Heterogeneities in carbonate reservoirs: detection and analysis using borehole electrical imagery
Authors: Nurmi R. , Charara M. , Waterhouse M. , Park R. 
Published: April 2008

Porosity heterogeneities in carbonate reservoirs occur at all scales, from the size of a reservoir delineated by seismic data and/or well-to-well correlations, down to microscales which can only be revealed by scanning electron microscopy. The origin of porosity heterogeneities is varied; some result from the depositional system, while others are products of burial, diagenesis and tectonism. Defining the three-dimensional distribution of textures and facies in a carbonate reservoir is exceedingly difficult. At present, carbonate specialists rely on cores to provide an understanding of decimetre to micron-scale textures and fabrics in carbonate reservoirs. Cores are rarely available from every well in a reservoir and usually do not cover the entire reservoir interval. Thus, the characterization of carbonate textures, fabrics, pore types, and porosity distribution using wireline well logs in uncored wells provides additional valuable information. Such an approach requires the integration of all of the available data, including acoustic, nuclear, electric, and dielectric measurements for reliable geological and petrophysical analyses. The wide range of possibilities coupled with the complexity of many carbonate reservoirs, however, generally means that both core and logs have to be acquired, at least in a few key wells. The evaluation of borehole electrical imagery in carbonate reservoirs from different depositional settings with a variety of diagenetic histories, indicates that this technique is providing valuable, and sometimes new, information about porosity heterogeneities. Electrical imagery, which has a high resolution and provides three-dimensional data, usually reveals more of the complexity of pore distributions than standard well logs are capable of. Large individual pores, fractures, and vugs are often directly visible in these images, although microscopic pores are below the resolution of the technique. The porosity fabric, or decimetre-scale distribution of microscopic integranular, intercrystalline and mouldic pores, is routinely defined, much as oil-saturated porosity in a core can be revealed by ultraviolet light. The correlation of electrical imagery with core samples, or drill cuttings in uncored intervals, can help to quantify the porosity fabric and also the geological interpretation of electrical texture and fabrics. On a decimetre-scale of examination, porosity is sometimes distributed homogeneously but is more commonly heterogeneous. In addition to vugs and fractures, there are four basic geometrical fabrics of decimetre-scale porosity. Layering is the most common, sometimes clearly associated with stratification. However, the thickness of porosity layers can be uniform or quite variable within the same formation. The next most common heterogeneous fabric is patchy porosity distribution. This fabric can either be patches of high porosity within low-porosity intervals or patches of low-porosity rock within porous reservoir zones. A common porosity fabric in Cretaceous and Tertiary carbonate shelf reservoirs is a three-dimensional convoluted continuous mixture of low- and high-porosity rock volumes. This porosity fabric is sometimes associated with isolated patches of either high- or low-porosity rock, although no obvious genetic transitions of these two fabrics have been observed in images, cores, or outcrops. The characteristics observed in electrical images must be summarized in a log format for large-scale correlation studies. Log summary plots of carbonate texture and fabric help to correlate long intervals of imagery with a variety of other data, including drill-cutting sample logs, other well-log data from the same well, and data from other wells. As specific porosity fabrics are generally associated with certain depositional or diagenetic facies, these log summaries also aid well-to-well correlation of facies. Many diagenetic and depositional facies with distinctive electrical fabrics are often not clearly defined by standard well logs because of the decimetre-scale averaging of rock properties by these logs.