The lake-margin canyon and its hydrocarbon potential

In a half-graben, the early-stage movement of the basin-controlling fault results in a great height difference along the lake margin. In an arid to semi-arid climate condition, spasmodic floods intensively eroded the fault's footwall basement so that an elongate incised valley, called the lake-margin canyon, is formed . The Y921 canyon in the northern steep slope of Dongying depression is such a canyon. In plane view, the 7 000-meter-long Y921 canyon cuts into the northern Chenjiazhuang footwall uplift and terminates at the Chennan fault in the south. In vertical profiles it is incised as deeply as 1 350 meters into the uplifted granite-gneiss footwall and has a “U”-shaped cross-section. The Y921 canyon is filled with conglomerate, arkose, lithic sandstone, mudstone, and carbonaceous shale of the Tertiary Shahejie Formation. The canyon was filled with braided river coarse-grained clastics, followed by cross-stratified sandstones of fan delta facies, and finally with shore-shallow-lake siltstone and mudstone.The paleotectonic analysis reveals that the Chennan fault's movement formed a large elevational difference, which led to incisement of the Y921 canyon. As the Dongying lake level rose with respect to the canyon bottom, the canyon filled until the differential elevation ceased. Black mudstones in the front of the lake margin canyon served as source rocks, and clastic deposits in the canyon served as reservoirs. In addition,the Chennan fault and the bedrock floor of the filled canyon served as conduits for hydrocarbon migration. Accordingly, many types of non-structural traps can be formed in such canyons as Y921.