The Middle Ordovician subsalt Majiagou Formation in the Ordos Basin comprises pervasively dolomitized shallow marine limestone and is a major reservoir rich in natural gas resources. Four types of dolomite matrix and cement were identified based on petrographic textures: (very) finely crystalline, non-planar to planar-s matrix dolomite (Md1); finely to medium crystalline, planar-s to planar-e matrix dolomite (Md2); microbialites comprising dolomite microcrystals (Md3); and finely to coarsely crystalline dolomite cement (Cd). The Md1 and Md2 dolomites were controlled by alternating lagoon-shoal facies and have δ¹³C values (− 1.89 to + 1.45‰ VPDB for Md1, − 1.35 to + 0.42‰ VPDB for Md2) that fall within or are slightly higher than the coeval seawater, suggesting the dolomitizing fluid of evaporated seawater. Md2 dolomite was then subjected to penecontemporaneous karstification by meteoric water and burial recrystallization by sealed brines during diagenesis, as indicated by its relatively lower δ¹⁸O values (− 8.89 to − 5.73‰ VPDB) and higher ⁸⁷Sr/⁸⁶Sr ratios (0.708920–0.710199). Md3 dolomite comprises thrombolite and stromatolite and is interpreted to form by a combination of initial microbial mediation and later replacive dolomitization related to evaporated seawater. Cd dolomite was associated with early-formed karst system in the Md2 host dolomite. The lowest δ¹⁸O values (− 11.78 to − 10.18‰ VPDB) and ⁸⁷Sr/⁸⁶Sr ratios (0.708688–0.708725) and fluid inclusion data (Th: 123–175 °C) indicate involvement of hydrothermal fluid from which the Cd dolomite precipitated during deep burial. These results reveal the multi-stage dolomitization history of the Majiagou Formation and provide new constraints on fluid origins and dolomites evolution during deep burial in old superimposed basins, such as the Ordos Basin and elsewhere.

鄂尔多斯盆地中奥陶纪中层盐下马家沟组由普遍白云石化的浅海灰岩组成，是天然气资源丰富的主要储集层。根据岩相织构鉴定了四种类型的白云岩基质和水泥：（非常）微晶，非平面至平面的基质白云石（Md1）；细至中晶，平面S至平面E基体白云石（Md2）；包含白云石微晶（Md3）的微斜石; 细到粗结晶的白云石水泥（Cd）。的MD1和MD2白云石通过交替泻湖浅滩相控制，并具有δ ¹³落入或略高于同期海水的C值（对于Md1为-1.89至+ 1.45‰VPDB，对于Md2为-1.35至+ 0.42‰VPDB），表明蒸发海水的白云石化流体。MD2白云石然后通过密封盐水成岩过程受到由大气水和埋藏重结晶准同生溶蚀作用，通过其相对较低的δ所示^18个O值（ - 8.89至- 5.73‰VPDB）和更高⁸⁷ SR / ⁸⁶ Sr比值（0.708920- 0.710199）。Md3白云石包括凝块岩和叠层石，并被解释为通过初始微生物介导和后来与蒸发海水有关的替代白云石化作用的组合而形成。Cd白云岩与Md2主体白云岩中早期形成的岩溶系统有关。最低δ¹⁸ O值（VPDB为− 11.78至− 10.18‰）和⁸⁷ Sr / ⁸⁶ Sr比（0.708688–0.708725）和流体包裹体数据（Th：123–175°C）表明热液参与了深部Cd白云岩的沉淀。葬礼。这些结果揭示了马家沟组的多阶段白云石化历史，并为鄂尔多斯盆地等老叠合盆地深埋期间流体成因和白云岩演化提供了新的约束条件。