Evaluation of low amplitude and low resistivity pay zones under the fresh drilling mud invasion condition

Abstract

Low amplitude and low resistivity oil reservoirs are developed in the Triassic Chang 2 oil layers in A block of Changqing Oilfield. They are controlled by palaeogeomorphology, have large oil-bearing areas, and may form large-scale low resistivity reservoirs. The petrophysical origin study of the reservoirs reveals that the main reasons for low resistivity are the low oil saturation caused by the gentleness of the structure, and the different influences of fresh drilling mud filtrate invasion on dual induction logging in oil pay and water zones. A step-by-step logging identification and quantitative evaluation method was established. Through the three steps of conventional logging cross-plot, invasion factor analysis based on the invasion principle, and comparison between wells, pay zones can be effectively identified and the ability of logging interpretation is enhanced. A quantitative evaluation of oil reservoirs can be made by the forward modeling of the fresh drilling mud invasion, the inverse interpretation under the constraint conditions of geological and reservoir engineering parameters, and the quantitative evaluation result verification, thus improving the accuracy and reliability of logging identification and quantitative evaluation of low resistivity pay zones in low amplitude reservoirs. 摘要: ： 长庆油田A区块三叠系长2油层组发育古地貌控制的低幅度低电阻率油藏，含油面积较大，可形成规模低电阻率油藏。经岩石物理成因机理研究发现，该区低电阻率油层的主要成因在于低幅度油藏背景造成其含油饱和度较低及淡水钻井液侵入对油层、水层双感应测井的不同影响。在此基础上，分别建立了淡水钻井液条件下分步骤的测井识别与定量评价方法。通过常规测井交会、基于侵入原理的侵入因子分析、多井对比等3个步骤对油、水层进行识别，提高了测井识别低电阻率油层的能力；通过淡水钻井液侵入影响正演建模、地质和油藏工程参数等约束的反演解释、定量评价结果检验等3个步骤开展油层定量评价，提高了低幅度油藏背景下低电阻率油层测井识别与定量评价的精度和可信度。此测井解释模式对低电阻率油气层勘探具有重要借鉴意义。图9表2参19 Key words: fresh drilling mud invasion, low amplitude reservoir, low resistivity pay, petrophysics, origin and mechanism, well logging identification and evaluation