四川盆地西部泥盆纪—石炭纪古坳陷存在的新证据及其油气地质意义

New evidence of Devonian-Carboniferous paleo-depression in the western Sichuan Basin and its implications for oil and gas geology

德阳—绵阳古生代坳陷是近年来四川盆地海相碳酸盐岩油气勘探领域的重要发现,但目前对坳陷内是否充填泥盆系—石炭系还存在较大的争议。为此,基于大量野外露头、钻井和地震资料,开展了德阳—绵阳坳陷沉积古地貌分析、岩性—生物地层对比和高分辨率地震精细引层,分析了古坳陷的地层接触关系、厚度变化规律和区域构造背景,恢复了古坳陷的演化过程,讨论了其形成的构造动力学背景,最后结合古坳陷的含油气地质条件分析,探讨了其油气地质意义和勘探价值。研究结果表明:①德阳—绵阳坳陷发育泥盆系—石炭系,其新证据包括:德阳—绵阳坳陷古地貌位置低于上覆泥盆系—石炭系的天井山古隆起,具有接受泥盆系—石炭系的可能;坳陷内二叠系下伏地层顶部具有典型的石炭系岩电特征,并产出晚石炭世䗴类化石;双鱼石地区向德阳—绵阳地区进行泥盆系—石炭系引层,可以识别出上超、削截等终止单元,并明确了古坳陷的边界。②古坳陷形成于加里东期华南板块与冈瓦纳大陆汇聚导致的区域挤压环境,海西期古特提斯洋和勉略洋扩张造成的拉张环境对古坳陷形态影响较小,但导致了四川盆地西部整体向西的加速沉降。③古坳陷位于寒武系优质烃源生烃中心和志留系烃源岩之上,烃源条件好;区内发育台内礁滩规模储层,储层条件好;兼具下二叠统梁山组致密泥岩直接盖层和三叠系膏盐岩间接盖层,盖层条件好,并发育大型地层—岩性圈闭,圈闭条件好。结论认为,德阳—绵阳地区泥盆纪—石炭纪古坳陷的发现进一步展示了该坳陷具有成为四川盆地海相油气勘探新区新领域的巨大潜力。

The Deyang-Mianyang Paleozoic Depression(DMPD)is an important discovery in the field of marine carbonate oil and gas exploration in the Sichuan Basin in recent years,but there is still controversy on whether the depression is filled with the Devonian and Carboniferous.Based on a large number of field outcrops,drilling and seismic data,sedimentological and paleogeomorphological analysis,lithological and bio-stratigraphic correlation,and high-resolution fine seismic tracking are conducted on the DMPD.Then,the stratigraphic contact relationship,thickness variation law and regional tectonic setting in the DMPD are analyzed.In addition,its evolution process is restored,and its structural dynamic background is discussed.Finally,the petroleum geological significance and exploration value of the DMPD are discussed by analyzing its petroleum geological conditions.The following research results are obtained.First,the new evidence for the development of the Devonian-Carboniferous system in the DMPD is as follows.The DMPD is paleogeomorphologically lower than the Tianjingshan paleo-uplift overlain by the Devonian-Carboniferous system,so it has the possibility to receive the Devonian-Carboniferous sediments;the top of the formation underlying the Permian in the DMPD has typical Carboniferous rock electrical characteristics and contains the fossil of Late Carboniferous fusulina;the seismic tracking of Devonian-Carboniferous system from the Shuangyushi area to the Deyang-Mianyang area reveals the termination units such as onlap and truncation,which define the boundary of the DMPD.Second,the DMPD was formed in the regional compression environment caused by the convergence of South China Plate and Gondwana Land in the Caledonian period.The extensional environment caused by the expansion of the Paleo Tethyan and the Mianlue Ocean in the Hercynian period had little influence on the morphology of the DMPD,but accelerated the westward subsidence of the western Sichuan Basin overall.Third,the DMPD is located above the hydrocarbon generation center of Cambrian quality source rocks and the Silurian source rocks,so it has good hydrocarbon source conditions.Large intra-platform reef flat reservoir is developed in the area to provide good reservoir conditions.It has good cap rock conditions with the direct cap rocks of Lower Permian Liangshan Formation tight mudstone and the indirect cap rocks of Triassic gypsum salt rock.Large stratigraphic-lithological trap is developed here,to provide good trap conditions.In conclusion,the discovery of the Devonian-Carboniferous paleo-depression in the Deyang-Mianyang area further displays the huge potential of this depression to become a new area and field of marine oil and gas exploration in the Sichuan Basin.