Page 121 - Synthetic Fuels Handbook

FUELS FROM TAR SAND BITUMEN 107

formations, which are older than both the Edna and Sisquoc reservoir rocks. The Santa
Cruz tar sands are discontinuous and overlie the pre-Cretaceous basement.
South Texas holds the largest tar sand deposits. The tar sand deposits occur in the San Miguel
tar belt (Upper Cretaceous) mostly in Maverick and Zavala counties as well as in the Anacadro
limestone (Upper Cretaceous) of the Uvalde district. The Kentucky tar sand deposits are located
at Asphalt, Davis-Dismal Creek, and Kyrock; they all occur in nonmarine Pennsylvanian or
Mississippian sediments. The three deposits appear as stratigraphic traps and are thought to
have received their bitumen or bitumen precursor from the Devonian Chattanooga shale. Tar
sand deposits in New Mexico occur in the Triassic Santa Rosa sandstone, which is an irregularly
bedded, fine- to medium-grainedmicaceous sandstone.
Finally, in the context of the tar sand deposits in the United States, the tar sand deposits
in Missouri occur over an area estimated at 2000 square miles in Barton, Vernon, and Cass
Counties and the sandstone bodies that contain the bitumen are middle Pennsylvanian in
age. The individual bitumen-bearing sands are approximately 50 ft (15 m) in thickness
except where they occur in channels which may actually be as much as 250 ft (76 m) thick.
The two major reservoirs are the Warner sandstone and the Bluejacket sandstone that at one
time were regarded as blanket sands covering large areas. However, recent investigations
suggest that these sands can abruptly grade into barren shale or siltstone.

4.1.3 Venezuela

Tar sand deposits in Venezuela occur in the Officina/Tremblador tar belt that is believed to con-
tain bitumen-impregnated sands of a similar extent to those of Alberta, Canada. The Officina
formation overlaps the Tremblador (Cretaceous) formation and the organic material is typical
bitumen having an API (American Petroleum Institute) gravity less than 10°. The Guanaco
Asphalt Lake occurs in deposits that rest on a formation of mid-Pliocene age. This formation,
the Las Piedras, is principally brackish sandstone to freshwater sandstone with associated lig-
nite. The Las Piedras formation overlies a marine Upper Cretaceous group; the Guanaco Lake
asphalt is closely associated with the Guanaco crude oil field that produces heavy crude oil from
shale and fractured argillite of the Upper Cretaceous group.

4.1.4 Other Countries
The Bemolanga (Madagascar) deposit is the third largest tar sand deposit presently known
and extends over some 150 square miles in western Madagascar with a recorded overburden
from 0 to 100 ft (0–30 m). The average pay zone thickness is 100 ft (30 m) with a total
bitumen in-place quoted at approximately 2 billion barrels. The deposit is of Triassic age
and the sands are cross-bedded continental sediments; the coarser, porous sands are more
richly impregnated. The origin of the deposit is not clear; the most preferred source is the
underlying shale or in down-dip formations implying small migration.
The largest tar sand deposit in Europe is that at Selenizza Albania. This region also
contains the Patos oil field throughout which there occurs extensive bitumen impregnation.
This deposit occurs in middle-upper Miocene lenticular sands, characterised by a brackish
water fauna. Succeeding Pliocene conglomerate beds, which are more generally marine,
are also locally impregnated with heavy crude oil. The Selenizza and Patos fields occupy
the crestal portions of a north-south trending anticline. Faulting also controls the vertical
distribution of the accumulation. The Miocene rests on Eocene limestone and it is these that
are thought by some to be the source of the tar.
The Trinidad Asphalt (Pitch) Lake situated on the Gulf of Paria, 12 miles west south
west of San Fernando and 138 ft (42 m) above sea level, occupies a depression in the