| The Keshen area in the Kuqa depression of the Tarim Basin is a hotspot for ultra-deep oil and gas exploration and research.It is still unclear to comprehend the main geologic factors and mechanisms influencing effective reservoirs in the ultra-deep buried sandstones of the depression for the multiple diagenetic alterlations,strong heterogeneity,various controlling factors etc.This paper mainly focuses on the scientific problems of the formation mechanism and distribution characteristics of ultra-deep effective reservoirs in the Keshen area of Kuqa Depression.The Lower Cretaceous Bashijiqike Formation in the Keshen 2 gas reservoir is taken as the research object.Scanning electron microscopy,cathodoluminescence,and fluid inclusion temperature measurement are used to study the petrology,diagenesis,pore-throat structure and organic fluid activities of ultra-deep sandstone reservoirs.The reservoir physical properties during the key hydrocarbon accumulation period were restored,the mai n controlling factors and formation mechanism of effective reservoirs were clarified,and the logging data were combined to clarify the distribution law of effective reservoirs,thus forming the effective reservoir.According to the difference of rock fabric and diagenesis,the sand reservoirs can be divided to four types of sandstones,respectively including the tight compacted(the plastic particle content greater than 15%),the tight carbonate cemented(the cement content is more than 15%,but the calcite cement is the main),the tight quartz cemented(The cement content is more than 15%,but the self-generated quartz is the main)and the low plastic particles-weak cemented.The Bashijiqike Formation of Keshen area has been interpreted as stage B of mesodiagenesis.After reservoir sedimentation,this formation experienced severe compaction in the stage A of early diagenetic.Afterwards,this formation uplift was subjected to atmospheric freshwater leaching.In the stage B of early diagenetic,the calcite cement was formed in large quantities,and after entering the middle diagenetic stage,strong quartz cementation occurred.After that,natural gas is charged into the reservoir during the 3?1Ma period.Different diagenetic evolution processes of different types of sandstones are obvious,the tight compacted sandstone has high content of plastic particles,which leads to its weak compaction resistance,it has been tight compacted before natural gas is charged;The tight carbonate cemented sandstone is mainly controlled by the early calcite cementation,and the calcite cement occupies a large amount of pore space to make the sandstone tight before the natural gas is charged;The tight quartz cemented sandstone developed large-scale quartz cementation before natural gas charging,resulting in such sandstone not experiencing natural gas charging;The low plastic particles-weak cemented sandstone still retains good physical properties during the natural gas filling period,and has undergone natural gas charging,thus becoming the effective reservoir.On the one hand,the low plastic particles-weak cementation sandstones has strong anticompaction ability due to the high content of rigid particles.On the other hand,most of the quartz particles in this type of sandstones are polycrystalline quartz derived from the genesis of metamorphic rocks.The growth rate of authigenic quartz crystal on the surface of polycrystalline quartz particles is significantly slower than that on the surface of single crystal quartz particles,effectively slowing the formation of authigenic quartz.Using natural gamma,acoustic time difference,density,neutron and resistivity log curves,the two-dimensional intersection graph method is used to explain the sandstone types of the Bashijiqike reservoirs in the study area.As a whole,the effective reservoir sandstone in the area where the Keshen 205 and Keshen 206 wells are located has the largest cumulative thickness and gradually becomes thinner around. |
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