Deposition-diagenesis Characteristics And The Favorable Reservoir Forecast Of Permian Major Gas-bearing Intervals In The Yanchang Exploration Area

The Yanchang exploration area is located in the southeastern Ordos basin.Since 2003,it has witnessed several great breakthrouths in the upper Paleozoic,resulting in the discovery of Yan'an Gas Field.By the end of 2016,the proven geological reserves are around 6650×10⁸m³ and the capacity is 32×10⁸m³,which means the capacity/reserve ratio is less than 0.5%.Huge amounts of natural gas resources have not been fully utilitized.In the aspect of geology,it is the unclear distribution of sedimentary systems and sand bodies,the complex diagenetic evolution and the vague main controlling facters regarding reservoir property that is responsible for this situation.With an integration of cores,logs,thin sections,SEM combined with Energy Disperse Spectroscope,X-ray diffractometor and so on,this thesis indentified the evolution of the sediment souce of Permian major gas-bearing intervals,examined the distribution of micro facies and sand bodies,analyzed the reservoir characteristics and diagenetic evolution,clarified the main controlling factors of the sandstone physical property and predicted the distribution of high quality reservoirs.Main results and innovation are as follows:?1?Based on cores,logs,seismic and geological dating data,Shanxi formation and the bottom of Xiashihezi formation can be devided into 4 3^rd sequence?SQ1^SQ4?.Detailed analysis of heavy minerals?Q type clustering analysis?shows that during SQ1^SQ4 the study area is controlled by two provenance systems,which situated in the north part?bigger?and the south part?smaller?,separately.The boundary line of the two provenance systems move to south?SQ1^SQ2?SQ3?and the move to north?SQ3?SQ4?.?2?According to plenty of core description,12 types of lithofacies and 8 kinds of lithoface associations are indentifed.Systemic analysis of lithofacies and facies associations indicate that the study area is dominated by meandering river delta front?SQ1^SQ3,9 lithofacies and 7lithofacie associations?and braided river delta front?SQ4,11 lithofacies and 7 lithofacie associations?.Then with the help of sedimentary parameters,the distribution of microfacies and sand body are examined.?3?By an integrated use of thin-section petrography,SEM,X-ray diffractometry and so on,6 types of reservoir spaces and 4 types of pore structures are indentified.The reservoir has experienced 3 period of hydrocarbon injection and 4 typical porosity evolution stages.It entered into the late diagenetic stage,with partly staying in middle diagenesis stage B.The porosity and permeability maninly range 3%⁷%and?0.01¹?×10^-3?m².?4?Symetic analysis and comparsion of reservoir physical property reveals that:?1?within the same set of sequence the physical property of the reservoir increases from the southern sedimentary system to the northern sedimentary system on average,?2?near the sequence boundary?SB2?SB3?SB4?,the physical property of the reservoir increases from the part below SB to the part above SB;?3?within the same set of sequence controlled by the same sedimentary systems,the reservoirs in LST and HST are better than that in the TST?SQ1^SQ4?;the reservoirs in LST are better than in HST?SQ1,SQ2,SQ4?,the reservoirs in LST are better than that in HST?SQ3?.Combined these difference with the initial the physical property dominated by provenance,base level and sedimentary environment and the burial stage diagenetic evolution,the machinism of the distribution of physical property within sequence framework is clarified,and mode is estiblised.?5?Six pramaters are applied to evaluate the quality of the reservoir by using grey relational analysis.Regarding the fact that the deposition and diagenesis characteristics within the northern and southern sedimentary systems are quite different,a method of?Score separately and map into one?is put forward to caluate the weight coefficient separately and merge the scores into one map.Quantitative evaluation of the reservoirs in LST and HST?SQ1^SQ4?indicates the superposition area of the high quality reaervoir?I type reaservoir?is located in Ansai area,the east part of Yanchuan-Yanchang and Zichang area.