Sedimentology and Reservoir Characteristics of He8 Member, in A Gas Field of Ordos Basin, Northern China

The rapid lateral variation of fluvial sand-bodies seriously affect the development of oil and gas field. One long-term base-level cycle(LSC), four middle-term base-level cycles(MSC) and nine short-term base level cycles(SSC) are recognized by researching the stacking patterns and volumetric partitioning according to the core, well log and seismic data in He8 member of A gas field, Ordos basin. The base level cycles which are mainly base level rise half cycles are mainly composed of braided and meandering river deposits. The evolution of depositional system has experienced three stages: braided river deposition in the earlier stage, braided and meandering river transition coexistence in the middle stage and only meandering river deposition in later period. The braided channel sand, mid-channel bar and point bar are mainly reservoirs which show belt and ribbon along the SEE trending. These sand-bodies are vertically and laterally stacked with good continuity in the early MSC1, MSC2 and MSC3, while isolated and with poor continutiy in the early MSC4. The sedimentary microfacies and diagenesis affect the fluvial reservoir quality including lateral continuity, porosity and permeability and the buried depth of the good reservoirs with development of secondary solution pores is less than 3500m in this area.