Research On Reservoir Characteristics And Diagenesis Evolution Of The Neogene Fine-grained Sedimentary Rocks In The Northwestern Qaidam Basin

Typical saline lacustrine facies were developed in the Neogene in the northwestern Qaidam Basin,where thick fine-grained sedimentary rocks are deposited.The fine-grained sedimentary rocks are characterized by strong heterogeneity and complex hydrocarbon generation and storage mechanisms,which is need to further research.In this thesis,150 core samples were collected,and X-ray diffraction tests,porosity and permeability tests,high-pressure mercury experiments,low temperature liquid nitrogen adsorption experiments,rock thin section and scanning electron microscopy were used to investigate their features including deposition distribution,lithology,mineral composition and reservoir conditions.Original evolution samples and artificial heating samples including low salinity to super high salinity were selected to analyze the characteristics of pore evolution and to determine the controlling factors of reservoirs which combined with fractal theory.Several favorable exploration areas were classified in the northwestern Qaidam Basin.The investigations indicate that the salinity of Neogene has relatively good regular changes in vertical and horizontal directions in the Northwestern Qaidam Basin.The salinity is low at early sedimentary stage while higher during the late sedimentary stage,and the salinity of the sedimentary center is relatively high on the plane.The salinity of sedimentary water and its lithologic characteristics are controlled by climate and migration of depocenter.Along with the deposition of younger strata,the contents of mixed fine-grained and carbonates increase obviously.In the depocenter,the thickness of fine-grained sedimentary rocks increase,but their porosity and permeability decrease.The fine-grained sedimentary rocks are heterogeneous and have complex pore structure.The intrusive mercury curves are divided into three types including pre-peak type(pore diameter more than 100 nm),post-peak type(pore diameter less than 100 nm)and mult-peak type of the samples.Mesopore is dominant in the fine sedimentary rocks with weak heterogeneity in the Shangganchaigou Formation,while most pores are macropore and mesopore with strong heterogeneity of the fine sedimentary rocks in the Shangyoushashan Formation and Xiayoushashan Formation.For the Neogene fine-grained sedimentary rocks in the study area,cementation and compaction are strong at the early diagenetic stage,and it develops pores with relatively smaller pore volume and larger pore diameter.At the middle diagenetic stage,the dissolution of organic acids is evident and the cementation of salts can resist the process of compaction,which can increase pore volume and porosity.When it comes to the late diagenetic stage,compaction becomes weaker,while the cementation of salt minerals is still strong and destroys the reservoir.Fine-grained sedimentary rocks with higher organic matter show the advantages of developing more organic pores during this stage.Salinity,mineral composition,total organic matter(TOC)content and thermal maturity are the main factors controlling the pores develop in fine-grained sedimentary rocks.TOC content has positively correlated with the mesopore volume.The carbonate mineral content was negatively correlated with specific surface area,macropore volume,mesopore volume and micropore volume.Clay mineral has positively correlated with specific surface area,mesopore volume and micropore volume,while has negatively correlated with macropore volume.Generally,salinity is the most dominant factor of pore distribution.Micropore is more complex in low-relatively high salinity samples,while micropore in super high salinity samples is relatively simpleBased on the spatial distribution,hydrocarbon generation conditions and reservoir characteristics of the Neogene fine-grained sedimentary rocks,the favorable exploration areas were classified in the northwestern Qaidam Basin.For the Shangganchaigou Formation,the Ganchaigou-Youquanzi-Nanyishan areas are favorable for oil and gas accumulation.The oil and gas favorable enrichment areas of the Xiayoushashan Formation are located in the Youquanzi-Nanyishan-Kaitemilike and west of Dafengshan.For the Shangyoushashan Formation,favorable for oil and gas enrichment areas are distributed in the Youquanzi-Kaitemilike and west of Dafengshan.