Origin of Phosphate-bearing Carbonate Concretions in the Upper Triassic Lacustrine Black Shales of the Southern Ordos Basin,China  

作　　者：ZHANG Long LIU Chiyang ZHANG Dongdong QU Shaodong MA Huanhuan JIANG Chong LEI Kaiyu 

机构地区：[1]Shaanxi Key Laboratory of Petroleum Accumulation Geology,Xi’an Shiyou University,Xi’an 710065,China [2]Shandong Key Laboratory of Depositional Mineralization&Sedimentary Mineral,Shandong University of Science and Technology,Qingdao 266590,China [3]School of Earth Sciences and Engineering,Xi’an Shiyou University,Xi’an 710065,China [4]State Key Laboratory of Continental Dynamics,Northwest University,Xi’an 710069,China [5]Shaanxi Provincial Land Engineering Construction Group,Xi’an 710075,China [6]Oil and Gas Exploration Company of Shaanxi Yanchang Petroleum(Group)Co.LTD,Yan’an,Shaanxi 716000,China

出　　处：《Acta Geologica Sinica(English Edition)》2021年第3期883-894,共12页地质学报（英文版）

基　　金：This work was supported by the National Natural Science Foundation of China(Program No.41330315);the Natural Science Foundation of Shaanxi Province(Program No.2020JQ-766);the Scientific Research Program Funded by Shaanxi Provincial Education Department(Program No.20JK0838);the Opening Foundation of Shandong Key Laboratory of Depositional Mineralization&Sedimentary Mineral,Shandong University of Science and Technology(Program No.DMSM20190034).

摘　　要：Carbonate concretions are conspicuous in organic-rich shales and are generally related to decomposition of organic matter.The black shales from the Chang 7 Member of the Upper Triassic Yanchang Formation of the southern Ordos Basin host abundant carbonate concretions,which provide a unique record of depositional and early diagenetic conditions of the paleo-lake sediments.However,little attention has been given to the genesis and growth processes of the concretions in these lacustrine petroleum source rocks.New petrographic observations and geochemical analysis show that the concretions are composed of calcite,phosphate fossil fragments,K-NH4-feldspar,quartz,bitumen,and minor Fe-dolomite.Phosphate minerals,mainly carbonate fluorapatite(CFA),show pervasive replacement by calcite,most of which contains phosphorus,ranging in concentration from 0.26 to 2.35 wt%.This suggests that the phosphate minerals are the precursors for concretion growth.Positiveδ13C(+5.6 to+12.4‰V-PDB)signatures and the absence of pyrite indicate that microbial methanogenesis was the dominant driver for concretion growth,rather than bacterial sulfate reduction.Quartz,bitumen,and Fe-dolomite are the last cements that occurred,at deep burial depths and high temperatures.The formation of phosphate minerals might have been induced by upwelling of phosphate-enriched deep water in the Late Triassic paleo-lake,which promoted phytoplankton blooms and further enrichment of organic matter.Extremely slow sedimentation rates of fine-grained detrital minerals,relative to dead organism accumulation,led to the high permeabilities of the organic-rich sediments and rapid concretion growth during shallow burial.The close association of phosphate-bearing carbonate concretions and organic-rich shales reflects that upwelling played a critical role in the formation of the high-quality petroleum source rocks in the Triassic paleo-Ordos lake.

关 键 词：carbonate concretion phosphate mineral UPWELLING PALEOENVIRONMENT lacustrine source rock Ordos Basin 

分 类 号：P618.13[天文地球—矿床学]