四川盆地及其邻区中上寒武统洗象池群沉积相与沉积模式特征研究

近年来,我国海相碳酸盐岩油气勘探正处于大发现期,其中四川盆地龙王庙组气藏的突破,使得寒武系碳酸盐岩储层越来越获得研究者的重视。洗象池群作为寒武系有利储集层段目前还处于勘探初期,地质研究程度相对滞后。为此,基于前人的研究成果,通过综合分析露头、钻井、地震和测井等资料,研究了四川盆地及邻区洗象池群沉积相、沉积体系模式,编制岩相古地理图,建立洗象池群“三滩夹两凹”沉积模式,指出了该区下一步天然气勘探的方向。研究结果表明:(1)四川盆地洗象池群呈现碳酸盐岩镶边台地的古地理格局,发育蒸发台地、局限台地、半局限台地、台地边缘、斜坡盆地等5种沉积相;(2)建立了洗象池群“三滩夹两凹”沉积模式,其中一条为台地边缘颗粒滩,分布于该盆地东缘,另外两条为台内颗粒滩,分布于局限台地相的东西两侧,分属局限台地相和半局限台地相;(3)台内颗粒滩发育主要受古地貌及高频海平面升降变化所控;台地边缘颗粒滩主要分布于盆地的东缘,发育受沉积古地貌坡折控制。结论认为:台内颗粒滩和台地边缘滩储层质量好,是天然气勘探的重要领域;洗象池群“三滩夹两凹”沉积模式和颗粒滩带分布的新认识,对古老地层的沉积体系研究具有重要的理论意义。

Sedimentary Facies and Model Characteristics of Middle Upper Cambrian Xixiangchi Group in Sichuan Basin and Its Adjacent Areas

In recent years, marine carbonate oil and gas exploration in China has made important discoveries. With the exploration breakthrough of gas reservoirs in the Longwangmiao Formation in Sichuan basin, increasingly more researchers are paying attention to the Cambrian carbonate reservoirs. The Cambrian Xixiangchi Group, as a favorable reservoir, is still in the initial exploration stage, with geological research degree development still lagging. Based on previous research results, the sedimentary facies and sedimentary system mode of Xixiangchi Formation in/around the Sichuan basin were studied, and lithofacies paleogeographic maps were produced. Besides, the “three shoal belts with two sags” sedimentary model of the Xixiangchi Formation was established, and the directions and fields of natural gas exploration were proposed. We discovered that: (1) the Sichuan Basin paleogeography in the Xixiangchi period was a carbonate rimmed platform, and the sedimentary facies of (semi-)restricted platform, evaporation platform, platform margin and slope basin were developed; (2) the “three-shoal belts with two sags” sedimentary model of Xixiangchi Formation is established. One of them is the platform-margin grain shoal, which is distributed along the eastern basinal margin. The other two are intra-platform grain shoals, which are distributed on the eastern and western side of the restricted platform facies, respectively, belonging to the restricted and semi-restricted platform facies; (3) the grain shoal of restricted platform facies is controlled by sea-level fluctuation and paleo-uplift. The platform-margin grain shoal is mainly distributed along the eastern basinal margin, with its development controlled by the slope break of sedimentary paleogeomorphology. To conclude, the grain shoal of restricted platform facies and of platform margin facies are good reservoirs and important targets of natural gas exploration. The new understanding of “three shoal belts with two sags” sedimentary model and grain shoal distribution of Xixiangchi Formation is of great theoretical significance to ancient sedimentary system studies.