Based on drilling cores, well logging and seismic data, source rocks and reservoirs are evaluated; and the natural gas genesis is identified through the analysis of natural gas isotopes, components and fluid inclusions, to study the gas accumulation conditions of the gypsum salt rock related strata of the Ordovician lower assemblage in Ordos Basin. (1) The natural gas from Ordovician lower assemblage is high thermal evolution dry gas from marine source rock, characterized by relatively light δ¹³C value of methane and heavy δ¹³C value of ethane. The natural gas is identified as gas cracking from crude oil according to component analysis. Thermochemical sulfate reduction (TSR) reaction has happened between the hydrocarbon fluid and sulfate as sulfur crystals are found in the cores, hydrogen sulfide is found in the natural gas, and hydrocarbon and hydrogen sulfide fluid inclusions are widespread in secondary minerals. (2) Around the gypsum-salt lows, argillaceous rocks are extensive in the Ordovician lower assemblage, reaching a cumulative thickness of 20–80 m. The effective source rocks include argillaceous rock rich in organic laminae, algal clump and algal dolomite. Analysis shows that the source rocks have a dominant TOC of 0.1%–0.5%, 0.31% on average and 3.24% at maximum. The source rocks have an average TOC of 0.58% after recovered through organic acid salt method, indicating the source rocks have high hydrocarbon supply potential. (3) In the sedimentary period, the palaeo-uplift controlled the distribution of reservoirs. The inherited secondary palaeo-uplift in Wushenqi–Jingbian east of the central palaeo-uplift and the low uplift formed by thick salt rocks near Shenmu–Zizhou area controlled the distribution of penecontemporaneous grain shoal dolomite reservoirs. The salinization sedimentary environment of gypsum salt rock can promote the development of reservoir. There are three types of dolomite reservoirs, the one with intercrystalline pore, with dissolution pore, and with fracture; intercrystalline and dissolution pores are main reservoir spaces. (4) There are two types of cap rocks, namely tight carbonate rock and gypsum-salt rock, constituting two types of source-reservoir-cap assemblages respectively. The general accumulation model is characterized by marine source rock supplying hydrocarbon, beach facies limy dolomite reservoir, small fractures acting as migration pathways, and structural-lithologic traps as accumulation zones. (5) The third and fourth members of Majiagou Formation are major target layers in the lower assemblage. The Wushengqi–Jingbian secondary paleo-uplift area and Shenmu–Zizhou low uplift are dolomite and limestone transition zone, there develops tight limestone to the east of the uplift zone, which is conducive to the formation of gas reservoir sealed by lithology in the updip. Two risk exploration wells drilled recently have encouraging results, indicating that the two uplift zones are important prospects.

根据钻井岩心、测井和地震数据，对烃源岩和储层进行评价；通过对天然气同位素、组分和流体包裹体的分析，确定天然气成因，研究鄂尔多斯盆地奥陶系下组合石膏盐岩相关地层的成藏条件。（1）从奥下组合的天然气是从海洋源岩高热演化干气，其特征在于，相对光δ ¹³甲烷和重δ的C值¹³乙烷的 C 值。天然气经成分分析鉴定为原油裂解气。烃类流体与硫酸盐发生热化学硫酸盐还原（TSR）反应，岩心中发现硫晶体，天然气中发现硫化氢，次生矿物中广泛存在烃类和硫化氢流体包裹体。(2) 膏盐低洼周围，奥陶系下组合广泛发育泥质岩，累计厚度达20～80 m。有效烃源岩包括富含有机层的泥质岩、藻块和藻白云岩。分析表明，烃源岩的优势TOC为0.1%～0.5%，平均为0.31%，最高为3.24%。烃源岩经有机酸盐法回收后平均TOC为0.58%，表明烃源岩具有较高的供烃潜力。(3) 沉积期古隆起控制了储集层的分布。中央古隆起以东的乌神旗—靖边次生古隆起和神木—紫州地区附近厚盐岩形成的低隆起控制了准同生颗粒滩白云岩储层的分布。膏盐岩盐化沉积环境可促进储层发育。白云岩储集层可分为晶间孔、溶蚀孔和裂缝三种类型；晶间孔和溶蚀孔是主要的储集空间。(4) 盖层有致密碳酸盐岩和膏盐岩两类，分别构成两类源-储-盖组合。一般成藏模式以海相烃源岩供烃、滩相石灰质白云岩储集层、小裂缝为运移通道、构造-岩性圈闭为成藏带为特征。(5) 马家沟组三段、四段为下组合主要目的层。乌胜旗—靖边次生古隆起区和神木—子州低隆起为白云岩与灰岩过渡带，隆起带东侧发育致密灰岩，有利于上倾岩性封堵气藏的形成。近期钻探的两口风险勘探井取得了可喜的成果，表明这两个隆起带具有重要的前景。一般成藏模式以海相烃源岩供烃、滩相石灰质白云岩储集层、小裂缝为运移通道、构造-岩性圈闭为成藏带为特征。(5) 马家沟组三段、四段为下组合主要目的层。乌胜旗—靖边次生古隆起区和神木—紫州低隆起为白云岩与灰岩过渡带，隆起带东侧发育致密灰岩，有利于上倾岩性封堵气藏的形成。近期钻探的两口风险勘探井取得了可喜的成果，表明这两个隆起带具有重要的前景。一般成藏模式以海相烃源岩供烃、滩相石灰质白云岩储集层、小裂缝为运移通道、构造-岩性圈闭为成藏带为特征。(5) 马家沟组三段、四段为下组合主要目的层。乌胜旗—靖边次生古隆起区和神木—紫州低隆起为白云岩与灰岩过渡带，隆起带东侧发育致密灰岩，有利于上倾岩性封堵气藏的形成。近期钻探的两口风险勘探井取得了可喜的成果，表明这两个隆起带具有重要的前景。小裂缝作为运移通道，构造-岩性圈闭作为聚集区。(5) 马家沟组三段、四段为下组合主要目的层。乌胜旗—靖边次生古隆起区和神木—紫州低隆起为白云岩与灰岩过渡带，隆起带东侧发育致密灰岩，有利于上倾岩性封堵气藏的形成。近期钻探的两口风险勘探井取得了可喜的成果，表明这两个隆起带具有重要的前景。小裂缝作为运移通道，构造-岩性圈闭作为聚集区。(5) 马家沟组三段、四段为下组合主要目的层。乌胜旗—靖边次生古隆起区和神木—紫州低隆起为白云岩与灰岩过渡带，隆起带东侧发育致密灰岩，有利于上倾岩性封堵气藏的形成。近期钻探的两口风险勘探井取得了可喜的成果，表明这两个隆起带具有重要的前景。乌胜旗—靖边次生古隆起区和神木—紫州低隆起为白云岩与灰岩过渡带，隆起带东侧发育致密灰岩，有利于上倾岩性封堵气藏的形成。近期钻探的两口风险勘探井取得了可喜的成果，表明这两个隆起带具有重要的前景。乌胜旗—靖边次生古隆起区和神木—紫州低隆起为白云岩与灰岩过渡带，隆起带东侧发育致密灰岩，有利于上倾岩性封堵气藏的形成。近期钻探的两口风险勘探井取得了可喜的成果，表明这两个隆起带具有重要的前景。