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Direct Paleo-overpressure Estimation, from Demonstration to Practical Application: The Sheep Mountain Anticline Example

Daniel, Jean Marc *1; Lacombe, Olivier 2; Callot, Jean-Paul 1; Bellahsen, Nicolas 2; Barbier, Mickael 1; Amrouch, Khalid 1; Beaudoin, Nicolas 2
(1) Structural Geology, IFPEN, Rueil Malmaison, France.
(2) UMR 7193, ISTEP, UPMC Sorbonne Universités, Paris, France.

Geomechanical concepts and models play an increasing role in geoscience workflows, from exploration to reservoir management. This implies that new equations and parameters govern our earth models. Such models therefore request new data to be controlled. When it comes to mechanics in particular, it is crucial to rely on observations that control paleostress magnitudes. In this presentation, we demonstrate how paleo-stress and paleo-pressures can be estimated in practice from the analysis of calcite twinning. This discussion is based on the study of an outcrop dataset, including structural and geochemical data to validate the proposed approach.

The studied outcrop is Sheep Mountain, an asymmetric basement cored anticline corresponding to a typical Laramide arch of the Bighorn Basin (Wyoming, USA). We show how paleo-principal stress magnitudes during the development of this fold can be estimated using the systematic analysis of calcite twins, a few rock mechanics data characterizing the carbonates from the Madison Fm outcropping in the anticline and the chronology of the main fracturing events defined using the standard tool of structural geology. This analysis demonstrates that the growth of Sheep Mountain anticline induces a brutal release of 15Mpa of overpressure accumulated before folding during layer parallel shortening.

This innovative direct quantification of paleo-overpressure is compatible with present day estimates of in-situ stress and overpressure in similar tectonic context. It is compared with the evolution of the fracture patterns over Sheep Mountain. In particular, we can demonstrate that the overpressure release is contemporary of the formation of a very pervasive fracture set connecting the whole sedimentary sequence. This connection is further compatible with the evolution of stable and Sr isotope geochemical signature of cements precipitating within the veins showing.

From a methodological point of view, we take advantage of this beautiful outcrop example to propose an operational workflow using mainly core data and log information to characterize paleo-stress and overpressure. We discuss the underlying hypothesis (paleo-depth hypothesis, calcite twin thresholds characterization) and the ongoing efforts to put this promising workflow on stream. From a geological point of view, this example is used to discuss the complex interplay between fracture network growth and overpressure build-up during the formation of fractured reservoirs.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California