Page 352 - Carbonate Facies in Geologic History

Stratigraphy and Tectonic Framework 339

Many researchers have pointed to differences as well as similarities between
the Mexican offshore banks and the Deep Edwards-Stuart City trend. No reef
talus has been found on the foreslope ofthe Deep Edwards trend but actually very
little subsurface data are available from this area. The measurable slopes along
the trend are considerably less than on the sides of the Mexican banks (about 2
degrees compared with 5-30 degrees in Mexico). The backreef Albian and Ceno-
manian beds in Texas grade into rather argillaceous strata at the outcrops. How-
ever, the Mexican area was far beyond the northern source of this material and is
essentially one of pure carbonate with any marl confined to very thin and styloli-
tic layers between basinal limestone strata.
Significant differences existed in the later geologic history of the two areas.
Carbonate bank development lasted longer in Mexico and was completed only at
the end of Cenomanian time rather than in Late Albian time as in Texas. Neither
was the Deep Edwards-Stuart City reef trend exposed to as much weathering and
karst-producing solution. No large cavernous oil pools are known in Texas as
there are in Mexico's Golden Lane. Tertiary orogeny, which tilted and periodi-
cally exposed the Golden Lane bank and uplifted the western banks into the
Sierra Madre, did not affect the Gulf coast. Only along the former topographic
crest is secondary leaching porosity developed in the Stuart City trend. Dolomiti-
zation is the important factor in all the backreef Texas fields.

Middle and Lower Cretaceous Facies in the Middle East

At the present time about two-thirds of the world's known petroleum reserves
outside of the USSR reside in the Middle East in areas surrounding the Persian
Gulf. The reservoir strata include chalk, carbonate sands, dolomite, and quartz
sand of Mesozoic and Tertiary age. An appreciable part of the productive section is
of Cretaceous age. The Burgan Sandstone in Kuwait, which presently forms the
world's largest oil field, is Albian in age. The equivalent Nahr Umr Shale in the
Middle East is conjectured to be the source bed for much of the Cretaceous oil
and possibly even for Tertiary production. Some of the most interesting reservoir
facies in the geologically intriguing Persian Gulf area are found in Cretaceous
rudist and lime sand shelf margin buildUps. Such strata have been extensively
studied petrographically, originally for paleontology and later for sedimentologic
interpretation, some results of which are given below.

Stratigraphy and Tectonic Framework
Table XI-2 illustrates the major stratigraphic units for the Early and Middle
Cretaceous. These are mapped on Fig. XI-H. The facies are largely controlled by
clastic influx off the Arabian shield. The terrigenous facies gives way progressively
east and south of the shield to massive, pure limestones deposited both north and
south of the center of the Persian Gulf.