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在衣阿华州,热未成熟到边际成熟的生油岩中,存在两种偏向生油的独特中奥陶世有机质组合。显然更易于生油的A组合产生独有的“奥陶系石油”指纹,与微化石Gloeocapsamorpha prisca的有机壳有关。B组合则产生了更加常见的标记。这两类组合在北美许多奥陶系沉积中混合或交替出现,可以解释来自奥陶系生油岩的多种原油类型。这种混合物也有助于解释G.prisca的化学性质。化验了衣阿华州三口井中钻入Decorah组古登堡段向下到St.Peter砂岩的原生碳酸盐岩层序的岩心样品。对生油岩的沥青抽提物和附近的石油进行显微镜、Roc-Eval热解和毛细管气相色层分离分析,用来评价两类组合。A组合占优势的岩石中溶剂抽提物有鉴别奥陶系的气相色谱标记,在C_(11)—C_(19)正构烷烃范围内存在奇数碳优势、较重正构烷烃含量低、实质上缺失包含姥鲛烷和植烷在内的类异戊二烯。尽管A组合只出现在极薄层中,仍有生油潜力,以致几英寸厚的岩层可能很重要。含B组合占优势的岩石抽提物显示了奇数碳优势减小、正构烷烃的全剖面特征以及明显地存在包括姥鲛烷和植烷的类异戊二烯。在北美、欧州波罗的海地区、澳大利亚大多数古生代盆地所见到的中奥陶生油岩中,衣阿华州的这些岩样具代表性。因此,从衣阿华州这些中奥陶岩石样品得出的结论,给在其它许多地区发现的奥陶系石油多样解释提供了依据。
In Iowa, there are two unique combinations of Ordovician organic matter that are biased toward oil production, from immature warm to marginal mature source rocks. Clearly easier to generate oil, the A combination produces unique “Ordovician oil” fingerprints associated with the organic shell of the microfossil Gloeocapsamorpha prisca. B combination is produced more common mark. These two types of assemblages, mixed or alternating in many Ordovician sediments in North America, explain a wide range of crude types from Ordovician source rocks. This mixture also helps to explain the chemical nature of G.prisca. Samples of cores from the primary Carbonate sequence of the Decorah Formation, down into St.Peter Sandstone, were drilled in three wells in Iowa. Microscopy, Roc-Eval pyrolysis and capillary gas chromatographic separation of the bitumen extract of the source rock and nearby oil were used to evaluate the two combinations. A solvent extraction of the dominant rocks in the A combination of gas chromatography to identify Ordovician mark in the C_ (11) -C_ (19) n-paraffins have the advantage of odd-numbered carbon, heavier n-alkanes content is low, the essence Isoprenoids, including pristane and phytane, are missing. Although the combination of A appears only in very thin layers, there is still potential for oil generation so that rock formations a few inches thick may be important. Rock extracts with a B-dominated composition show a decrease in odd-numbered carbons, a full profile of normal paraffins, and the obvious presence of isoprenoids including pristane and phytane. In North America, the Baltic Sea region of Europe, and the Middle Ordovician source rocks seen in most of the Paleozoic basins in Australia, these rock formations in Iowa are representative. Thus, the conclusions drawn from these Iowa rock samples in Iowa provide the basis for the diverse interpretation of Ordovician found in many other areas.