致密储层孔隙结构相测井表征及对经济可采储量的约束——以柴达木盆地英西地区湖相混积岩为例

Characteristics and logging prediction methods of pore structure facies in the tight reservoir and its constraints on economic recoverable reserve:Case study of mixed rocks in Yingxi area,Qaidam Basin

柴达木盆地英西地区下干柴沟组上段是一套典型的湖相混积岩致密储层,油气资源潜力巨大,孔隙结构相测井表征方法的建立对约束该区的油气经济可采储量有重要意义。研究区储集空间类型以晶间孔隙为主,核磁共振孔径T2谱分布普遍集中在10 ms以内,孔喉总体趋于小孔、细喉特征。根据可动流体有效孔隙度大小,将研究区孔隙结构相划分为大孔粗喉型、小孔细喉型和致密型。其中,小孔细喉型和致密型的孔隙结构相发育广泛,大孔粗喉型为有利孔隙结构相。石英及碳酸盐矿物有利于微裂缝的发育,高含量的泥粉晶碳酸盐矿物和黏土矿物是研究区储层致密的根本原因。孔隙结构相越好,测井伽马值和密度值越小,白云石含量越高,黏土含量越少。基于孔隙结构相响应敏感的测井参数,运用主成分分析法建立了该区孔隙结构相测井分类图版,图版结果与核磁共振测井结果匹配良好。单井纵向孔隙结构相连续分类表明,大孔粗喉型常在碳酸盐矿物含量高的层段发育,构造裂缝的发育程度对孔隙结构相类型无明显的控制作用。单井经济可采储量受储层孔隙结构相约束较为明显,其中大孔粗喉型储层单井经济效益较好,大孔粗喉型+小孔细喉型储层具有一定经济效益,而小孔细喉型储层在井深较大的情况下盈利较为困难。

The upper section of the Lower Ganchaigou Formation in the Yingxi area of the Qaidam Basin is a typical compact reservoir of mixed siliciclastic-carbonate lacustrine successions,which has tremendous potential for oil and gas resources.Classification characteristics of pore structure facies and logging characterization meth⁃od are critical to economic recoverable reserves in this area.The main types of reservoir space are intercrystal⁃line pores,intergranular pores,dissolution pores and microfractures.The distribution of aperture T2 spectrum is generally concentrated within 10 ms,and the pore and throat tend to be small and fine.According to the mov⁃able fluid effective porosity,the pore structures in the study area are divided into large pore-throat type,small pore fine throat type and tight type.Among them,the pore structure of small pore fine throat type and tight type is widely developed,and the large pore-throat type is good pore structure facies.Sandy or sandy limy dolostone are favorable for the development of microfracture.The dissolution facies is mostly developed in the high carbon⁃ate minerals.The commonly developed micritic carbonate minerals and high-content clay minerals are the rea⁃sons why the reservoir is tight.Core calibration log analysis shows that dolomite content tends to increase and clay content tends to decrease as the pore structure gets better.Based on the well logging parameters sensitive to pore structure facies,the log identification chart of pore structure facies in this area was established by principal component analysis,and the results of log identification chart were consistent with those of the NMR log re⁃sults.The results of pore structure facies in single well showed that the large pore-throat type often developed in the strata with high carbonate mineral content,and the development of fractures had no obvious control on the types of pore structure facies.The economic-recoverable reserves of single wells are obviously constrained by the pore structure facies of the reservoir.Among them,the large pore-throat type facie reservoirs have good eco⁃nomic benefits from single wells,large pore-throat+small pore fine throat type facie reservoirs have certain economic benefits,but small pore fine throat type facie reservoirs are more difficult to get economic profit in the case of deep well depths.