Quantitative Evaluation and the Geochemical Responses of Gas Washing in Lunnan Petroleum Province
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摘要: 自喜山期, 过量干气自东向西对轮南低凸起原始油藏的大规模侵入、冲刷, 诱发本区持续性气洗分馏作用的发生.在各组分气液溶解平衡的格架下, 气洗作用将导致残余油中正构烷烃的大量损失以及次生凝析气藏的形成.地球化学研究表明, 残余油与凝析油中轻烃组分(C6-C8) 的变化趋势具有典型的气洗分馏特征.同时, 基于气洗作用模型, 通过对各层系原油正构烷烃相对蒸发量(Q) 的计算, 定量描述了轮南地区气洗作用的强度.计算结果显示, 奥陶系的油气藏曾遭受强烈的气洗作用, 原油中正构烷烃的相对蒸发量由东向西表现出依次递减的特征.其中, 轮古东地区Q值高达97%;位于轮南低凸起中部的轮古2井以及轮古18井地区, 正构烷烃相对蒸发量较小(Q=20%~76%); 而在更加靠近西部的轮古17及轮古100井, 其原油中正构烷烃分布正常(Q=0), 并未发生气洗分馏作用.另一方面, 石炭系以及三叠系的油气藏均表现出未遭受气洗作用的特征, 证实了喜山期大量干气的优势运移通道被限制于海西早期运动所形成的奥陶系岩溶缝洞体系中的事实.因此, 奥陶系, 尤其是位于临近气源区的轮古东地区, 是轮南地区气藏的有利靶区.Abstract: Lunnan hydrocarbon province has recently experienced intensive gas invasion since the Himalayan movement, and some of the oils in the Ordovician reservoir show characteristics of progressive fractionation by gas moving.Based on the framework of the components phase equilibrium, the process of gas washing can deplete the n-alkanes of the oil and produce the subsequent condensates.Geochemistry analysis shows that both the residual oil and the condensates indicate the phase fractionation characteristics on their light ends (C6-C8). Moreover, gas chromatogram data show that the altered oils are depleted of light n-alkanes. Through calculating the mass depletion of oil in n-alkanes relative to an unfractionated oil (Q), it determines that oils in the Ordovician reservoir experienced great alternation of gas washing, and the Q value decreases westward.However, oils in the Carboniferous and Triassic reservoir show no depletion of the n-alkanes.
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图 3 相分馏作用中组分F、B变化示意图(据Van Graas et al., 2000,补充修改; 图中箭头方向表示由残余油向凝析油的变化)
Fig. 3. The alternation of index F, B during the phase fractionation
图 4 气洗作用后正构烷烃分布示意图(据Losh et al., 2002修改)
Fig. 4. Molar fracion plot, showing depleted and reconstructed n-alkane profile
表 1 轮南地区原油MPI指数折算成熟度
Table 1. Vitrinite reflectance calculated by MPI
表 2 轮南地区凝析气组分
Table 2. Condensate gas composition in Lunnan area
表 3 轮南低凸起原油气洗作用参数
Table 3. Depths, break numbers, and n-alkanes mass depletion fractions for oils in Lunnan area
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