Sequence stratigrapgy and its bearings on the gas accumulation in the Lower Ordovician Majiagou Formation,Shaanxi-Gansu-Ningxia Basin

The Lower Ordovician Majiagou Formation, Shaanxi-Gansu-Ningxia Basin may be divided into three sequences, each of which consists of both transgressive and highstand systems tracts. The sea-level changes are considered to be the main factors controlling the formation of the sequences. In addition, the basal structures of the basin also have important effects on the distribution of sequence stratigraphy in the formation. Broadly speaking, the shallow-water sequences occur in the middle part, whereas the deep-water sequences appear in the western and southern parts of the basin. The source-reservoir-cap rock associations are preferred in the highstand systems tracts of the shallow-water sequences in the middle part of the basin, which indicate significant hydrocarbon potential.