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 Correlation of High-Resolution Sequence Stratigraphy and Mechanical Stratigraphy for Improved Carbonate Reservoir Characterization

Eberli, Gregor P.1; Al Kharusi, Layaan 2; Morettini, Elena 3
1 University of Miami, Miami, FL.
2 Oxy, Muscat, Oman.
3 Dirección Exploraciòn y Desarrollo de Negocio, Repsol-YPF, Buenos Aires, Argentina.

The assessment of fracture patterns in the subsurface is a difficult task because seismic data do not resolve fractures and information from cores and boreholes is volumetrically limited. Examining the fracture pattern within a high-resolution sequence stratigraphic framework is a tool to establish mechanical stratigraphy for a better estimate of the fracture distribution within the reservoir.

In shallow-water carbonates, high-frequency sea-level changes produce a stratigraphic succession of depositional cycles here called genetic units. A genetic unit consists of a transgressive and a regressive hemicycle that differ in facies content and often diagenetic overprint. Fracture analysis within these genetic units reveals that generally hemicycles have different facture behavior, resulting in two mechanical units per genetic unit. For example, in the Lower Mississippian Madison Formation in the Sheep Mountain anticline, many dolomitized transgressive hemicycles display a higher fracture density than the calcareous regressive hemicycles. As a result, the stratigraphic units boundaries and the turnaround from transgressive to regressive hemicycle are also mechanical unit boundaries. Likewise in the Pennsylvanian mixed carbonate/siliciclastic strata of the Paradox Basin, 90% of the genetic unit boundaries coincide to mechanical boundaries. This pattern is observed in the limb of the Raplee anticline and in the flat-lying strata at Honaker trail, indicating that mechanical boundaries have a good correlation to high-resolution sequence stratigraphic boundaries, independent of strain. In the Calcare Massiccio Formation, Italy, high-resolution sequence stratigraphic cycles determine mechanical heterogeneities producing fracture density variations up to 80% or more within the same cycle.

The results from these outcrop studies show that fracture distribution within mechanical units are predictable within a framework of high-resolution sequence stratigraphy. Consequently, a core-derived sequence stratigraphic analysis can help identify the mechanical layering in the subsurface. For example in the Fahud field of Oman, the integration between hierarchies of sequence stratigraphic units and fracture systems has proven to be crucial to explain the distribution of flow and mechanical units, and ultimately guided reservoir exploitation.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009