Noble gas constraint of CO2 source and connectivity in a CO2-rich gas reservoir, offshore Angola

Subsalt prospects in the South Atlantic margin contain potential hydrocarbon plays as exemplified by significant discoveries on the Brazilian margin. On the conjugate east side of the South Atlantic Ocean, pre-salt sections of the offshore Kwanza Basin, Angola also contain potentially economic reservoirs. Here we report the results of an exploration well which encountered gas containing high portions of CO 2 (82 – 84 mol%) and CH 4 (14.2 – 16.6 mol%) from offshore Angola. Using bulk gas, stable carbon isotopes and noble gas compositions of gas samples collected via downhole sampling of the exploration well, we constrain the source and connectivity of the CO 2 -rich gas reservoir. Measured CO 2 / 3 He ratios indicate that the CO 2 is primarily mantle-derived, with the samples from the deepest portion of the reservoir exhibiting evidence of CO 2 loss, most probably due to CO 2 dissolution into the groundwater. Using neon isotopes, we show that noble gas measurements constrain that the CO 2 has a Mid-Ocean Ridge Basalt (MORB) source, related to the rifting of the Atlantic margin at approximately 110 Ma. We additionally identify that a recently outlined methodology utilising noble gases can be applied to show a disconnection between the two upper and lower sections of the reservoir encountered in the well. This corroborates the evidence of disconnection provided from the δ 13 C gas fraction (C1 to C5) and provides further verification of the use of noble gases to determine static vertical connectivity within a hydrocarbon prospect. • Bulk gas, stable C isotopes and noble gas geochemistry used to constrain source and connectivity of a CO 2 -rich gas reservoir. • Measured CO 2 / 3 He ratios indicate that CO 2 is primarily mantle-derived. • Samples from deepest part of well exhibit evidence of CO 2 loss, most likely via CO 2 dissolution. • Neon isotopes show that a Mid-Ocean Ridge Basalt (MORB) source for CO2. • Noble gas geochemistry shows a disconnection between the two upper and lower sections of the reservoir.