Abstract:

The pyroclastic rocks and sandstones of Tongbomiao-Nantun Formation of Early Cretaceous are an important oil reservoir in the Beier depression, Hailarer basin, China and in the Tanan depression, Tamtsag basin, Mongolia. The reservoir in Tongbomiao Formation consists of mainly pyroclastic rocks with a less proportion of sandstones, in contrast, the reservoir in Nantun Formation is predominantly sandstones, although significant percentages of pyroclastic rocks are present. The pyroclastic rocks are gradually decreased from Tongbomiao Formation to Nantun Formation. The pyroclastic rocks of Tongbomiao Formation are composed of tuff in Beier depression, and composed of tuffite and tuffaceous sandstone in Tanan depression. According to the data of vitrinite reflectance and rock maxima pyrolysis temperature，the diagenetic stage of the pyroclastic rocks mainly  belong to period A of middle diagenetic stage. The pore type in pyroclastic rock reservoir mainly is secondary pore as well as primary pore. The measurement of porosity and permeability indicate that pyroclastic rock is middle-low porosity, extra low permeability reservoir. Two abnormal high porosity belts are found respectively in Beier depression and Tanan depression. Porosity of reservoir is mainly controlled by the component of detrital material, the sedimentary environment and the diagenesis.

Key words: Beier depression, Tanan depression, reservoir characteristics, processes controlling porosity, porosity, petroleum gas