Genetic Mechanism For Deep Buried Tight Sandstone Reservoir And Its Evalution

Deep buried sandstone typically underwent strong diagenesis which makes it very tight,but it may also become effective reservoir whose physical property is well preserved under certain geological conditions.Deep tight clastic reservoirs,occurring widely with huge potential reserves,are the most important alternative exploration targets in the future in China.Silurian Kepingtage Formation is the major exploration target and prospective hydrocarbon zone,which is a typical deep tight sandstone reservoir characterized by very deep burial,tightness,strong heterogeneity,complex diagenesis,low-amplitude structure,and thin sand bodies.It is of great significance to the exploration and development of such reservoirs in central Tarim or even in the whole Tarim Basin to improve reservoir evaluation system by analyzing the evolution and confirming the reservoir genetic mechanism.Referring to the previous studies and based on theories like petroleum geology,reservoir geology,sedimentology,structure geology,and relevant industrial standards,this work studies the reservoir and diagenesis characteristics of Kepingtage Formation adopting techniques multiple technologies including core observation,thin section identification,scanning electronic microscope,cathodoluminescence spectroscopy,X-diffraction,mercury injection,inclusion thermometry,and high-temperature and high-pressure dissolution experiment and making good use of general geologic data,and data from logging,drilling,and production so that the diagenetic facies are classified,and the effects of diagenesis of different types on porosity are calculated quantitatively.Pore development mechanism is analyzed in detail concerning generation of secondary pores and preservation of primary pores,and the controlling factors influencing reservoir development are discussed.Reservoir physical property lower limit is determined according to the real situation of the study area.The reservoirs are classified into types and evaluated after the reservoir evaluation indexes are optimized and the evaluation standards are set up.The study shows that Kepingtage Formation is low in compositional maturity and moderate in textural maturity.The reservoir space is mainly intergrain dissolved pore,and then primary intergrain pore and intragrain dissolved pore,with porosity averagely 7.08%and permeability averagely 0.6 1mD,which,overall,is very low porosity and ultra-low permeability porous reservoir.It mainly experienced compaction,cementation,dissolution,metasomasis,hydrocarbon charging,and cataclasis.The sand body with higher maturity is strong in anti-compaction and favorable for dissolution.Low geo-temperature gradient inhibits,to some extent,widespread development of late phase cementation.Hydrocarbon charging inhibits the happening of calcareous cementation,and in the meanwhile promotes dissolution.Kepingtage Formation is classified into 6 types of diagenetic facies,including tight facies from strong compaction,tight facies from calcareous cementation,tight facies from bitumen charging,porosity preservation facies from weak diagenesis,porosity enhancement facies from medium dissolution,and porosity generation facies from strong dissolution.Analysis of porosity evolution of each sandstone member shows that mechanic compaction during the phase of early diagenesis is the upmost cause of the tightness of the reservoir and early calcite cementation is the critical cause of the tightness while bitumen charging during Phase B of early diagenesis intensifies its tightness,and the reservoir became tight during the beginning period of Phase A of middle diagenesis.The secondary pores mainly result from dissolution of feldspar,clastic debris and slight calcite cements by organic acid.The dissolution started from Phase B of early diagenesis,peaked at the end of Phase A of middle diagenesis,and became weak afterwards.Dissolved quartz is seldom seen,which mainly occurs in alkaline environment.High temperature and high pressure experiment suggests that organic acid mainly dissolves feldspar minerals and carbonate minerals,whose dissolution not only creates a large number of secondary pores but also enlarges the primary pores and effectively improves the pore textures.Sedimentary environment is the fundamental condition for primary pore preservation.The sandstone with high compositional maturity and textural maturity,formed under strongly hydrodynamic force,is favorable for primary pore preservation under deep burial condition.The coating film of chlorite prevents intergrain pore of authigenic quartz from freely growing,which is an important mechanism for primary pore preservation.Hydrocarbon charging evidently inhibits calcareous cementation,which is of significance to primary pore preservation.Favorable burial mode and low geothermal gradient provide favorable conditions for primary pore preservation.Diagenesis determines the effectiveness of the deep tight sandstone reservoir whereas sedimentation mainly dominates the position of effective reservoir macroscopically.Channel mouth bar sand body of Kepingtage Formation that has the best physical property is ready for high quality reservoir,and next to it are the under-water distributary channel body and foreshore sand body.Compaction is the upmost cause of reservoir densification,and next to it is calcareous cementation while dissolution effectively improves reservoir property.Hydrocarbon charging preserves primary pores effectively and boosts dissolution whereas residual bitumen reduces porosity.The fractures greatly increase permeability.The lower limit of porosity is 5.5%,and the limit of permeability is 0.1mD.Classification valuation shows that type ?a(moderate)is most common and next to it is type?b(poor).Type ?(good)and type ?(non-reservoir)are not common.Vertically,the lower member mainly contains type ?b reservoir whereas the upper member contains type ?a reservoir.Laterally,the reservoir quality is good to the west of Well Shun903H,to the north of Well Shun901 and to the northeast of Well Shunl,where potential targets of exploration and development lie.