Abstract: The Changxing Formation reservoirs of Yuanba gas field are dominated by ultra-deep bioherm developed at ramp-type platform margin.Development appraisal of the gas reservoir is challenged by the diversity of reef development model,unclear reservoir distribution and great difficulty of precise prediction in ultra-deep and thin reservoir rocks.In order to solve these problems,we studied the relationship between sea-level changes and bioherm development model,analyzed the main controlling factors of reefal dolomite reservoirs development,proposed the ideas and methods of reefal reservoir prediction,and predicted the bioherm reservoir distribution.The results show that there are four-stage bioherms in the Changxing Formation of Yuanba area.For single bioherm,there are two reef development models which are controlled by paleo-geomorphology and changes of sea-level.One is single-stage and the other is double-stage(multi-stage).For reef groups,there are five development models including vertical progradation,vertical retrogradation,horizontal migration,horizontal paralleling as well as composite and superimposed model.The major factors controlling the development of favorable bioherm reservoirs in Yuanba area are sedimentary background,eustacy,constructive diagenesis and tectonic fracture.Vertically,the reservoirs mainly occur in the reef cap,while laterally,the reservoirs mostly occur in the reef crest,followed by reef back and reef front.

Key words: bioherm, reef characterization, reservoir prediction, Changxing Formation, Yuanba gas field, Sichuan Basin