克拉通盆地内油气垂向微渗漏的烃类地球化学──以鄂尔多斯盆地城川1井为例

GEOCHEMISTRY OF HYDROCARBON FOR OIL─GAS MICROLEAKAGE IN INNER OF CRATON BASIN ──TAKING CHENGCHUAN No.1 WELL OF ORDOS BASIN AS AN EXAMPLE

通过对鄂尔多斯盆地腹部城川１井（０─４５００ｍ）酸解烃、热解烃及部分层段抽提烃的系统研究，结合剖面上有机质含量的变化及产油气层的分布，讨论了油气垂向微渗漏在浅层沉积层中留下的可识别的地球化学痕迹。该井浅层０─１０２０ｍ井段，有机碳含量低，在０．０６％─０．３％之间，平均为０．１１％，ｔ（ｍａｘ）值小于４００℃，有机质处于未成熟热演化阶段，岩性发育以红色、砖红色粉砂岩和浅灰色砂质泥岩和粉砂岩为主，这样一套沉积组合显然不利于有机质转化成烃。但与其下伏相邻成熟层段相比，Ｓ１／ＴＯＣ，Ｓ１／（Ｓ１十Ｓ２）和Ｃ（２＋）／ＴＯＣ均异常偏大，饱和烃的ＯＥＰ值为１．１１─１．１３，饱／芳值为５．２─６．４，显示了不具有奇偶优势的成熟烃的特征；经丙烯均一化之后，浅层甲烷和重烃异常并未消失，表明有深部热成因气态烃的加入。盆地内断裂不发育，则通过岩石中微裂隙和孔隙系统的油气微渗漏是深部油气运移的主要方式。由此可见，油气垂向微渗漏在浅层沉积层中留下了可识别的地球化学痕迹。

Acidolysis, pyrolysis and extracted hydrocarbon samples from a deep well in the inner of Ordos basin, eastern China are studied, and the recognizable geochemical traces of oil-gas microleakage in the shallow section are discussed combining the variation of concentration of total organic matter and the distribution of oil and gas layers developed in the whole section.The section from 0 to 1220m depth in the well has a low concentration of total organic matter, which ranges from 0.06% to 0.3% with an average of 0.11%, and a low t(max) value,which is less than 400℃, showing that the organic matter is in immaturity stage. The lithology is mainly red or lateritic siltstone and greyish sandy mudstone and siltstone, indicating that it is unfavorable for organic matter to generate hydrocarbon. But the section has much larger ratios of C1/ TOC, S1/ (S1+S2) and C(2+) / TOC than those of the underlying mature sections. The OEP value of saturated hydrocarbon is 1.11─1.14 and the ratio of saturated HC/aromatic HC 5.2─6.4, showing the feature of mature HC without odd─even predominance. The shallow anomaly of methane and heavier hydrocarbon still exist after homogenized by propylene, indicating that such an anomaly is caused by thermal genetic hydrocarbon migrating from deep mature section other than by biogenic gases in situ. There is no fault in this basin, so the main migration mechanism for deep oil-gas should be the microleakage of oil─gas through the microfracture and pore system of sedimentary rocks.Therefore, it can be seen that the microleakage of oil─gas may leave recognizable geochemical traces in the shallow sedimentary layers.