Sedimentary characteristics and genesis of the Carboniferous Kalashayi Formation sand-shale sequence in Tahe oilfield are analyzed through integrating palaeontological

log

core

seismic and geochemical data.Overall

the sandstones in Kalashayi Formation are of progradational sedimentary sequence that get coarser and thicker upwards.The upper Ⅰ-Ⅲ oil reservoir groups are mainly of fluvial facies

while the lower Ⅳ-Ⅴ oil reservoir groups are of tide-effected fan delta front facies and plain facies.The evidences of tidal influences include:(1)rich in charophyta fossils with individual brackish Ostracoda and Acritarch microfossils;(2)local wavy

lenticular

bioturbate and scour-and-fill beddings;(3)a strontium/barium ratio ranging between 0.021 and 0.716

suggesting an environment from brackish to fresh water;(4)the Ⅳ-Ⅴ oil reservoir groups

as a whole

are the transition facies from tidal flat "double peak limestone" in the Bachu Formation to fluvial deposition of Ⅰ-Ⅲ reservoir groups.