Determination of the bottom boundary of the Middle Triassic Leikoupo Formation in the Huangjinkou area of Xuanhan, Northeast Sichuan

Abundant energy and mineral resources are widely distributed in the Huangjinkou area of Xuanhan, Northeast Sichuan.The Middle Triassic Leikoupo Formation is one of the host beds of potassium and lithium resources, and also an important natural gas reservoir in the area.The bottom boundary of the Leikoupo Formation in the Huangjinkou area was previously mainly defined by rock association and logging curve characteristics, rather than "mung bean rock" widely distributed in the area as a mark of bottom boundary.The mung bean rock in one of the potash exploration wells in this area was discovered recently with a thickness of about 2~3 cm, which was identified as potash rich feldspar glassy tuff under the microscope.XRD analysis shows that the main mineral components are quartz, microcline and muscovite.The discovery of the mung bean rock makes us reacquaint the correctness of the original bottom boundary of Leikoupo Formation.Based on the comparative study of lithologic association and logging curve, it is considered that the bottom boundary of the Leikoupo Formation in the Huangjingkou area should be moved down to be in the original Jialingjiang Formation.