试论四川盆地晚二叠世沉积环境与礁滩分布

通过地面地质、钻井、地震等资料的综合研究认为：早二叠世后，四川盆地经历又一次广泛海退，加之断块的差异升降，使二叠系茅口组受到不同程度的剥蚀，晚二叠世沉积格局发生很大变化，康滇古陆上升、扩大，成为主要物源区，沉积相带由陆到海呈南西、北东向展布；依次发育冲积扇-河流，含煤沼泽相环境；向东、向南至自贡、泸州、叙永、古蔺、重庆一带，这套河流相侧变为海陆交互相的滨海潮坪含煤沼泽环境；向北、向西在龙门山前的大邑、彭县一带相变为滨海混积潮坪相；再往北及北东至绵阳、阆中、南充、达州、宣汉、开江、开县、万州、云阳，东至湘、鄂西一线之间发育以石灰岩和燧石灰岩为主的开阔台地到局限台地。在上述沉积环境之北、之东发育硅质岩为主的大隆组海槽相沉积，即广（元）旺（苍）、城口-鄂西海槽，形成多槽围台的沉积格局。在海槽与台地之间的台地边缘，发育斜坡相浅水礁滩环境。分析了礁滩发育程度与水面上升及造礁生物生长的关系，进而认识到礁滩环海槽两侧的肩部分布规律；在台地缓坡发育生物鲕粒滩，在陡坡发育生物礁，尤其是经历白云化作用的礁滩对油气富集有利。在川东北地区采用礁滩的地震响应与喜山、燕山期构造叠合分析，预测了环海槽分布的巴中、仪陇莲花状构造；绵阳旋卷构造、中台山旋卷构造砥柱的复兴场、中台山、老君庵背斜可能是又一个礁滩型气藏。

A DISCUSSION ON DEPOSITIONAL ENVIRONMENT IN LATE PERMIAN AND DISTRIBUTION OF REEF-BANK IN SICHUAN BASIN

A comprehensive study of surface geologic, drilling, seismic and other data shows that the Sichuan Basin experienced another extensive regression after the Early Permian, which in combination with the differential elevation and subsidence of fault blocks resulted in the erosion of the Permian Maokou Formation to various degrees and the great change of depositional pattern in the Late Permian. The Kangdian ancient land elevated and enlarged and became the main source area, and the SW NE trending deposits of alluvial fan channel and coal bearing paludal facies were developed successively from land to ocean. To the north and west, the sedimentary facies changed to coastal mixed tidal flat facies along Dayi and Pengxian counties in the foothill belt of Longmen Mountain. Open platform to restricted platform facies consisting mainly of limestone and cherty limestone were developed further north and northeast to many districts like Mianyang, Langzhong, Daxian, Xuanhan, Kaijiang, Kaixian, Wanxian and Yunyang and further east to the western part of Hunan and Hubei provinces. To the north and east of the above sedimentary environment, the Dalong Formation of ocean trough facies dominated by silicic rock was deposited, forming a depositional pattern of several troughs surrounding a platform. Shallow water reef bank of slope facies was developed on the platform margin between the ocean trough and platform. The relationship between the development of reef bank with the rising of water level and the growth of reef building organisms is also analyzed. The large amount of basalt flow blowing out along the submarine rift provided rich SiO2 to the platform and ocean troughs. The reef bank was distributed along the shoulders on both sides of the troughs. Organic oolitic beach was developed on the gentle slope of the platform, while organic reef occurred on the steep slope. The dolomitized reef bank is especially favorable for the accumulation of oil and gas. In the northeastern Sichuan Basin, Bazhong and Yilong lotus form structures are recognized through integrated analysis of the seismic responses of reef bank and the Himalayan and Yanshan tectonism. Fuxingchang, Zhongtaishan and Laojun'an anticlines in relatively stable parts of Mianyang and Zhongtaishan convolute structures may be gas reservoirs of reef bank type.