Reservoir Characteristics And Genetic Mechanism Of Lacustrine Carbonate

The fourth member of Shahejie formation(Sha 4th member)mainly deposited a set of oil shale,mudstone and argillaceous carbonate interbeds during the rifting period of lacustrine basin,in western sag,Liaohe depression,Bohai Bay basin.It is a major set of hydrocarbon source rocks in Liaohe depression.Recently,with the development of unconventional oil and gas exploration at home and abroad,the tight oil and gas resources have drawn people's attention.It has a great theoretical and practical significance to study in detail on its deposition characteristics and the performance of the reservoir.Under the theoretical guidances of sequence stratigraphy,sedimentology,reservoir geology and unconventional oil and gas geology,making full use of drilling,logging,seismic and other laboratory analytical data,this thesis used the combine method of macroscopic and microcosmic,geology,well logging and seismic data to make a detailed study on stratigraphy and sedimentary characteristics in Sha 4th member.According to core analysis,thin section identification and logging analysis,combining with some research measures such as casting thin sections,scanning electron microscopy,mercury intrusion experiment CT scanning and rock mechanics analysis to describe the reservoir basic features of Sha 4th member,such as petrological characteristics,storage geometry types,physical property distribution,pore structure and rock brittleness characteristics.It discussed the controlling factors on reservoir development,like sedimentation,diagenesis and tectonic stress,etc.Synthesizing all the outcomes above and combined with seismic reservoir fracture prediction,brittle development prediction and hydrocarbon source rocks condition,this thesis evaluated and predicted the distribution and development of tight oil in Sha 4th member with the multi-factor overlapping probability method.During the diagenesis research,it studied deeply on the genetic mechanism of special rocks discovered in tight oil reservoir,such as dolomite and analcite.The specific results are as follows:1)The Sha 4th member of western sag has clear boundaries with underlying Fangshenpao formation,they are contacted as disconformity and angular unconformity,and it still has comparatively clear boundaries with overlying Sha 3rd member around the edge of lacustrine basin,while in the center of lacustrine basin,there are not apparent dividing characteristics.The stratum of Sha 4th member can be divided into four sequences on the basis of cycle characteristics and seismic data,corresponding to Gaosheng layer,Du 3rd layer,Du 2nd layer and Du 1St layer from bottom to top.2)When Sha 4th member was depositing,western sag is a single break dustpan faulted basin,with faulting at the eastern boundary and overlapping at the western boundary.There were several underwater low uplift and islands out of the water.The main body developed chromocratic sediment of semi-deep lacus or deep lacus intrafacies,which can be further divided into argillaceous lakebed microfacies,argillaceous-dolomite lakebed microfacies,marlaceous lakebed microfacies,lime mud lakebed microfacies and dolomitic-argillaceous lakebed microfacies.In the edge of the basin and highlands under water,a set of grain beach and arenaceous beach Microfacies of shore-shallow is developed.There is a belt developing fan delta around the eastern edge,whose coarse fragment comes from central bulge.3)Based on all kinds of data,including core observation,thin section authentication,scanning electron microscopy,analysis and test data,it can be concluded that the dolomite of Sha 4th member forms early which relates to the upwelling basement hydrothermal fluid and evaporation.Through the comparison of domestic exhalite sedimentation and based on data of X diffraction and macroscopic and microscopic petrology,it is found that the relationship between analcite and clay mineral in Sha 4th member is not obvious,and barely related to materials,like feldspar and volcanic debris,etc.So it can be speculated that the analcite of Sha 4th member are hot water sediments.4)The reservoir of Sha 4th member can be classified into conventional reservoir and unconventional reservoir.The conventional reservoir rocks mainly are grain dolomite or grain limestone,and pore reservoir is the primary reservoir type;while the unconventional reservoir rocks are argillaceous dolomite and argillaceous limestone,calcareous mudstone and dolomitic-argillaceous can be also developed as preferable reservoir on the condition that it exists crack.All kinds of cracks and a bit of secondary dissolution pore are the main reservoir space of unconventional reservoir,it has complex pore structure and strong aeolotropism,and the reservoir physical property closely relates to the development of microcrack.On electric property,the Sha 4th member reservoir has good response characteristics.Due to the development of mixed rocks in Sha 4th member,the difference of mineral compositions and their different contents can lead to a big difference in the brittleness.Based on the experimental results of the mechanics,both of carbonate minerals and analcite can improve the brittleness of reservoir,and the area with high brittleness often develops fractures.5)During development process,the reservoir of Sha 4th member was controlled by the following aspects.? The zoning of sedimentary facies controls the distribution of conventional reservoirs and unconventional reservoirs.For unconventional reservoirs,the lithologic difference controlled by sedimentary microfacies is an important factor to the distribution of tight reservoir.?Several types of diagenesis have contribution to reservoir of Sha 4th member.Among these,compaction and pressolution contribute most to the densification of unconventional reservoir.The later filling effect has a certain destructive effect on the development of the reservoir.Dissolution is the main constructive diagenesis of Sha 4th member.It formed the secondary pores in a certain extent and it is also favorable for the preservation of them.?Tectonic fracture is an important factor that affects the distribution of Sha 4th reservoir.6)On the basis of the above researches,combining with the results of hydrocarbon source rock condition analysis,the development zone of tight oil in Sha 4th member was evaluated and predicted by the multi-factor overlapping probability method.The result shows that the Du 3rd is the most favorable development layer of tight oil and Type I development zone is the most favorable area which is mainly located in Lei53-Leiping2 well area.What's more,the type II predict favorable area locates in Lei2-Lei55-Lei11 well area.