四川盆地前震旦纪沉积岩新认识与油气勘探的意义

四川盆地前震旦纪地层至今未被钻井揭示,前人对该盆地的基底特征进行过很多有益的探索性研究,认为四川盆地的基底具有双重性。通过对近年来该盆地新采集的二、三维地震资料解释成果的分析研究,得出以下新认识:四川盆地震旦系之下仍发育厚度介于3 000～5 000m含古生物化石的巨厚沉积岩,地层层位属于新元古界南华系澄江组;按地震反射时间推算,盆地基底埋深(沉积岩厚度)介于8 000～15 000m;前震旦纪沉积岩地震反射连续性较好、能量较强,推测其岩性主要为碎屑岩,在盆地内可能未变质或仅轻微变质,有可能成为有价值的、新的勘探领域。鉴于四川盆地震旦系之下的沉积岩是天然气勘探值得探索的新领域,目前处于尚待认识的阶段,建议钻探超深井以揭示深部地层的奥秘,井位宜选择在埋深相对小、无火山岩刺穿的区域。

New understandings of Pre-Sinian sedimentary rocks in the Sichuan Basin and the significance of oil and gas exploration there

No satisfactory discovery has ever been found by the previous drilling data in the Pre-Sinian formations in the Sichuan Basin,although it was considered that the bottom basin is of double-layer structure from its characteristic by the precedent researchers in their exploratory studies.Through interpretation and analysis of the newly collected 2D and 3D seismic data from this basin in recent years,new understandings are obtained as follows.First,the sedimentary rocks with huge thickness of 3000-5000 m are well developed beneath the Sinian in this basin,and the formation is classified as the Chengjiang Formation of Nanhua system in the Neoproterozoic.Second,according to the calculation by the seismic reflection time,the burial depth of the bottom basin,i.e.,the thickness of the sedimentary rocks,is between 8000 and 15000 m.Third,due to the good continuity and reflection energy from the seismic inversion,it is presumed that there mainly exist the clastic rocks,and the possible non-metamorphism or low-grade metamorphism does exist within this basin.In that sense,it is concluded that there is great gas exploration potential in the deeper formation of this basin.Based on the above understandings,it is suggested that ultra deep wells be drilled to explore the deeper formations in this basin,and the well sites be better selected at such areas with rather shallow depth and without being pieced by the volcanic rocks.