四川盆地及其周缘下志留统龙马溪组层序与沉积特征

Stratigraphic sequence and sedimentary characteristics of Lower Silurian Longmaxi Formation in the Sichuan Basin and its peripheral areas

开展四川盆地及其周缘下志留统龙马溪组高精度沉积环境研究是目前该区页岩气勘探开发面临的重要课题。以野外露头剖面、钻井资料为基础,依据特定的笔石带和重要标志层建立了等时地层格架,并结合岩相、古生物群落、钙质含量、测井响应特征、地球化学特征等地质信息开展了龙马溪组早、晚期沉积微相研究,揭示了其地层层序、沉积演化以及优质页岩分布等特征:①页岩地层具有顶、底界区域穿时性,自下而上可划分为SQ1和SQ2两个三级层序,SQ1主体为最大海侵期形成的深水相沉积组合,而SQ2总体为海平面下降期形成的半深水—浅水相沉积组合;②发育钙质深水陆棚、泥质深水陆棚等8种沉积微相,富有机质页岩主要形成于SQ1深水域;③从SQ1到SQ2,沉积中心自川南—川东坳陷区向川北地区迁移,川中古隆起为持续的水下隆起。结论认为:①SQ1是龙马溪组主力产层的沉积期,川南—川东坳陷为SQ1期深水陆棚中心区,是页岩气富集区;②川北地区的黑色页岩形成于SQ2晚期,且分布局限,页岩气勘探潜力相对不足,而鄂西地区黑色页岩的沉积规模和勘探潜力则介于川南—川东和川北地区之间。

High-precision sedimentary environment study of Lower Silurian Longmaxi Formation is an important subject for shale gas exploration and development in the Sichuan Basin and its surrounding areas. On the basis of outcrops and drilling data, its isochro- nous stratigraphic framework was built according to a particular graptolite zone and an important marker bed, and lithofacies, pale- ontology, calcareous content, well logging, geochemistry and other geologic information were combined to describe the sedimentary microfacies of Longmaxi Formation and its stratigraphic sequence, sedimentary evolution process and high quality shale distribution features：①with regional diachronism of the top and the bottom, I.ongmaxi Formation is divided into two third-order sequences （SQ1 and SQ2）, in which SQ1 is mainly an abyssal sedimentary assemblage deposited in the maximum marine transgression period, and SQ2 is a bathyal to shallow sea sedimentary assemblage deposited in the marine regression period; ② there are eight microfacies such as deep calcareous shelf, deep argillaceous shelf, etc. in this formation and the organic-rich shale was mainly deposited in the deep water area of SQ1; ③ from SQ1 to SQ2, the depocenter moved from the depression area in southern-eastern to northern Si chuan Basin, but the central Sichuan uplift remained an underwater one. It is concluded from this study that：① the gas shale mainly deposited in SQ1, the southern-eastern depression area was the depocenter in SQ1, as well as a shale gas enriched area; ② the black shale in northern Sichuan deposited in late SQ2, with limited distribution and relatively insufficient exploration potential, but the po- tential of shale gas exploration in western Hubei area is between southern-eastern and northern Sichuan Basin.