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Characteristics and genesis of analcites in different occurrence states: A case study of the fourth member of Shahejie Formation in the Leijia area of the Western Liaohe Depression TEXT SIZE: A A A
(Vol. 40, No.6, 2020)

FANG Rui1,DAI Zong-yang1,CHEN Zhi-jun2,SHAN Jun-feng3,JIN Ke3, ZHANG Yun-feng1

(1. School of Earth Science and Technology, Southwest Petroleum University, Chengdu 610500, China;
2. Sinopec Northwest Oilfield Company, Bayingolin 841000, China; 3. Liaohe Oilfield Exploration and Development Research Institute, Panjin 124000, China)

Abstract: The study of analcites in different occurrence states is of great significance for understanding sedimentary environment, diagenetic fluid type and reservoir. Based on the observation of a large number of cores and their thin sections of rocks in the fourth member of Paleogene Shahejie Formation in the Leijia area of the Western Liaohe Depression, the cathodoluminescence imaging, and geochemical studies of trace elements, isotopes, and fluid inclusions of analcites in different occurrence states from those rocks have been systematically carried out in this paper. Three occurrence states , namely, lamellar, fissure filled, and nodular, of analcites have been identified. The lamellar analcite mainly occurred as lamellar layer in dolomite and mudstone, often with various degrees of crumpled deformation characteristics and disordered and broken occurrence. Its characteristics of high Si/Al values, LREE enrichment, HREE depletion, relatively high 87Sr/86Sr values, low δ18O values, high δ13C values, rich in SiO2, Al2O3, Na2O, TFe2O3, and poor in TiO2 indicate that this kind of analcite was formed by the interaction between the deep volcanic hydrothermal fluid and the high salinity and alkalinity water. The fissure filled analcite usually has relatively large semi-euhedral to euhedral crystals, whose sizes are gradually increased and whose crystalline degrees are getting better from the edge to the center of fissure It has various phases and often contains asphaltene residue. It was formed in the latest time among 3 kinds of analcites. The characteristics of its low Si/Al values and elemental compositions, and various peaks of homognization temperatures of its fluid inclusions indicate that it could be formed by the crystallization of analcite from the infilled fluid, which contains dissolved lamellar analcite from the consolidated rocks, at the fissure edge, or by the crystallization of analcite from the slow cooling deep sourced hydrothermal fluid infilled into fissures of rocks. The nodular analcite is mostly scattered in the host rocks, with occasionally bead-like distributed along the layer of rocks. The characteristics of its distribution and occurrence state indicate that it could be formed by the differentiation and redistribution of lamellar analcite in the diagenetic process.

Keywords: Characteristics of analcite; analcite origin; the fourth member of Shahejie Formation; Paleogene system; the Western Liaohe Depression

ACTA MINERALOGICA SINICA Vol. 40, No.6, 2020, page 734-746

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