Shale: widely distributed intergranular clay-size microquartz cement is a major factor controlling rock strength. This correlation also applies to other mudrock successions of various geological ages, tectonic histories and lithologies.     

Interaction between aeolian, fluvial and playa environments in the Permian Upper Rotliegend Group, UK southern North Sea

The Permian Upper Rotliegend Group in offshore UK Quadrants 42, 43, 47 and 48 comprises a sequence of mixed aeolian/fluvial/playa deposits. These deposits are up to 300 m thick and contain a record of the interaction between desert fluvial systems and adjacent aeolian and playa environments. The relative dominance of water vs. wind transport and deposition in this stratigraphic package was a function of fluctuations in the discharge of ephemeral fluvial systems and changes in water table/playa level driven by a combination of climatic change and syndepositional tectonics. The Rotliegend sedimentary record is punctuated by numerous surfaces recording erosion by wind and water. The origin of these surfaces is mostly climatic, with periods of increased runoff resulting in fluvial incision, especially near active faults. During periods of reduced runoff, wind erosion of fluvial deposits occurred, with fluvially derived sand being reworked into expanding aeolian dune fields. Wind erosion also occurred as a rising water table isolated dunes from their sediment supply, resulting in deflation of dunes down to the water table. These surfaces formed in a basin that was subsiding. Thus, even in a background of overall increasing accommodation space, climatically driven falls in the water table allowed for periods of erosion. The occurrence of significant erosion, especially near syndepositional fault zones, resulted in a sedimentary record that shows pronounced lateral as well as vertical facies variations.     

南海天然气水合物钻孔自生黄铁矿硫同位素特征

南海神狐海域天然气水合物钻探岩心含有大量自生黄铁矿,主要为长条状、短柱状。黄铁矿主要出现在沉积物浅部和含水合物层,含量主要为20%~90%。SH5C岩心(不含水合物)的黄铁矿硫同位素δ34SV-CDT变化范围为-40.488‰~-19.538‰,SH7B岩心(含水合物)的黄铁矿δ34SV-CDT为-38.922‰~37.660‰。尤其在水合物层的黄铁矿硫同位素组成偏重,δ34SV-CDT在22‰~27‰,这是水合物盖层形成的封闭体系和AOM持续发生共同作用的结果,可能是水合物层中独特的硫同位素特征。     

陕北青化砭油田长2砂岩储层物性的控制因素

青化砭油田上三叠统长2辫状河砂岩储集层物性主要受沉积相及成岩作用的控制。沉积相通过控制储集岩的颗粒粒度及粘土矿物含量来控制储集物性,河道中心砂岩的物性好于河道侧翼砂岩,河道间沉积的储集物性最差。压实作用是造成砂岩原生孔隙丧失的主要原因,碳酸盐矿物和石英的次生加大胶结是降低储层物性的主要胶结作用,经溶解作用改造的辫状河河道砂体成为储集性能良好的油气储集体,是今后研究区长2油层组的主攻目标。     

伊拉克HF油田上白垩统碳酸盐缓坡相储层发育特征

伊拉克HF油田上白垩统Hartha组灰岩沉积在碳酸盐缓坡环境,储层发育特征及其主控因素尚不明确。通过钻井、测井、岩芯和地震资料,并结合薄片观察、分析化验,研究了该油田Hartha组的岩石学特征、储集空间特征及物性特征,并分析了沉积相和成岩作用对储层发育的控制。结果表明,厚度约50 m的Hartha组的储层岩石类型主要为粒泥灰岩、泥粒灰岩和颗粒灰岩,孔隙度10%～22%,渗透率集中在(0.1～0.5)×10^(-3 )μm^2和(1～5)×10^(-3 )μm^2,基质物性较好。Hartha组上段储集性能相对较好,Hartha组下段发育外缓坡灰泥,储集性较差。沉积作用对储层的物性起着主要控制作用,滩相储层的物性明显好于非滩相。该套灰岩的储集空间为粒间孔、铸模孔、生物体腔孔,以及粒内溶孔和微裂缝。晚白垩世暴露剥蚀期,在表生作用下Hartha A层溶孔部分被胶结物充填,存在局部的强烈胶结带。     

南海台西南区碳酸盐岩矿物学和稳定同位素组成特征——天然气水合物存在的主要证据之一

南海台西南区是中国南海中天然气水合物赋存的最有利场所。研究表明,该区的碳酸盐岩主要以结壳、烟囱的形式出现,结壳的裂隙或孔洞中常常充填有淡黄-白色的文石晶体。碳酸盐岩中自生碳酸盐矿物主要为文石、高镁方解石,少量白云石、铁白云石和菱铁矿。扫描电子显微镜(SEM)分析表明,文石主要呈针状、长柱状、放射束状,高镁方解石呈颗粒状。碳酸盐岩的碳同位素δ13C值主要在-56·878‰~-32·829‰PDB之间,大多数小于-40‰PDB,显示了生物甲烷成因碳源的特征;氧同位素δ18O值在2·1875‰~5·045‰PDB之间,主要在4‰PDB以上,这种较重的氧同位素比值表明,天然气水合物分解产生的富18O水体可能是碳酸盐岩沉淀的流体源。矿物学和碳氧稳定同位素研究表明,南海台西南区的碳酸盐岩为细菌性甲烷成因碳酸盐岩,可能与天然气水合物有关,显示了该区水合物存在的可能性很大。     

