Abstract
This paper presents an approach for permeability characterization of a thin, high-K dolomitized interval based on application of depositional and diagenetic concepts. The interval is 3.5 feet in average thickness and has higher permeability values than surrounding limestones. It is an important drilling target and the permeability shows significant heterogeneity vertically and laterally which impacts future well placement and completion designs of long laterals. Detailed core observations from 85 wells suggests that the intensity of dolomitization is laterally variable at an inter-well scale, but that there is a clear vertical trend of dolomite content increasing towards the middle part of the interval at each well location. The dolomitization is often concentrated within the grain-dominated facies and, in general, it leads to increased permeability. However, in some instances it can result in an intense overdolomitization yielding degraded reservoir quality. Permeability can also be improved in some areas by leaching of non-dolomitized material, particularly in the crestal area of the field.
Core and petrographic observations combined with plug analysis data suggest that the dolomitized zone can be divided into three sublayers and characterized by two rock types:
Dolostone with well-developed intercrystalline pores (high-K) and highly overdolomitized dolostone that occur in the middle sublayer.
Calcareous dolostone characterized by weaker dolomitization and/or dissolution that are in the upper/lower sublayers. This rock type is also occasionally assigned to the middle sublayer where dolomitization is locally weak.
Regression analyses of plug porosity-permeability values are characterized by one regression line for the dolostone rock type, which represent a higher permeability trend (Trend 1) that contrasts the calcareous dolostone rock type (Trend 2).
The proposed fine-scale layering scheme and rock typing can be used to model the vertical and lateral petrophysical heterogeneity and permeability contrasts of the high-K dolomitized interval.
