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Integrating Fracture Systems into a Carbonate Sequence Stratigraphic Framework

Zahm, Chris 1; Kerans, Charles 2; Janson, Xavier 1
1 Bureau of Economic Geology, The University of Texas at Austin, Austin, TX.
2 Department of Geological Sciences, The University of Texas at Austin, Austin, TX.

Integrating fractures within a sequence stratigraphic framework increased the predictability of fractures in three dimensions. This study describes and contrasts the stratigraphic framework, fracture style, size and intensity in two outcrops of Albian-age carbonates of Texas. Both study areas were deformed as a result of extensional faulting. We compared the fault-related fracture response within a sequence stratigraphic framework to leverage the predictive aspects of systems-tract-specific facies, lithology and rock mechanical variations within this framework.

The Bulverde site is a platform interior succession, located approximately 25 km landward of the Glen Rose-equivalent (lower Albian) Stuart City Margin. Low amplitude base level changes produced a laterally continuous, vertically stacked set of subtidal facies/cycles that were bundled into a high-frequency sequence representing the highstand systems tract (HST) of the the Albian 6 and the transgressive system tract (TST) of Albian 7 high-frequency sequences.

The fracture systems at Bulverde form in proximity to an extensional fault system associated with a regionally mapped fault with 5.5 m of offset. Fracture intensity varies with both fault proximity and facies variations at the HFS system tracts scale.

The second exposure is from the Albian 19 HFS, developed along the northern margin of the Maverick Basin. These strata were tectonically modified by obliquely-sheared extensional faults along the Lower Pecos River. The succession consists of high frequency cycles of slightly retrogradationally stacked chondrodont-radiolitid buildups within the TST, overlain by thick grainstones during the HST and then capped by a regional exposure horizon.

In the high frequency sequences of both outcrops, HST facies have a higher proportion of grainer facies and more competent rock strength resulting into a lower overall fracture intensity, but the curvilinear fractures have a greater surface area (taller and longer). TST facies are thinner, muddier, and less competent leading to higher deformation intensity, but the fractures have less surface area (shorter in height and length).

Because of the fracture geometry, there is clear segregation of the fracture permeability between the HST fractures that are more capable of conducting fluid leading to higher permeability compared to the TST fracture permeability with fractures of less surface area resulting in only slightly increased fracture permeability.


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