Abstract
The Wu 2 section of the Ke017 well block is a low-resistance gas reservoir with ultralow porosity and low permeability. The comprehensive analysis of rock lithology, physical properties, sedimentary characteristics, and gas content demonstrated that the development of micropores in illite/smectite dominated clay minerals together with the resulted additional conductivity capability and complex reservoir pore structures, as well as the enrichment of self-generating conductivity minerals like zeolites and pyrite which were the formation mechanisms of low-resistance gas layers in the Wu 2 section. A low-resistance gas reservoir has poor physical property, and it is difficult to distinguish the oil layer from the dry, gas, or water layers. In this paper, based on well/mud logging data and laboratory data, by taking advantages of the “excavation effect” of neutron gas and the dual-lateral resistivity difference between different depths, we successfully established a set of low-contrast log response methods for the identification and evaluation of oil layer and formation fluids. For a gas layer, the difference between neutron porosity and acoustic (or density) porosity is smaller than 0 and the difference in dual-lateral resistivity is greater than 0. For a water layer, the neutron porosity is similar to the acoustic (or density) porosity and the dual-lateral resistivity difference will be less than 0. While for a dry layer or a layer with both gas and water, the difference in porosity as well as dual-lateral resistivity is very small. The proposed method effectively solves the technical problem of oil layer and formation fluid identification in low-resistance gas reservoirs.
Highlights
A low-resistance oil/gas reservoir is defined as the oil/gas reservoir with a resistivity ratio between oil/gas layer and water layer smaller than 2 [1]
The low-resistance gas layer is formed by the comprehensive response of many factors such as structure, sedimentation, lithology, physical properties, gas-bearing properties, and formation water
(2) When the reservoir is a water layer, the neutron has no “excavation effect,” three porosity data changes are essentially the same, the interpretation results of the three porosity data are essentially the same, and the deep and shallow bilateral resistivity shows a “negative difference”
Summary
A low-resistance oil/gas reservoir is defined as the oil/gas reservoir with a resistivity ratio between oil/gas layer and water layer smaller than 2 [1]. Most researchers tended to pay more attention to special logging technologies so as to find out a quantitative method for fluid identification in low-resistance reservoirs, and there was scarce information on using conventional logging data. Due to the effects of poor reservoir conditions and low oil recovery, the researches on the formation mechanism and log identification and evaluation of the low-resistance gas reservoir in upper Wuerhe formation of Permian in Zhungeer basin are still insufficient. We established a set of low-contrast log response methods for the identification and evaluation of oil layer and formation fluids by analyzing the formation mechanisms and governing factors
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