次生孔隙带预测新技术及其在大庆长垣以西地区的应用

New Prediction Technology of the Secondary Porosity Zone and Its Application in the Western Part of Daqing Peripheral

次生孔隙成因机制复杂,控制次生孔隙发育的因素众多,综观各种理论,从机理上看,砂岩的溶蚀及次生孔隙的形成都可归结到与有机酸及无机酸有关。目前,国内外对次生孔隙带的定量研究还只处于起步阶段。碎屑岩层的埋藏作用,对储层物性有双向的作用:一类是负向作用,使储层物性变差;另一类是正向作用,即使储层物性变好。本研究结合大庆长垣以西地区的石油地质特征,分析了次生孔隙的主控因素,认为研究区次生孔隙主要受机械压实作用、成岩胶结作用、大气水淋滤作用以及有机酸、碳酸等酸性流体的溶蚀作用控制。针对机械压实作用、有机酸和碳酸三种主控因素,确定了他们控制次生孔隙发育的门槛值,并建立了相应的定量评价模型。研究表明,压实作用和胶结作用使储层物性变差,而大气水淋滤作用和碳酸(HCO3-+CO32-)、有机酸等酸性流体的溶蚀作用都可以形成次生孔隙,使储层物性变好。当有机酸值高于1 000 mg/l时或者当HCO3-+CO32-值高于3 000 mg/l时或者某深度现今孔隙度大于其正常压实曲线公式计算的孔隙度时,次生孔隙度发育,根据这些门槛值,可以定量研究有机酸、碳酸和异常高孔的纵向和平面展布特征;沉积相对储层的原始物性具有控制作用,而成岩作用对储层物性的后期改造作用也很明显。因此,研究次生孔隙发育不能忽视对沉积相和成岩相的研究。以异常高孔、有机酸值、碳酸根+重碳酸根离子浓度平面等值线预测图叠合为主,并结合沉积相和成岩相,对次生孔隙带进行预测。利用该方法,对大庆长垣以西地区萨尔图油层、葡萄花油层、高台子油层以及扶杨油层的次生孔隙进行预测,预测区域和区内含油面积吻合的较好;薄片资料也佐证了本方法对次生孔隙带预测的可靠性。

The genetic mechanism of secondary porosity is complex,there are a lot of factors that control the development of secondary porosity.Looking at the various theories,regardless of the theory of formation water,geothermal field,precipitation,or the theory of organic matter evolution.From the mechanism point of view,dissolution of sandstone and formation of secondary porosity can be attributed to the organic acid and inorganic acid.The burial effect of clastic rocks on reservoir properties has two-way effect,one is the negative effect,that is the deterioration of reservoir properties,mainly are mechanical compaction and diagenetic cementation;the other is a positive effect,that is making reservoir properties better,mainly are the dissolution of acid fluid such as leaching of meteoric water,carbonate and bicarbonate,organic acids and so on.Combining with the oil geological features of the western part of Daqing Placanticline,the controlling factors of secondary porosity are analysesed in this paper.The author believe that secondary porosity in the study area is mainly controled by mechanical compaction,diagenetic cementation,meteoric water filtering effects and dissolution of acid fluid such as organic acids.The three controlling factors such as compaction,organic acids and carbonate,confirmed their threshold value of controling development of secondary porosity and the corresponding quantitative assessment model were set up. Porosity is greatly influenced by depth,Athy（1930） and Hedberg（1936） believed that porosity and depth are an exponential relationship under the normal effect of compaction.This paper established normal compaction curve formulas for three lithologies such as coarse sandstone,fine sandstone and siltstone in the study area,the formulas are （z）=44exp（-0.000 65·z）,（z）=47.8exp（-0.000 68·z）,（z）=48exp（-0.000 7·z）,where Z represents the depth,（z） represents the porosity at present. During the process of thermal evolution and degradation of organics,organic acid and CO2 are generated.They could form organic acid thermal fluid in water,which could corrode aluminosilicate mineral and carbonate cement in reservoir and thus form the secondary porosity（Surdam,1989）.According to the cross-plot analysis of the abnormal high porosity data and organic acid content in the formation water and the cross-plot analysis of the abnormal high porosity data and the value of HCO3-＋ CO2-3 in the research area,data shows that they have a good correlation,and the four abnormal high porosity areas on vertical（500~850 m,1 050~1 350 m,1 500~2 050 m,2 125~2 350 m;） correspond to high value areas of acid organic and HCO-3＋ CO2-3 respectively. Then how to give a plane quantitative prediction to the secondary porosity？ It has been proved that when the value of organic acid is higher than 1 000 mg/l or the value of HCO-3＋ CO2-3 is higher than 3 000 mg/l the secondary porosity is extremely developed.Then what is the threshold of the porosity？ The threshold is a normal compaction curve formula and it varies with depth.The developed area of the secondary porosity can be drawn on plane by using the value higher than the thresholds.In order to predict the plane characteristics of the secondary porosity better,reservoir group can also be divided.The area where the value is higher than zero is the relative developed area of the secondary porosity,and the area where the value is lower than zero is the relative area of diagenesis cementation and zero area is the normal compaction area or cancellation area.The higher the value is,the more developed the secondary porosity is. Sedimentary facies is a congenital factor that controls the original physical properties of the reservoir,while diagenetic facies,as an acquired factor,is the consequence of late reformation.Consequently,study of sedimentary facies and diagenetic facies should not be neglected when the secondary porosity is researched.Based on the predicted isoline map of abnormal high porosity,organic acid value and the concentration of HCO-3＋ CO2-3,combined with the sedimentary facies and the diagenetic facies,secondary porosity can be predicted. In order to check applicability of this method,the predicted results and the reserve areas are superimposed in this paper.The reserve areas in each formation coincide well with the results predicted in the paper.Thin section analysis also gives convincing evidence that there is a lot of secondary porosity in this area.Therefore,this method is convincible to predict the secondary porosity.It also provides a direct indication method for exploration deployment and the submission of reserves.