鄂尔多斯盆地奥陶系马家沟组四段优质白云岩储层发育特征及成因机制

Development characteristics and genetic mechanisms of high-quality dolomite reservoirs in the 4^(th) Member of Ordovician Majiagou Formation in the Ordos Basin

鄂尔多斯盆地米探1井在奥陶系马家沟组四段(以下简称马四段)白云岩获得天然气勘探新发现,展示出马四段良好的天然气勘探前景,但马四段白云岩储层发育特征及其分布规律不清,制约了该层系下一步的天然气勘探部署。为此,基于岩心、薄片、工业CT、U-Pb定年、物性分析及地球化学特征等资料,系统研究了马四段白云岩储层发育特征及成因机制,并预测了该盆地白云岩储层分布和天然气有利勘探区带。研究结果表明:(1)马四段主要发育丘滩相白云岩储层和斑状白云岩储层,其中丘滩相白云岩储层岩性为砂砾屑白云岩、细—中晶白云岩、粉—细晶白云岩和叠层石白云岩,斑状白云岩储层岩性为斑状粉—细晶白云岩、斑状粉晶白云岩和斑状灰质白云岩。(2)丘滩相白云岩储层储集空间主要为晶间(溶)孔,含少量溶蚀孔洞、残余粒间孔和微裂缝,平均孔隙度为5.06%;斑状白云岩储层储集空间以晶间孔为主,含少量晶间溶孔和微裂缝,平均孔隙度为2.46%。(3)丘滩相白云岩储层发育主要受微生物丘和颗粒滩控制,并经历了准同生白云石化作用和埋藏白云石化作用的改造,斑状白云岩储层发育主要受生物扰动作用、准同生白云石化作用和埋藏白云石化作用控制。(4)构造古地理格局和海平面升降变化共同控制了丘滩体发育、生物扰动作用程度和白云石化流体的形成。结论认为,马四段优质白云岩储层主要沿中央古隆起和神木—志丹低隆起规模分布,其中神木—志丹低隆起马四段白云岩储层位于岩性相变带上,其储盖配置关系好,是天然气富集区,预测有利区储层面积18 000 km^(2)。

Well Mitan 1 in the Ordos Basin achieves a new discovery of natural gas in the dolomite section of the 4^(th) Member of the Ordovician Majiagou Formation (hereinafter referred to as the Ma 4 Member),showing good natural gas exploration prospect,but the development characteristics and distribution laws of the dolomite reservoirs of Ma 4 Member are not understood clearly,which restricts their exploration deployment in the next step.Based on the data such as core,thin section,industrial CT,U-Pb dating,physical property analysis and geochemical characteristics,this paper systematically studies the development characteristics and genetic mechanisms of the dolomite reservoirs in the Ma 4 Member,and predicts the distribution of dolomite reservoirs and the favorable natural gas exploration zones in the Ordos Basin.And the following research results are obtained.First,in the Ma 4 Member,mound-beach facies dolomite reservoir and porphyry dolomite reservoir are mainly developed,among which,the former is lithologically composed of psammiticpsephitic dolomite,fine to medium crystalline dolomite,powder to fine crystalline dolomite and stromatolitic dolomite,and the latter is lithologically composed of porphyry powder to fine crystalline dolomite,porphyry powder crystalline dolomite and porphyry limy dolomite.Second,the reservoir space in the mound-beach facies dolomite reservoir is dominated by intercrystalline (dissolution) pores with a small number of dissolution cavities,residual intergranular pores and microfractures,with an average porosity of 5.06%.The reservoir space in the porphyry dolomite reservoir is dominated by intercrystalline pores,with a small number of intercrystalline dissolution pores and microfractures,with an average porosity of 2.46%.Third,the development of mound-beach facies dolomite reservoir is mainly controlled by microbial mounds and granular beaches,and has undergone penecontemporaneous dolomitization and burial dolomitization.The development of porphyry dolomite reservoir is mainly controlled by biological disturbance,penecontemporaneous dolomitization and burial dolomitization.Fourth,the development of mound-beach bodies,the degree of biological disturbance and the formation of dolomitized fluids are under the joint control of the tectonic paleogeographic pattern and sea level fluctuation.In conclusion,the high-quality dolomite reservoirs of Ma 4 Member are distributed in a large scale along the central paleo-uplift and the Shenmu-Zhidan low uplift.In addition,the dolomite reservoirs of Ma 4 Member in the Shenmu-zhidan low uplift are located in the lithological facies change zone,where reservoir-caprock configuration is go_to_(hd) and natural gas is enriched with a predicted favorable exploration area of 18000 km^(2).