T23D-2988
Real Time Mud Gas Logging During Drilling of DFDP-2B

Tuesday, 15 December 2015
Poster Hall (Moscone South)
Loren Ariel Mathewson1, Virginia Toy1, Catriona Dorothy Menzies2, Martin Zimmer3, Joerg Erzinger4, Samuel Niedermann5 and Simon Cox6, (1)University of Otago, Dunedin, New Zealand, (2)University of Southampton, Southampton, SO14, United Kingdom, (3)GFZ Potsdam, Potsdam, Germany, (4)Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Potsdam, Germany, (5)Deutsches GeoForschungsZentrum GFZ, Potsdam, Germany, (6)GNS Science-Institute of Geological and Nuclear Sciences Ltd, Lower Hutt, New Zealand
Abstract:
The Deep Fault Drilling Project (DFDP) aims to improve our understanding of the Alpine Fault Zone, a tectonically active mature fault system in New Zealand known to rupture in large events, by deep scientific drilling. The borehole DFDP-2B approached the Alpine Fault at depth, reaching a final depth of 892 m (820 m true vertical depth). Online gas analysis (OLGA) while drilling tracked changes in the composition of gases extracted from the circulating drill mud. The composition of fluids from fault zones can provide information about their origins, flow rates and -paths, fluid-rock interactions along these paths, and the permeability structure of the faulted rock mass.

Apart from an atmospheric input, the gases in drilling mud derive from the pore space of rock, crushed at the drill bit, and from permeable layers intersected by the borehole. The rapid formation of mud wall cake seals the borehole from further fluid inflow, hence formation-derived gases enter mostly at the depth of the drill bit. OLGA analyses N2, O2, Ar, CO2, CH4, He, and H2 on a mass spectrometer, hydrocarbons CH4, C2H6, C3H8, i-C4H10, and n-C4H10 on a gas chromatograph, and Rn using a lucas-cell detector. Gas was sampled for offline analyses on noble gas and stable isotopes to complement the OLGA dataset.

The principle formation-derived gases found in drilling mud during drilling of DFDP-2 were CO2 and CH4, with smaller component of H2 and He2. High radon activity is interpreted to reflect intervals of active fluid flow through highly fractured and faulted rock. 3He/4He values in many samples were extremely air-contaminated, i.e. there was almost no excess of non-atmospheric He. The 3He/4He values measured at 236 m and 610 m, which are the only analyses with uncertainties <100%, are very similar to those measured in hot springs along the Alpine Fault, e.g. Fox River (0.64 Ra), Copland (0.42 Ra), Lower Wanganui (0.81 Ra). We will compare these data to those gathered using OLGA and discuss the implications. 

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