Sedimentary Condition of Lacustrine Dolomite in the Lower Tertiary Biyang Depression

HUANG Xing zhen; SHAO Hong shun; YAN Cun feng; TUO Jin cai; HE Zu rong; ZHANG Xin she; WANG Shou qing; LI Yu lan

Most of lacustrinc carbonate rocks of Mesozoic-Cenozoic era consist of limestone mainly.However in some lake basins such as the Nan-Xiang hasi, it mainly deposited dolomite with a net accumulative thickness of 900 meters.Whilt the entire dolomite sedimentarv, sequence can he more than 2100 meters which formed a set of good oil-generation source rocks, and a small and rich Nan-Yang oil field iashccn found there.This paper descrihes, taking the Bivang depression as an example the genetic conditions of the thick lacustrinc dolomite of stable sedimentary fac ics: 1.A hot palcoclimatc which is semi-wet to wet;2.A water body of carbonate type which contains plenty of K⁺,Na⁺,HC0₃^- and C0₃^-but less. Ca²⁺ and S0₄^2- ions; 3.A medium of palcowatcr with a pH value of ≥ 9, and a low water salinity of generally 3‰-10‰, which belongs to the range of slight salty<0.5‰-5‰>to semi-salty<5‰-30‰>,or even higher salinity although the deposition of dolomite may not he so strict on the palcosalinitv: 4.A stable shallow lake and deep lake condition of a broad lake basin: 5.The biological and chemical effect may he strong so that the contents of C0₃^2-and HC0₃^-,as well as the pH value will he increased while the content of 0₄^2- decreased.

Lanzhou Institute of Geology,Chinese Academy of Sciences Lanzhou 730000

Received Date: 2000-06-20

Key words:

lacustrine dolomite /
sedimentary genetic condition

Abstract: Most of lacustrinc carbonate rocks of Mesozoic-Cenozoic era consist of limestone mainly.However in some lake basins such as the Nan-Xiang hasi, it mainly deposited dolomite with a net accumulative thickness of 900 meters.Whilt the entire dolomite sedimentarv, sequence can he more than 2100 meters which formed a set of good oil-generation source rocks, and a small and rich Nan-Yang oil field iashccn found there.This paper descrihes, taking the Bivang depression as an example the genetic conditions of the thick lacustrinc dolomite of stable sedimentary fac ics: 1.A hot palcoclimatc which is semi-wet to wet;2.A water body of carbonate type which contains plenty of K⁺,Na⁺,HC0₃^- and C0₃^-but less. Ca²⁺ and S0₄^2- ions; 3.A medium of palcowatcr with a pH value of ≥ 9, and a low water salinity of generally 3‰-10‰, which belongs to the range of slight salty<0.5‰-5‰>to semi-salty<5‰-30‰>,or even higher salinity although the deposition of dolomite may not he so strict on the palcosalinitv: 4.A stable shallow lake and deep lake condition of a broad lake basin: 5.The biological and chemical effect may he strong so that the contents of C0₃^2-and HC0₃^-,as well as the pH value will he increased while the content of 0₄^2- decreased.

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Sedimentary Condition of Lacustrine Dolomite in the Lower Tertiary Biyang Depression

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