GEOCHEMISTRY TAGS UPPER JURASSIC SOURCE FOR MOST OF MEXICO'S OIL, GAS

Raul Gonzalez Garcia, Noel Holguin-Quinones
Petroleos Mexicanos SA
Mexico City

Mexico is 75% covered by six sedimentary sequences, but most of the oil and gas production is restricted to basins under the Gulf of Mexico coastal plain.

The exceptions are Sabinas basin gas fields and the newly discovered oil and condensate fields of the Sierra de Chiapas.

Gas and condensate are also known to exist in the Gulf of California but have not been commercially produced, and the amount of reserves is not known.

Petroleos Mexicanos SA has developed a basic geochemical data bank with more than 10,000 pyrolyses and organic carbon values from producing basins and a similar amount from nonproductive areas,

With these data, the Mexican technicians defined source rock units with broad distribution although acknowledging that there is not yet sufficient knowledge of their quantitative contribution to the creation of Mexico's reservoirs.

Main conclusions:

Geochemical studies indicate that Mexico's oil and gas were generated mostly by Upper Jurassic calcareous shales and shaly limestones and to a lesser degree by Tertiary and Cretaceous source rocks.
Petroleum in the Sureste basin, the most significant for its production and reserves, was generated by the Tithonian shaly limestones, with the Mesozoic and Tertiary oils in the Tampico-Misantla basin, second in importance, coming from the Taman and Pimienta formations of Kimmeridgian and Tithonian age, respectively. In both cases kerogen is predominantly of Type II.
There is no definite evidence about the origin of the gas and condensate produced in the Burgos basin from the Paleogene; however the geologic setting suggests that they come from the same enclosing Tertiary shales since the Upper Jurassic is found at great depth and a migration through the thick overlying shale section is not likely.
In the gas bearing Sabinas province, generation is ascribed to the Kimmeridgian-Tithonian shales with presently overmature Type II kerogen.
In a new petroleum province of Mexico named Sierra de Chiapas, which produces oil and condensate from a Cretaceous carbonate and evaporate section, the source rocks probably are thin shaly beds present in the same lagoonal facies.
Sureste basin encompasses onshore provinces better known as Chiapas-Tabasco, Salina del Istmo, and Macuspana, as well as the offshore part known as Campeche Sound. The Sureste basin is Mexico's most important petroleum region, with production of 2,414,379 b/d (386,300 cu m/d) of oil, 96% of the country's daily average output. Sureste basin contains 67% of the national proved reserves. Most of its production comes from the Chiapas-Tabasco fields and Campeche Sound.
Tampico-Misantla basin contains three oil producing provinces: the basin proper, the Golden Lane, and the Chicontepec Paleocanyon. The basin contains 30% of the country's national reserves.
Burgos basin, producer of gas and condensate from Paleocene, Eocene, Oligocene, and Miocene, contains hydrocarbons equivalent to 2.5% of the proven reserves of Mexico.
Veracruz basin has produced from the time hydrocarbons were discovered in 1953. Proved reserves for this basin are 0.5% of Mexico's total.
Sabinas basin is the only productive basin outside the Gulf of Mexico's Coastal Plain. Gas was discovered in 1972, although in very limited quantities. Sabinas has only 0.05% of the country's total proven reserves.
Sierra de Chiapas basin is Mexico's latest discovered petroleum province. In 1986 Nazareth gas/condensate field was found in a Lower Cretaceous limestone and evaporate sequence. In 1990 oil was discovered in Lacantun field in similar lithologies but of Albian-Cenomanian age.
In the Chihuahua and Tlaxiaco basins no commercial hydrocarbons have been discovered. They are presently being explored, and the preliminary evidence seems to point to source rocks as the critical factor for existence of oil and gas. Upper Jurassic sourcing seems most likely, although in Chihuahua these rocks appear to be overmature.