鄂尔多斯盆地南部延长组热水沉积作用与烃源岩发育的关系

Relationship between hydrothermal sedimentation process and source rock development in the Yanchang Formation in southern Ordos Basin

鄂尔多斯盆地三叠系延长组优质烃源岩的形成可能与热流体活动密切相关,但一直缺乏直观且有效的证据支持。通过连续采样的高分辨率元素地球化学研究,结果显示延长组整体上是具有较多陆源输入的非典型热水沉积,但在长7-3段优质烃源岩沉积期热流体输入显著增强。热液指标Al/(Al+Fe+Mn)、(Fe+Mn)/Ti比值与TOC丰度之间的相关系数分别高达-0.89和0.89,表明热水沉积与烃源岩之间具有密切关系,即热液输入强度越大,烃源岩有机质丰度越高。Al/(Al+Fe+Mn)比值与P/Al、P/Ti、TS含量的相关系数分别为-0.81、-0.80和-0.89,指示热水沉积与烃源岩之间存在的关系可能缘于热水活动能够促进湖面古生产力的提高和底水缺氧环境的形成,在烃源岩发育过程中提供充足的有机质来源和良好的有机质保存条件。研究表明,热水沉积是潜在的富有机质烃源岩,元素定量指标相关性分析是揭示两者之间关系的一种有效途径。

The hydrocarbon source rock development of the Yanchang formation in the Ordos Basin is closely related to the hydrothermal fluid activities. A total of 87 core samples were collected continuously for the high resolution element geochemical analysis. The results show that the high-quality hydrocarbon source rocks generally belong to atypical hydrothermal sedimentation with more terrigenous input, but the intensity of hydrothermal input became stronger during the Chang 7-3 period when the high-quality source rocks deposited. The correlation coefficients between the hydrothermal index A1/（A1 ＋ Fe ＋ Mn） or （Fe ＋ Mn）/Ti ratio and TOC abundance are as high as --0. 89 and 0. 89, respectively, indicating that the hydrothermal sedimentation is closely related to the source rocks, i.e. increasing hydrothermal input intensity increases organic matter abundance in the hydrocarbon source rocks. The correlation coefficients between A1/（Al-＋-Fe＋Mn） ratio and P/A1, P/Ti, TS content are -0. 81,-0. 80 and -0. 89, respectively, suggesting the correlation of the hydrothermal sedimentation and hydrocarbon source rock may be due to hydrothermal activities. Such activities can promote surface productivity and anoxic condition in lake bottom, providing sufficient organic matter source and good preservation conditions in the process of hydrocarbon source rock development. Studies show that hydrothermal sedimentation are potential organic-rich hydrocarbon source rocks, and correlation analysis of quantitative elemental indices is an effective way to tmcover the relationship between hydrothermal sedimentation and source rocks.