Abstract

The Middle Devonian Lucas Formation of southwestern Ontario, Canada, forms the main hydrocarbon reservoir rocks in the region. Integrated field, petrographic, fluid inclusion and isotope geochemistry reveal a variety of diagenetic processes that include the formation of dolomite, calcite, celestine and fluorite under diverse geochemical conditions. The host carbonate strata display two types of replacive dolomite fabrics: 1) fine-crystalline matrix dolomite (5–20 μm); and 2) medium-crystalline dolomite (50–150 μm) with an average size of 100 μm.

Stable oxygen and carbon isotope results for both dolomite fabrics show a wide range of values (δ¹⁸O = −9.4‰ to −3.9‰ VPDB, δ¹³C = 0.4‰ to 3.4‰ VPBD). For late fracture-filling calcite, δ ¹⁸O and δ ¹³C values range from −9.4 to −5.9‰ VPDB and −7.7‰ to −0.04‰ VPDB, respectively. The ⁸⁷Sr/⁸⁶Sr ratios of selected dolomite and calcite samples range from 0.70798–0.70838, preserving Middle Devonian seawater values while some samples have slightly more radiogenic ratios than contemporaneous seawater.

Field and petrographic evidence and Sr isotope ratios of dolomite suggest early dolomitization from evaporated seawater in a sabkha setting (shallow seepage reflux or evaporitive pumping). However, the depletion in δ¹⁸O (varying from −3.9 to −9.4‰ VPDB), and radiogenic Sr isotope values of the Lucas Formation dolomite are considered to be the result of recrystallization, which is interpreted to occur at elevated temperatures in a burial environment.

The ¹⁸O-depleted values and slightly radiogenic ⁸⁷Sr/⁸⁶Sr ratios of calcite cements suggest precipitation from hot, saline basinal/hydrothermal brines. In addition, the ¹³C-depleted calcites indicate that carbon was possibly formed by oxidation of organic matter. Fluid inclusions in some of the late calcite cements fluoresce under UV light, indicating the presence of hydrocarbons. Homogenization temperatures of these inclusions vary from 87.1°C to 171.9°C with an average value of 126°C. Average salinity of fluid inclusions is 19.7 wt. % NaCl eq. Celestine samples have lower salinities relative to calcite, averaging 14.5 wt. % NaCl eq. with an average homogenization temperature of 219.5°C. Based on the geochemical and fluid inclusion results, hot, saline basinal/hydrocarbon bearing brines originated from central part of the basin and migrated to Devonian carbonates via fractures and faults. Celestine is a product of thermochemical sulfate reduction and formed due to mixing of saline, Sr and sulfate bearing brines and meteoric waters.