Using Cathodoluminescence to Map Regionally Zoned Carbonate Cements Occurring in Diagenetic Aureoles above Oil Reservoirs, Velma Oil Field, Oklahoma

BARKER, CHARLES E., and MARY C. DALZIEL, U.S. Geological Survey, Denver, CO

Strongly reducing chemical conditions related to the microseepage of petroleum above oil reservoirs appear to cause mappable variations in the trace element content of resulting late-diagenetic carbonate cements. Published geochemical studies show that changes in Fe and Mn concentration in carbonate cements are a cause of chemical zonation in some diagenetic aureoles related to oil reservoirs. Fe and Mn are, respectively, the common quencher and activator of cathodoluminescence (CL) in carbonate minerals. Thus, CL can be used as a sensitive detector of changes in Fe/Mn ratio, if other trace element CL sensitizers/quenchers have a negligible effect in the samples. Our samples seem to respond to changes in Fe/Mn suggesting that in this case CL can be used to make a quick visual estimate of Fe/Mn content in the carbonate cements.

Application of the method to Velma oil field, Oklahoma, suggests that the cathodoluminescent properties form a regionally mappable zoned pattern seemingly related to the production field. The diagenetic aureole at Velma, Oklahoma, is particularly well developed. It consists of dark reddish-brown sandstone containing abundant iron sulfide and ferroan carbonate cements over the reservoir, surrounded by iron-poor country rock. The regional cathodoluminesence pattern observed in the carbonate cements roughly correlates with the observed changes in iron content with dull- or nonluminescent cement (high Fe/Mn) over the field and dull to bright luminescence (low Fe/Mn) on its flanks.

The regional CL zonation is interpreted to be caused by vertical microseepage of petroleum producing reducing conditions over the reservoir. These strong reducing conditions produce Fe and Mn in the 2+ state allowing these elements to easily substitute for Ca(2+) in the carbonate crystal lattice. CL in such high Fe/Mn carbonates will be dull- or nonluminescent. The less reducing conditions conjectured to exist off the reservoir produce Fe(3+) but Mn remains in the 2+ state producing a low Fe/Mn carbonate cement that is highly luminescent.

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