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.