FRACTURE CARBONATE RESERVOIR

The topic of fractured carbonate reservoirs is important for SE Asia : however, it is truly of global significance to the industry. As such, we are pleased at the breadth of the presentation. Not only will be discussing studies from Italy, France, the Midle East, Brazil, Colombia, Canada and The USA. Studies presented for this workshop range from small to large smallm; from thin section to logs to seismic: from outcrop work from fractured shales and fractured basement. 

Our understanding of fractures in carbonate reservoirs is hindered by low data density,which prevents effective fracture attribute mapping. Despite the advent of image logs and new core-recovery techniques, fracture data are frequently incomplete and sometimes misleading. Sparse sampling of large fractures remains unavoidable. From the interwell region, information is confined to seismic identification of faults and layer curvature. Although there may be a link between layer curvature and fracture intensity in situations where fractures develop during or after layer flexure, if fracturing predates flexure thereis no such relationship. Collection of meaningful, systematic data at the well bore and extrapolation into the interwell volume are significant challenges.

Macrofractures in limestones and chalks show many characteristics similar to those of the observed in siliciclastic rocks (e.g., power-law aperture-size distributions), but populations of microfractures have proved more difficult to observe in the samples we have studied. In the case of the Austin Chalk, the fracture intensity is so low that even the microfractures have very low intensities on the scale of a thin section. Low fracture intensity limits the technique we have developed to fracture-quality prediction and determination of fracture orientation. Scaling work in these cases requires the use of outcrop analogs. However, information gained from the fracture quality assessment can help to steer outcrop-analog selection because structural diagenesis histories of rocks at outcrop can be matched to those of subsurface rocks as closely as possible.

For subsurface studies, we quantify fracture attributes and flow potential on a bed-by-bed basis using samples from whole core, if available, and wireline-sidewall cores if whole core has not been taken. Horizontally orientated thin sections are taken in order to establish the fracture orientation(s), crosscutting relationships, and structural diagenesis of each sample. Structural diagenesis encompasses the relative timing of cementation and fracturing and the relative proportions of different cements that have precipitated before, during, and after each fracturing event, termed pre-, syn- and postkinematic cements, respectively. Because fracture systems in carbonate reservoirs are diverse in origin, a new emphasis on structural diagenesis research is essential. All too often, unjustified assumptions are made about relationships between fractures and large-scale structures, without reference to rock properties at the time of fracturing, as affected by diagenetic history. Patterns of cementation within developing fractures have been established through studies of siliciclastic rocks, allowing us to predict whether macrofractures are likely to be open to fluid flow.

The complexity of carbonate diagenesis makes this work a special challenge in carbonate reservoirs, but we see some parallels in the basic pattern of fracture mineral fill. It is common for a synkinematic cement to line the fracture, with bridges of that cement extending across the fracture from one wall to the other. The porosity of that fracture after this phase of cementation either remains open or becomes occluded by a postkinematic cement, either partly.