--> ABSTRACT: Characterization of Subsurface Syndepositional Fractures in Carbonates Using Borehole Image Logs, by Zarian, Pedram; Guidry, Martha; Cavallerano, Edward; #90135 (2011)

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Characterization of Subsurface Syndepositional Fractures in Carbonates Using Borehole Image Logs

Zarian, Pedram 1; Guidry, Martha 1; Cavallerano, Edward 1
(1)Geoscience, Baker Hughes Inc., Houston, TX.

Syndepositional fractures are a significant component of many carbonate systems. These fractures are observed in numerous carbonate reservoirs around the world, and their presence definitely challenges our classic view of fractured reservoirs. Syndepositional fractures form pathways for diagenetic fluid flow in early stages of carbonate lithification. As these pathways are exposed to constant modification by different paleokarst processes, they inherit certain attributes that facilitate their identification. The very few studies that have been published on the characterization of syndepositional fractures were largely focused on outcrop analysis; while identification of these fractures in subsurface has since become very challenging. Borehole image logs are one of the leading technologies for the identification and characterization of subsurface geological features and are well suited for studying such fractures.

We have examined the characteristics of several karst-modified syndepositional fractures on a number of borehole image examples from different carbonate reservoirs. Various fracture attributes such as orientation, fill material and morphology, are studied in depth. Moreover, subtle textural variations resolved on the borehole image logs allowed determination of likely sedimentary architecture which, in turn, helped in the interpretation of the paleokarst processes associated with syndepositional fractures. Where possible, comparisons with outcrop analogs are presented and discussed in detail.

The results of this study demonstrate that syndepositional fractures are characterized on borehole image logs by non-planar to semi-planar morphology, moderate to steep dip magnitudes, varying fracture apertures and heterogeneous fill material (such as breccia, cement, and siliciclastic infilling). The fracture analyses are conducted with a specific focus on determining fracture trends and their relationship to the platform-margin trajectory, as syndepositional fractures are commonly oriented parallel to the platform margin. The illustrated criteria for discrimination between tectonic and atectonic syndepositional fractures can consequently help with the predictions of diagenetic patterns affecting reservoir properties in subsurface.

 

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