Shallow-Burial Fractures in Steep-rimmed Carbonate Platforms: Outcrops in Canning Basin, NW Australia, as Analog for Tengiz Reservoir, Kazakhstan
Wayne Narr¹ and Eric Flodin²
¹Chevron Energy Technology Company, San Ramon, CA
²Tengizchevroil, Atyrau, Kazakhstan
Natural fractures have a strong impact on productivity in Tengiz field, which is one of several giant light-oil accumulations trapped in isolated, mid to late Paleozoic carbonate platforms in the Pricaspian Basin of Kazakhstan. Outcrop analogs are particularly important for understanding reservoir fracture systems because many aspects of fracture character (height, length, connectivity, etc.) are impossible to measure with subsurface data.
The Devonian margin of the Canning Basin in NW Australia presents a well-exposed outcrop analog for steep carbonate margin and slope deposits of Tengiz field. Fracture data gathered from Tengiz core and image logs suggest affinity to fractures in the Canning outcrops in terms of origin, orientation, and range of density (surface area/volume). Inclusion of additional information – gained through outcrop study – into reservoir fracture descriptions leads to improved understanding of stratigraphic influence on their occurrence and character, and thus improved ability to predict fracture occurrence.
Shallow-burial (SB) fractures – those formed in carbonate strata prior to significant burial, including neptunian fractures (i.e. fractures open to the sea-floor) – have the greatest impact on reservoir productivity at Tengiz field. The characteristics and formation mechanisms of SB fractures are generally less well established in the literature than deep-burial or “tectonic” fractures. In contrast to tectonic fractures that generally form in the presence of a strong guiding tectonic force, SB fractures form in the near-surface environment under the dominant influence of gravitational forces, and thus tend to align with respect to local topography and express less-planar traces. Furthermore, the host rock mechanical properties may differ substantially from the present state, and the fractures have enjoyed the full diagenetic history experienced by the host. Thus, SB fractures are distinguished based on their orientations, diagenetic character, and fill.
Both at Tengiz and the Canning Basin SB fractures dip steeply and strike dominantly parallel and/or normal to the local orientation of the depositional margin, which indicates the importance of depositional morphology on their development. The highest fracture density is in boundstone-dominant facies of the platform-margin to upper slope environment, which were lithified and able to sustain fracture penecontemporaneous with deposition. Dissolution by corrosive fluids following burial led to enlargement of fracture apertures, which range from small to cavernous. In Tengiz field, cavernous fractures pose high lost-circulation risk as well as the reward of highly productive wells.
AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012