October 31, 1975 - Gerald R. Ford Presidential Library and Museum

The Conroe oil field is located in Montgomery County, Texas in the
Eocene belt of the Texas Gulf Coast.
The principal producing members are in the Yegua Formation of the
Claiborne GrouD of Eocene age. These are the Upper Cockfield sand
at 4,750 feet and the Main Conroe sands at 5,000 feet.
The structure at Conroe is a highly faulted, broad, ovate anticline
which is underlain by a deep seated salt dome. The productive sands
are overlain dnd separated by shale members. Bottom water is present.
The productive area is somewhat in excess of 17,000 acres (27 square
miles).
The Upper Cockfield sand had a large gas cap--having an area some five
times the oil rim. It produced under the influence of a dissolved gas
drive and an expanding gas cap. Water influx into the Upper Cockfield
is limited. The Main Conroe sands had original gas caps. However,
an active water drive is in effect for these zones. The reservoir
pressure in the Upper Cockfield has been reduced considerably by production
from gas wells. This has caused migration of oil from the
Main Conroe sands to the overlying gas caps up through fault planes
and through the reservoir. In an attempt to control this situation,
oil rates have been increased recently; gas production has been reduced
considerably; and efforts are underway to unitize the field. Water and
gas will be injected to maintain pressure and to prevent oil migration.
Hydrocarbons originally in place have been calculated using the
volumetric method. Although the field was discovered in 1932, there
have been many infilling wells drilled recently to improve upon
structural position in the various fault segments and to prevent oil
migration up the fault planes. The modern data so obtained have aided
considerably 1n understanding the geometry of the reservoir and the
rock properties of the producing sands. Material balance analyses to
determine hydrocarbons in place have been inconclusive because of
complex geology, migration of hydrocarbons, numerous blow-outs, and
lack of early gas and water production records.
The oil recovery efficiencies have been estimated as 52.5 percent and
57 percent in the Upper Cockfield and Main Conroe sands respectively.
Following oil depletion the gas caps will be rapidly blown down and
a 70 percent gas recovery efficiency has been assumed.
The FEA report on the Conroe Field has been prepared by the U.S.
Bureau of Mines, Department of Interior, under Interagency Agreement
CG-05-50058-00.
38