--> ABSTRACT: Calculated Geologic Oil Generation Temperatures of the Miocene Monterey Formation, by Daniel M. Jarvie and Leeland L. Lundell; #91019 (1996)

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Calculated Geologic Oil Generation Temperatures of the Miocene Monterey Formation

Daniel M. Jarvie and Leeland L. Lundell

The hydrocarbon generation characteristics of the marine Monterey formation source rocks were evaluated using open and closed system pyrolysis techniques. Hydrocarbon generation rates from open system pyrolysis were used to calculate the level of conversion rates from well data in the Santa Maria basin.

Temperatures for the onset of generation for certain sulfur and oxygen rich Monterey samples are lower than for other labile Type II kerogens such as those from the Phosphoria and Kimmeridge shales. Overall, the kerogen in these Monterey samples reaches 10% conversion at approximately 105°C at a 10°C/million year (my) constant heating rate. Peak generation temperature is approximately 140°C at this same heating rate which is comparable to the aforementioned Type II kerogens. Other Monterey samples having lower sulfur and oxygen contents decompose at rates similar to other more refractory Type II source rocks such as the Woodford and Barnett marine shales in both the temperatures of onset and peak generation.

The Tmax scale for Monterey formation rocks is considerably different than classical maturity interpretations. The onset of generation for the sulfur/oxygen rich Monterey samples is reached at Tmax values of approximately 400°C while 50% kerogen conversion is achieved at 430°C Tmax. The onset of generation of typical marine source rocks is usually interpreted to occur at about 430°C Tmax.

Understanding the kinetics of the chemically unique marine source rocks of the Monterey formation will help us better interpret kerogen conversion to hydrocarbons, where geologic control is limited in the early exploration of a basin.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California