Medium-deep clastic reservoirs in the slope area of Qikou sag, Huanghua depression, Bohai Bay Basin

Abstract

To get a better understanding of the distribution pattern of favorable reservoir belts in the medium-deep clastic rocks in the Qikou sag of the Huanghua depression, and find out premium reservoir zone, the characteristics and controlling factors of the medium-deep reservoirs in the Qikou sag were analyzed. Comprehensive study into the structural setting, provenance, depositional system, and reservoir properties shows that the Paleogene in the Qikou sag has multi-sags and multi-slopes, with slopes accounting for over 70% of the total sag area. A number of large braided river (fan) delta front - gravity flow sand bodies matching with multi-slope zones lay a good material foundation for the formation of medium-deep effective reservoirs there. Generally buried at over 2500 m, the medium-deep clastic rocks are lithic feldspathic sandstone, with secondary pores and cracks as major reservoir space. Being in the medium diagenesis evolution stage, the medium-deep clastic rocks in the Qikou sag span over a large depth. Nine major factors affecting reservoir properties have been identified by mathematical geology. Although the medium-deep clastic rock interval is poor in physical properties and complex in pore structure, high sedimentation rate, medium geothermal field, high fluid pressure, high feldspar content, relatively high dissolution rate and early oil and gas charge have given rise to three to four belts with abnormally high porosity in it, which are all favorable hydrocarbon reservoir zones. Key words: slope area, medium-deep clastic, oil and gas reservoir, diagenetic evolution, Paleogene, Qikou sag, Bohai Bay Basin