致密砂岩成藏期后古孔隙度反演研究

Inversion of paleo-porosity of tight sandstone during post-accumulation period

以鄂尔多斯盆地十里加汗地区二叠系下石盒子组致密岩屑砂岩为研究对象,通过逆向思维利用反演方法建立成藏期后砂岩孔隙度演化数学模型。首先利用流体包裹体均一温度和前人自生伊利石测年成果综合判断成藏期次并确定出成藏期后的界限;在砂岩压实与胶结综合减孔模型的基础上建立成藏期后砂岩孔隙度反演模型;以锦32井气藏岩屑砂岩为例从砂岩现今孔隙度出发进行单点砂岩孔隙度反演模拟,重建成藏期后孔隙度演化过程,将反演模拟结果与已有的Athy模型和双元函数模型以及前人的压实实验结果进行对比分析。研究结果表明:1)研究区下石盒子组成藏期后的界限对应的地质年代是距今130 Ma左右,该界限是根据与天然气包裹体伴生的盐水包裹体均一温度的上限值130℃确定的;2)锦32井的反演模拟结果显示,现今孔隙度为7.7%的砂岩储层在距今100 Ma达到最大埋深(3500 m左右)时为7.4%,而在成藏末期(埋深2700 m左右)为11.1%;3)在成藏之后的继续深埋过程中,现今孔隙度为10.7%与3.7%的两个砂岩点的反演模拟结果表明成藏末期砂岩孔隙度越大,其压实减孔幅度也越大;这一结果更加接近已有的压实实验结果:高孔砂岩和低孔砂岩的减孔趋势会逐渐靠近;而已有的Athy模型和双元函数模型的模拟结果显示高孔砂岩和低孔砂岩的减孔趋势是相互平行的;4)与已有的两个模型相比,只有反演模型能够模拟出抬升过程中的砂岩压实回弹(锦32井所选砂岩的回弹量为0.3%),这更符合研究区后期抬升剥蚀厚度与砂岩孔隙度正相关关系的地质认识。与已有模型相比,所建立的反演模型能够更客观地反映砂岩储层在成藏期后沉降和抬升过程中的孔隙度演化趋势,然而孔隙度变化幅度的恢复精度仍有待于进一步提高。

With tight litharenites in the Permian Lower Shihezi formation,Shilijiahan area,Ordos Basin being the research object,the model of porosity evolution pattern of sandstone reservoir during post-accumulation period has been put forward with a converse thinking inversion method.Firstly,inclusion homogenization temperature and the authigenic illite dating from former studies were used to find out the period of hydrocarbon accumulation and determine the end of it;then the inversion model of porosity evolution of sandstone were put forward on the basis of comprehensive pore reduction model of sandstone compaction and cementation.Taking the tight litharenite gas reservoir in well Jin 32 as an example,inversion of single-point sandstone porosity was carried out based on the present porosity of sandstone to obtain porosity evolution process during post-accumulation period.a comparison of the inversion model with the pre-existing models(i.e.,Athy model and binary function forward model)was made to show which one is more reliable and reasonable.The results show:1)The corresponding geological age of the beginning of post-accumulation stage in target layers was around 130 Ma,determined by the upper-limit homogeneous temperature of two-phase aqueous fluid inclusions coexisted with natural-gas fluid inclusions,i.e.,130℃;2)The inversion result of well Jin 32 data indicates that the porosity of the sandstone reservoir with present-day porosity of 7.7%was 7.4%at 100 Ma with the maximum burial depth of approximately 3,500 m,and was 11.1%at the end of post-accumulation period with the burial depth of around 2,700 m;3)The inversion results of the two sandstone samples with present-day porosities of 10.7%and 3.7%,respectively,show that the higher the sandstone porosity at post-accumulation period is,the higher the reduction of porosity by compaction is.This conclusion is more coincident with results of pre-existing compaction experiments,viz.,the porosity reduction trends of high-and low-porosity sandstones gradually converge with increasing burial depths;while those of pre-existing Athy model and binary function forward model show that the porosity reduction trends of high-and low-porosity sandstones are parallel;4)Compared with the two other models,the inversion model herein could reveal compaction rebound of sandstone in the uplift(the rebound magnitude of the selected sandstone in well Jin 32 is 0.3%),which is more consistent with known geological knowledge of the positive correlation between late uplifting erosion thickness and sandstone porosity.Consequently,the inversion model in this research is more reasonable than the pre-existing models because it can more objectively reflect the porosity evolution pattern of sandstone reservoir in the subsidence and uplift during post-accumulation period.However,the precision of reconstructed paleo-porosity still needs to be further improved.