First Research on Marine and Nonmarine Sedimentary Sequences and Micropaleontologic Significance across Permian/Triassic Boundary in Iran (Isfahan and Abadeh)

Abundant ichthyoid remains, conodonts and holothurians sclerites were recovered near the Permian/Triassic boundary from a section south of Isfahan. Recovered ichthyoid remains include shark micro teeth and scales. The ichthyolith material is similar to a Fasanian ichthyolith from the Zakazane area in the Slovak karst of the Western Carpathians, which represents a subspecies of Acodina triassica . Conodont species are mostly neogondolellids. This fauna indicates that the sedimentary environment was marine, while to the north of localities near Isfahan and Zagross, terrestrial deposition was dominant at that time. Aluminasilicate and kaolin are present in a continental unit in Dopolan refractory main (Shahid Nilchian mine) and a section south of Chahriseh Village, north of Isfahan. Pisolitie, ironstone facies and bauxite clay are common near the Permian/Triassic boundary in the Chahriseh region.

Main Controlling Factors on Hydrocarbon Accumulation and Distribution in Marine Sedimentary Sequences in South China

Multiple source rock assemblages were deposited in the sedimentary provinces in South China in geologic history, and some of them were destructed by and some survived against multiple tectonic movements. Therefore, multiple sources, mixed sources, and uneven distribution of sources occurred in the marine sedimentary basins in South China during the late stage of hydrocarbon pooling. Epidiagenesis of the marine carbonate reservoirs and its modification to reservoir poroperm characteristics determined the formation and the scale of natural gas pools. The exploration practices show that the large to medium gas fields mainly occur in areas with high-quality reservoirs. Detailed study of the paleo-oil accumulations and typical oil and gas reservoirs reveals that the basins experienced multiphase superimposition and modification, leading to the distribution of the Paleozoic paleo-oil accumulations and bitumen in the peripheral areas. The phenomenon that oil and gas production concentrates in the Sichuan basin indicates that the overall sealing conditions of a basin determine the oil/gas potentials and the scale of oil and gas production. This is a critical factor controlling the accumulation and distribution of gas in the marine sequences in South China. The early oil and gas pools in the Yangtze platform left billions of bitumen in the peripheral areas due to the destruction of seals. Since the Himalayan, ＂late-generation and late-accumulation＂ gas pools represented by the gas pools in the Sichuan （四川） basin were formed in the marine sedimentary sequences in South China as a result of the change of the sealing conditions. Current gas discoveries appear to be ＂paleo-generation and paleoaccumulation＂ gas pools but actually are ＂late-generation and late-accumulation＂ gas pools. These patterns of hydrocarbon pooling clearly depict themselves in western Sichuan basin and Weiyuan （威远） gas field. It is revealed that the gas pools in the Sichuan basin were mainly formed as a result of hydrocarbon phase change （thermal cracking of oil to gas）, miscible migration, and dynamic equilibration since the Himalayan. A large number of gas pools were formed in the Himalayan and the gas pools in the marine sequences are characterized by late pooling; this kind of gas fields/pools are controlled by： （1） effectiveness of modification and superimposition of the marine basins, （2） effectiveness of the source rocks, （3） effectiveness of the overall preservation conditions, and （4） effectiveness of plays.