Using Isotope Technology to Identify Oil and Gas Reservoir Sweet Spots

Abstract

Abstract Porosity and permeability of rocks are crucial parameters utilized to assess the quality of oil and gas reservoirs. Diagenesis in carbonate reservoirs commonly results in dissolution, and creates secondary porosity and permeability. At the same time, geochemical records (e.g., isotopes and elements) in the carbonate rocks are diagenetically altered. This study is to utilize the isotope technology to evaluate diagenesis and its impact on oil and gas reservoirs. Micro-samples were collected from marine carbonate rocks in a studied reservoir for carbon and oxygen isotope analyses, plus analyses of Sr isotopes and element concentrations. The analytical results show outstanding negative shifts of oxygen isotopes in some intervals, whereas carbon and strontium isotopes have a minor or little change. These intervals also contain lower element contents including strontium. It is believed that these intervals experienced diagenetic dissolution with abundant secondary porosity, and are the best potential for oil and gas reservoirs. Petrographic study indicates minor cements but higher porosity developed within these intervals. This is confirmed by high porosity measured within these intervals by using other methods including gas injection and well logging. Furthermore, the degrees of water-rock interaction were quantitively assessed by modeling covariations of isotope pairs, which can help evaluate the quality of reservoirs relating to the intensity of diagenesis. Intensive diagenesis with a high water-rock ratio particularly in a closed system may result in additional cementation, alteration of most geochemical parameters, and also reduce porosity and permeability. Thus, the isotope technology can be utilized as an effective method to assess reservoir potential and determine reservoir sweet spots. Note that carbonate cementation and recrystallization commonly occur during diagenesis, and commonly show negative oxygen isotope values. If sampling happens to contain a certain amount of cements and recrystallized carbonate minerals, the analytical results will show lower oxygen isotopes and trace elements than their primary values in the studied marine carbonates. Petrographic study was conducted to help micro-sampling, and verify that cements were not collected in the studied carbonate rocks for isotope analysis. The implementation of this study is that the isotope results obtained from one studied well can be applied in the whole field or/and the region, as diagenesis rarely occurs only around one well. This will save cost for repeatedly measurements from different wells and fields by using other methods.