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Will a reservoir produce hydrocarbons? This is a particularly troublesome question in carbonates because, frequently, the answer is anything but straightforward.

Despite the best geology put together from carefully crafted depositional and seismic models, only after a well is drilled into a carbonate reservoir, can a geologist decide whether or not the well will give up commercial quantities of hydrocarbons or, indeed, any hydrocarbons at all. Besides information from surrounding wells, data from drill stem tests, cores, cuttings, and open-hole logs ensure the best basis for making a decision about a well's productivity; unfortunately, drill stem tests or core data are not always available so the geologist is forced to fall back on open-hole logs for most of his or her information.

Because of unique pore characteristics in carbonate rocks and their affect on resistivity logs, geologists can easily make some incorrect judgements. They sometimes decide a well is productive when it's not, or they sometimes overlook a good well.

Problems occur because carbonate reservoirs can have several types of porosity which include intergranular, intercrystalline, vuggy, moldic, and fracture. In addition to these different types of porosity, the pore size may be large (megaporosity) or very small (microporosity). The different pore types and sizes result from both depositional and diagenetic processes.

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