High coincidence rate of fluid identification of carbonate reservoirs in Yan'an gas field by gas logging bright spot -resistivity dark spot technology

Abstract

The main gas layer of the Lower Paleozoic in the Yan'an gas field in the Ordos Basin is the carbonate rock of the Majiagou Formation, which is a low-porosity and low-permeability reservoir. Its storage space is mainly composed of intercrystalline pores, intercrystalline dissolved pores, dissolved caves and micro-fractures, with strong heterogeneity. The reservoir generally contains pyrite, which makes the resistivity value negatively abnormal, which interferes with the identification of low-resistivity fluids in the high-resistivity background. This leads to the failure of traditional logging interpretation methods and the low coincidence rate of reservoir interpretation. Through the statistical analysis of 240 drilling and gas testing data, the sensitive parameters of logging total hydrocarbon and logging resistivity are selected from many reservoir parameters. On this basis, after effective technical treatment, a “gas logging bright spot-resistivity dark spot technology” for carbonate reservoirs is formed. Through the field application and gas test results of 35 wells in Yan'an gas field in 2021, the interpretation coincidence rate is as high as 83%, which is much higher than the previous traditional method coincidence rate (44.3%). This study organically combines the advantages of mud logging and logging technologies, and establishes a reservoir interpretation chart and standard, which solves the long-standing problem of low coincidence rate in the identification of carbonate reservoir fluids in the geological community. An effective method for predicting favorable carbonate oil and gas areas is provided.