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.     

云南思茅盆地景谷地区下白垩统曼岗组风成砂岩沉积学特征及其古气候意义

本文通过野外剖面的观察和实测,室内薄片鉴定、阴极发光分析、粒度分析、扫描电镜及能谱测试分析,研究了云南思茅盆地景谷地区下白垩统曼岗组风成砂岩沉积学特征,详细了解了风成砂岩的发育特征,以期为研究区白垩纪古气候研究提供新的资料,为我国钾盐勘探提供一定的地质依据和新的思路。研究结果表明:1研究区曼岗组主要出露一套中—细粒石英砂岩、岩屑石英砂岩,发育中—大型交错层理、平行层理,碎屑颗粒分选中等-好,以次圆状-圆状为主,孔隙不发育,杂基含量少;2曼岗组中段发育具平行层理的石英砂岩,粒度分布呈双众数,带状分布,石英颗粒为浑圆状,颗粒表面见Fe、Mn质薄膜,颗粒表面具明显的碟形坑等特征,属于典型的风成砂岩,初步判定其沉积环境为风成沙丘和丘间沉积;3景谷地区下白垩统曼岗组风成砂岩,整体表现为干热型气候,曼岗组中段为丘间沉积,曼岗组上段和下段为沙丘沉积。其在搬运沉积成岩过程中经历了强烈的物理、化学风化作用,整体表现为氧化环境,风力较大,风速稳定,持续时间长。此外,可能还经历了一段时期的湿润性气候环境,这可能与大气降水、大气环流带的迁移,以及白垩纪海侵作用等有关。     

Evidence of multi-stage faulting by clay mineral analysis: Example in a normal fault zone affecting arkosic sandstones (Annot sandstones)

Fault affecting silicoclastic sediments are commonly enriched in clay minerals. Clays are sensitive to fluid–rock interactions and deformation mechanisms; in this paper, they are used as proxy for fault activity and behavior. The present study focuses on clay mineral assemblages from the Point Vert normal fault zone located in the Annot sandstones, a Priabonian-Rupelian turbidite succession of the Alpine foredeep in SE France. In this area, the Annot sandstones were buried around 6–8 km below the front of Alpine nappes soon after their deposition and exhumed during the middle-late Miocene. The fault affects arkosic sandstone beds alternating with pelitic layers, and displays throw of about thirty meters. The fault core zone comprises intensely foliated sandstones bounding a corridor of gouge about 20 cm thick. The foliated sandstones display clay concentration along S–C structures characterized by dissolution of K-feldspar and their replacement by mica, associated with quartz pressure solution, intense microfracturation and quartz vein precipitation. The gouge is formed by a clayey matrix containing fragments of foliated sandstones and pelites. However, a detailed petrographical investigation suggests complex polyphase deformation processes. Optical and SEM observations show that the clay minerals fraction of all studied rocks (pelites and sandstones from the damage and core zones of the fault) is dominated by white micas and chlorite. These minerals have two different origins: detrital and newly-formed. Detrital micas are identified by their larger shape and their chemical composition with a lower Fe–Mg content than the newly-formed white micas. In the foliated sandstones, newly-formed white micas are concentrated along S–C structures or replace K-feldspar. Both types of newly formed micas display the same chemical composition confirmed microstructural observations suggesting that they formed in the same conditions. They have the following structural formulas: Na_0.05 K_0.86 (Al _1.77 Fe_0.08 Mg_0.15) (Si_3.22 Al_0.78) O₁₀ (OH)_2. They are enriched in Fe and Mg compared to the detrital micas. Newly-formed chlorites are associated with micas along the shear planes. According to microprobe analyses, they present the following structural formula: (Al_1,48 Fe_2,50 Mg_1,84) (Si_2,82 Al_1,18) O₁₀ (OH)₈. All these data suggest that these clay minerals are synkinematic and registered the fault activity. In the gouge samples, illite and chlorite are the major clay minerals; smectite is locally present in some samples.In the foliated sandstones, Kubler Index (KI) ((001) XRD peak width at half height) data and thermodynamic calculations from synkinematic chlorite chemistry suggest that the main fault deformation occurred under temperatures around 220 °C (diagenesis to anchizone boundary). KI measured on pelites and sandstones from the hanging and footwall, display similar values coherent with the maximal burial temperature of the Annot sandstones in this area. The gouge samples have a higher KI index, which could be explained by a reactivation of the fault at lower temperatures during the exhumation of the Annot sandstones formation.     

