| It is the first time to construct age spectrum of detrital zircons in the He 8 and Shan 1 Group of the Low Permian, Upper Paleozoic in Ordos basin, and research on chronology and characteristics of parent rocks in provenance tracing, using techniques of LA-ICPMS in situ U-Pb isotopic dating and trace element analysis of detrital zircons, accompanied with comparative study on petrology of skeleton and heavy minerals and their assemblages in the sandstones, reconstruction via magnetic fabric paleomagnetic original orientation. The depositional system and sedimentary facies models for the He 8 and Shan 1 Group are constructed, temporal and spatial distribution of depositional microfacies are investigated in this paper, base upon solid research on facies indicators from cores, study on well log facies accompanied by regional depositional background. Characteristics of petrology, types of pores and their assemblage, size of pore and throat and their coordination relationship, as well as the horizontal and vertical distribution of these pore structure micro features, based on observation of drilling cores, identification of thin-sections, X-ray diffraction (XRD)of clay minerals, cathodoluminescence image and electron scan micrpscope (ESM) analysis, sandstone grain, porosity, permeability and capillary pressure measurement of the sandstones, homogenization temperature of inclusions in authigenic cements. Impact of parent rock characteristics in provenence, depositional facies and hydrodynamic conditions and diagenesis on formation and development of excellent reservoir is also discussed in this study.Research result indicates that the Early Proterozoic Khondalite (2300Ma-1800Ma) zone and TTG gneisses (2300Ma-2600Ma) in the Yingshan Block located in north of the Ordos basin was the major provenence, and subordinate ones came from the Devinian-Carboniferous (300Ma-400Ma) granites and volcanic rocks. Part of the sedimentary sources in the eastern Ordos may derived from the Archean old metamorphic rock series in the northeastern Yingshan Block.Types of sandstones, skeleton and heavy minerals and their horizontal distribution of He 8 and Shan 1 group are controlled by chronology and stratigraphy, and characteristics of parent rocks, and the latter also resulted in heterogeneity of reservoir internally. Direction of sedimentary sources, stratigraphy and characteristics of parent rocks, depositional condition controlled the spatial distribution of sedimentary facies and sandbodies. Meandering deltaic plain sedimentary environment was domineering in the study area with the deltaic front in the southern part during the Shan 1 period. "Two-element structure" of the distributary channel sediment developed equally in vertical section. Horizontal distribution of zonal sand bodies dominated and sheet sand bodies subordinated occurred. While during the He 8 period, richer sources from the north provenence and more intense hydrodynamic condition compared with the Shan 1 phase resulted in alluvial plain in the north part, and shallow-water braided deltaic plain environment in vast region of study area, which was characterized by multiple network of distributary channels.Distributary channel course's frequent. changes resulted in multi-period channels overlapped, thus joint-sheet sandbodis horizontally and very thick sand body vertically occurred. Depositional facies and hydrodynamic condition controlled distribution of the sand bodies, influenced the structural maturity of sandstones, thus lead to the primary pore features of sandstones.Different sandstone types contributed to different diagenetic paths and various authigenic products, thus resulted in deviation in cements and their occurrences, pore types and their development, pore-throat assembledges and different mico pore structures, and this in turn induced heterogeneity and various reservoir quality. The excellent reservoir for natural gas is quartzarenite with litharenite poor ones, quality of sublitharenite is between the former and the latter.This study may not only answer problem of Upper Paleozoic provenance and parent chronology of the Ordos basin, but is very important theatrically and practically in a thorough exploration in characteristics of provenance in the northern part and deposition and tectonic evolution of the Ordos basin, and predict prospecting natural-bearing area and promote exploration and development for hydrocarbon resources in the Ordos basin. |
PDF Full Text Request