A braided river delta deposited at the lower member of the Lower Ganchaigou Formation in the Kunbei step-fault zone of southwestern Qaidam basin is considered to be of great importance to the study of oil and gas accumulation in the region.Analyses based on drilling core

logging and seismic data

as well as grain size statistics

were employed to perform a detailed study on the sedimentary characteristics of the delta.Three subfacies were recognized in the delta:braided-river delta plain

braided-river delta front and prodelta.The plain subfacies can be further divided into two micro-facies:braided channels and flood plain.The front subfacies can be subdivided into five micro-facies:under water distributary channel

mouth bar/distal bar

sheet sand and under water distributary bay.Vertically

the delta evolved successively from braided river delta plain

braided delta front to shore-shallow lake. The plain facies experienced superimposition of multiple channels

while the front facies had only a weak influence over the channels.Horizontally

it was controlled by two major provenances and developed two main channels with horizontal heterogeneity.The syngenetic faults were consi-dered as the major controlling factors upon the development of the delta

while the provenance constraints were thought to have affected the distribution and composition of the delta sandbodies and the paleogeomorphology helped forming the present landform of the delta.A sedimentary model was then established for the delta based on comprehensive analyses of all geological factors.