Baja California Sur, an Untapped Great Valley of California
Minch, John A.; Ledesma-Vásquez, Jorge; and Guzmán, Alfredo E.
Baja California Sur is underlain by a Cretaceous basin that is essentially identical to the Cretaceous basin of the Great Valley of California, containing similar petroleum bearing structures. This basin is bounded on the east by major faults, on the west by the mélange of the Coast Ranges, and is largely covered by Oligocene to Miocene and Pliocene sediments and volcanics. The fragmenting of North America by the East Pacific Rise moved this basin along faults from central Mexico to the present position in Baja California.
The Cretaceous basin is thousands of feet thick with an axis parallel to the peninsula. it is offset some 50 kilometers by a right lateral trans-basin fault and truncated by a fault along the Cape Massif to the south. This trans-basin fault formed folds in the lower Miocene rocks. There is an arch similar to the Stockton Arch and Miocene intrusive plugs similar to the Marysville Buttes. The Paleocene sediments contain a micro-fauna identical to the Anita Shale at Cantu Creek. The Cretaceous and Paleocene sections are similar to those of the Great Valley.
During the Oligocene and lower Miocene much of Baja Sur was a shallow continental shelf with the shoreline somewhere east of the present Gulf of California. This allowed the Marine waters to enter the tectonic basins of the northern Proto-Gulf prior to the opening of the present mouth of the Gulf. Miocene sediments identical to the Monterey of California were deposited on this shelf before being overwhelmed by coarser marine clastic sediments and finally fluvial, volcani-clastic sediments and tuffs. Seismic data suggests that there are thicker Miocene basins immediately offshore.
The similitude of these Cretaceous to Miocene sedimentary sequences with oil and gas bearing rocks north of the border, plus documented gas in Baja are indicators that there could be commercial hydrocarbons accumulations in Baja. In the 50`s to late 70`s Pemex, drilled several wells in the central Sebastiàn Vizcaino Basin and the Purìsima Iray – Magdalena Basin to the south, having obtained noncommercial flows of natural gas.
The fact that these results were gas and coincided with the discovery of the mega provinces of Chiapas–Tabasco in 1974 and offshore Campeche in 1976 made Pemex focus its resources on Southeastern Mèxico, virtually abandoning the exploration in Baja. Applying an exploratory effort with modern concepts and tools as well as new drilling and completion technologies, would result today in the discovery of economically producible hydrocarbons in Baja.
AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013