Seal Risking and Petroleum Leakage - A Case Example from Taglu Gas/Condensate Field, Richards Island, Beaufort Mackenzie Delta, Northern Canada

Jokanola, Olufemi A.¹, Dale R. Issler², Andrew C. Aplin³, Steve R. Larter⁴, Kuncho D. Kurtev³ (1) Petroleum Reservoir Group (PRG), Department of Geology and Geophysics, University of Calgary, Calgary, AB (2) Geological Survey of Canada (Calgary), Calgary, AB (3) University of Newcastle upon Tyne, Newcastle upon Tyne, United Kingdom (4) University of Calgary, Calgary, AB

The Taglu gas/condensate field in the Beaufort Mackenzie Delta has an estimated gas reserve of 3.2 Tcf. It is one of the anchor fields intended to serve the proposed Mackenzie valley gas pipeline. Structurally, Taglu is an Eocene fold truncated by a NW-SE normal fault. Three pressure compartments have been mapped in the field's Reindeer reservoir sands: normally pressured (top); 22 MPa (middle) and 40 MPa (down-dip) overpressured compartments. A gas/condensate column of ~680m is found only in the normally pressured compartment on the footwall while the overpressured compartments have no petroleum accumulations.

We present the integrated results of pore pressure, geochemical and petrophysical analyses suggesting that the Taglu accumulation represents a dynamic charge-leak system potentially involving both capillary and hydraulic leakage in different parts of the structure. Our data suggest that petroleum has leaked by capillary flow into the Richards Formation cap rock. Capillary pressure data for the Richards Formation cap rock shows that it cannot sustain the 680 m condensate column observed in the field. Leakage of thermogenic gas is confirmed by direct evidence of thermogenic gas all the way from reservoir through the cap rock, to the gas hydrate and permafrost zones above. Log data suggest an average gas saturation of ~15 % in the Richards Formation. Around 8 % of the volume of the gas in-place in the Taglu field is currently in the overburden, although much more may have leaked. Basin modeling results suggest that the leakage could have occurred in the last 9 Ma with a proposed leakage rate of ~1,000 m3/yr.