--> ABSTRACT: Paleokarst Breccia-Pipe Reservoir Analogue, Carboniferous, Svalbard, by Wheeler, Walter H.; Tveranger, Jan; Heincke, Björn; Stemmerik, Lars; Breesch, Liesbeth; Braathen, Alvar; Maher, Harmon D.; Rossi, Giuliana <sup>6</sup>; Buckley, Simon J.; #90135 (2011)

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Paleokarst Breccia-Pipe Reservoir Analogue, Carboniferous, Svalbard

Wheeler, Walter H.1; Tveranger, Jan 1; Heincke, Björn 2; Stemmerik, Lars 3; Breesch, Liesbeth 3; Braathen, Alvar 4; Maher, Harmon D.5; Rossi, Giuliana 6; Buckley, Simon J.1
(1)Center for Integrated Petroleum Research, Uni Research, Bergen, Norway. (2) Norwegian Geological Survey, Trondheim, Norway. (3) Institute for Geography and Geology, Copenhagen University, Copenhagen, Denmark. (4) University Centre in Svalbard, Longyearbyen, Norway. (5) Dept. of Geography and Geology, University of Nebraska at Omaha, Omaha, NE. (6) OGS, Trieste, Italy.

Paleokarst introduces reservoir challenges such as 3D heterogeneity in nearly all reservoir petrophysical properties, and, as many critical paleokarst elements are below 3D seismic resolution, a cryptic architecture offering little basis for prediction. To address this, we present a reservoir-analogue study that covers the basin context, the oil-field scale and the well scale. This study focuses on the geometry and statistical facies architecture of breccia-filled collapse pipes in platform-type carbonates. Breccia pipes form strong heterogeneities in rock properties, affecting fluid-flow characteristics on a meter to hundred-meter scale.

The study covers the Carboniferous northern Billefjord half-graben basin on Spitsbergen. Cliff exposures show near-vertical breccia pipes cutting a gently-dipping 200-m-thick carbonate sequence. Geological field studies were supplemented with helicopter lidar and photo coverage of 60 linear km of outcrop. A detailed study area covering 1 km2 used 2D GPR (25 m line spacing, 30-40 m penetration) to map breccia-pipe size and spacing, and to identify local and basin-scale faults. Pipe density is about 40 per km2, with diameters ranging from 20 to 100 m with an average of 60 m. Pipe-height frequency is unconstrained, as the GPR study lies 200 m above the base of the pipes (short pipes not addressed). Two pipes were further studied using 3D GPR, geo-electric and seismic methods, resolving aspects of the 3D pipe shape and pipe-margin deformation. The pipes have been inferred to originate from the dissolution of underlying gypsiferous strata and represent the latest of several karstification episodes. Although outcrop relations indicate the pipes could have formed at a burial depth of less than a kilometre (Permian), recent isotope studies of the calcite cement in the breccia indicate precipitation from high temperature formation fluids (>120°C) suggesting a much later event at over three kilometres burial depth.

The resulting reservoir-analogue model quantifies the 3D breccia-pipe facies architecture and, by proxy, the porosity-permeability architecture. The basin-scale model constrains the relationship to basin structure interpretable from 3D seismic. Such studies are fundamental for understanding karst and collapse processes, and yield petrophysical models that can be applied predictively to hydrocarbon E&P in selected paleokarst settings.

 

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.