Presence of carboxylate salts in marine carbonate strata of the Ordos Basin and their impact on hydrocarbon generation evaluation of low TOC, high maturity source rocks
Presence of carboxylate salts in marine carbonate strata of the Ordos Basin and their impact on hydrocarbon generation evaluation of low TOC, high maturity source rocks
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DOI：
10.1007/s11430-013-4713-3
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发表时间：
2013-10
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通讯作者：
Quanyou Liu;Zhijun Jin;Wenhui Liu;Longfei Lu;Qianxiang Meng;Ye Tao;P. Han
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作者：
Quanyou Liu;Zhijun Jin;Wenhui Liu;Longfei Lu;Qianxiang Meng;Ye Tao;P. Han
鄂尔多斯盆地海相烃源岩中总有机碳（TOC）大多在0.2%~0.5%之间。行业标准普遍规定TOC值不低于0.5%（高成熟或过成熟烃源岩为0.4%）才能形成大型油气藏。但气源对比表明靖边气田天然气确实受到海相烃源岩的贡献。为了确定低TOC值高成熟烃源岩中羧酸盐（或称有机酸盐）对TOC的影响，我们在鄂尔多斯盆地南部一口钻井中对奥陶系海相烃源岩和二叠系过渡相烃源岩进行了采样，并进行了红外和GC-MS分析。研究发现，海相和过渡相烃源岩中均存在干酪根衍生有机酸和羧酸盐转化有机酸。羧酸盐转化的有机酸主要来自于羧酸盐的完全酸化，证实了海相烃源岩中存在羧酸盐。尽管C16:O峰是酸化前后有机酸的主峰，但与干酪根基有机酸和游离有机酸相比，羧酸盐转化的有机酸在C16:O之前的相对丰度要低得多。这一观察结果表明，干酪根基有机酸和游离有机酸主要脱羧形成低级羧酸，而羧酸盐转化的有机酸主要分解成石蜡。以十六酸钙为参照物定量干酪根衍生有机酸和羧酸盐转化有机酸，发现高TOC（>2.0%）海相烃源岩羧酸盐含量低，低TOC（0.2%~0.5%）海相烃源岩羧酸盐含量高。因此，对于TOC为0.2%～0.5%的海相烃源岩，羧酸盐可能是高成熟阶段的潜在气源。
The total organic carbon (TOC) in the marine source rock of the Ordos Basin mostly ranges from 0.2% to 0.5%. The industrial standard commonly states that the TOC value has to be no less than 0.5% (0.4% for high mature or over-mature source rock) to form large petroleum reservoirs. However, gas source correlation indicates that the natural gas in the Jingbian gas field does receive contribution from marine source rocks. In order to determine the effect of carboxylate salts (or called as organic acid salts) on TOC in highly mature source rocks with low TOC value, we sampled the Ordovician marine source rock and the Permian transitional facies source rock in one drilled well in the southern Ordos Basin and performed infrared and GC-MS analysis. It is found that both kerogen-derived organic acids and carboxylate salt-conversed organic acids exist in both marine and transitional facies source rocks. The carboxylate salt-conversed organic acids mainly come from the complete acidification of carboxylate salts, which confirms the presence of carboxylate salts in the marine source rocks. Although the C16:Opeak is the main peak for the organic acids both before and after acidification, the carboxylate salt-conversed organic acids have much less relative abundance ahead of C16:Ocompared with that of the kerogen-based and free organic acids. This observation suggests that the kerogen-based and free organic acids mainly decarboxylate to form lower carboxylic acids, whereas the carboxylate salt-conversed organic acids mainly break down into paraffins. By using calcium hexadecanoate as the reference to quantify the kerogen-derived and carboxylate salt-conversed organic acids, the high TOC (>2.0%) marine source rocks have low carboxylate salt content and the low TOC (0.2%–0.5%) marine source rocks contain high content of carboxylate salt. Therefore, for the marine source rocks with 0.2%–0.5% TOC, the carboxylate salts may be a potential gas source at high maturity stage.