This study uses synthetic sonic sections to evaluate the hydrocarbon potential of the carbonate reservoirs of the Lower and middle Cretaceous Thamama Group, namely the Mishrif, Nahr Umr, Shuaiba, and Kharaib formations of the North field, Qatar. The North field area was a regional high throughout Early and middle Cretaceous time. Oil and gas accumulations are found on the crestal portions of this paleohigh in structural/stratigraphic traps.

The synthetic sonic program produces a series of synthetic sonic logs from real seismic traces. It is a powerful addition to conventional seismic section because it monitors the additional parameters of seismic continuity and rock properties in what otherwise is a relatively structureless subsurface carbonate terrain.

Detailed studies of seven regional synthetic sonic lines across the North field area indicate that significant decreases in interval velocities occur in all of the studied carbonate reservoir formations. Three factors effect the interval velocities on both a regional and local basis. These are (1) variation of carbonate facies - higher energy wackestone/packstone and possible grainstones flanked by predominantly mudstones, (2) secondary porosity developed near the top of unconformity surfaces, and (3) the existence of hydrocarbons in the reservoir.

Many local lateral and vertical variations in interval velocities were noted. Areas of tight or porous reservoir development, permeability barriers, and subtle faulting were observed on the synthetic sonic sections that would have otherwise been undetected using more conventional processing. In these formations, there are many examples of low interval velocity zones that are known to contain hydrocarbons whereas equivalent higher interval velocity on the seismic sections at other well locations do not contain hydrocarbons. In many cases, these lateral variations are of sufficient magnitude to be mapped as intraformational permeability barriers. These variations were useful in explaining the occurrence of different oil-water and gas-water contacts within the same formation that could n t be explained solely on structural criteria.

It can be concluded from this study of North field, Qatar, that the synthetic sonic technique is a particularly useful exploration tool in carbonate reservoir environments because it is able to delineate areas of higher exploration potential. The geologist can use this tool in areas of known well control to project carbonate reservoir variation in between or away from bore holes where log or petrographic information is unavailable.

The SEISON-II modeling was found to be a well-suited modern seismic technique in this offshore marine environment, where it was used to investigate relatively shallow reservoir depths (2,500 to 3,700 ft [762 to 1,128 m]), and relatively thick formation subzones (40 to 90 ft [12 to 27 m]) having a fairly consistent regional development.