塔河油田天然气特征对比与成因分析

塔河油田东西部奥陶系天然气地球化学特征存在明显的差异.依靠多年勘探开发及研究积累的大量天然气分析测试数据,结合塔河油田油气成藏地质条件与主要油气成藏期次,从天然气组分特征及碳、氢同位素特征等方面进行研究,认为塔河油田西部地区奥陶系储层天然气以溶解气或伴生气为主,以甲烷碳同位素较轻为特征,其成因主要为原油初期裂解.塔河中部主体区奥陶系储层天然气主要以伴生气为主,部分是凝析气,为与原油同期次充注的成熟-高成熟天然气.塔河东部地区奥陶系储层天然气具有高干燥系数和甲烷碳、氢同位素较重的特征,成熟度高于塔河中、西部地区,部分天然气出现甲烷、乙烷碳同位素倒转,表明天然气具两期充注成藏特征.第一期天然气充注成藏时间与塔河中部主体区油气成藏时间一致;第二期充注以过成熟裂解气为主,为原油裂解气与干酪根裂解气的混合气,充注成藏期为喜马拉雅期晚期.不同成因的天然气及不同期次天然气的混合叠加,是形成塔河油田不同地区天然气特征差异的主要因素.

Characteristics comparison and origin of natural gas in Tahe oilfield

The geochemical features of natural gas in the Ordovician vary obviously from east to west in Tahe oilfield. After years of exploration, a large amount of testing data for natural gas accumulated. In combination with analyses of the geologic conditions and stages for natural gas accumulations in Tahe oilfield, studies of natural gas compositions and C/H isotopic features, led to the following conclusions. In the western part of Tahe oilfield, the natural gas in the Ordovician reservoirs was mainly dissolved or associated gas, featuring in lighter methane carbon isotopes, and mainly originated from the initial cracking of crude oils. In the central part of Tahe oilfield, the natural gas in the Ordovician reservoirs was mainly associated gas and partially condensate gas, and was mature/high-mature gas which was charged at the same time as crude oils. In the eastern part of Tahe oilfield, the natural gas in the Ordovician reservoirs feature in highly dry coefficient and heavy methane C/H isotopes, with higher maturity than those samples from both the western and the central Tahe oilfield. For some natural gas samples, δ¹³C₁ >δ¹³C₂, indicating that the natural gas were charged and accumulated in stages. The first stage of natural gas charging and accumulation took place when hydrocarbon accumulated in the central part of Tahe oilfield. In the second stage of natural gas charging, the cracking of over-mature hydrocarbon played an important role. The nature gas mainly was sourced from the cracking of crude oil and kerogen, and charged during the late Himalayan period. The different origins of natural gas and the mixing of natural gas during different stages explained the obviously different features of natural gas among different regions in the Tahe oilfield.