The critical point in the evaluation of a well occurs after the first logging Trip or reconnaissance run. When the logs are reviewed. If nothing is identified, further evaluations is not considered. By acknowledging the limitations of conventional logging measurements (induction, sonic, density and neutron) in defining the reservoir and, more importantly, in recognizing its presence, we realize the need to enhance the recognition and evaluation process.
Three types of reservoirs which are difficult to interpret using conventional logging data are those which 1) have high radioactivity, 2) are mineral dominated, or 3) have low resistivity. Probably the most common type in the Gulf of Mexico, low-resistivity pay can be further classified as laminated sands, fine-grained sands, or dispersed clays.
Conventional logging tools have a vertical resolution from 1 1/2 ft to 5 or 6 ft (from 0.116 to 1.5 or 1.8 m). In a highly laminated reservoir. the data are averaged with the shale laminations and a pessimistic interpretation results. A fine-grained sand may appear to be water productive when, in fact, it is a commercial, low resistivity reservoir. Likewise, a dispersed clay reservoir mayappear to be 100% shale. Zones which appeared marginal at best, have produced 2000 or 3000 barrels of oil per day. In addition, highly radioactive sands and mineral-dominated reservoirs can be very confusing and difficult to interpret. Therefore, it is essential to recognize and identify by type all potential hydrocarbon reservoirs on the first logging trip. Then, by selecting the tools suitable for subsequent runs (electromagnetic, lithology-density, spectral gamma ray, dipmeter, formation tester, core sampler, neutron), one can properly evaluate the formation.
The method now being used combines a microresistivity tool tool with the induction/sonic/GR tool string on the first logging trip. Having a vertical resolution of 4 to 5 in. (10–13cm) and the capability to output a synthetic microlog, this tool combination can satisfy the logging needs. An initial presentation which shows:
Moved hydrocarbons RXO/Rt
Synthetic microlog for permeability
MSFL for laminations
In the past log interpretation in the Gulf of Mexico was thought to be very simple and straightforward. After all, the Gulf was nothing more than continual sequences of relatively thick sand and shale layers. What could be easier to interpret? During the last 5 to 10 years, this oversimplification proved to be inadequate. More complex geologies were discovered, and the interpretation was not so straightforward. As a result, many commercial pay sands were overlooked because volcanics, highly radioactive sands, and low-resistivity pay sands were not understood. This paper will explain a method now in use in the Gulf of Mexico that greatly enhances the interpretation process in these types of reservoirs.
Any log interpretation is a two-step process: recognition and evaluation. First, a zone must be recognized as having some potential before it; can be considered for further evaluation.