High-Resolution Imaging of a Karstified Carbonate Reservoir Analog, Late Carboniferous, Spitsbergen
Walter H. Wheeler1, David Hunt2, Jan Steinar
Rønning3, Einar Dalsegg3, and Alvar Braathen4
1 Center for Integrated Petroleum Research, University of Bergen, N-5007 Bergen,
Norway
2 Norsk Hydro Research Centre, Bergen, Norway
3 Geological Survey of Norway, N-7491 Trondheim, Norway
4 Center for Integrated Petroleum Research, Unifob, University of Bergen, N-5007 Bergen, Norway
Karstified platform carbonates are major exploration targets, yet their
complex reservoir architecture poses significant exploration and production
challenges. We present 3D reservoir-analog images combining outcrop studies and
high-resolution geophysical images of the geobody interior. Foci include the
geometry, frequency and interconnectivity of breccia bodies, dolines and faults
at reservoir and seismic scales, as well as the relation between karst formation
and deformation. The study area is a kilometre-wide mesa with cliff-side
exposures of karst breccias, faults, and flexures in Late Carboniferous platform
carbonates. The geophysical imaging consists of ground-penetrating radar (GPR)
profiles, reaching 50 meters into the outcrop, and electrical
-
resistivity
tomography (ERT) profiles, modelled to 120 m depth. Hence the GPR, ERT and
outcrop ground-truth constrain the karst geometry at scales relevant to both
reservoir evaluation and seismic interpretation. For example, a cliff-side
exposure shows a 100m-wide, “V”-shaped, flexural collapse structure in cross
section. Closely-spaced (12m) GPR profiles adjacent to this cliff exposure show
the “V” to be one of a family of adjacent, doline-like circular depressions. The
“V” and adjacent depressions lie towards the upper side of a monocline
(500m-wavelength). The toe of this monocline is characterised by local
axis-parallel faulting (imaged using GPR) coincident with a steep, deep
low-resistivity zone (imaged using ERT) suggesting karst. Elsewhere, a cliff
face along a flat part of the mesa exposes a field of breccia pipes. Here,
coincident GPR and ERT profiles show lower strata locally disrupted by chaotic
reflections (GPR image), and a vertical, pipe-like resistivity structure (ERT
image).
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005