Main Controlling Factors of the Chang 9 Oil Reservoirs in the Beiliang Area, Western Ordos Basin

Abstract

The Chang 9 interval of the lower Yanchang Formation in the Ordos Basin is an important exploration target. Integrating stratigraphic correlation of 163 wells, sedimentary facies analysis of 10 wells, reservoir testing, and fluid pressure–temperature data, this study investigated reservoir characteristics and the main controlling factors of hydrocarbon accumulation in the Beiliang area of Wuqi. The K0 marker bed allows the Chang 9 Member to be subdivided into Chang 91–93. Delta-front subaqueous distributary channel sandbodies form the principal reservoirs, composed mainly of fine-grained feldspathic and lithic feldspathic sandstone with residual intergranular and dissolution pores. Average porosities are 8.23–9.66%, and permeabilities are 4.28–5.64 × 10−3 μm2, indicating low-porosity, low-permeability, strongly heterogeneous reservoirs. The lithologic reservoirs belong to a normal-temperature, low-pressure system (pressure coefficient 0.78), with CaCl2-type formation water. Hydrocarbons were mainly derived from Chang 7 source rocks, with possible supplementary supply from Chang 9 dark shales. Accumulation is controlled by hydrocarbon supply, subaqueous distributary channel distribution, and reservoir effectiveness and connectivity; low-amplitude nose structures only adjust local oil–water distribution, while traps are mainly lithologic, controlled by updip pinch-out and tight-sand sealing.