Tight Sandstone Reservoir Evaluation Based On Xujiahe Formation Of Western Sichuan Depression

Xujiahe Formation of Western Sichuan Depression has been one of the stratas that have the greatest exploration potential.It was deposited during the period of transition from the marine cratonic basin to the continental basin since late Triassic.Based on pre-existing researches,this study summarizes the characteristics of tight sandstone reservoirs,establishes evaluations for different types of reservoirs in TX2and TX4,defines the primary physical properties required for reservoir in both industrial and non-industrial gas wells,and finally predict the lateral extent of reservoirs with highest in Xujiahe Formation of Western Sichuan Depression.This thesis utilizes core data and gas testing data,combined with statistical analyses to define basic characteristics of tight sandstone.The gas generation,accumulation and migration pathway of the targeting reservoir are summarized.Types of accumulation spaces,diagenesis,sedimentary facies and associated physical characteristics are analyzed in detail.This depositional information allows the identification of facies associations,combined with distribution of fracture development,a systematic evaluation of reservoir quality is established.By analyzing gas testing and statistical results,the primary physical properties required for reservoir in industrial and non-industrial gas wells can be confirmed.Based on the evaluation of reservoir quality for prospecting facies,three layers of sediment are picked to predict the extent of profitable reservoir.Several conclusions can be made from this study:1.Xujiahe Formation of Western Sichuan Depression is dominated by terrigenous clastic rocks.There is a significant decrease in quartz content,and increase in lithics content observed from TX1 to TX5.Quartz is observed as the most abundant content of TX1,and lithics dominates the TX5.Lithics are mainly sedimentary type,and metamorphic type comes the second in abundance.There is minor igneous lithics observed.The mineralogical contents vary from reservoir to reservoir.TX2 contains much more feldspars than TX4,TX4 has more lithics in composition.Rock facies penetrated by wells with moderate production consists of quartzite as the dominant content,and the one penetrated by wells with low production is dominated by more feldspar and lithics.2.According to the distribution of micro-sedimentary facies,the subaqueous distributary channels of TX2 to TX4 has the greatest reservoir potential with great porosity and permeability in large extent.These sandstones commonly contain charcoal,and are consisted of light grey to whitish coarse to fine sandstone.Major sedimentary structures include massive stratification,parallel stratification and cross stratification.In terms of mineralogical composition,the sandstone contains abundant lithics which indicate better maturity.The porosity and permeability of mouth bar sand are worse than distributary channels.Interdistributary facies has the worst permeability and porosity,which can not contribute to the formation of prospecting reservoir.The best reservoir is typically observed in subaqueous distributary facies,since it has coarser grain size and matured quartzite and lithic arenite which are resistant to weathering.The best mouth bar reservoir quality is observed in TX3,and the best interdistributary facies reservoir quality is founded in TX2.3.The most heterogeneous physical characteristics are observed in Type I tight sandstone in Xujiahe Formation of Western Sichuan Depression.It is dominated by sandstone with extremely low porosity and permeability.Most wells drilled into this type of sandstone has moderate to high production performance.The lithology is dominated by medium sand-sized lithic quartz arenite to granule/pebble-bearing lithic quartz arenite.The sandstone is cemented by pressure dissolution.The diagenesis stage is from period B of early diagenetic stage to period A of middle diagenetic stage.The lithofacies configuration is mainly type C or combination of type C and type D.Type II tight sandstone typically contributes to low production performance of drilled wells.This type of reservoir contains thick massive sandstone,among which part of the facies has great potential for gas accumulation.The sandstone is dominated by sandstone and sandy conglomerate,which are cemented by pressure dissolution to pore-filling types.It gets matured at period A of middle diagenetic stage.The lithofacies configuration is dominate by type C+type D and type D+type E.Wells drilled into type III tight sandstone are characterized as gas flow well.Its cementation type is mainly pore-filling to pressure dissolution.It became mature at middle diagenetic stage with type A+type C and type C+type D lithofacies configuration.The above three types of reservoir are defined as effective reservoirs,and type IV marked as ineffective reservoir which contains dry or water-swept layers.4.The primary physical properties required for reservoir in industrial and non-industrial gas wells can be confirmed based on gas testing and statistical analyses.The base porosity limit for industrial gas well production for TX2 is defined as 2.34%,and base permeability limit is 0.4�10^-3?m².In the case of non-industrial gas well,the lower porosity and permeability limits are founded as 1.63%and 0.034�10^-3?m².For TX4,the base porosity and permeability limits for industrial wells are 1.47%and0.042�10^-3?m².1.22%and 0.0043�10^-3?m² are observed for non-industrial wells.5.The major gas producing layers of Xujiahe Formation of Western Sichuan Depression are the upper TX2 and lower TX4.By combining the reservoir evaluation result and gas testing data,the prospecting reservoirs are mainly concentrated in well-preserved sandstone,especially in areas in adjacent to coal laminae.The major gas producing layers in TX2 are TX₂¹?TX₂² and TX₂³.The profitable facies within TX₄⁷ gives sand ratio of 50%⁸0%.The lithofacies of which are dominated by C1 and D.The major gas producing layers in TX4 are TX₄⁷?TX₄⁸ and TX₄⁹.The profitable facies in TX₄⁷ reflects sand ratio between 40%and 80%,and lithofacies are mainly C1 and D.TX₄⁸ is dominated by C1,C2,D and E,and the sand ratio observed is from30%to 90%.Finally,TX₄⁹ contains C1,C2,D and E lithofacies with sand ratio between 40%and 100%.