High Frequency Sequence And The Development Regularity Of Siderite-Bearing Strata Of Late Paleozoic Coal Measures In Xingxian Area

In this paper,the theories and methods of sedimentary petrology,sedimentary geochemistry,lithofacies palaeogeography and sequence stratigraphy are applied.Through comprehensive analysis of lithological characteristics,vertical sedimentary sequence characteristics,element geochemical characteristics,frequency spectrum analysis of logging data and wavelet analysis characteristics,physical characteristics of more than 20 drilled coal-bearing strata in Xingxian area.Researches on sedimentary facies types and characteristics of Late Paleozoic coal measures in Xingxian area,identification and division of Milankovitch cycle,high frequency sequence stratigraphic analysis,sedimentation of siderite-bearing mudstone beds,physical characteristics and development regularity were carried out.According to the analysis of sedimentary environment,three sedimentary facies types of carbonate tidal flat,barrier-lagoon and Delta were identified in the coal-bearing strata of Carboniferous-Permian in the study area,and the sedimentary characteristics of each subfacies and microfacies were summarized.The stable Milankovitch cycle has been recorded in the Carboniferous-Permian strata,in which the long eccentricity cycle has the strongest control and influence on the sedimentary cycle,and has a good correspondence with the high-frequency sequence.Based on the spectrum analysis and wavelet analysis of logging data,combined with cyclic sedimentary sequence and cyclic interface characteristics,the Carboniferous-Permian coal-bearing strata are divided into 12 third-order sequences(SQ1-SQ12).The high-frequency sequence division is carried out within the third-order sequence framework.Each third-order sequence corresponds to 2-4 fourth-order sequences,and the high-frequency sequence stratigraphic framework is established,and the distribution characteristics of sedimentary facies are traced internally.From the study of sedimentary characteristics of siderite-bearing mudstone beds,the sedimentary environment and sedimentary sequence of siderite-bearing mudstone beds are studied.The results show that siderite in siderite-bearing mudstone beds occurs mostly in layers,nodules and granular small grains.It is yellow-brown under a planepolarized light and can be divided into colloidal aggregates,siderite microcrystallinepowder aggregates,spherical grains and siderite in micro-morphology.Siderite is mainly formed in weak oxidation-reduction environment with weak hydrodynamic conditions.The physical properties of siderite-bearing mudstone samples show that the pore structure and pore size distribution will be affected by different stages of siderite development,resulting in different sealing properties.Compared with ordinary mudstone,siderite-bearing mudstone bed(colloidal aggregate of siderite)has the characteristics of low porosity,low permeability and high breakthrough pressure.The calculation results of comprehensive evaluation parameters show that its sealing performance is better than that of ordinary mudstone bed.Based on logging identification of siderite-bearing mudstone formation,the vertical and horizontal distribution characteristics of siderite-bearing mudstone formation are discussed.Many layers of siderite-bearing mudstone beds have been developed steadily in Xingxian area,and different scale of siderite-bearing mudstone beds are controlled by the third-order and high-frequency sequence stratigraphic framework.The distribution of siderite-bearing mudstone layers is controlled by sedimentary environment,and develops in tidal flat-lagoon subfacies with weak hydrodynamic conditions and weak oxidation-reduction sedimentary environment and interdistributary bay microfacies with delta front subfacies.