塔里木盆地下寒武统玉尔吐斯组烃源岩分布预测及油气勘探的意义

下寒武统玉尔吐斯组是塔里木盆地深层碳酸盐岩沉积中重要的烃源岩之一,因资料稀少,目前其整体分布情况尚不明确,制约了该区深层白云岩储层油气勘探的进程。为此,通过对该盆地区域地质资料的综合分析,认为玉尔吐斯组烃源岩的分布受海平面变化、古构造及沉积环境的控制,是震旦纪一早寒武世时期海平面快速上升的产物,整体上为缓坡一盆地沉积模式,优质烃源岩发育在中、下缓坡相中。同时,根据野外露头剖面和钻井资料揭示出玉尔吐斯组与下伏震旦系是一种"伴生"的沉积关系,钻井地震层位标定进一步明确了两者之间的地震反射伴生组合特征,进而利用覆盖全盆地的大量二维地震资料首次预测出了玉尔吐斯组的分布范围及厚度变化趋势。结果表明:玉尔吐斯组主要分布在该盆地北部坳陷及南部地区(存在南、北2个生烃中心),预测厚度为5~50 m,其中巴东2井北部厚度最大,可达50 m;在巴东2、玉北2、巴探5南部分别有"海槽"伸入巴楚隆起内部,向巴楚隆起尖灭。结论认为:巴东低梁带位于玉尔吐斯组烃源岩分布线与古构造低梁叠合部位,生储盖组合条件优越且具备长期继承性古隆起的构造背景,应是该盆地寒武系白云岩储层下一步油气勘探的有利区。

Distribution prediction of Lower Cambrian Yuertusi Formation source rocks and its significance to oil and gas exploration in the Tarim Basin

The Yuertusi Formation of Lower Cambrian is one of the important source rocks deposited in deep carbonate formations in the Tarim Basin. However, due to the limited well data, its distribution has not been understood so clearly that the oil and gas exploration of deep dolomite reservoirs in this area is highly restricted. After a comprehensive analysis was conducted on regional geological data, it is believed that the distribution of Yuertusi Formation source rocks is controlled by sea level variation, palaeostructures and sedimentary environments. As the product of rapid sea-level rise during Sinian - Early Cambrian, it is generally in the pattern of slope-basin sedimentation, with high-quality source rocks developed in the middle and lower slope facies. Besides, field outcrop sections and drilling data show that there is an "associated" sedimentation relationship between the Yuertusi Formation and its underlying Sinian. The associated seismic reflection characteristics between them are further confirmed by drilling-seismic horizon calibration. Then, it is the first time that distribution range and thickness change tendency of the Yuertusi Formation are predicted by using sufficient 2D seismic data across the whole basin. It is shown that the Yuertusi Formation is mainly distributed in the south and in the depressions in the north of the basin and there are two hydrocarbon generation centers in the north and the south respectively. Its predicted thickness is 5 - 50 m, with the thickest part (50 m) in northern Well Badong 2. In Badong 2, Yubei 2 and south of Batan 5, the ocean troughs run into Bachu uplift and pinch out towards it. It is concluded that the Badong low ridge zone is located at the superposition position of Yuertusi Formation source rock distribution line and palaeostructural low ridge, where source rock-reservoir-cap rock assemblage is ascendant with the structural setting of long-term inherited palaeohighs, so it is the favorable exploration target of Cambrian dolomite reservoirs in the Tarim Basin.