Geochemical characteristics of lacustrine carbonate rocks in lower part of first member of Shahejie Formation in Qijiawu area of Huanghua Depression, Bohai Bay Basin
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摘要: 黄骅坳陷齐家务地区沙一下亚段主要以碳酸盐岩储层为主。通过该段10余口钻井岩心的岩石化学组分、碳氧同位素及微量元素分析,认为该区沙一下亚段碳酸盐岩的岩石化学组分的SiO2、Al2O3、CaO等含量相对较高,而MgO含量则随岩性不同而变化,同时,SiO2与Al2O3、K2O+Na2O均具有正相关性,而与MgO则呈负相关。SiO2富集显示出该区碳酸盐岩的湖相沉积环境特征;碳、氧同位素测定结果表明,δ18OPDB值在-4.73‰～9.46‰,δ13CPDB值则在-0.08‰～7.71‰,具有局限湖盆碳、氧同位素变化特征,反映研究区碳酸盐岩具有湖水咸化与淡化的沉积环境;微量元素测试结果表明,微量元素组合及含量变化复杂,以石灰岩为主的碳酸盐岩,其Sr含量大都高于1000×10-6,而含泥砂较高的碳酸盐岩,其Sr含量大都低于1000×10-6。B的含量大多分布在(7.77～122)×10-6,泥质样的B/Ga比值均大于4,Sr/Ba比值均大于1;碳酸盐岩样的Sr/Ba主要分布在2.3～20.33之间,最高为33.98,B/Ga比值均小于10,主要分布在2～5之间,总体反映出淡水与半咸水之间变化的沉积环境。Abstract: Carbonate reservoirs dominate in the lower part of the first member of the Shahejie Formation in Qijiawu area of the Huanghua Depression, the Bohai Bay Basin. Through analyzing the chemical composition of rock, carbon and oxygen isotope and trace element of the core samples from over 10 wells, the results show that the contents of SiO2, Al2O3 and CaO are relatively higher, while that of MgO varies with the differences of lithology. Meanwhile, SiO2 is positively correlated with Al2O3 and K2O + Na2O. SiO2 is negatively associated with MgO. The enrichment of SiO2 shows that the sedimentary environment in the study area is lake carbonate facies. Carbon and oxygen isotope tests show that δ18OPDB varies from -4.73‰ to -9.46‰ and δ13CPDB varies from -0.08‰ to 7.71‰, which is in line with the carbon and oxygen isotope characteristics of limited lake, reflecting the salinization and dilution effects happened in the study area. Trace tests show that the composition and content of trace element are so complex, that is to say, the content of Sr is higher than 1 000×10-6 in the limestone-based rock, but the content of Sr is less than 1 000×10-6 in the limestone with higher argillaceous. The content of B mostly distributes in (7.77-122)×10-6. For mud samples, the B/Ga ratio is greater than 4, and the Sr/Ba ratio is greater than 1. For carbonate samples, the Sr/Ba ratio ranges from 2.3 to 20.33, up to 33.98, and the B/Ga ratio is less than 10, mainly from 2 to 5, reflecting the alteration between freshwater and brackish sedimentary environments.
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