Research On Effectiveness Of Oil Accumulation In Gaotaizi Tight Sandstone Reservoir In Qijia Area,Songliao Basin

Songliao Basin is one of the non-marine sedimentary basins which is the richest of oil and gas resources in the world.In recent years,more than one hundred million tons of tight oil reserves have been found in the Gaotaizi oil layer in the Cretaceous Qingshankou Formation and Fuyu oil layer in the Quantou Formation.Therefore,it has bright prospects for tight oil exploration in the Songliao Basin.Taking Gaotaizi tight oil layer in Qijia area,Songliao Basin as an example,this research focuses on the characteristics and source of tight oil,reservoir characteristics and their effect on flow potential of tight reservoir,controlling factors of oilness heterogeneity in the tight reservoir,as well as the process and effectiveness of tight oil accumulation.This study helps people know more about the accumulation mechanism and enrichment law of tight oil,and thus make a further guide and more accurate predictions for tight oil exploration.There developed two sets of source rocks in Gaotaizi oil layer,namely the first member of Qingshankou Formation(K₂qn₁)and the second and third member of Qingshankou Formation(K₂qn₂₊₃).Both of the source rocks are characterized by wide distribution,large thickness and high organic matter abundance.The source material of both of them mainly comes from the mixtures of bacteria,plankton and terrestrial higher plants.The maturity of K₂qn₁ source rock is higher than that of K₂qn₂₊₃ source rock,and thus was used as an essential index for oil-source correlation.By comparison of biomarker fingerprint and parameters of both sets of source rocks and oil sand extracts,it has been found that the tight oil in Gaotaizi reservoir comes from both of the K₂qn₁ and K₂qn₂₊₃ source rocks.The lithology of Gaotaizi tight reservoir is mainly siltstones,followed by fine sandstone.Overall,the porosity and permeability of Gaotaizi reservoir are poor.By a combination of mercury instrusion capillary pressure(MICP)and nitrogen gas adsorption(N₂GA),the full-range pore-throat size distribution of Gaotaizi tight sandstone reservoir has been established.By combination of rock constituents,mineral composition,physical property,pore types,pore-throat structures and pore-throat distribution,Gaotaizi reservoir has been divided into three types,namely unimodal pattern with big pores and medium throats(Type I),bimodal pattern with medium pores and thin throats(Type II)and mixed pattern with small pores and tiny throats(Type III).On this basis,pore structure models for each type of reservoir has been constructed,and the formation mechanism of different types of the reservoir has been discussed.The percolation capacity of the reservoir has a good positive correlation with the mean pore-throat radius.To investigate the range of pore-throat radius that make the best contribution to the percolation capacity of Gaotaizi reservoir actually,a new classification scheme of the pore-throat size has been proposed:micropore(<10 nm),transition pore(10?100 nm),mesopore(100 nm?0.625?m)and macropore(>0.625?m).The result shows that the percolation capacity of Gaotaizi tight reservoir is highly dependent on the content of mesopore and macropore(pore-throats larger than 0.1?m),and the main pore types are dissolution pores and residual intergranular pores.The main pore types of the micropores and transition pores are intercrystalline and intergranular pores within clay minerals,and they make less contribution to the percolation capacity of tight reservoir.Oil is distributed unevenly from sand body scale to thin-section scale in the Gaotaizi tight reservoir.Many factors are affecting the oil heterogeneity in the tight reservoir.The area with a high value of hydrocarbon-generating intensity and hydrocarbon generation pressure is in accordance with the enrichment area of tight oil because source rocks with high quality have supplied both sufficient material source and dynamic source for tight oil accumulation.When the hydrocarbon generation pressure of source rocks is higher than 30 MPa,large amounts of oil could be charged into the tight reservoir.The pore-throat structure of the reservoir has significant impact on the fluid property and oil content in Gaotaizi tight reservoir.The values of porosity,permeability and pore-throat radius of oil-water layer,water layer,tight oil layer and dry layer decrease successively while the displacement pressure of them increase in succession.In addition,with the increase of pore-throat radius,the oiliness grade of tight reservoir is higher.The pore-throat space with a radius between 0.1?1?m holds most of the tight oil.Wettability has some impact on the oil content in the reservoir,especially for the tight reservoir with poor pore-throat structures.It has some promoting effect on tight oil accumulation.Gaotaizi oil reservoir began to form at the end of Nenjiang Formation deposition and entered into the major hydrocarbon accumulation period at the end of Mingshui Formation depostion.Overall,the tight oil accumulation was a relatively long and continuous process without obvious staging features.The porosity evolution history of the three types of reservoir shows that during the major oil accumulation period,the porosity of Type I reservoir was so high that buoyancy was larger than the capillary force.Therefore,it formed the“sweet spot”of conventional oil in the tight oil layer.Type II and Type III reservoir had become tight in the oil accumulation period due to the strong compaction and carbonate cementation in the early stage.Thus buoyancy was smaller than capillary force.The residual pressure difference between source rocks and reservoir was the main driving force,and it overcame the capillary force when oil stepped into the reservoir.Comparatively,the porosity and pore-throat radius of Type II reservoir were larger,and thus capillary force was smaller than Type III tight reservoir.Therefore,Type II reservoir had much more percentage of pores bearing oil and formed a good tight oil layer.Type III reservoir had a few percentages of pores bearing oil and formed a poor oil layer or a dry layer under the same dynamic condition.Based on the study above,combined with the main controlling factors of the effectiveness of Gaotaizi tight oil accumulation,the accumulation model of“two sources supply,differences of accumulation”has been established for Gaotaizi reservoir.