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我们在利比亚苏尔特盆地Gialo高地的二口油井中采集了200多个Nubian砂岩岩芯样品进行了微体古生物研究。在这些样品中我们首次发现了众多的硅化海洋微体化石,如有孔虫、介形类等。硅化化石表明,在Gialo地区,遭受中等至强烈二氧化硅胶结和重结晶的Nubian砂岩是潮下带和潮上带的过渡相产物。根据新发现的硅化微体化石及超微化石的年代测定,Nubian砂岩主体是早白垩纪Albian期的沉积,其中井Ⅰ底部可能有少量Aptian期的产物。硅化化石的发现修正了上世纪50年代以来一直沿用至今的Nubian砂岩是“前Neocomian(早白垩世)或古生代陆相沉积”的错误年代及岩相判测。硅化微体化石可分为四种类型:Ⅰ型硅化化石表面保留有钙质外壳,缝合线,螺旋线清晰可见,遇10%盐酸即起泡溶解,化石内部及某些原始特征被非晶质二氧化硅部分充填和交代;Ⅱ型则被非晶质二氧化硅完全交代,在光滑的表面上可以观察到保留清晰的缝合线和结合线;Ⅲ型化石表面为微晶石英交代,化石特征保存不佳,但缝合线尚清晰可辨;Ⅳ型则为粗晶石英所取代,表面上仍隐约可见某些化石的缝合线特征。这些硅化微体化石类型被认为是代表成岩作用的不同阶段。Ⅲ型和Ⅳ型(特别是Ⅳ型)与粗砂中强烈的二氧化硅交替有关。在原始阶段发育良好的颗粒间隙可能会允许大量的二氧化硅液体进入岩体,从而产生而后的大量二氧化硅沉淀,胶结和重结晶作用。由于Gialo地区的频繁的断层作用和火山活动,化石硅化可能与区域和局部构造活动和热液活动有关。硅化微体化石表明,在后白垩纪时期,Nubian砂岩的石英胶结作用和石英增生造成了明显的孔隙度和渗透率减少或消亡,对油藏质量和储油条件产生负面影响。总之,Gialo地区硅化微体化石的发现为研究苏尔特裂谷盆地,以及世界其他地区的裂谷盆地之生物地层,岩相,古环境重建,孔隙度和渗透率预测,盆地勘探和盆地建模开辟了一个新窗口。
We collected more than 200 Nubian sandstone core samples from two oil wells in the Gialo Heights, Sirte Basin, Libya, for micro-organism paleontology research. For the first time in these samples we discovered a large number of silicified marine microfossils, such as foraminifera, ostracods and the like. Silica fossils show that in the Gialo region Nubian sandstones subjected to moderate to intense silica cementation and recrystallization are transitional products of the subtidal and tidal zones. Based on the dating of newly discovered silicified microfossils and superficial fossils, the Nubian sandstone body was deposited during the Early Cretaceous Albian period, in which there may be a small amount of Aptian period at the bottom of well I. The discovery of silicified fossils amends the erroneous dating and lithofacies dating of the Nubian sandstones that have been in use since the 1950s and are “pre-Neocomian (Early Cretaceous) or Paleozoic terrestrial sediments.” Silica fossils can be divided into four types: Type I silicified fossils retain the surface of calcium shells, sutures, spirals clearly visible in case of 10% hydrochloric acid bubbled dissolution, fossil internal and some of the original features of amorphous Silica partially filled and confiscated; Type II was completely replaced by amorphous silica, the smooth surface can be observed to retain a clear suture and bond lines; Ⅲ type fossil surface of microcrystalline quartz account, fossil features Poor preservation, but the suture is still clear and legible; Ⅳ type is replaced by coarse quartz, the surface is still vaguely visible suture characteristics of some fossils. These silicified microfossil types are thought to represent different stages of diagenesis. Type III and type IV (especially Type IV) are related to the intense silica alternation in grit. A well-developed particle gap in the original phase may allow a large amount of silica liquid to enter the rock mass, resulting in a large amount of subsequent silica precipitation, cementation and recrystallization. Due to frequent faulting and volcanic activity in the Gialo region, fossil silicification may be related to regional and local tectonic and hydrothermal activities. Silicified microfossils show that during the Late Cretaceous, quartz porosity and permeability decreased or disappeared due to quartz cementation and quartz accretion in the Nubian sandstone, negatively affecting reservoir quality and reservoir conditions. In summary, the discovery of silicified microfossils in the Gialo area was used to study the biogenic stratigraphy, lithofacies, paleoenvironment reconstructions, porosity and permeability prediction of the Sirte rift basin and other rift basins in other parts of the world. Basin exploration and basin construction Model opened up a new window.