Depositional environment and characteristic comparison between lacustrine mudstone and shale: A case study from the chang 7 Member of the Yanchang Formation, Ordos Basin

被引：0
作者：
Liu Q. [1 ]
Yuan X. [2 ]
Lin S. [2 ]
Guo H. [3 ]
Cheng D. [2 ]
机构：
[1] University of Aberdeen, Aberdeen
[2] Research Institute of Petroleum Exploration & Development, PetroChina, Beijing
[3] Southern Petroleum Exploration & Development Corporation, PetroChina, Haikou, 52000, Hainan
来源：
| 2018年 / Editorial Department of Oil and Gas Geology卷 / 39期
关键词：
Chang; 7; Member; Depositional environment; Mudstone; Ordos Basin; Organic-rich shale; Shale;
D O I：
10.11743/ogg20180310
中图分类号：
学科分类号：
摘要：
Argillaceous rocks(or argillite)can be subdivided into two categories, mudstone and shale, which differ significantly in genetic mechanism, content of organic matters and mode of occurrence, constituent minerals, etc. A study on distribution patterns of lacustrine mudstone and shale as well as comparisons of their distinguished characteristics would be of great help in building up an organic-rich shale depositional model. The Chang 7 Member of the Yachang Formation is represented by a large set of mudstone and shale, which was deposited in the period of maximum lake depth of the Ordos Basin during the Triassic. Their differences, which are ascribed into five aspects (distribution pattern, geochemical cha-racteristics, rock composition, fabric, and main and trace elements), have been disclosed by typical vertical profile analysis, core-thin section observation, geochemical analysis, etc. That is, (1)distribution pattern: mudstone and shale are complimentarily deposited, with mudstone mainly deposited in delta front, deep-lacustrine-turbidite facies, and semi-deep lacustrine facies, while shale mainly in deep lacustrine facies; (2)fabric feature: shale is characterized by laminae and organic matters occurring along layers, while mudstone exhibits no laminae and its organic matters scattered; (3)rock composition: shale is rich in pyrite, eleven times the content of mudstone pyrite, whereas mudstone holds more dolomite and si-derite; (4)organic geochemistry indices: the TOC of shale with an average of 6.6%, is three times as much as that of mudstone, and the value of S1 averaging at 3.12 mg/g, is 2.5 times that of mudstone; (5)main and trace elements: shale is significantly higher in trace elements content like U, Th, Mo, Co, Ni, and in ratios like U/Th, V/Cr and P/Al than those in mudstone, while Ti/Al and K/Al ratios are higher in mudstone. In a word, the formation of mudstone and shale in the Chang 7 Member is mainly controlled by sedimentary facies, lake depth, and lake basin productivity. Shale is deposited further away from sediment source area with higher productivity and less oxygen in water. © 2018, OIL & GAS GEOLOGY Editorial Board. All right reserved.
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页码：531 / 540
页数：9
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