Special bedrock buried hill and the reservoiring process in Qijia-Yitong basin in northeastern China

The bedrock buried hill is a mountainous peak formed by the arching up of the basement rocks in a sedimentary basin. The mountainous peak could be the ancient buried hill, known as buried-hill drape structure, present before the formation of sedimentary cover. In contrast, the late-formed buried hill comes into being after the deposition of the sedimentary cover due to the fold, fracture, volcanic eruption and other tectonic events in later stages. No matter what type of buried-hills, the reservoiring is comparable, with the dissolved pores formed by weathering and leaching of bedrocks as the reservoir, and the overlying sedimentary rocks as the source rocks and cover rocks. These are known as ancient reservoir but newborn sources. We present here, however, a different situation of the buried hill in Yitong basin in northeastern China. The bedrock in Yitong basin is the Yanshanian granite, which occurs as a sill underlain by Paleozoic marine strata of low electric resistivity. A right-lateral strike-slip extrusion of Yitong basin in Himalayan period leads to the diapiric ascent of the Lower Paleozoic argillite, which in turn causes the arching up of the granite bedrock to form the buried hill. It is concluded, on the basis of drill No. Chang 37, that the natural gas is sourced from Carboniferous-Permian argillite, and reservoirs in the cracks developed beneath 300m of the granite sill, with the upper part of granite as the cover.