Sedimentary and Petroleum Geological Characters of Lower Cretaceous in West Sub-Sag, South Buir Sag of Tamtsag Basin
Advances in Geosciences
The evolution of the West Sub-Sag, South Buir Sag of Tamtsag Basin in Mongolia is mainly controlled by extension of the contemporaneous NE-orientated faults. In addition, it is superimposed with the influences of the base-mental relief and the transferring of NEE-/near WE-orientated shear fracture. The early Early Cretaceous was a fault-depressing stage of the West Sub-Sag's tectono-sedimentary evolution, and the facies developing experienced a complete cycle of
... erentiation-re-universal. In the Tongbomiao and Nantun Formations, the major sedimentary facies-types are fan delta, inshore underwater fan and lacustrine facies. The distribution of fan bodies and each sub-facies occurring of fan delta and inshore underwater fan should be constrained by the source regions to both the eastern and western sides. To the southwestern side of the South Hollow, there existed a "temporary" source region, which resulted in the subsidence center being not superposition with the depocenter during the sedimentary period of the Tongbomiao Formation. Owing to spatial changes of the thickness, lithology and petrofacies, the source rocks and reservoirs in the West Sub-Sag display distinct inhomogeneity. In the South Hollow, the source rocks are better occurred, with the organic materials being mainly II2-type. The organic thermal evolution can be divided into three stages: unmatured, matured and oil-generating peak stages. The sand bodies are better developed in the Central-North Hollow. The available data demonstrate the reservoir's physical property being relatively poor, namely, the majority being reservoir with extra-lower porosity and super-lower permeability.