关键词: 牛驼镇凸起, 潜山内幕, 方解石脉, 稳定同位素, 流体包裹体

Abstract: The inner buriedhills with low degree of exploration, widely occur in Niutuozhen uplift of Baxian Sag. Vein calcites and host rocks were investigated using petrology, isotope and fluidinclusion to determine the origin, stage and evolution character of paleofluids. Combining the characteristics of present formation water, the influence of fluid flow on petroleum entrapment were summarized. The results show that three types of calcite veins have been found(Cal1, Cal2 and Cal3). The stable isotopes of the earliest calcites (Cal1), formed in the early stage of the Caledonian Orogeny, were equivalent to the host marine carbonates, which indicated that Cal1 were associated with host rocks. The subsequent calcite veins(Cal2) formed in the later stage of the Caledonian Orogeny could be divided into two periods. First, the early calcite veins (Cal21)in the edge of the fissure displayed isotopic compositions similar to those of the host rocks, which suggested that Cal21 were also associated with the host rocks. The late calcite vein (Cal22) in the middle of the fissure displayed similar δ13C, depleted δ18O, and higher 87Sr/86Sr relative to host rocks. The characteristic of fluidinclusion suggested the entrapment temperature was less than 50 ℃, and the saline of fluid inclusions in Cal22 were relative low, ranging from 13wt% to 35wt%NaCl. The outcomes have been interpreted as the result of the mixing of host marine carbonates with atmospheric fresh water. The latest calcite veins(Cal3)with hot homogenization temperature (Th=142 ℃ to 210 ℃)fluid inclusions, formed at the stage of the Es3 to Es4 during the Himalayan Orogeny, displaying depleted δ18O and δ13C, and higher 87Sr/86Sr relative to the host rocks, were derived from magmation. Based on the above data, the fluid flow history could be broken down into three stages which were from host marine carbonates, atmospheric fresh water and magmation, respectively. The evidence of hydrocarbon fluid flow has not been found and the present formation water has a low total dissolved solids(2 to 3 g/L), high rNa/rCl(1 to 12). There are large differences between Niutuozhen uplift and its surrounding hydrocarbon generation subsags in the chemical characteristics of present formation water. These proved that the condition of oil and gas preservation is poor and there is no connection between Niutuozhen uplift and Baxian Sag in dynamic fluid field. Combined comprehensively the evolution of the fluid flow and the chemical characteristics of present formation water, the paper argues that the feature of fluid flow is unsuitable for petroleum entrapment.

Key words: Niutuozhen uplift, inner buried hill, calcite vein, stable isotope, fluid inclusion