鄂尔多斯盆地渭北地区上三叠统延长组长7油层组碳酸盐结核中自生碳酸盐矿物的特征

早期形成的碳酸盐结核在埋藏期间会经历多种碳酸盐矿物相沉淀的复杂胶结作用,岩石学研究是探究结核成因的关键。通过野外剖面观察、岩石学观察和阴极发光技术,分析了鄂尔多斯盆地渭北地区上三叠统延长组长7油层组泥页岩中各种形状的方解石和白云石结核中自生碳酸盐矿物的特征。这些结核为成岩早期的产物,构成结核的自生碳酸盐矿物特征显著:(1)球粒方解石结核中,方解石呈纤维状或刃片状,球粒间充填晶粒方解石或因压实呈贴面结合,纤维状方解石发桔红色和暗红色2种光,刃片状方解石发暗红色光;(2)粉晶方解石结核中,方解石呈他形粒状,含有机质包裹体或纤维状晶形残余,晶间含沥青和纤维状方解石残余,主要发暗红色光;(3)白云石结核有泥晶和粉晶2种晶体类型,粉晶白云石结核含较多泥质,泥质条带或有机质条带处常见纤柱状白云石;(4)沿裂缝充填的方解石和白云石常呈纤维状或纤柱状结构,发暗红色光或不发光。研究区长7油层组碳酸盐结核中的方解石和白云石具有不同的成因类型和复杂的胶结作用:球粒方解石和泥晶白云石代表了结核开始形成时的胶结作用,可以准确地反映结核的成因;粉晶方解石、粉晶白云石反映了交代成因;裂缝中纤维状、纤柱状方解石和白云石集合体则为结核经历了较强压实作用之后充填裂缝而成。     

自生绿泥石对储集层质量影响的定量评价:以北部湾盆地涠西南凹陷涠洲组三段为例*

为了定量研究自生绿泥石对储集层质量的影响,在限定控储因素前提下,通过常规岩心分析、扫描电镜、薄片鉴定、激光粒度分析和黏土矿物X衍射等资料,以北部湾盆地涠西南凹陷涠洲12-X油田古近系涠洲组三段碎屑岩储集层为例,探讨了自生绿泥石对石英次生加大、孔隙结构及储集层物性的影响,并提出了综合得分评价法,定量评价自生绿泥石对储集层物性的控制作用。研究结果表明,当溶蚀作用积极意义大于绿泥石沉淀的消极影响时,石英加大和方解石含量较高(7.3%)及自生绿泥石相对含量较低(20%~30%)的细砂“控孔喉”作用,等同于石英加大和方解石含量较低(2.4%)及自生绿泥石含量较高(30%~40%)的细砂,二者的“控孔喉”作用具有互补性。自生绿泥石的“控孔、控渗”作用与碎屑颗粒粒度、分选、胶结物及泥质含量等因素的“控孔、控渗”作用相互制约,存在互补性和差异性,绿泥石相对含量较低的中砂“控孔、控渗”作用等同于绿泥石相对含量较高的细砂(互补性);泥质含量较低的细砂岩中绿泥石“控孔、控渗”作用弱于泥质含量较高的细砂岩(差异性)。总之,在岩相一定的情况下,包膜或衬里状绿泥石与孔渗呈正相关关系,孔隙充填状绿泥石与孔渗呈负相关关系。     

Diagenetic and Depositional Impacts on the Reservoir Quality of the Upper Jurassic Arab Formation in the Balal Oilfield,Offshore Iran

The Kimmeridgian‐Tithonian aged Arab Formation, as the main reservoir of the Jurassic succession in the Balal oilfield, located in the offshore region of the Iranian sector of the Persian Gulf, is investigated in this study. The formation is composed of dolomites and limestones with anhydrite interbeds. Based on detailed petrographic studies, six microfacies are recognized, which are classified in four sub‐environments including supratidal, intertidal, lagoonal and the high energy shoal of a homoclinal carbonate ramp. The main diagenetic features of the studied succession include dolomitization, anhydritization, cementation, micritization, fracturing and compaction. Based on stable isotope data, dolomitization of the upper Arab carbonates is related to sabkha settings (i.e. evaporative type). In terms of sequence stratigraphy, three shallowing‐upward sequences are recognized, based on core and wireline log data from four wells of the studied field. Considering depositional and diagenetic effects on the reservoir quality, the studied facies are classified into eight reservoir rock types (RRT) with distinct reservoir qualities. Dolomitization has played a major role in reservoir quality enhancement, whereas anhydritization, carbonate cementation, and compaction have damaged the pore throat network. Distribution of the recognized RRTs in time and space are discussed within the context of a sequence stratigraphic framework.     

