鄂尔多斯盆地米脂地区奥陶系马四段储层成因与分布

Genesis and distribution of reservoirs of the 4^(th) Member of Ordovician Majiagou Formation in the Mizhi area of the Ordos Basin

近期,鄂尔多斯盆地米脂地区米探1井在奥陶系马家沟组四段(以下简称马四段)首次获得35.24×10^(4)m^(3)/d的高产工业气流,取得了天然气勘探的新发现,展示出米脂地区奥陶系马四段具有良好的勘探前景,然而该区马四段储层研究却较为薄弱,储层成因及分布规律不清制约了下一步天然气勘探部署。为此,基于野外剖面、地震、测井、钻井岩心、岩石薄片、场发射扫描电镜以及微区碳氧锶同位素等资料,对该区储层特征、成因及分布进行了深入研究。研究结果表明:①该区马四段储层岩性主要为斑状粉晶白云岩、斑状灰质白云岩和斑状云质石灰岩,三类岩石储集空间均以晶间孔为主,含少量溶孔和微裂缝,平均孔隙度分别为3.08%、1.51%和1.29%;②储层发育主要受生物扰动作用、白云石化作用、断裂作用和溶蚀作用控制,生物扰动作用为储层发育奠定了基础条件,白云石化作用有利于孔隙的保持,断裂作用和溶蚀作用形成的裂缝和溶蚀孔进一步改善了储层的储集性能;③有利储层分布主要受海平面升降变化、古地理格局和断层控制,纵向上发育于高频向上变浅上部,横向上主要沿地势相对较高的凸起带分布。结论认为,鄂尔多斯盆地米脂地区马四段有利储层主要分布在瑶镇—大保当、神木—花石崖、万镇—米脂和绥德—碛口地区,有利面积达7200 km^(2),为下一步米脂地区的天然气勘探指明了方向。

Recently,Well Mitan 1 in the Mizhi area of the Ordos Basin obtains a high-yield industrial gas flow(35.24×10^(4) m^(3)/d)from the 4^(th) Member of Ordovician Majiagou Formation,which means a new exploration discovery and reveals a good exploration prospect there in the Mizhi area.However,reservoirs in this area are less researched and its genesis and distribution laws are not understood clearly,which restricts the next deployment of natural gas exploration.In this paper,the characteristics,genesis and distribution of the reservoirs in this area are studied deeply based on the data of field profile,seismic,logging,drilling core,rock thin section,field emission SEM and in-situ C/O/Sr isotopes.And the following research results are obtained.First,these reservoirs are lithologically dominated by porphyritic power crystalline dolomite,porphyritic limy dolomite and porphyritic dolomitic limestone,whose reservoir space is mainly acted by inter-crystalline pores with a small number of dissolution pores and micro-fractures,with an average porosity of 3.08%,1.51%and 1.29%,respectively.Second,the growth of these reservoirs is controlled by bioturbation,dolomitization,faulting and dissolution,among which,bioturbation forms the basic conditions for the growth,dolomitization is favorable for the preservation of pores,and the fractures and dissolution pores induced by faulting and dissolution improve the reservoir capacity further.Third,the distribution of favorable reservoirs is controlled by sea level change,paleogeographic patterns and faults.These favorable reservoirs are vertically developed in the upper part of high-frequency shallowing upward and laterally distributed mainly along the uplift belt with higher terrain.In conclusion,favorable reservoirs in this area are mainly distributed in Yaozhen-Dabaodang,Shenmu-Huashiya,Wanzhen-Mizhi and Suide-Qikou areas,covering an area of 7200 km^(2).These research results point out the direction of next natural gas exploration in the Mizhi area.