Paleocene sandstones in the Kupe Field of Taranaki Basin, New Zealand, are subdivided into two diagenetic zones, an upper kaolinite–siderite (K-S) zone and a lower chlorite–smectite (Ch-Sm) zone. Petrographic observations show that the K-S zone has formed from diagenetic alteration of earlier-formed Ch-Sm sandstones, whereby biotite and chlorite–smectite have been altered to form kaolinite and siderite, and plagioclase has reacted to form kaolinite and quartz. These diagenetic zones can be difficult to discriminate from downhole bulk-rock geochemistry, which is largely due to a change in element-mineral affinities without a wholesale change in element abundance. However, some elements have proven useful for delimiting the diagenetic zones, particularly Ca and Na, where much lower abundances in the K-S zone are interpreted to represent removal of labile elements during diagenesis. Multivariate analysis has also proven an effective method of distinguishing the diagenetic zones by highlighting elemental affinities that are interpreted to represent the principal diagenetic phases. These include Fe-Mg-Mn (siderite) in the K-S zone, and Ca-Mn (calcite) and Fe-Mg-Ti-Y-Sc-V (biotite and chlorite–smectite) in the Ch-Sm zone.Results from this study demonstrate that the base of the K-S zone approximately corresponds to the base of the current hydrocarbon column. An assessment with 1D basin models and published stable-isotope data show that K-S diagenesis is likely to have occurred during deep-burial diagenesis in the last 4 Myr. Modeling predicts that CO2-rich fluids were generating from thermal decarboxylation of intraformational Paleocene coals at this time, and accumulation of high partial pressures of intraformational CO2 in the hydrocarbon column is considered a viable catalyst for the diagenetic reactions. Variable CO2 concentrations and residence times are interpreted to be the reason for different levels of K-S diagenesis, which is supported by a clear relationship between the presence or absence of a well-developed K-S zone and the present-day reservoir-corrected CO2 content.

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地球化学和盆地模拟在新西兰库佩油田古新世砂岩成岩评价中的应用

新西兰塔拉纳基盆地库佩油田的古新世砂岩被细分为两个成岩带，上部高岭石-菱铁矿 (KS) 带和下部绿泥石-蒙脱石 (Ch-Sm) 带。岩石学观察表明，KS 带是由早期形成的 Ch-Sm 砂岩的成岩蚀变形成的，黑云母和绿泥石-蒙脱石被蚀变形成高岭石和菱铁矿，斜长石反应形成高岭石和石英。这些成岩带很难与井下大块岩石地球化学区分开来，这主要是由于元素-矿物亲和力的变化而没有元素丰度的整体变化。然而，一些元素已被证明可用于划定成岩带，特别是 Ca 和 Na，KS 带中低得多的丰度被解释为代表在成岩过程中去除了不稳定元素。多变量分析也证明了一种通过突出被解释为代表主要成岩阶段的元素亲和性来区分成岩带的有效方法。其中包括 KS 带中的 Fe-Mg-Mn（菱铁矿），以及 Ch-Sm 带中的 Ca-Mn（方解石）和 Fe-Mg-Ti-Y-Sc-V（黑云母和绿泥石-蒙脱石）。这项研究表明，KS 带的底部大致对应于当前烃柱的底部。对一维盆地模型和已发表的稳定同位素数据进行的评估表明，KS 成岩作用很可能发生在最后 4 Myr 的深埋成岩作用期间。建模预测，此时地层内古新世煤的热脱羧会产生富含 CO2 的流体，烃柱中地层内 CO2 高分压的积累被认为是成岩反应的可行催化剂。不同的 CO2 浓度和停留时间被解释为不同程度的 KS 成岩作用的原因，这得到了发育良好的 KS 带的存在与否与当今储层校正的 CO2 含量之间的明确关系的支持。