RELATIONSHIP BETWEEN ORIGIN AND DISTRIBUTION OF DOLOMITES IN LOWER PALEOZOIC, JIZHONG DEPRESSION, NORTH CHINA

摘要

The origin and distribution of the Lower Paleozoic dolomites in the Jizhong Depression, North China, are related and affect the distribution of the reservoir. The Lower Paleozoic dolomites can be divided into four types: Type-A is gypsiferous mud- silt-sized crystalline dolomites; Type-B is non-gypsiferous mud-silt-sized crystalline dolomites; Type- C is silt-fine-sized crystalline dolomites; Type-D is medium-coarse-sized crystalline dolomites. Type-A dolomites contains gypsum, with stratiform stromatolites, bird's eye structures and brecciated structures. The ordering of dolomites averages 0.56. δ~(13)C averages -0.155‰ (PDB) and δ~(18)O averages -5.669‰ (PDB). Type-B dolomites are gypsum-free and not associated with gypsum bed. δ~(13)C averages -2.114‰ (PDB). The rest of the characteristic are similar to type A. Type-C dolomites are subhedral and euhe- dral. Metasomatic relict structure are common. The orderingofdolomitesaverages0.75.313Caverages- 0.693‰ (PDB) and δ~(18)O avenges -6.079‰ (PDB). Cathodoluminescence is orange and represents zonal structure. Type-D dolomites are subhedral or anhe- dral. Mosaic structure, recrystallization and residual porphyritic features are common. The ordering of dolomites averages 0.86. δ~(13)C averages -1.433‰ (PDB) and δ~(18)O averages -0.149‰（PDB). The homogenization temperature of inclusions ranges between 90-330°C. Cathodoluminescence is dark brown. Type-A dolomites and Type-B dolomites are thin, with a small horizontal distribution, but the number of dolomite layer is more. Type-C dolomites have large thickness and large horizontal distribution. It is generally distributed under Type-A and Type-B dolomites. Type-D dolomites have a massive distribution with a large vertical thickness. The range of horizontal distribution is relatively small. These characteristics can be used to analyze the original rock of dolomite, dolomitization temperature, stratigraphic burial history, the occurrence time of dolomitization, and the source of magnesium ions. It is indicated that Type-A and Type-B dolomites are formed in tidal flat environment. Type-A dolomites are formed as a result of evaporative pumping. The origin of Type-B dolomites are similar to that of dolomite in Andros Island, Bahamas. Type-C dolomites are formed as a result of seepage reflex dolomitization, and have a sandwich structure. Type-D dolomites are formed as a result of burial hydrothermal dolomitization, closely related to the deep faults of the Himalayan Movement and Cenozoic magmatic rocks. These dolomitizations result in different patterns of dolomite distribution. Type-A, Type-B and Type-D dolomites underwent dissolution during the formation process, and could form good reservoirs for oil and gas.