Most fractures below 2,500 ft (762 m) are nearly vertical or of high inclination. Even if there are horizontal fractures, they probably tend to close owing to the effect of the overburden. What are the possibilities of intercepting a vertical fracture with a vertical hole? Most likely those probabilities are very slim. Therefore, when looking for naturally fractured reservoirs, the chances for success would be better if, instead of drilling vertical holes, directional holes, perpendicular to the orientation of the fractures were drilled. If the fractures are not intercepted, it is nearly impossible to detect potential hydrocarbon intervals from logs utilizing conventional techniques, as most methods are based on invasion of the fractures by mud filtrate. Furthermore, a drill stem test will probably recover only some mud and the pressure will not increase significantly because of the very tight characteristics of the primary porosity system. Under these circumstances, the prospect probably would be abandoned. The problem is that a potential hydrocarbon reservoir might be abandoned. This paper discusses exploration methods which are giving encouraging results. The importance of recognizing water saturation distributions in each matrix and fractures is stressed. An equation is presented which allows an estimate of fracture porosity as a function of depth for reservoirs under structural folding. The effects of large hydraulic fracturing treatments when the natural fractures are not intercepted by the borehole are analyzed. Finally, some thought is given to the well spacing concept in naturally fractured reservoirs. It is concluded that conventional exploration and evaluation methods might leave behind substantial amounts of undiscovered hydrocarbons in naturally fractured reservoirs.

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