Reservoir limits and grading evaluation criteria of tight glutenite:a case study of Cretaceous Shahezi Formation in Xujiaweizi Fault Depression,Songliao Basin
The Lower Cretaceous Shahezi Formation glutenite reservoirs in the Xujiaweizi Fault Depression are important tight gas reservoirs in the deep strata of the Songliao Basin.Their complex porosity and permeability relation-ships pose challenges to determining reservoir boundaries and evaluating reservoir grades.For the Shahezi Forma-tion glutenite reservoirs,the water film thickness method,charge dynamics method,gas testing productivity method,and buoyancy balance method were used to determine theoretical lower limits,gas accumulation lower limits,effective flow lower limits,and reservoir-forming upper limits.The petrophysical values of the reservoir boundaries were determined based on the porosity and permeability relationships under different diagenetic controlling factors.On this basis,reservoir types were classified according to differences in microscopic pore structures,and a grading evaluation standard for tight reservoirs was established.This standard was then applied using logging data to provide a basis for selecting sweet spots in the tight gas exploration areas.The Shahezi Formation glutenite was divided into conventional reservoirs,Class Ⅰ-Ⅳ tight glutenite reservoirs,and non-reservoirs.The reservoir boundaries and classification evaluation results were well matched.Conventional reservoirs had porosity greater than 9％and permeability greater than 0.05×10-3 μm2.Class Ⅰ tight reservoirs had porosity of 8％-9％and permeability of(0.01-0.05)× 10-3 μm2.Class Ⅱ tight reservoirs had porosity of 5％-8％and permeability of(0.001-0.01)×10-3 μm2.Class Ⅲ tight reservoirs had porosity of 3.5％-5％and permeability of(0.2-1)×10-3μm2.Class Ⅳ tight reservoirs had porosity of 2％-3.5％and permeability of(0.05-0.2)×10-6 μm2.Non-reservoirs had porosity less than 2％and permeability less than 0.05×10-6 μm2.The gas production of tight glutenite was controlled by the reservoir type.Class Ⅰ and Class Ⅱ tight reservoirs were favorable high-yield layers.The thick-ness of favorable reservoirs in the Anda-Songzhan area of the northern Xujiaweizi Fault Depression was large,making it a sweet spot for tight gas exploration and development.
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