Depositional systems in the Lower Cretaceous Xiagou Formation, Ying'er depression, Jiuquan Basin, Gansu

The Ying'er depression, Jiuquan Basin, Gansu appears as a Mesozoic-Cenozoic sedimentary depression on the northern margin of the Qilian fold zone. Four third-order sequences have been distinguished, on the basis of cores, well logs and seismic data, for the Lower Cretaceous Xiagou Formation as a main exploration target in the depression. Six sedimentary facies types are also discriminated, including the braided delta, fan delta, nearshore submarine fan, offshore submarine fan, fluxoturbidites and lake facies. The delta front subfacies and offshore submarine fan sandstones are believed to be the most favourable area for the hydrocarbon accumulation due to excellent physical properties and reservoir-seal associations. In contrast, the things are different for the delta plain subfacies and nearshore submarine fan deposits owing to coarser-grained desposits, and poor sand-mud differentiation and physical properties of the reservoir rocks. The spatial distribution and structural arrangement of the favourable facies zones play an important role in the spatial distribution of prolific areas.