黄骅坳陷齐家务地区储层地球化学特征及其相关问题讨论

Geochemical Characteristics and Dicussion on Related Problems of Sha 1 Sub-member Carbonate Reservoirs in the Qijiawu Area in Huanghua Depression

黄骅坳陷齐家务地区沙一下段主要以碳酸盐岩储层为主。通过该区10余口井岩心的岩石化学组分、碳氧同位素及微量元素分析,揭示了该区沙一下段碳酸盐岩的岩石化学组分主要以SiO2、Al2O3、CaO等含量相对较高为特征,而MgO含量则随岩性不同而变化。SiO2与Al2O3、K2O+Na2O均具有正相关性,而与MgO呈负相关,显示出SiO2的富集受陆源物质及湖盆水体pH值变化的影响,并与Al2O3、K2O+Na2O呈线性上升的特点,而受蒸发作用制约的MgO含量的增加不利于SiO2的富集。各小层不同岩类碳、氧同位素"Z"值计算结果大都在120以上,显示了频繁海侵使湖相碳酸盐岩沉积的水体具有咸化的性质。而δ18O值相应变轻,说明是研究区沙一下沉积期时沿断裂上升的热液中所含的CO2、CO导致湖相碳酸盐岩的δ18O值在高温高压下分馏而进一步贫化的结果。各类碳酸盐岩Cu、Pb、Zn、Cr、Ni、Ga等金属元素含量偏高的原因与频繁海侵及强还原环境有关,而Sr、Ba等亲碱性元素及亲生物元素B的变化受陆源混入物及古气侯条件制约。上述湖相碳酸盐岩岩石化学组分、碳氧同位素及微量元素的深入研究,为重建研究区沙一下段岩相古地理环境、预测有利储集相带提供了地球化学依据。

The lower first member of Shahejie Formation （Sha 1 Sub-member） in the Qijiawu area in Huahua depression is dominated by carbonate reservoirs. Lithochemical analysis of 10 cores, carbon and oxygen isotopic and trace element compositions show that the content of SiO2, AI2 O3 and CaO are relatively high, while the content of MgO varies with the difference of lithology. SiO2 is positively correlated with A12O3, so SiO2 and K20 ＋ Na2 O, but is negatively associated with the MgO, indicating that SiO2 enrichment is subject to terrestrial material source and pH value of water in lake basin. With linearly increasing of A12O3 and K2O＋Na2O, increasing MgO is against SiO2 enrichment. Most of C and O isotopic Z-value in each layer are higher than 120, indicating that frequent marine transgression resulted in salinization of water in which lacustrine carbonatic rock deposited. Decreasing δ^18O value shows that CO2 and CO contained in hydrothermal fluid migrating along fault during deposition of Shal sub-member in the study resulted in further depletion of δ^18O value in lacustrine carbonatic rock due to fractionation under the high temperature and pressure. The reason for relatively high contents of metals （such as Cu, Pb, Zn, Cr, Ni, and Ga） in various carbonatic rocks is related to frequent marine transgression and strong reduction environment, while change of Sr, Ba and B are restrained by addition of terrestrial material and ancient weather condition. Detailed studies of petrochemical composition, C and O isotopic analysis, and trace elements contents establish the geochemical basis for reconstructing ancient geographical environment of Sha 1 sub- member rock facies and predicting favorable reserves zones in the studying area.