川西坳陷中段侏罗系次生气藏地层水化学特征及与油气运聚关系

川西坳陷侏罗系地层水化学研究结果表明,侏罗系地层水以CaCl2型为主,其中上侏罗统蓬莱镇组见较多Na2SO4型地层水,结合地层水离子参数,总体上地层水封闭条件较好,有利于油气的聚集和保存。侏罗系地层水化学特征在垂向和平面上表现出明显的垂直分带性和平面分区性。纵向上,侏罗系地层水总体表现出断层越流淡化地层水的特征;平面上,地层水矿化度、主要离子浓度受构造位置以及断裂系统控制。构造位置较高或邻近烃源断层的地区,侏罗系地层水受大气水下渗淡化作用和须家河组五段湖相泥岩黏土矿物脱水淡化作用影响。与中国大部分油气田不同,川西坳陷侏罗系次生气藏主要分布在矿化度小于30 g/L,HCO-3浓度大于300 mg/L的区域。须家河组五段烃源岩黏土矿物转化析出的大量低矿化度层间水与有机质热演化和硫酸盐还原作用形成的烃类和CO2气体,在断层沟通下上涌进入侏罗系储层,导致低矿化度、高HCO-3浓度地层水的分布范围与气藏分布有较好的一致性。在断层欠发育地区,地层垂向连通性差,侏罗系次生气藏难以形成,地层水具有较高的矿化度和较低的HCO-3浓度。

Chemical characteristics of Jurassic formation water and its relationship with hydrocarbon migration and accumulation in middle part of Western Sichuan Depression

Studies on the chemistry of the Jurassic formation water indicate that the formation water is dominantly characterized by CaCl₂ type with relatively low salinity and Na₂SO₄ type water is common in the Upper Jurassic Penglaizhen Formation. In general, the Jurassic formation water is in a closed system, demonstrating its favorableness for hydrocarbon accumulation and preservation. There is a distinct difference in the chemistry of formation water vertically and horizontally. Water composition varies with depth and three vertical variation patterns might be observed, showing the diluting effect caused by the cross-formation flow through faults. Regionally, formation water chemistry is mainly controlled by elevation and faults. In regions of high elevation and fault zones, formation water dilution can occur by meteoric invasion and diagenetic dehydration reactions of lacustrine facies clay minerals in the 5th section of the Xujiahe Formation within the main hydrocarbon source rocks. The Jurassic secondary gas pools in the Western Sichuan Depression correspond generally to the regions with TDS lower than 30 g/L and bicarbonate concentration higher than 300 mg/L, which is different from most oil and gas fields in China. The low-salinity water released from the transformation of smectite to illite and the hydrocarbon and carbon dioxide generated by kerogen-cracking reaction and sulfate-reducing reaction can flow upward through permeable fault zones and therefore cause the coincidence between the distributions of hydrocarbon and formation water with low TDS and high bicarbonate concentration. In regions without faults, it is unlikely to form hydrocarbon accumulation and the formation waters are often characterized by high TDS and low bicarbonate concentration due to poor vertical connectivity.