库车坳陷大北地区下白垩统巴什基奇克组储层成因地质分析

塔里木盆地库车坳陷大北地区白垩系巴什基奇克组储层以细-中粒岩屑砂岩为主夹少量长石岩屑砂岩,杂基为铁泥质和泥质(2%～10%),胶结物以方解石(3%～15%)为主。粘土矿物以伊利石、绿泥石和伊/蒙混层为主,不含高岭石,伊/蒙混层中的蒙皂石含量一般为(15～20)%;主要孔隙组合为残余原生粒间孔—溶蚀孔—微孔隙,占储集空间总量的(50～90)%,其次为构造缝—溶蚀孔,主要的孔隙类型为溶蚀孔和裂缝,总体属于低孔低渗—特低孔特低渗储层。综合分析认为:阶段性的前陆构造逆冲推覆作用对沉积旋回性、水体盐度变化及物源区距离变化的控制至关重要;构造活跃期扇三角洲前缘环境中形成的差分选中-粗砂岩、近源快速堆积造成的高岩屑含量、干旱咸湖导致的高碳酸盐胶结和早成岩期相对深埋—后期持续快速深埋压实作用,是特低孔特低渗储层的主要成因;而较优质储层是构造平缓期辫状河三角洲前缘环境中形成的分选好的中-细砂岩、早期表生溶蚀及弱碳酸盐胶结作用、早期构造破裂的复合作用及其叠加效应的结果。

Geological analysis on reservoir mechanism of the Lower Cretaceous Bashijiqike Formation in Dabei area of the Kuqa Depression

The reservoirs in the Lower Cretaceous Bashijiqike Formation in Dabei area of the Kuqa Depression,Tarim Basin,consisted mainly of fine-middle sized lithic sandstones with a little feldspar-lithic sandstone,and contents of Fe-mud and mud matrices being(2～10)%,calcite cement being(3～15)%.The clay minerals were illite,chlorite and illite/smectite,with contents of the smectite being(15～20)%,and without kaolinite.The main pores composed of residual primary intergranular pore,solution pore and microvoid,with the contents being(50～90)%,the minor pores composed of structural fissure and solution pore.The main pore types were solution pore and fissure,the reservoir generally belonged to lower porosity and permeability to extralower porosity and permeability.By comprehensively analysing,the paper thought that episodic thrusting played an importance role to control the sedimentary cycles,alteration of water salinity and distance from the provenance.The mechanisms for the reservoir of extralower porosity and permeability might be as follows:middle-coarse sandstones of poor sorting deposited in fan delta front during tectonically active period,sandstones of high content debris deposited proximally and quickly,sandstones of high carbonate cement deposited in dry saline lake,sandstones experienced relatively deep buried in early diagenesis period and a sustained,rapid,deep-buried compaction in later period.The reservoirs with high quality were middle-fine sandstones deposited in braided river delta front during tectonically stable period,which experienced supergene solution and week carbonate cementation in early period,structural rupture in early period and their compound action.