鄂尔多斯盆地靖边气田区中奥陶统马家沟组五_1—五_4亚段古岩溶型储层分布特征及综合评价

Distribution Characters and Comprehensive Evaluation of Middle Ordovician Majiagou 5_1-5_4 Submembers Reservoirs in Jingbian Gas field Area,Ordos Basin

鄂尔多斯盆地中部中奥陶统马家沟组最上部的马五1—马五4亚段是靖边气田区主要的产气层之一。按岩性及天然气勘探潜力每个亚段又可划分出2～4个小层,共12个小层。其中最佳产层为马五12和马五13层,其次有马五11、马五14、马五22和马五41层。储层最主要的岩石类型是含硬石膏结核或含硬石膏柱状晶和结核的粉晶白云岩,其次为含原生晶间孔隙的粗粉晶白云岩。中奥陶统至中石炭统经历了140Ma以上的沉积间断,马五1—马五4地层遭受强烈的表生裸露期古岩溶作用,形成了大量的溶蚀孔、缝、洞。埋藏成岩期的次生溶解孔隙,主要叠加在古岩溶作用形成的孔隙和含孔隙的填隙物,以及含原生晶间孔隙的粗粉晶白云岩上,由此发育成三者叠合的孔隙系统。印支运动和燕山运动形成的良好孔隙网络成为以表生成岩裸露期古岩溶孔隙为主的天然气储渗体。综合399口井各小层孔隙度、渗透率、地层厚度、测井有效厚度、含气饱和度及已有井的无阻流量等,制定储层评价标准,然后对各小层进行了详细的储层评价,在平面上圈出了不同储层级别的区块。

Ma51 to Ma54 Submembers that are at the uppermost part of Middle Ordovician Majiagou Formation are ones of the main nature gas pay horizons in Ordos Basin,According to the lithology and gas exploratory potential,each submember can be further divided into 2-4 layers which are 12 layers in total with total thickness of 70～80m.The best pay horizons are Ma51^2 and Ma51^3 layers,and the next ones are Ma51^1,Ma51^4,Ma52^2,Ma54^1 layers.In lithological types of reservoirs,the predominant one is silt-sized crystalline dolostone with anhydrite nodules or with anhydrite columnar crystalline and nodules,and the next is coarser silt-sized crystalline dolostone that contain intercrystal pores.Since experience for over 140 m.y.of sedimentary discontinuity form late Ordovician to Middle Carboniferous,the Ma5^1-Ma5^4 strata that situated at the topmost intensively suffered karstification of weathering palaeocrust during epidiagenetic exposure phase and formed a great amounts of solution pores,fissures,caves and channels.The secondary solution pores that formed during burial digenesis phase mostly stacked at the early paleaokastic pores and porous filling,or stacked at the coarser silt-sized crystalline dolostone with intercrystal pores so as to the stacked pore system developed.The good pore network that formed during Indo-Sinian and Yanshan movements formed in natural gas reservoir beds.By synthesizing the data of porosity,permeability,strata thickness,effective logging thickness,gas saturation and well flow from 399 wells,a criterion of reservoir evaluation is established.According to the criterion,the detailed reservoir evaluation does for each layer of Ma 51 to Ma 54 submembers and different reservoir grades of gas areas are cycled and mapped.