陕北上三叠统延长组含油砂体的沉积环境

陕北上三叠统延长组的含油砂体主要是与湖泊三角洲相关的河道沉积。本区的三角洲是河流注入到克拉通盆地的大型淡水湖泊中建造的。当时的陕北位于湖盆北翼的浅水台地。因此,三角洲前缘河口坝沉积不发育,且多为后继的河水强烈冲刷,以致河道砂体常常与浅湖泥岩直接呈冲刷接触。因此,河道砂体是该区的主要储集层,其中发育在上三角洲平原的曲流河道砂体是最有利的储集相带。

SEDIMENTARY ENVIRONMENT OF THE OIL-BEARING SAND BODIES IN THE UPPER TRIASSIC YANCHANG FORMATION OF NORTHERN SHAANXI

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.