Characteristics of weathering crust at top of the Upper Permian Changxing Formation in Fuling area, eastern Sichuan Basin and its geological significance
Li Hongwei1,2, Tang Hao1,3, Su Chengpeng1,3, Cheng Hongyu1,3, Liu Hong2,3, Hu Guang2,3, Li Ling2,3, Tan Xiucheng1,2,3
1 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Southwest Petroleum University,Chengdu 610500,Sichuan
2 Sichuan Province Key Laboratory of Natural Gas Geology,Southwest Petroleum University,Chengdu 610500,Sichuan
3 Branch of Deposition and Accumulation, PetroChina Key Laboratory of Carbonate Reservoir, Southwest Petroleum University,Chengdu 610500,Sichuan
Whether there exists exposed erosion near the top of non-reef facies strata of the Upper Permian Changxing Formation in eastern Sichuan Basin has not been reported. In this paper,we take the non-reef strata at the top of Changxing Formation at Fuling area in eastern Sichuan Basin as an example,comprehensively studying the macro- and micro-features,mineralogical and geochemical elements characteristics. The result shows that there exists a weathering crust at the top of Changxing Formation. The weathering crust is 3-7cm thick and shows very clear vertical zonation, i.e.,from bottom to top,it is divided into four zones:the bedrock(micritic bioclastic limestone),saprolite layer,partially￣weathered layer which is characterized by enrichment of Si and decalcification,completely weathering layer which are characterized by enrichment of Fe.Since almost all the clay minerals available are illites, secondary minerals such as kaolinites and gibbsites of strong weathering stages have not yet been found, and iron oxide minerals are concentrated, it reflects an overall moderate weathering stage,corresponding to the ferromanganese-rich phase of weathering crust evolution stages. Further analysis reveales the presence of abundant typical Permian macroscopic fossils buried in situ,such as corals,brachiopods(Oldhamina),bryozoans and trilobites, etc., in the 80cm thick interbed of shale and limestone at the bottom of Feixianguan Formation which directly overlies the weathering crust,indicating that the study area uplifted and exposed during the Late Permian. At the same time,outcrops and cores in the study area exhibit plenty of karst fabrics such as solution grooves and karst caves, which are filled by mixture of porous carbonate sands,breccias,bauxitic mudstones and carbonaceous mudstones,indicating that the non-reef facies strata are modified by supergene karstification. The regressive event in the Late Permian was not only of seawater shallowing,but may lead to the long-term exposure of platform area,which modified reefs and beach reservoirs, and even the non-reef facies strata of the Changxing Formation by supergene karstification. This result is not only a reference for the study of the Late Permian eustatic events,but also points out that non-reef facies modified by supergene karstification inside platforms may also be a favorable exploration zone,thus exhibits important practical significance for exploration.
Li Hongwei,Tang Hao,Su Chengpeng et al. Characteristics of weathering crust at top of the Upper Permian Changxing Formation in Fuling area, eastern Sichuan Basin and its geological significance[J]. JOPC, 2015, 17(4): 477-492.
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