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some of which ran wild, culminating in 1916 with the completion of Cerro
Azul 4 for nearly 41.500 m3/day (260,858 bbl/day), setting a world record
that probably still stands. In 1919 the first warning signs appeared, with
water encroachment in some fields. Mexico had been the world’s second larg-
est oil producer after the United States of America, and production peaked
in 1921 at 840,240 m3/day, with a severe decline after that. By 1930, Mexico
had fallen to 6th largest oil producing country, and the expropriation of for-
eign and domestic oil companies in 1938 was followed by 20 years of diffi-
culties (see Hewins, 1957; Young, 1966; Owen, 1975;for interesting accounts
of these events).
Although Mexico soon ceased to be a major oil producing country, she
was not inactive. The application of improved geological and geophysical tech-
niques led to a number of significant discoveries in the years following the
War of 1939-1945, and the Golden Lane was found to extend to the south-
east, the extension being called the New Golden Lane. Late in the 1950s, ex-
ploration moved offshore, and the discovery of the Marine Golden Lane began
in 1963. During the 1970s, Mexico’s successful exploration onshore and off-
shore raised her once again to be one of the world’s largest producers of crude
oil, with the added advantage of geographical position close to North America
and far from the Middle East. By the end of 1980, proven recoverable reserves
were estimated at 44 X lo9 bbl (7 X lo9 m3), ranking 5th by reserves after
Saudi Arabia, Kuwait, USSR and Iran - and well ahead of the U.S.A. Produc-
tion in 1980 averaged 1.96 X lo6 bbl/day (312,000 m3/day), giving her a
reserves/production ratio of 62 : 1 (data from Oil and Gas Journal, 78 (52),
p. 79).
Important production comes from middle Cretaceous limestones, probably
reefal, that form a huge atoll 70 km wide in east-west direction, 130 km
long in northsouth direction (Fig. 12-10), with a perimeter of about 350
km.
The Old Golden Lane is remarkable not only for its production, but also
for the shallow depths from which it was obtained: from 500 m in Cerro
Azul to 800 m in San Isidro. Production comes from the El Abra limestone
(Albian+enomanian) in a series of culminations that are reefal in origin, or
erosional. The top surface tends to be karstic, and the limestone is fractured
and cavernous (several caves having been found when drilling). The productive
trend is very narrow, less than 3 km. The oil columns are thin relative to the
1 km relief on the top of the El Abra, so little is known about the deeper
parts of the reef. The cap rock is shale that ranges in age from Early Creta-
ceous in the central and southern areas to Late Oligocene in the northern
Golden Lane. These overlying formations are draped over the reef trend to
form an anticline, and this originally led to the belief that the Golden Lane
was a tight fold.
The New Golden Lane fields are rather deeper than the old, below 2 km in
the south-east, but their character is the same. However, the largest onshore