---

UNCERTAINTY IN VOLUME OF SHALE CALCULATED FROM GAMMA RAY LOG: A COMPARATIVE STUDY BASED ON LINEAR AND NON-LINEAR MODELS

Bayonle A. Omoniyi*, Lucky O. Imagbe

---

Download PDF

---

Abstract

      In this study, we evaluated volume of shale from gamma ray log based on three techniques: gamma ray index, Larionov pre-Tertiary, and Thomas-Steiber, and compared results to evaluate variability in volume of shale.

      Twelve zones were recognised in the reservoir interval penetrated by B1 based on qualitative analysis of gamma ray and sonic logs. Volume of shale was calculated for the zones to build facies-constrained object models in a three-dimensional grid. The grid comprises 1306 cells and 1180 cells in the horizontal directions with cell dimensions of 5 ft by 5 ft (1.5 m by 1.5 m) and 68.5 ft (21 m) along depth.

In all the zones, gamma ray index is linear, whereas Larionov pre-Tertiary and Thomas-Steiber techniques yield deflected curves from the gamma ray index. Two-dimensional models of volume of shale show that gamma ray index technique produces overestimated volume of shale unlike Larionov pre-Tertiary and Thomas-Steiber techniques. Findings reveal that the Thomas-Steiber model with higher constant value produces significantly lower volume of shale. This variability in volume of shale reflects large contrast in volume of shale across lateral extent of channel elements that dominate the internal architecture. To reduce this uncertainty and the attendant risk of overestimation or underestimation of average net-to-gross value used in resource/reserve calculation and flow simulation, it is imperative to adopt a multi-technique approach that compares volume of shale from these techniques with volume of shale computed from spontaneous potential and combined neutron and density logs.