Abstract:

 The Xu-6 reservoir of Guang'an gas field is a large scale sandstone complex of distributary channels of braided delta plain sub facies, and the sandstones of reservoir are mainly medium grained feldspar detritus sandstone and detritus sandstone. It has low porosity and ultra low permeability, with a porosity distribution range of 3％ to 15％ and permeability distribution range of 0. 01×10^-3～1×10^-3 μm². Main and sub-main distributary channels are effective reservoirs, and the alternate distribution of permeable zones and compact layers in sand complex lead to complicated reservoir distribution. Based on above results, and combined with diagenesis, the controlling factors of reservoirs are discussed. The reservoir development is controlled by three sedimentary factors of different scales: distinctive rock composition of sandstones from different source regions which affects enrichment areas of reservoirs; sedimentary evolution which controls reservoir distribution sections; sedimentary micro facies which determines the heterogeneity in sandstones of favorable sections. Strong compaction of sandstones in acid environment of coal measures layer is the main cause of general low permeability. Medium and coarse grained feldspar detritus sandstones deposited in main distributary channels, which have relatively high secondary dissolved pores, are the main sandstone type of reservoirs.

Key words:  Central Sichuan, Xujiahe Formation, low permeability sandstone reservoir, braided delta, reservoir quality, controlling factors of reservoir