鄂尔多斯盆地北岔沟砂岩沉积特征及其对天然气成藏的控制作用*

北岔沟砂岩为华北地台二叠系山西组与太原组划分对比的主要标志层,在鄂尔多斯盆地呈大面积连片分布,是上古生界重要的天然气储集层。依据地面露头调查结果与勘探资料,结合前人有关古构造和岩相古地理研究成果,笔者对鄂尔多斯盆地北岔沟砂岩的沉积特征进行了详细分析。结果表明,受益于物源区大量高石英含量的粗碎屑物质在短时间内的集中供应,北岔沟砂岩在地层中位置稳定,以含砾粗粒石英砂岩为主,石英含量高达90%左右,砂体叠置紧密,单砂体之间很少见泥岩隔夹层。这些砂体分别隶属于冲积扇、砾质辫状河和辫状河三角洲沉积体系,在鄂尔多斯盆地北部高、南部次高、中南部低的古地理背景控制下,由盆地南、北边缘向中部依次推进,沉积中心区以北发育4个规模宏大的三角洲沉积主砂带,以南发育6个规模较小的主砂带。受各主砂带砂体规模、矿物组分含量及其成岩作用差异影响,各砂带的储集物性具有较大变化,其中砂体规模最大、石英含量最高、长石等易溶矿物在酸性地层水作用下溶蚀彻底的杭锦旗—榆林—子洲—清涧—永和—吉县主砂带,储集能力最好,最有利于天然气的大规模聚集成藏,是鄂尔多斯盆地北岔沟砂岩天然气进一步勘探开发的有利场所。

Roots of fossil and extant ferns: Morphological evolution and paleoenvironmental implications

Roots are important vegetative organs of plants,and play essential roles in plant physiology as well as in the nutrient cycle and energy flow of an ecosystem. Fern roots are known to arise from the shoots in a predictable pattern,and thus are called shoot-borne roots,or more commenly adventitious roots. In contrast to the extensive studies of seed plant roots,relatively little information is available on fern roots. Based on a compilation of the available data and our own observations on both fossil and extant fern specimens,this study presents a review of root morphology of several fossil and extant ferns. In the early ferns that flourished during the Middle-Late Devonian,dense shoot-borne roots are attached on stems,or inferred rhizomes,or on expanded bases of trunks;such roots seldom branch,but when they do they display an isotomous branching pattern. In the crown groups of ferns,however,shoot-borne roots may be either densely or sparsely arranged,and show a monopodial branching pattern with two or more orders of lateral roots. The morphological evolution of fern roots may be interpreted by“the telome theory”: shoot-borne roots seem to be evolutionary conservative;primitive roots in early ferns can be considered as telome trusses that occasionally branch isotomously;and lateral rooting systems in crown groups of ferns evolved through overtopping of telome trusses. Information on fern root architecture,morphology,anatomical structure,and allocation ratios is documented in the fossil record,and is of significance in understanding the evolution of plant rooting systems and plant-soil interactions over geological time. As a case study,fossils of fern roots and root-soil systems from the Jurassic of Western Hills,Beijing,are briefly introduced.