Abstract:

The Shahezi Formation in the Xujiaweizi Fault Depression is the main target for recent natural gas exploration in the northern Songliao Basin.It is of great significance to study the reservoir characteristics and controlling factors of such tight glutenite gas reservoir.The deposits in the fault depression formed in an early Cretaceous extensional tectonic environment,can be divided into four third-order sequences vertically,each of which develops depositions of fandelta and braided delta in the steep and gentle slope zones of the rifted lacustrine basin.The reservoir isdominated by coarse clastic rocks such as glutenites and pebbly coarse sandstones.The reservoir pores of the Shahezi Formation consist of primary intergranular pores and secondary pores,with porosity mainly in the range of 2% to 8% and with permeability usually less than 0.1×10^-3μm²,indicating a tight reservoir.The controlling factors for reservoir development in the Shahezi Formation include burial depth,sedimentary facies belt,rock type and epigenesis.The fan delta front(braided delta)with shallow depth and glutenite reservoir of plain faciesare characteristic of good reservoir quality,while the sandstones and glutenites with pyroclastic materials,tend to form dissolution pores and improve the reservoir performance.The favorable reservoirs of the Shahezi Formation in the Xujiaweizi Fault Depression are mainly located in the fan-delta front and the plain area with a burial depth less than 4 000 m at the east and west sides of Anda,as well as in the higher part of fan development zone of Xudong and Xuxi areas.The shore-shallow lake facies with shallow depth in central Anda and the Xudong and Xuxi slope fan development zones with deep depth are also favorable zones for reservoir development.

Key words: reservoir characteristic, favorable region prediction, tight glutenite, Shahezi Formation, Xujiaweizi Fault Depression, Songliao Basin