Relationship between natural gas accumulation and overpressure in Xujiahe Formation, Xiaoquan—Fenggu structural belt, Western Sichuan Depression
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摘要: 川西坳陷为典型的前陆超压盆地,但目前关于超压的形成机制、超压与天然气的成藏关系仍存在较大争议,在研究过程中。该文采用声波时差法对孝泉—丰谷构造带上29口井地层压力进行了计算,并结合37口井的实测地层压力数据,对研究区超压形成机制、超压带类型划分、超压与天然气成藏关系及超压环境下的保存条件进行了分析,研究认为:1)孝泉—丰谷构造带须家河组存在明显超压,须二段和须四段分属不同压力系统,并且邻源超压特征明显;2)欠压实作用、生烃作用是研究区超压形成的主要原因;3)发现了3种类型的超压分布模式,即新场型、合兴场型和丰谷型;4)孝泉—丰谷构造带须家河组气藏分布与超压关系密切,紧临超压仓顶界面的泄压层为天然气最有利富集区;5)盖层突破剩余压力研究表明,须二段气藏整体保存条件较好,须四段在新场主体构造东北部及丰谷地区东南部保存条件较差,不利于天然气聚集。Abstract: The Western Sichuan Depression is a typical overpressure foreland basin.The forming mechanism of overpressure and its relationship with natural gas accumulation still remain discussing in researches.Interval transit time has been applied to calculate formation pressures of 29 wells in the Xiaoquan—Fenggu structural belt.Combined with the measured formation pressures of 37 wells,it has been concluded in this paper the forming mechanism and classification of overpressure as well as the relationship between overpressure and natural gas accumulation and preservation.1) Obvious overpressure has been found in Xujiahe Formation of the Xiaoquan—Fenggu structural belt.The 2nd and the 4th members of Xujiahe Formation belong to different pressure system.The neighbor-source overpressure mechanism is obvious.2) Undercompaction and hydrocarbon generation effects are the main causes for overpressure in the study area.3) 3 types of overpressure distribution have been found,including the Xinchang,the Hexingchang and the Fenggu ones.4) Natural gas accumulation is closely related to overpressure in Xujiahe Formation of the Xiaoquan—Fenggu structural belt.The PLS (pressure leakage strata) close to the top of OPC (overpressured compartment) is rich in natural gas.5) Studies of residual pressure for caprock breakthrough have indicated that the 2nd member of Xujiahe Formation is favorable for natural gas pre-servation all around the study area while the 4th member is unfavorable for preservation in the northeast of Xinchang and the southeast of Fenggu.
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