THE GENETIC TYPES AND ASSOCIATION EVOLUTION OF DEEP LACUSTRINE FACIES MUDSTONES

Deng Hongwen (China Univerity of Geology) Qian Kai (Geological Research Institute of Shengli Oilfield)  

The deep lacustrine mudstones are the important source rocks, and the significant evidence of reconstructing evolutional history of basin and tracing the associated reservoirs as well. In recent ten years, the abundant information contained in deep lacustrine mudstones has been revealed more and more with the development of science and technology. It is actual necessary to research and use the information included in mudstones and form the sysmetric theory directing practice. The research of the lacustrine mudstones in Dong Ying Basin of Bohai Gulf area indicates that the deep lacustrine mudstones comprise at least eight lithofacieses: black shale, calcareous sliced shale, calcareous laminated shale, homogenerous massive mudstone, graded laminated mudstone, inhomogenerous massive mudstone, chaotic mudstone and deformed mudstone. These mudstone facieses assemble two categories on the basis of self-generating characteristics, i.e. autogenic mudstones, including black shale, calcarous sliced shale, calcarous laminated shale and homogenerous massive mudstone; allogenous mudstones, including graded laminated mudstone, inhomogenerous massive mudstone, chaotic mudstone and deformed mudstone. There are significant differences in six aspects between autogenic mudstones and allogenous mudstones, such as petrologic physics, organic geochemistry, inorganic geochemistry, mineralogic constituent, palaeontologic association and geophysics. The autogenic mudstones fully reflect the characteristics of deep lake, whereas the features of allogenous mudstones show that they are exotic to their depositiona! environments. The genetic characteristics of two types of mudstones indicate that eight lithotypes can be divided into three groups according to their genetic processes. The lithof'aeieses in the same group are closely related to each other in origin, but they are formed in the different evolutional stages of the lake. Therefore, the above- mentioned two categories of mudstones comprise three genetic serieses: lake- controlling mudstones, fiow- controlling mudstones and slumping mudstones. The lake-controlling mudstones were formed by slow grain -by -grain vertical accretion of fine-grained suspended particles. The direct origin which resulted in the different lithotypes was change of lake water nature. The autogenic mudstone, black shale and calcareous sliced shale, calcareous laminated shale, homogenerous massive mudstone, were formed successively as the lake was changed from permanent stratified lake, seasonal stratified lake with bottom anoxic environment into fully mixed aerobic lake. The flow- controlling mudstones consist of turbidity mudsetones and debris flow mudstone. The former comprise graded laminated mudstone which is the products of turminal stage of sandy tarbidite and inhomogenerous massive mudstone formed by flood turbidity current rich in mud or by redeposition of unconsolidated muddy deposits in the shallow lake. The graded laminated mudstone commonly show four types of sequences similar to marine classical muddy turbidite. Debris current mudstone series is chaotic mudstones introduced by redeposition of consolidated or semiconsolidated mudstones. The slumping mudstone series is mostly deformed mudstone initiated by slide or slamp. The association patterns of deep lacustrine mudstones of above-mentioned three genetic types depend on the different evolutional stages and palaeotopography of the basin. The starved stage of the basin was characterized by interbeded black shale and calcareous laminated shale with local mud turbidil:es (prevailing inhomogenerous massive mudstone) , covering an extensive area. Association of homogeiaerous massive mudstone with mud turbidite (prevailing graded laminated mudstone) and debris flow mudstone occurred in setting-compensation equilibrium stage of the basin. In the over-compensation stage ol' the basin, the lithofaeieses introduced by gravity flows gradually increased because the basinal slope background was formed. During this period slumping mudstone series was of wide di

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