Heterogeneity Of Diagenesis And Reservoir Quality In Tight Sandstone

Tight sandstone oil,as an important unconventional oil and gas resource,has become an important target of oil and gas exploration and development.The marine reservoir of tight sandstone has been developed successfully in recent years,,however,the reservoir of tight sandstone oil in China is mainly distributed in the continental basin.The sedimentary facies change fast and sandstones show complex distribution.There are large differences of diagenesis and reservoir quality within the tight sandstone.The scale of "dessert" in continental tight reservoir is small and the distribution is complex,which results in low efficiency of development.Therefore,it is of great significance to study the mechanism of diagenatic heterogeneity and reservoir quality difference in tight sandbodies for the effective development of continental reservoir.The Chang 8 reservoir of Honghe Oilfield in the Ordos basin is composed of many sets of tight sandstones(the average porosity is 9.06% and permeability is 0.23mD).The reservoir quality of the same sandbody is strongly heterogeneous and the internal productivity is different.Taking the Chuankou belt sandbody(distributary channel in shallow water delta)of Chang 8 reservoir in Honghe oilfield as an example,this paper focus on the mechanism of heterogeneity of diagenesis and reservoir quality in tight sandstone based on seismic,well logging,test data of core samples.This research has great practical and theoretical significance for the development of oilfield and the development of reservoir geology.The main achievements of this research are as follows:The architecture and petrological distribution characteristics within Chuankou distributary channel belt were clarified in this study.The distributary channel belt in sublayer is stacked vertically and laterally by multistage distributary channel sandbodies.The distributary channel,which is composed of two architectural elements(braided channel and bar)in study area and the planar combination pattern is ?wide bar-narrow channel?.The width ratio of braided channel and bar is about 1:3,and the length to width ratio of braided bar is about 2.6:1.The distributary channel is mainly fine-grained sandstone and the medium sandstone shows lentoid distribution in the middle of the distributary channel.Along the direction of the source,the content of ductile components gradually decreases.The content of ductile components in distributary channel is lower than that of overbank sand.The classification method of reservoir quality in tight sandstones based on pore throat configuration was proposed.The pore throat configurations are classified by the size of throat based on fluid mobility and pore type,and then reservoir quality is classified by pore throat configuration with similar por-perm relationship.The sheet-like throat dominates the throat system and there are wide sheet-like throat,narrow sheet-like throat and extremely narrow sheet-like throat according to fluid mobility.The reservoir quality is divided into four classifications based on six types of pore throat configuration.The reservoir quality of sandstone with intergranular pore and wide sheet-like throat is the best(I type of reservoir).The second best are reservoirs with the intragranular pore and the narrow sheet-like throat(II and III type of reservoir).The poorest reservoir is sandstone with the group of interstitial cement pores and extremely narrow sheet-like throat(IV type of reservoir).The mechanism of the diagenatic heterogeneity and reservoir quality difference within the tight sandstone of the distributary channel in shallow water delta was revealed.The diagenatic heterogeneity and reservoir quality difference within tight sandstone is influenced by rock fabric,composition and architechture under the same tectonic setting.The coarser and lower ductile component content sandstone,which is away from the sandstone-mudstone boundry,goes through weaker compaction and weaker carbonate cementation,and favours the lining chlorite rim,resulting in the intergranular pore--wide sheet-like throat and the good reservoir quality.The dense carbonate zones are favoured in sandstone near the sandstone-mudstone boundry.The finer and higher ductile component content sandstone goes through stronger compaction,and then the original pores are damaged severly,resulting in the group of interstitial cement pores-extremely narrow sheet-like throat and the poor reservoir quality.The strong cementation by kaolinite favoured in the delta plain sandstone owing to the strong feldspar dissolution by meteoric water,resulting in the decrease of permeability.The spatial distribution of diagenesis and reservoir quality in the tight sandstone of distributary channel belt of shallow water delta was established.The good reservoir is favoured in the middle of the sandbody vertically,and there are dense carbonate zones at the top and bottom of the sandbody,which severly decrease the reservoir quality.Along the provenance to lake,the content of ductile component is reduced(rigid component increases),compaction intensity decreases,lining chlorite rim increases,kaolinite cementation decreases,and reservoir quality improves generally.Crosscutting provenance to lake direction,reservoir quality of the central part of single distributary channel is good.The compaction is strong and reservoir quality is poor in the edge of the distributary channel because of the fine-grained and high ductile component content.In the convergence central area of two distributary channel sandbody,sandstone is coarse,the thickness of sandstone is large,the compaction is weak,the non carbonate cementation zone is thick,and the reservoir quality is good.