四川盆地德阳-安岳凹槽形成机理及灯影组沉积新格局

Formation mechanism of Deyang-Anyue trough and new sedimentation pattern of Dengying Formation in the Sichuan Basin

四川盆地德阳—安岳凹槽两侧陡坎带震旦系灯影组天然气勘探不断取得新突破,是盆地内天然气增储上产的重要领域。为了明确该凹槽的形成机理、形成期次及沉积新格局,进一步指导灯影组气藏的深化勘探,采用最新大连片三维地震成果、结合钻井等资料,分析了各期陡坎之间、陡坎与台缘带之间的空间展布关系,探索了陡坎的演化规律,进一步明确了天然气的勘探方向。研究结果表明:①德阳—安岳凹槽陡坎总体表现为侵蚀谷特征,同沉积小断层的作用不明显;②灯影组二段(以下简称灯二段)发育Ⅰ、Ⅱ两个陡坎,灯影组四段(以下简称灯四段)发育一个陡坎,分别在桐湾Ⅰ和Ⅱ、Ⅲ幕侵蚀作用形成,这些陡坎与灯二段、灯四段台缘带无关;③灯二段沉积时期,受控于基底垒堑格局,发育由南向北进积的4~5排台缘带,呈北东向展布,其中蓬莱地区发育两个大型台缘带,台缘带丘滩体被陡坎Ⅱ截断,叠加岩溶作用形成优质储层。结论认为:①高石梯—磨溪地区灯四段与灯二段的陡坎叠合区发育灯四段陡坎型台缘带,而在灯四段槽上带,丘滩体仍呈北东向发育,表明该地区受陡坎Ⅱ与北东向古地貌双重因素控制;②蓬莱地区陡坎Ⅱ坡度变小,对灯四段台缘带控制作用减弱直至消失,主要发育2排北东向展布的缓坡型台缘带,即蓬莱①、②台缘带,蓬莱地区可作为今后天然气勘探重点关注目标区。

New breakthroughs have been achieved in the exploration of the Sinian Dengying Formation natural gas in the steep slope zones at both sides of Deyang-Anyue trough in the Sichuan Basin,making it an important area for increasing reserves and production within the basin.To clarify the formation mechanisms,formation stages and new sedimentation patterns of Deyang-Anyue trough and guide the further exploration of the Dengying Formation,this paper analyzes the spatial distribution relationships between steep slopes of different ages and between the steep slope and the platform margin belt by using the latest continuous 3D seismic results and drilling data.In addition,the evolution laws of steep slopes are investigated,and the natural gas exploration direction is pointed out further.And the following research results are obtained.First,the steep slopes of the Deyang-Anyue trough generally exhibit the characteristics of erosion valleys,while the role of synsedimentary minor faults is not significant.Second,there are two steep slopes(I and II)in the second member of Dengying Formation(Deng 2 Member)and one in the fourth member(Deng 4 Member).These steep slopes were formed due to erosion during the Tongwan I andⅡ-Ⅲepisodes and are unrelated to the platform margins of Deng 2 and Deng 4 Members.Third,during the deposition of the Deng 2 Member,controlled by the basement horst-graben background,4 or 5 rows of platform margin belts prograding from south to north were developed.These platform margin belts are distributed along northeast.Among these platform margin belts,two large ones are developed in the Penglai area,whose mound-shoal complexes were truncated by steep slopeⅡ,and superimposed with karstification processes,high-quality reservoirs were formed.In conclusion,in the Gaoshiti-Moxi area,the steep slope-type platform margin belt of Deng 4 Member is developed in the overlapping region of the steep slopes of Deng 2 and Deng 4 Members,but in the above-trough zone of Deng 4 Member,mound-shoal complexes are still developed along the northeast direction,indicating that this area is controlled by both steep slopeⅡand the NE paleogeomorphy.What's more,in the Penglai area,the gradient of steep slopeⅡdecreases,resulting in a weakening and even disappearance of its controlling effect on the platform margin belt of Deng 4 Member.Instead,two rows of gently sloping platform margin belts oriented northeast,are mainly developed,namely Penglai①and②platform margin belt.It suggests that the Penglai area can be taken as the key concerned target of natural gas exploration in the future.