对大川中地区上三叠统须家河组二、四、六段砂岩沉积相的再认识

There are strong disagreements in the academic circle as to such understandings of the late Triassic channel facies in the Xuiahe Formation of the Sichuan Basin. In view of this, this paper conducts a deep investigation in this study area into the outcrop profile observation, line description of logging from cored wells, per-well profile analysis, and chips identification at laboratory. On the basis of this and in combination with research results from a number of logging data, this paper concludes that shore-shallow lake facies dominate the sand bodies in the 2(superscript nd) member of the Xujiahe Formation while fluvial fades of sand bodies are mainly distributed in the areas to the north of Longgang-Wangcang and to the east of Guang'an. The influence of marine transgression can be shown from the pure quality quartz sandstone in the areas of the western Middle Sichuan Basin and the northwestern Sichuan Basin. In the sedimentary period, apart from the well-developed delta branch channels in the lower 4(superscript th) and 6(superscript th) members of the Xujiahe Formation of the Yingshan-Bajinochang and Guang'an-Hebeochang areas, the channel sand bodies in most other areas advanced and retreated along the lake shoreline, and alter being reformed and re-distributed by currents and waves, the shore-shallow lake fades came into being in the end. The following conclusions are achieved in this study. (1) Influenced by the paleo-uplift in the Leikoupo period, the lower beds in the 1(superscript st) and 2(superscript nd) members of the Xujiahe Formation are absent in the southern study area. (2) Influenced by the direction of lake currents, the sand bodies in the lake basin are in the N-E trending distribution in the southern study area, while those in nearly E-W trending distribution in the northern study area. (3) The quality porous formations are well developed in the area dominated by the shoal-bar sand bodies.