Origin of Hemipelagic Source Rocks During Early and Middle Miocene, Monterey Formation, Salinas Basin, California

Karl A. Mertz, Jr.

Organic-rich hemipelagic shales of the lower Monterey Formation in the Salinas basin of central California record intervals of enhanced productivity and accumulation of fine-grained biogenic and clastic sediment under dysaerobic to anoxic conditions during the late early Miocene and early middle Miocene. Generally low oxygen conditions helped preserve abundant hydrogen-rich organic matter in laminated hemipelagic shales. These deposits are good potential oil source rocks, and consist primarily of (1) pelagic components (biogenic carbonate and silica, and marine organic matter), (2) fine-grained siliciclastic material transported mainly by sediment gravity processes, and (3) an associated assemblage of diagenetic phases. The following paragenetic sequence is observed: (1) hallow burial, with formation of glauconite, carbonate fluorapatite, pyrite, and iron-dolomite, and (2) deeper burial, with transformation of opal-CT to quartz, and precipitation of mixed layer illite/smectite, kaolinite, and chlorite. Sulfur and carbon isotope ratios from apatite suggest formation just beneath the sediment-water interface under anoxic to dysaerobic conditions. Kaolinite postdates, or is cogenetic with, diagenetic quartz and may be a dissolution product associated with the onset of thermal maturation and carboxylation of organic matter. Although conventional geochemical maturity measures suggest that these source rocks have not entered the main oil window, relatively early hydrocarbon generation is well documented from Monterey shales, and these rocks are likely a signif cant source of much Salinas basin oil.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.