Abstract:

Oil-bearing sandbodies in the upper Triassic Yanchang Formation of north Shaanxi are mainly channel deposits related to lacustrine delta. The delta was builted by the river which discharged into a large freshlake. At that time, north Shaanxi was located on a vast shallow platform of the north flank of the lake basin, the water depth of which ranges from several metres to more thanten metres. In this setting, the river waters often flowed in by a density underflow, so that it was very difficult to form river-mouth bar. Even though thin progradational facies could be formed, it would have been scoured intensely by advancing river waters. As a consequence,channel sandbodies are commonlyin contact with lacustrine mudstone by scouring. The platform deltas are characterized by widespread banded sandbodies, the distribution map of which mainly reflects fluvial and distributary channel de-posits. These channel sandbodies are subdivided into three types according totheir sedimentary characters: sand-rich low-sinuosity channel, mud-rich low-sinuosity channel and mixed load meandering channel deposits, in which the mixed load meandering channel deposits occurred in upper delta plain are the most important reservoirs.