Features and genesis of carbon-oxygen isotopes in calcite cement from sandstone in oil-bearing Fuyu layer of Daqing Placanticline

Song Tushun; Ma Feng; Liu Li; Meng Qi'an; Yu Yanlong; Liu Na; Yu Miao

doi:10.11743/ogg20150210

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Features and genesis of carbon-oxygen isotopes in calcite cement from sandstone in oil-bearing Fuyu layer of Daqing Placanticline
Vol. 36, Issue 2, Pages: 255-261(2015)
Online First：08 April 2015，

Published：2015
DOI： 10.11743/ogg20150210
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DOI：

Song Tushun, Ma Feng, Liu Li, et al. Features and genesis of carbon-oxygen isotopes in calcite cement from sandstone in oil-bearing Fuyu layer of Daqing Placanticline[J]. 2015, 36(2): 255-261. DOI： 10.11743/ogg20150210.

摘要

方解石是大庆长垣深层砂岩中广泛分布的一种胶结物，同时方解石胶结作用是大庆长垣扶余油层致密储层的控制因素之一。以方解石胶结物为研究对象，运用薄片鉴定，扫描电镜观察、X-射线衍射分析、包裹体测试和碳、氧同位素分析，查明方解石碳、氧同位素组成及其成因。研究结果表明，储层砂岩类型为长石岩屑砂岩和岩屑长石砂岩，粘土以I/S（伊/蒙混层）+I（伊利石）+K（高岭石）组合为代表。方解石的碳同位素（δ

C，PDB）分布范围是-20.3‰～-3.2‰，氧同位素（δ

O，PDB）的分布范围是-25.3‰～-20.3‰，平均值为-22.67‰。碳、氧同位素分析数据的计算表明，碳酸盐的相对古盐度值（Z）介于73.92～109.89，表明其形成时的古流体均为矿化度较高的淡水；方解石的形成温度为80～100 ℃。方解石胶结物的同位素显示，其形成与有机酸脱羧作用有关，暗示其“碳”来源为有机碳。

Abstract

Calcite is one the most widely spreading cements in deep sandstone reservoirs in Daqing Placanticline.It has also been regarded as the main factor for facilitating the formation of tight reservoirs in oil-bearing Fuyu layer.Thin section

scanning electron microscope

X-ray diffraction

fluid inclusion and carbon/oxygen isotope analyses were carried out on calcite from cored wells penetrating the layer to identify its isotope composition and genesis.The results show that the reservoirs are mainly feldspar lithic and lithic feldspar sandstone with clay minerals represented by I/S+I+K assemblage.The cement has its δ

C(PDB)value ranging between -20.3‰ and -3.2‰ and the δ

O(PDB)value varying from -25.3‰ to -20.3‰ with an average value of -22.67‰.Calculation shows that the paleosalinity Z is between 73.92 and 109.89

indicating a fresh water environment with high salinity for the formation of carbonate.The temperature for the calcite to be formed is between 80 ℃ and 100 ℃ based on converted temperature of isotopes and homogenization temperature of fluid inclusions.Isotope diagrams reveal that the formation of the calcite cement is probability connected with organic acid decarboxylation

inferring an “organic carbon” origin for the carbon in the cement.