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

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 isochronous stratigraphic framework was built according to a particular graptolite zone and an important marker bed, and lithofacies, paleontology, 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: (1) with regional diachronism of the top and the bottom, Longmaxi 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 Sichuan 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 potential of shale gas exploration in western Hubei area is between southern-eastern and northern Sichuan Basin. ©, 2015, Natural Gas Industry Journal Agency. All right reserved.