HGS International and HGS Joint Dinner Meeting - <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />December 15, 2003
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Styles, Mechanisms and Hydrocarbon Implications of Syndepositional Folds in Deepwater Fold Belts;
Examples from Angola and the Gulf of Mexico
Frank J. Peel, BHP Billiton Petroleum, Houston
Abstract:
This presentation combines recent published material with new ideas as to provide a review of how the structural geology of deepwater fold belts influences the distribution of hydrocarbons within them. How do deepwater foldbelts differ from orogenic fold belts? What factors control the location of the fold belt? What is the significance of early-formed precursor folds? Why are these factors important in exploration for hydrocarbons?
First, we consider the significant differences between passive-margin and orogenic fold belts, then, the application of Coulomb wedge theory to passive margins (to explain where and why fold belts form), and lastly, explore a critical factor-- whose significance has only recently been recognized-- namely the influence of early-formed folds on the later-formed large structures, and how hydrocarbons are trapped within them.
Part 1: Comparison of passive margin fold belts with orogenic fold belts
Fold and thrust belts occur primarily in two settings: either linked to an orogenic belt forming due to plate convergence, or in the compressional toe of a system of gravity-driven movement on passive margins. While mixed-mode fold belts also exist, and other scenarios for fold belt formation are also observed, it is instructive to compare and contrast the two end members and consider the implications of the differences for the hydrocarbon systems, which can trap in either setting.
Orogenic fold belts
The ultimate driving mechanism of orogenic fold belts (including accretionary prisms) is relative plate movement. The rate of convergence is effectively fixed, and the main variable affecting the rate of movement in the frontal thrust belt is the partitioning of shortening between the frontal thrust system and contraction within the body of the orogenic belt. Shortening will occur whether or not there is a good decollement. The nature of the decollement does, however, have a strong influence on the structural style. The total shortening in the orogenic fold belt can be 100s of km, and, as a result, most of the thickening of the orogenic wedge occurs by tectonic thickening of the accreted mass.
Passive-margin fold belts
The ultimate driving force of passive-margin fold belts is gravity. This may take the form of gravity sliding, driven by the existing slope of the margin, plus continued tilting (as seen in the outer Kwanza basin, and the GOM Cretaceous/Paleogene strata), or as gravity spreading of the sediment wedge (like in the Niger Delta, Africa, and Neogene GOM). The rate-limiting factors are the rheology of the wedge, decollement level, and the rate of sediment input to the shelf and upper slope. As a result, passive margin fold belts are commonly intimately linked to the pattern of depositional systems on the margin.
While most passive margin fold belts shorten at slower rates than