塔中地区良里塔格组海平面变化与烃源岩的非均质性

Sea-level change and heterogeneity of source rocks of Lianglitage Formation in the central Tarim area

塔中地区上奥陶统良里塔格组沉积的灰泥丘是目前保存下来的海相成因工业性烃源岩之一。沉积相研究及碳、氧同位素与锶同位素分析表明,在良里塔格组沉积时期,塔中地区经历了3期大规模的海平面变化。根据3期海平面变化特点将良里塔格组划分为3个层序。烃源岩的非均质性与海平面变化有如下关系:①层序1中的烃源岩只发育在第1期海平面上升最大时期,即密集段;②层序2中的烃源岩形成于第2期海平面上升期与下降早期,即海侵体系域与高位体系域下部;③层序3中的烃源岩主要发育于第3期海平面上升初期,即海侵体系域的下部。沉积相与海平面升降变化对烃源岩非均质性具有控制作用,台地边缘灰泥丘相中有机质丰度较高的优质生油岩分别赋存于丘间洼地、层状灰泥丘和层状生物灰泥丘亚相中,块状灰泥丘的有机质丰度较低。海侵速率影响了烃源岩有机质丰度,快速的海平面上升易使台地边缘发育高有机质丰度的烃源岩。

The lime-mud reef of Lianglitage Formation of the Ordovician in the central Tarim area is one of the industrial source rocks with marine origin. The results of facies study and carbon,oxygen and strontium isotope analyses showed that there were three peri ods of sea-level change. According to the characteristics of three periods of sea-level change, Lianglitage Formation in the central Tar im area had been classified into three sequences. The heterogeneity of source rocks had some relations with the sea-level change. The source rocks in the first sequence developed in the greatest rise stage of sea level in the first period. In the second period of sea-level change,the source rocks were in the transgressive systems tract and the bottom of highstand systems tract. The source rocks in the third rise stage of sea level were in the bottom of transgressive systems tract, and the well-developing period of source rocks was in the first transgressive time. The sedimentary facies and sea-level change influenced the heterogeneity of source rocks. The high-grade source rock was located in the subfacies including depression of lime-mud reef and the bedded reef. The source rocks with low organic abundance were in the massive lime-mud reef. The transgressive velocity had an influence on the organic abundance of source rocks. In the rapid rise stage of sea level, the source rocks with high organic abundance would deposit in the margin of platform.