Provenance and burial history of cement in sandstones of the Northbrook Formation (Carboniferous), western Newfoundland, Canada: A geochemical investigation

Abstract

The Carboniferous North Brook Formation (western Newfoundland, Canada) consisting of fluvial/lacustrine arkosic litharenites has undergone two cementation cycles. The first cycle comprises chlorite followed by minor quartz and calcite. Mono-phase aqueous fluid inclusions hosted in first-cycle calcite reflect precipitation in a near-surface meteoric diagenetic environment. Chlorite linings and cements generally inhibit precipitation of quartz overgrowths, thus maintaining open pore space until a subsequent cycle when temperatures are within the oil window. The second cycle comprises chlorite followed by calcite which hosts two-phase fluid inclusions with homogenization temperatures ranging from 91.7 to 120.7°C and salinity from 6.3 to 8.4eq. wt.% NaCl. These results suggest conditions in-line with a deep burial environment, and calculated fluid inclusion isochors stipulate a trapping temperature of around 132°C and pressure of 500bar (5km hydrostatic pressure) for these cements. Quantitative fluid inclusion gas analysis further confirms that fluids were sourced from evolved meteoric fluids with fluctuating CO2:CH4:N2 ratios. Manganese analysis confirms an oxidizing environment for the first-cycle calcite cement but a reducing one for the infilling second-cycle cement. Both cements have similar REE signatures, are slightly LREE-depleted, and during migration the fluids were influenced by the sedimentary host rock. Although there is a prominent negative Ce anomaly, there is no Eu anomaly in the cements. The δ13C and δ18O values are mostly in the negative in the calcite cement hosted in the coarse-grained clastics, which were influenced by migrating brines and/or hydrocarbons.In the diagenetic history of this sedimentary basin, the first cementation event is characterized by chlorite, quartz and calcite emplaced under shallow, oxidizing meteoric water conditions and appropriate temperatures. A diagenetic hiatus ensued until the units were more deeply buried. At this stage, a second generation of cement characterized by minor chlorite and major calcite, infilled vugs and voids under higher temperatures equivalent to the upper limit of the oil window and 5km burial depth. The lack of porosity precludes the North Brook Formation from being a petroleum reservoir although limited petroleum may have been sourced during the geologic history.