鄂尔多斯盆地长北气田山西组二段低孔低渗储层特征及形成机理

Characteristics and Mechanism of Permian Shanxi Tight Reservoir of Changbei Gas Field,Ordos Basin

长北气田山西组二段主要为三角洲平原分支河道沉积的石英砂岩和岩屑砂岩,其埋藏深度中等(2 700～2 950 m),但物性较差(孔隙度和渗透率平均分别为5.2%,0.7×10-3μm2),属于低孔低渗的致密储层。造成低孔低渗的主要原因是:该套砂岩形成于三角洲平原含煤酸性环境,砂岩中原始孔隙水介质呈酸性,碳酸钙处于不饱和状态,难以形成早期碳酸盐胶结,后来埋藏过程中压实作用损失了大量原生孔隙,使得粒间孔隙体积大大减少。同时,酸性成岩环境导致后来压实过程中大规模的石英次生加大,充填砂岩中未被压实损失的残余孔隙,使岩石孔隙度进一步降低。这是造成该地区砂岩低孔低渗致密化的主要原因。显微镜下岩石薄片观察证明了这种长北气田山二段砂岩在埋藏过程中的酸性成岩环境:砂岩孔隙中碳酸盐胶结尤其是早期碳酸盐胶结不发育,平均碳酸盐胶结物含量1%～3%,自生高岭石胶结发育,含量达7%～10%,伊利石胶结少,<3%,石英次生加大胶结发育,普遍达Ⅱ—Ⅲ级,平均为5%～10%,个别层段达10%～18%,最多达23%。另一方面,长北气田所处的鄂尔多斯盆地在大地构造上属于比较稳定的构造背景,盆地自上古生界二叠系山西组沉积以来,未经历大规模的构造运动,构造活动弱,地层平缓,褶皱少,断裂不发育,地层埋藏史分析表明山二段沉积后未经历明显的抬升与沉降。地下水尤其是含有机酸或CO2的酸性水不活跃,加之砂岩因压实作用和二氧化硅胶结后原始孔隙度和渗透性差,酸性地下水不能有效地对岩石进行溶蚀改造,不利于孔隙流体中物质的带入与带出;而且岩石中可溶组分(如长石、碳酸盐胶结物)少,长石含量<3%。因此,岩石在后期的埋藏过程中,不能形成大量的溶蚀次生孔隙,这是造成研究区目的层段砂岩低孔低渗的另一个原因。

Changbei gas field is located in Yishan Slope of the northern center of the Ordos Basin,China.The main pay zone reservoir is Upper Paleozoic Lower Permian Shanxi P1S2 quartz arenite,which deposited in distributary channel of delta plain.The reservoir is low porosity low permeability tight reservoir with average porosity of 5.2% and average permeability of 0.7×10-3 μm2 in a current depth of 2 700～2 950 m.Integrated study on depositional environment,lithology,diagenesis and tectonic background has been conducted.The reasons cause this low porosity low permeability tight reservoir features are considered as the following two factors.（1） On one hand,the reservoir was deposited in a coaly bearing acid environment of deltaic distributary channel,the original pore water is acid water with non saturated calcium carbonate,which can not form earlier cementation within the sandstone reservoir.However,compaction has significantly reduce the original pore and pore volume.On the other hand,this acid diagenesis environment cause large amounts of secondary quartz cementation fill the pores left out during compaction of sandstone reservoir.This is the main reasons cause the low porosity low permeability tight reservoir features in the region.Observation from thin section indicate that calcium carbonate cementation is not developed in the pore of sandstone reservoir especially in the earlier stage,the average component of calcium carbonate cementation is 1%～3%,authigenic kaolinite cementation is well developed with an average component of 7%～10%,illite segmentation is also less developed 3%,secondary quartz cementation well develop,mostly reach Ⅱ—Ⅲ level with an average component of 5%～10%,some layer can reach 10%～18% and maximum is 23%,this phenomenon indicated the acid diagenesis environment of P1S2 quartz arenite reservoir in Changbei gas field.（2） Tectonic study indicated that Changbei gas field is located in Ordos Basin tectonically stable at the time when Lower Permian Shanxi Formation was deposited.No tectonic movement and significant structure activities developed so that the formation layers are flat and rare faults and folds seen in the Changbei field.Buried history analysis also indicated that there is no apparently uplift and subsidence after P1S2 Shanxi Formation deposited.Underground water especially organic acid and CO2 acid water is not active,and pore is reduced by compaction and silicon dioxide cementation,acid water can＇t effectively dissolve the reservoir and displacement of materials in the pore.On the other hand,there are few dissolvable components e.g.feldspar（3%）,lithic and carbonate cementation in the rock,hence limited amount of secondary pores generated,this is the second reason that cause poor quality reservoir in Changbei gas field.