| Sichuan Basin is an important oil and gas bearing basin in southwestern China,nature gas resources are very rich, Longwangmiao FM is very important reservoir in Lower Cambrian, but we are unknonw on it, recent research results show that mixed sediments are common. Mixed sediment of terrigenous clastic and carbonate arises general concern due to its special sedimentary mechanism and crucial theoretical and practical significance. Nowadays, there is no related study about mixed sediment on petrology scale in Sichuan Basin, especially in the deep strata. Combining with regional geological information, well logging data, profile date, the paper recounts sedimentary characteristic,source significance,genetic type and its model, and the influence of the oil and gas geological conditions of the mixed sediment. During the deposition of Longwangmiao Fm in central Sichuan Basin, the basin is shallow in the west and deep in the east. Longwangmiao Fm occurred in gentle plateform depositional environment, the composition of mixed sedimentary is almost carbonate. Combine with drilling wells date, we find mixed sediments are mainly silt-bearing dolarenite, silt-bearing oolitic dolomite, silt-bearing crystal powder dolomite, the types of mixed sedimentary sequence are layer system of carbonate rock-hunji rock and layer system of hunji rock. The composition of terrigernous in the mixed sediments are silt-grained terrigernous quartz, sorting of particles are in good grade-level, rounding of particles are in poor grade-level. The occurence of terrgenous quartz could be divided to 5 types, which indicated different sediment and diagenetic events. Mixed sedimentary type could be divided into two categories, gentle mixed sedimentation and complex mixed sedimentation Ⅱ. The content of quartz is controlled by sea level on the longitudinal, mixed sediments are developed with transgressive cycle and regressive cycle, the strength of mixed sedimentary of transgressive cycle is stronger than regressive cycle, in the three kinds of sedimentary microfacies, the strength of mixed sedimentary from high to low is beach between depression microfacies, mixed grain beach microfacies, grain beach microfacies, the content of quartz is controlled by paleogeomorphy on the horizontal, the thickness and strength of mixed sedimentary of Gaoshiti area is stronger than Moxi area. Combining with characteristic of mixed sediments of two areas, author find the terrigernous quartz of north of Sichuan Basin are more and bigger than central of Sichuan Basin, the strength of mixed sedimentary of north of Sichuan Basin is stronger than central of Sichuan Basin, the types of mixed sedimentary sequence of north of Sichuan Basin are more than central of Sichuan Basin, the evidences show that environment of central of Sichuan Basin changes slowly, with no diffences between pure carbonate sedimentary environment, while north of Sichuan Basin has environmental catastrophe phenomenon. Five storm stages have been identified in Longwangmiao FM in the central of Sichuan Basin, storm depositional structures are including scouring filling structures, chrysanthemum-like and storm tearing structures in storm flow-generated gravelstone, we statistics storm calcirudite thickness and find the tempestites are distributing in northeast to southwest direction horizontally,and the stidal currents flowing from northeast to southwest. So we deduces that siliciclastic origin from the northern part of Sichuan Basin.Based of the abovementioned opinions, the author come up with the tidal flat model, and further discussed controlling factors of the mixed sedimentation. By contrast to its oil significance of former studies, author conclude mixed sedimentation has a negative efffct on reservoir, it must develop cracks to form good reservoirs. The author speculate grain beach sediments of Longwangmiao Fm in Tongnanba area is very rare and thin. |
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