TOC evaluation using variable-coefficient ΔlogR model

Key words:  well logging  source rocks  total organic carbon(TOC)  variable-coefficient ΔlogR model  Wuerxun sag  Hailaer Basin

Abstract:

The source rocks in the First Member of Nantun Formation of Wuerxun sag, Hailaer Basin are characterized by strong organic heterogeneity with large variations in total organic carbon(TOC) values, resulting in the organic abundance of source rocks which cannot be represented by limited and discrete test samples. Source-rock sections can be easily recognized by high natural gamma ray, high interval transit time and medium resistivity logging response, but it is difficult to establish a universal TOC prediction formula, because the key parameters in ΔlogR model vary from well to well. Unlike in conventional approaches where ΔlogR is approximated empirically, an new approach that assumes ΔlogR as unknow parameters (k and a) is applied. In the variable-coefficient ΔlogR model, "k" is predicted according to single well, whereas "a" is predicted in the light of zones and facies. This variable-coefficient ΔlogR model is applied to 120 wells. Sources rocks in the First Member of Nantun Formation are divided into three grades according to the relationship between measured TOC and Hydrogen index (IH). TOC limit for poor source rocks, medium source rocks and excellent source rocks are 1.0％ and 2.0％, thus logging-derived TOC is used to classify source rocks. Within the sequence stratigraphic framework, the depth of source rocks of various grades is determined by calculating thickness ratio of various-grade source rocks in system tracts of single well and by using the Kriging interpolation between all the wells. The results show that logging calculated TOC by the variable-coefficient ΔlogR model agrees well with the measured values. The TOC predicting error by the variable-coefficient ΔlogR model is about 16.6％, with 19.9％ reduction than the traditional ΔlogR model. For the three rock types, poor source rocks, medium source rocks, and excellent source rocks, their depths are 15-20 m, 10-20 m, and 20-45 m; and their volumes are 0.46×10¹¹m³,0.46×10¹¹m³, and 1.08×10¹¹m³, respectively. These three types account for 14.7％, 34.5％, and 50.8％ of total source rocks in volume.