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.