BOOK REVIEW

Eolian landforms as sandy dunes and dune heaps were mapped by the author in 1979–1980 in a zone where vegetation had been destroyed by a forest fire on 30 June 1976, in an area to the east of Elverum, south-east Norway. The mapped eolian landforms are discussed together with information from 40 samples of the eolian sediment. Winds, mostly from the north and north-west, were responsible for the development of these eolian landforms, utilizing the fluvioglacial sediments accumulated as a sandur during the retreat of the inland ice approximately 10,000 years ago. The winds were active during a short period of time, only a century or so before the area became vegetated.     

罗布泊全新世沉积特征及其环境意义

罗布泊西湖属开阔湖盆,上世纪30年代其面积达1900km2,但湖心剖面粒度分析的结果表明,其粒度组成与一般开阔湖盆有着显著的不同:沉积物常具黄色与灰色互层结构,层理清晰,黄色层的粒度组成中常含较多粗颗粒砂,灰与黄色层的混合样是以粉砂黏土为主,同时含有一定量的中、细砂,甚至粗砂。根据与风成砂和尘暴沉积的粒度组成对比,可知这种沉积特征与现代尘暴沉积有很大的相似性,因而表明风尘是罗布泊沉积的重要物源,并可据此来恢复10000a以来本区尘暴作用的强弱变化。     

脱气温度和样品粒径对致密砂岩低温氮气吸附实验结果的影响

基于静态体积法的低温氮气吸附实验已广泛用于分析致密储集层的孔隙结构。本文探索了脱气温度和样品粒径对松辽盆地高台子致密砂岩低温氮气吸附实验结果的影响。结果显示,110℃的脱气条件并不能清除束缚水而使得孔隙体积、比表面积均偏低,300℃的脱气条件容易破坏样品中黏土矿物的结构令孔隙体积、比表面积减小,200℃是比较合适的脱气温度,既能去除束缚水又不破坏黏土矿物结构。样品粒径从5～10目减小至10～30目,氮气探测的孔隙数量增多使比表面积、孔体积增大。从10～30目减小至180～200目,黏土矿物相对含量降低令比表面积、孔体积显著减小;小于200目的样品中减少的黏土矿物主要集中在大于200目的岩样中,因此大于200目岩样的测定结果最高。因此,10～30目是利用低温氮气吸附实验寻求分析致密砂岩储集层特征的最佳粒径范围。     

Holocene history of lacustrine and marsh sediments in a dune-blocked drainage, Southwestern Nebraska Sand Hills, U.S.A.

As many as 2500 interdune lakes lie within the Nebraska Sand Hills, a 50000 km stabilized sand sea. The few published data on cores from these lakes indicate they are typically underlain by less than two m of Holocene lacustrine sediments. However, three lakes in the southwestern Sand Hills, Swan, Blue, and Crescent, contain anomalously thick marsh (peat) and lacustrine (gyttja) sediments. Swan Lake basin contains as much as 8 m of peat, which was deposited between about 9000 and 3300 years ago. This peat is conformably overlain by as much as 10.5 m of gyttja. The sediment record in Blue lake, which is 3 km downgradient from Swan lake, dates back to only about 6000 years ago. Less than two m of peat, which was deposited from 6000 to 5000 years ago, is overlain by 12 m of gyttja deposited in the last 4300 years. Crescent Lake basin, one km downgradient from Blue Lake, has a similar sediment history except for a lack of known peat deposits. Recently, a 8-km long segment of a paleovalley was documented running beneath the three lakes and connecting to the head of Blue Creek Valley. Blockage of this paleovalley by dune sand during two arid intervals, one shortly before 10500 yr BP and one in the mid-Holocene, has resulted in a 25 m rise in the regional water table. This made possible the deposition of organic-rich sediment in all three lakes. Although these lakes, especially Swan, would seem ideal places to look for a nearly complete record of Holocene climatic fluctuations, the paleoclimatic record is confounded by the effect dune dams have on the water table. In Swan Lake, the abrupt conversion from marsh to lacustrine deposition 3300 years ago does not simply record the change to a wetter regional climate; it reflects the complex local hydrologic changes surrounding the emplacement and sealing of dune dams, as well as regional climate.     

