四川盆地合川地区栖霞组-茅口组白云岩储层微观表征

Microscopic Characterization of Dolomite Reservoir of Qixia-Maokou Formation in Hechuan Area,Sichuan Basin

近年来,合川地区栖霞组-茅口组钻获多口高产工业气流井,表明其具有良好的勘探前景。然而该地区整体勘探程度较低,储层特征及成因仍不明确,导致储层评价、预测难。基于潼深4井、合深6井等8口钻井的资料,开展岩芯、薄片、成像测井及孔渗数据的储层特征分析,并优选样品开展基于氩离子抛光扫描电镜(scanning electron microscopy,SEM)及工业计算机断层扫描(computed tomography,CT)的储层孔喉结构微观表征。结果表明:四川盆地合川地区栖霞组-茅口组主要发育晶粒白云岩、含灰白云岩和灰质云岩,晶粒白云岩为主要的储层岩性,也是主要的储集空间载体。栖霞组白云岩储层以相对均匀发育的基质孔为主,整体具有中高孔、中渗的特征;茅口组白云岩储层主要以非均质分布的溶蚀孔洞和溶缝为主,也存在一定量的基质孔,整体具有中高孔、中高渗特征。基于工业CT的储层微观表征揭示栖霞组晶间(溶)孔分布相对均匀,而茅口组白云岩则非均质性强,晶间(溶)孔分布与裂缝无关,反映这些孔隙为早期孔隙的继承;基于扫描电镜的储层微观表征揭示栖霞组-茅口组都存在与晶体缺陷相关的微孔隙,表明除溶蚀孔洞外,晶间(溶)孔与微孔隙对储层物性同样具有重要贡献。研究成果对合川地区栖霞组-茅口组白云岩储层评价提供了一定的科学依据。

In recent years,multiple high-yield economically viable gas flow wells have been drilled in the Qixia Maokou Formation in the Hechuan area,indicating that it has good exploration prospects.However,the overall exploration level in the area is relatively low,and the characteristics and genesis of reservoirs are still unclear,leading to difficulties in reservoir evaluation and prediction.Based on the data of 8 drilling wells,including Tongshen 4 well and Heshen 6 well,reservoir characteristic analysis was conducted on core samples,thin sections,imaging techniques,and porosity-permeability measurements.Samples were selected for microscopic characterization of reservoir pore throat structure using scanning electron microscopy(SEM)with argon ion milling and industrial computed tomography(CT)testing.The results show that the Qixia Formation-Maokou Formation dolomite in the Hechuan area of the Sichuan Basin predominantly consist of three dolomite types:crystalline dolomite,dolomitic limestone and calcareous dolomite,with crystalline dolomite being the most crucial reservoir lithology and the main reservoir space carrier.The dolomite reservoir of the Qixia Formation is chiefly by intercrystalline pores which are relatively uniform in development,exhibiting medium to high porosity and medium permeability overall.The dolomite of the Maokou Formation primarily features with a varied distribution of intercrystalline pores,resulting in overall medium-high porosity and permeability.Reservoir microscopic characterisation based on industrial CT reveals that intercrystal pores are relatively homogeneous in the Qixia Formation dolomites,whereas the Maokou Formation dolomites are strongly heterogeneous,and both develop independently of fractures,reflecting that these pores are the inheritance of earlier pores.Reservoir microscopic characterisation based on SEM with Ar-ion milling reveals that micro-pores associated with crystal defects exist in both the Qixia and Maokou Formations,it suggests that in addition to vugs,intercrystal pores and micro pores also significant contributions to the reservoir properties.The research results provide a certain scientific basis for the evaluation of dolomite reservoirs in the Qixia Maokou Formation in the Hechuan area.