Lithofacies-palaeogeography And Ancient Karst Characteristics Of Carboniferous In Easten Sichuan Basin

Upper Carboniferous Huanglong Formation , in eastern Sichuan Basin , has been found natural gas since 1977 , that make the Sichuan Basin become the most important one of the gas producing formation. After 30 years of exploration and development , Huanglong Formation has yielded very fruitful results of petroleum geology , however, the reservoirs into the possession of natural gas development and the role of the direct control of sedimentary facies and sequence stratigraphy characteristics and favorable reservoir facies distribution law and other basic geological research, especially karst breccia research in Carboniferous is still weak, which block the effectiveness of exploration and development of Eastern Sichuan. Therefore , Sequence - lithofacies palaeogeography in east Sichuan Huanglong Formation is not only theoretical but more practical significance.Based on the geological setting in East Huanglong, rock characteristics, sedimentary facies, karst features, geochemistry, and diagenetic detailed study of epigenetic changes, made the following points understand:(1) According to lithology, lithofacies and sedimentary evolution, Huanglong Formation , form bottom to top , can be divided into three sedimentary systems tracts of three lithologic: 1 (C₂hl¹), equal to LST, to secondary and secondary gray limestone karst breccia composition; 2 (C₂hl²), quite a TST, as the grain dolomite powder - fine-grained dolomite and microcrystalline dolomite each layer combination; 3 (C₂hl³), HST, the micro - light grain calcarenite, microcrystalline limestone with grain dust and mud - microcrystalline limestone interbedded combination.(2)Huanglong Formation deposition is a complete "transgression - regression" deposition, the early Carboniferous seawater intrusion into the internal to the Yangtze plate slowly, in eastern Sichuan, the scope of seawater intrusion is still limited to the east Yunyang; to Late Carboniferous Huanglong in East since the early invasive, low-lying parts of the basin to the basin and gradually on high ground over the margin, forming a low system Huanglong Formation domain Sabkha deposition; to Huanglong sections in the second end into the "maximum flooding" , and to generally accepted Huanglong region three paragraphs of a high deposition; Late Late Carboniferous movement affected by the Yunnan Province, Huanglong Formation formed at the top of rugged erosion surface and associated karst landscape; to the early occurrence of the Early Permian Liangshan Formation of coal deposits overlap the role of Huanglong Formation and then buried diagenetic transformation of re-entering the stage.(3) Carboniferous Huanglong Formation in East general development of the type identified as karst breccia breccia. And in accordance with the structure of karst breccia - the genesis, is divided into four kinds of different genesis of breccia types, namely: net joint mosaic dolomite karst breccia, brecciated dolomitic karst breccia-like support , breccia-like secondary support karst breccia and matrix support gray dolomitic karst breccia-like. Dissolution and karst breccia environment is divided into 4 different solution intervals, respectively: the top section of dissolution, the upper section of dissolution, the lower section of corroded sections and the bottom of the dissolution. Division of the four kinds of karst: karst highlands and residual hill, slope, karst valley and karst depressions.(4) The features of karst rock C, O, Sr isotope analysis shows that, with the vadose zoneâ†'active undercurrent zoneâ†'Inactive active zone, the dissolution increased strength, corrosion by the formation of karst rock appearanceδ¹³C andδ¹⁸O weak positive offset, while the product formed by the dissolution of secondary limestone, and cement fluid freshwater cementation was a strong negative shift, the corresponding ⁸⁷Sr/⁸⁶Sr ratio also has an active subsurface flow from the seepage zoneâ†'active undercurrent zoneâ†'Inactive phreatic zone with offset the negative trend.