南京下蜀土的地球化学特征及其物源指示意义

风尘堆积的物源研究对于揭示物源区的环境演化状况、重建古风场强度和古大气环流格局等都具有重要的意义。目前长江下游地区下蜀土的物质来源问题仍然存在争议。为了解决这一争议问题,本文以南京泰山新村下蜀土剖面为研究对象,开展了系统的地球化学研究。通过对南京下蜀土的常量元素、微量元素、Nd同位素进行测试,并与黄土高原同期黄土进行对比分析,探讨其对长江下游下蜀土的物源指示意义。南京下蜀土与北方黄土具有相似的常、微量元素UCC标准化曲线和稀土元素球粒陨石标准化曲线,但是它的Zr、Hf含量较高,Tl、Pb含量较低,且具有较高的SiO2/Al2O3、TiO2/Al2O3、Nb/Ta和GdN/YbN,较低的SiO2/TiO2、Zr/Hf、Y/Ho、Lu/Hf和εNd(0)值,说明南京下蜀土的物质来源明显有别于黄土高原黄土。南京下蜀土与长江中游下蜀土在地球化学特征方面有较多的相似性,指示其物质来源与中游下蜀土的物质来源相似,主要来源于长江中下游地区的松散沉积物。     

Geochemistry and detrital zircon U–Pb dating of Pliocene-Pleistocene sandstones of the Chittagong Tripura Fold Belt (Bangladesh): Implications for provenance

The Bengal Basin originated during the collision of India with Eurasia and Burma. The provenance analysis of the Chittagong Tripura Fold Belt (CTFB), which is the folded eastern flank of the Bengal Basin as well as the Neogene belt of the Indo-Burman Ranges (IBR) is key to better understand the possible sources of sediment input from the complex interplay of the Indian, Eurasian and Burma plates. We report new whole rock geochemical and detrital zircon U–Pb data from the upper Neogene sandstones of Tipam-Dupi Tila formations (Pliocene to Plio-Plestocene succession) from the CTFB. Detrital zircon U–Pb age spectra show three predominant peaks at <200 Ma, 480–650, ∼800–1000 Ma. The geochemical discriminations and elemental ratios of Eu/Eu* (∼0.70), La/Sc (∼16.13), La/Co (∼15.76), Th/Sc (∼2.95), La/Th (∼5.67), Th/Co (∼2.87), Cr/Th (∼4.63) as well as Chondrite-normalized REE patterns with flat HREE, LREE enrichment, and negative Eu anomalies for the Tipam and Dupi Tila formations are suggestive of a dominantly felsic source area experiencing moderate to intensive chemical weathering (Chemical index of alteration, CIA - 57 to 81) and have a recycled provenance orogen related to active continental or passive margin settings. Integrated geochemical and zircon U–Pb studies reveal that the main sediment input might have been from the Himalayan orogen with significant arc-derived detritus, possibly from the Gangdese arc as well as from the Burma magmatic arc.     

李希霍芬与黄土的风成学说

第四纪古气候重建在最近半个世纪的时间里取得了前所未有的进展,这在很大程度上与对陆相黄土地层,特别是中国黄土-古土壤序列的研究,以及对深海沉积氧同位素的研究密不可分。现在我们已经可以在黄土-古土壤序列的基础上,建立起260万年来地球轨道尺度的古气候变化。一个不容否认的事实是,这些进展都是在黄土风成成因的基础上,结合土壤地层学、年代学、磁性地层学、古生物学等方面的进展逐渐发展起来的。只有解决了黄土的风成成因,才可能将堆积于各地的黄土,通过搬运粉尘的风力系统与区域、甚至全球的古气候变化联系起来。文章回忆了黄土的早期研究历史,回顾了黄土风成成因的奠基人李希霍芬教授在黄土成因方面的杰出贡献以及其后黄土成因的完善与发展。