Plagioclase Dissolution Related to Oil Residence Time, North Coles Levee Field, California

BOLES, JAMES R., University of California, Santa Barbara, CA

Diagenetic mineral zones characterize an originally homogeneous turbidite sandstone reservoir at North Coles Levee (NCL). The medium to coarse-grained sands in the reservoir contain a 500 ft oil column with high water saturation. Extensive plagioclase porosity and associated diagenetic kaolinite occur in the lower third of the oil column. Plagioclase dissolution is greatest near the oil-water contact. Kaolinite and plagioclase porosity are very rare in the upper third of the oil column next to the gas cap.

Previous work has shown that at NCL plagioclase dissolution occurred in the last several million years of the reservoir burial history, nearly contemporaneous with oil emplacement. Clearly if plagioclase alteration was independent of the hydrocarbons, a uniform alteration pattern would be expected. The zonation pattern described here appears to be related to the presence of the oil. Thus diagenesis was most intense where the residence time of the hydrocarbon was greatest (in the lower part), whereas near the gas cap, plagioclase diagenesis was minimal where the accumulation of gas effectively shut down feldspar dissolution. These new results indicate that plagioclase porosity at NCL was an effect of oil emplacement, perhaps due to acid components solubilized from the oil.

Important implications of these findings are: (1) Maximum porosity enhancement may be off-structure away from the gas cap, (2) tends to confirm the importance of acid components associated with hydrocarbons on feldspar dissolution, and (3) diagenetic interpretations may differ if sample locations relative to gas/oil/water zones are not known.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)