鄂尔多斯盆地环县-西峰地区延长组长9油层组自生绿泥石发育特征研究

本文以鄂尔多斯盆地环县-西峰地区延长组长9油层组为研究对象,利用钻孔岩心、薄片鉴定、扫描电镜等资料,对长9油层组砂岩储层中自生绿泥石的产状、成因及其与形成环境的关系进行研究。结果表明,自生绿泥石主要以孔隙的环边衬里形式出现,其沉淀作用发生在沉积期后而不是沉积阶段或沉积作用以前,其分布对应于辫状河三角洲前缘水下沉积环境。     

粤北金鸡岭丹霞群砂岩矿物表面结构初步研究

本文对丹霞群砂岩中的石英、长石、电气石、锆石４种碎屑矿物的表面结构特征作了初步研究，并与现代河流和海岸砂质沉积物中的同类矿物的表面结构特征进行类比。文章指出，由于成岩作用的影响，古代砂岩中的碎屑矿物比现代砂的同类矿物具有更为复杂的表面结构，原因在于成岩环境下，溶解、溶蚀和压溶作用对古代砂岩的矿物表面有更深刻的改造。     

鄂尔多斯盆地陕北地区长101高渗储层的控制因素

鄂尔多斯盆地中部的陕北地区,是盆地中生界油藏最为丰富的区带之一,主要含油层系为三叠系延长组长6层,其次是侏罗系延安组,以往的研究也多集中于以上含油层.长101层是近几年勘探评价的新层系,利用现代测试技术和数理统计研究了长101层的沉积、成岩特征,分析高渗储层控制因素.结果表明,沉积环境控制长101高渗储层分布,成岩作用对长101高渗储层的形成起主导作用.主河道砂岩厚度大、粒度粗、孔喉大、孔隙连通性好,河道侧翼砂岩厚度薄、粒度细、孔喉小、孔隙连通性差.因此,压实作用后主河道砂岩粒间孔含量多于河道侧翼砂岩,导致主河道砂岩胶结作用弱于河道侧翼砂岩,而溶蚀作用强于河道侧翼砂岩,主河道砂岩物性明显好于河道侧翼.沉积、成岩作用共同造成长101高渗储层发育于河道主带和分流交汇部位.     

Detrital K‐feldspar 40Ar/39Ar Ages: Source Constraints of the Lower Miocene Sandstones in the Pearl River Mouth Basin,South China Sea

The South China Sea began to outspread in the Oligocene. A great quantity of terraneous detritus was deposited in the northern continental shelf of the sea, mostly in Pearl River Mouth Basin, which constituted the main paleo-Pearl River Delta. The delta developed for a long geological time and formed a superimposed area. Almost all the oil and gas fields of detrital rock reservoir distribute in this delta. Thirty-three oil sandstone core samples in the Zhujiang Formation, lower Miocene (23–16 Ma), were collected from nine wells. The illite samples with detrital K feldspar (Kfs) separated from these sandstone cores in four sub-structural belts were analysed by the high-precision 40Ar/39Ar laser stepwise heating technique. All 33 illite 40Ar/39Ar data consistently yielded gradually rising age spectra at the low-temperature steps until reaching age plateaus at mid-high temperature steps. The youngest ages corresponding to the beginning steps were interpreted as the hydrocarbon accumulation ages and the plateau ages in mid-high temperature steps as the contributions of the detrital feldspar representing the ages of the granitic parent rocks in the provenances. The ages of the detrital feldspar from the Zhujiang Formation in the four sub-structural belts were different: (1) the late Cretaceous ages in the Lufeng 13 fault structural belt; (2) the late Cretaceous and early Cretaceous-Jurassic ages in the Huizhou 21 buried hill-fault belt; (3) the Jurassic and Triassic ages in the Xijiang 24 buried hill-fault belt; and (4) the early Cretaceous – late Jurassic ages in the Panyu 4 oil area. These detrital feldspar 40Ar/39Ar ages become younger and younger from west to east, corresponding to the age distribution of the granites in the adjacent Guangdong Province, Southern China.     

鄂尔多斯盆地陕北地区长101高渗储层的控制因素

鄂尔多斯盆地中部的陕北地区,是盆地中生界油藏最为丰富的区带之一,主要含油层系为三叠系延长组长6层,其次是侏罗系延安组,以往的研究也多集中于以上含油层。长101层是近几年勘探评价的新层系,利用现代测试技术和数理统计研究了长101层的沉积、成岩特征,分析高渗储层控制因素。结果表明,沉积环境控制长101高渗储层分布,成岩作用对长101高渗储层的形成起主导作用。主河道砂岩厚度大、粒度粗、孔喉大、孔隙连通性好,河道侧翼砂岩厚度薄、粒度细、孔喉小、孔隙连通性差。因此,压实作用后主河道砂岩粒间孔含量多于河道侧翼砂岩,导致主河道砂岩胶结作用弱于河道侧翼砂岩,而溶蚀作用强于河道侧翼砂岩,主河道砂岩物性明显好于河道侧翼。沉积、成岩作用共同造成长101高渗储层发育于河道主带和分流交汇部位。