AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

Datapages, Inc.Print this page

Hybrid margins and holistic exploration; The role of variable magma budgets in creating successful petroleum systems.

Abstract

The role of magmatism in relation to subsidence and heatflow on our passive margins and its impact on the ultimate success of a petroleum system is slowly starting to be absorbed by the industry. In the distal sectors of the Atlantic passive margins magmatism is becoming a frequently cited observation. Its existence relates to the final stages of breakup and has been the basis for defining two endmember margin types based on the actual volume of magma generated at breakup; the so-called Magma-poor and Magma-rich margins. Many beautifully illustrated examples of each end-member margin are presented in the literature detailing their characteristic features and highlighting differences in their evolution. However, this paper will investigate the middleground between these two end-member type margins. In contrast to their end member siblings, these margins are poorly described and are often vary in their form. We term these margins “hybrid margins”, and here describe some of the features and exploration challenges presented when faced with variable magmatic budgets. Our examples will come from proven world class hydrocarbon provinces around the Atlantic including the Sergipe Alagoas-North Gabon/Rio Muni Conjugate margin. Unlike magma rich margins, it does not appear that hybrid margins developed from a large igneous event or hotspot. Yet many features we associate with magma rich margins can be observed (SDRs, paleo-volcanoes, sills and dykes in the overburden), however the source of the magma remains an enigma. In some cases, the magmatism is episodic, occurring during the late stages of break-up, immediately after break-up and well after break-up, covering a period of 10s of Ma, without an obvious mechanism. The Atlantic hybrid margins are often associated spatially with cratonic edges (i.e ancient foldbelts), leading to the theory that it is the highly deformed and reworked rock present at the edge of a craton that is more prone to melting and significantly contributing to the magmatic budget. The complexity in distribution of magma in a hybrid margin means that care needs to be taken when approaching exploration as classic approaches, techniques and technology can often be misleading. One of the hardest tasks in a hybrid margin will be describing the composition and thickness of the crust and the landward position of true oceanic crust. Public available gravity data does not provide the resolution to see the sharp and rapid variations, higher resolution marine gravity and magnetic data is a must. In addition, highly thinned, infiltrated or altered continental crust approaches densities similar to oceanic crust, so assigning densities for gravity inversion can be tricky. Unusual subsidence has been observed along hybrid margins. Notably, shallow water conditions can coexist in the distal domain with deep water sedimentation in the proximal domain, an observation that contradicts many of our classic rifting models and makes extrapolation of historical well data extremely problematic. In additional to creating unusual subsidence the magmatism also contributes to heatflow for the necessary maturation of the shallow water marine source rocks. We will show examples from the Atlantic where the role of magmatism associated with the final break up has contributed to the creation of world class Petroleum System in the distal domains. We will show the spatial relationship and variations in volume of magmatic budget in addition to well and seismic examples highlighting its role in play based exploration.