Research On Coal Accumulation Of The Jurassic In Chaoshui Basin

Considering the replaced coal mining crisis increasingly series in East China, the exploration and development of reserved Jurassic coal resources in Northwest China are more and more counted by the relative government administration. Forming in Mesozoic Era, nonmarine Chaoshui basin stored productive coal resources in Qingtujing formation and Jijigou formation growing in Early and Middle Jurassic. On basis of outcropping rock, drill samples, logs and seismic data, this dissertation integrates sedimentology, sequence stratigraphy, tectonic geology and seismic geology to study the reservation regularity of Jurassic coal resources in Chaoshui basin.The researcher recognized 17 synsedimentary faultings,4 palaeohighs and 2 underwater bulges, divided the Chaoshui basin with 4 uplift zones and 6 depressions(including 13 bulges and sags), rebuilded the basin's tectonic subregion and constructed structural framework of the basin during the coal-bearing strata depositional stage. On the study of outcropping rock and observation of drill sample, researcher identified 6 lithofacies configurations and 16 lithofacies forms of the coal-bearing strata, proved Alluvial Fan-Braid River- Braid River Delta sedimentary system developed on the gentle slope of the faulted basin, and Fan Delta-Lacustrine sedimentary system developed on the steep slope of the faulted basin in the region of interest, made the conclusion that braid river delta and offshore sedimentation is profitable coal-forming environment. The study showed there were 6 sequence boundary in the Jurassic coal-bearing strata which can be divided into 2 2nd-order sequences, 5 3rd-order sequences, and constructed the sequence framework of coal-bearing strata.Application of dominant facies mapping recovered the lithofacies paleography according to the sequences in Chaoshui basin. During the coal-bearing strata depositional stage, Chaoshui basin developed latitudinally and sedimentary facies varied longitudinally. The provenances mainly were the Beida Mountain lying in the north of the basin, the Longshou Mountain lying in the south of the basin, the central uplift zone and the Minqin uplift zone. In Early Jurassic(SIII1&SIII2), center of subsidence located in the western Pingshan Lake area where developed the braid river paleogeographical unit, formation grew sporadically in Hongshagang and Hongliuyuan areas where developed the Alluvial-Braid River paleogeographical unit; In Middle Jurassic, a extensive transgression occurred, while the center of subsidence migrated to the east, where braid river delta and offshore paleogeographical units which were the main coal-forming environments developed in 2-nd order sequence transgressive system tract depositional stage(SIII3,SIII4);euprofundal paleogeographical unit formed in highcast system tract depositional stage(SIII5), coal-forming effect drooped and vanished in the end. Synthetically analyzing the sedimentary environment, sequence stratigraphy, paleogeography, geophysics and through-going characteristics of coal-bearing strata helped the researcher to summary the coal-forming model of the nonmarine faulted basin.Analyzing the basin's subsidence-burial history and balanced cross section assisted to recover the history of structural evolution in and after the coal-forming depositional stages: the first stage occurred in Jurassic, coal-bearing strata was buried quickly, coal's metamorphism advanced gradually, the second stage occurred in Late Jurassic, coal-bearing strata was uplifted and denuded, and rarely effected by regional heat metamorphism of magmatic rocks; the third stage occurred in Cenozoic Era, coal-bearing strata was buried again, but the burial elevation was shallow and coal's metamorphism was not enhanced. On basis of study above, the areas of coal resources of profitability is proposed in Chaoshui basin.