Print this pageFracture-Controlled Dolomite Reservoirs — From Outcrop Comparison Between Extensional, Compressional and Strike-Slip Settings
Fracture-controlled dolomites are increasingly recognised as globally important reservoirs. They occur in the Ghawar field (worlds largest oil field), and form a “hot” play in North America. Whilst best documented as occurring in association with transtensional-transpressional tectonics and MVT mineral deposits, they are also recognised in extensional (rift) and compressional (fold/thrust) settings. We describe two well exposed fracture-controlled dolomite reservoirs, one from an extensional setting (Suez Rift), and one from a compressional setting (Zagros Fold Belt), and compare them to strike-slip related dolomites (Canada). The aim of our study is to compare dolomite/reservoir development in these differing tectonic settings.
By definition, fracture-controlled dolomite reservoirs are interpreted to be directly controlled or strongly influenced by fractures. In this study, fractures are defined as faults, joints and stylolites. Importantly, fractures are also recognised as conduits for dolomitising fluids. Fluid inclusions studies indicate fracture-controlled dolomites are often, but not always, hydrothermal in nature, hence also termed High Temperature/Hydrothermal Dolomites (HTD).
In both the extensional and compressional example, dolomite bodies show discordant and stratabound development. Dolomite bodies are 100m to several km in width, have plume-like geometry, with both fracture and diagenetic contacts with undolomitized country rock. Dolomite “fingers” extend away from dolomite bodies along steeply dipping fracture zones, and as strata-bound bodies following permeable depositional facies. Vertical barriers to dolomitisation are low perm mudstones, below which dolomitising fluids moved laterally. Where these barriers are cut by fault/fracture zones, dolomitisation is observed to have advanced upwards, indicating that fractures were fluid migration conduits.
Resultant dolomite reservoir geometry is complex in 3D, but well connected. Dolomitisation caused porosity redistribution, enlargement, and formation of new porosity. An understanding of which fault/joint systems were active during dolomitisation, coupled with a facies control on stratabound dolostones, allows semi quantitative prediction/modelling of dolomite distribution in the subsurface.
Comparisons between extensional, compressional and strike slip settings indicate striking textural, paragenetic and outcrop-scale similarities, implying a common emplacement mechanism.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil