The Reservoir Reconstruction Mechanism Of Ichnofabric Of Majiagou Formation In The North-west Of Henan Province

The trace fossils of Majiagou Formation are the research objects in this paper which develop in Qinglongxia, Putaoyu, Wangyao and Mujiazhai of Jiaozuo area, the northwest of Henan province. Combining with the previous research, the preliminary classification of ichnofabric and sedimentary environment analysis were made by using methods of outcrop observation, thin section analysis and scanning electron microscope observation. The trace fossils and ichnofabrics’ diagenesis in the process of diagenetic evolution and its property modification to host rock are studied on the basis of this. The results are as follows:(1)Trace fossils consist of about 10 ichnogenera and 17 ichnospecies from Ordovician Majiagou Formation in northwest of Henan province. According to the symbiotic relationship and the tier distribution, combining with the petrology characteristics of host strata, the trace fossils can be devided into 7 ichnofabrics: ①Psilonichnus Inchnofabric. It mainly developed in the upper intertidal flat environment of the tidal-flat zonation, the depth of the water is between the mean high water spring tide and the sea level; ②Psilonichnus-Thalassinoides Inchnofabric. It mainly developed in the middle intertidal flat environment of the tidal-flat zonation, the depth of the water is between the mean high water spring tide and the sea level; ③Thalassinoides Inchnofabric. It mainly developed in the lower intertidal flat environment of the tidal-flat zonation, the depth of the water is between the sea level and the mean low water spring tide; ④Thalassinoides-Helminthopsis Inchnofabric. It mainly developed in the lagoon and the beach interact sea, the depth of the water is between the mean low water spring tide and the mormal wave base; ⑤Helminthopsis Inchnofabric. It mainly developed in the grain banks of restricted carbonate platform, the grain banks of open carbonate platform, and the grain banks of platform-margin, the depth of the water is between the normal wave base and the storm wave base; ⑥Helminthopsis-Chondrites Inchnofabric. It mainly developed in the depression of open carbonate platform and the deep-water above REDOX interface of Carbonate platform slope, the depth of the water is between the storm wave base and REDOX interface; ⑦Chondrites Inchnofabric. It mainly developed in the deep-water beneath REDOX interface of Carbonate platform slope.(2)The diagenesis types ocurred in burrow fillings of trace fossils are mainly of dolomitization, cementation, dissolution, compaction/pressolution and dedolomitization. The process of dolomitization can be divided into three stages: ①the preparation stage before dolomitization; ②the formation stage of dolomites; ③the adjustment stage of dolomites. The burrow fillings of different trace fossils formed different dolomitization status in this process. The dolomitization of early diagenetic stage is the foundation of the subsequent diagenetic transformation which has the largest contribution according to the pore development. Intercrystalline pores, intercrystalline dissolution pores and intracrystal dissolution pores are the main pore types of burrow fillings. The porosity of burrow fillings experienced two cycles from lower to higher along with the change of diagenetic environment.(3)Diagenetic transformation of trace fossils can greatly improve the reservoir property of its host rock. Different ichnofabric types have different intensity of reservoir reconstruction effect according to the differences between them, such as trace fossils types of ichnofabric, intersection relationship, bioturbation index, the proportion of each trace fossil and the intensity of diagenetic transformation.