This study uses geologic and synthetic sonic sections to evaluate the hydrocarbon potential of the Lower and middle Cretaceous Thamama Group carbonates of the Mishrif, Nahr Umr, Shuaiba, and Kharaib Formations in the North field, Qatar. The North field area, a regional high throughout Lower and middle Cretaceous time, is documented by depositional thinning and by

End_Page 415------------------------------

higher energy carbonate facies development. 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 the conventional seismic section because it monitors 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 affect the interval velocities on both a regional and local basis. These are (1) variation of carbonate facies-higher energy wackestone/packstone and possibly 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 on the synthetic sonic sections that would have otherwise been undetected, such as areas of tight or porous reservoir development, permeability barriers, and subtle faulting. In these studied formations, there are many examples of low interval velocity zones that are known to contain hydrocarbons whereas equivalent higher interval velocity zones on the seismic sections at other well site locations do not contain hydrocarbons. In many places, these 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 not be explained solely on str ctural criteria.

It can be concluded from this field study of the 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 these data in areas of known well control to project carbonate reservoir variation in areas where log or petrographic information is not available.

End_of_Article - Last_Page 416------------