ABSTRACT: Water-Mineral-Organic Matter Interactions during Clastic Diagenesis of Cretaceous Heavy Oil Reservoirs, Cold Lake Area, Alberta

F. J. Longstaffe, M. A. Racki, A. Ayalon, L. M. Wickert, D. M. Wightman, G. W. Bird

Water-organic-mineral interactions have controlled the formation of diagenetic minerals in bitumen-saturated and water-saturated sands of the Lower Cretaceous Clearwater Formation in the Cold Lake area, Alberta. In the bitumen-saturated sands, early diagenesis is typified by Fe-rich, grain-coating clay minerals, including berthierine, smectite, smectite/chlorite, and smectite/illite. Other early diagenetic phases include calcite and framboidal pyrite. Intermediate diagenesis is characterized by extensive dissolution of feldspars and rock fragments and by the crystallization of K-feldspar overgrowths and silica cement. Late diagenesis involves patchy crystallization of diagenetic kaolinite, siderite, clinoptilolite, Fe- and Ti-oxides, and calcite. Calcite-cemented lenses are intercalat d with the bitumen-saturated sands and contain early calcite (FeO ^approx 1.5-3.5%) ± late calcite (FeO ^approx 3.5-5.0%). Samples containing both generations are generally devoid of bitumen.

Early diagenetic, Fe-rich minerals are rare in the water-saturated sands. Berthierine, in particular, is confined to water sands that also contain traces of bitumen. The early diagenetic minerals most commonly encountered are illite and dioctahedral smectitic clays, pyrite, and quartz overgrowths. Late diagenetic phases include kaolinite, pyrite, siderite, and clinoptilolite.

Oxygen-isotope data for these minerals suggest that early diagenesis occurred in fresh to brackish water at close to surface temperatures. Porewaters of meteoric origin also dominated burial diagenesis, resulting in dissolution of detrital grains and precipitation of diagenetic K-feldspar and quartz at temperatures no higher than 70°C. Carbon- and oxygen-isotope data indicate that calcite cementation began at low temperatures, utilizing inorganically derived carbon dioxide. However, the generation of carbon dioxide and the precipitation of calcite cement became dominated by microbial fermentation as temperatures rose during burial. Some very late stage calcite cement also formed from downward percolating water bearing soil-derived carbon dioxide.

Heavy oil formed in these deposits as the result of water-washing and bacterial degradation of existing hydrocarbons. Such processes have also been important in the formation of diagenetic minerals. The role of microbial fermentation in calcite formation is clear. We further speculate that microbial activity may also have mediated nucleation of diagenetic clay minerals such as berthierine. Bacterial templating of clay minerals may be an important diagenetic process in such systems.

AAPG Search and Discovery Article #90998 AAPG Eastern Section Meeting, London, Ontario, Canada, September 10-12, 1990