The Geological Society

Cratonic Basins - the missing data

Outcrops of the fluvial sediments of the Silurian Jaicós Formation in the Capivara National Park on the SE margin of the Parnaiba basin. (Famous Brazilian geophysicist, Vander Andrade, in the foreground).

Mike Daly* on a BP-underwritten basin analysis project that aims to improve our understanding of how cratonic sedimentary basins form.

Geoscientist 22.04 May 2012

Absence of high quality, regional, deep crustal seismic reflection data is a significant constraint on our understanding of cratonic sedimentary basins, and increasingly fuels a controversy around the subsidence mechanism of these large basins, the tectonic processes driving their formation and also the reasons for the remarkable variation in the scale of the oil and gas resources they may contain.

To inform this debate, BP Exploration Plc (through BP Energy do Brasil) is underwriting an integrated basin analysis project on the Parnaiba cratonic basin of NE Brazil. The project will combine deep crustal seismic reflection data, seismology and geological fieldwork to study the evolution and driving mechanism of this classic cratonic basin. It will also draw in partners from a range of universities.

Image: Outcrops of the fluvial sediments of the Silurian Jaicós Formation in the Capivara National Park on the SE margin of the Parnaiba basin. (Famous Brazilian geophysicist, Vander Andrade, in the foreground).

Cratonic basins: petroleum resources and exploration maturity. Note the immaturity of the South American and African basins compared to those of North America and Russia.

CONTEXT

Image: Cratonic basins: petroleum resources and exploration maturity. Note the immaturity of the South American and African basins compared to those of North America and Russia. See below for comparison.

There remains little doubt that lithospheric extension followed by mantle cooling is the primary driving mechanism of the world’s Phanerozoic rift basins. Regional seismic reflection data showing rotated fault blocks and stratigraphic expansion into faults, followed by relatively passive thermal subsidence demonstrates this repeatedly.

Similarly convincing is the conclusion that the flexural loading of a visco-elastic lithosphere is the primary driving mechanism behind the wide (~100km) foreland basins that develop adjacent to Phanerozoic mountain belts. In this case, regional seismic reflection data characteristically show a sedimentary basin thickening markedly towards a mountain range, with sedimentary depocentres younging away from the mountains.

Far less clear is the driving mechanism of the world’s cratonic basins. Characteristically these basins are large (0.5 – 1.5 x106km²) with a present day sub-circular shape that does not necessarily represent their original basin form. They are developed on relatively thick, Precambrian lithosphere and typified by negative Bouger gravity anomalies. Sedimentary fill varies considerably in thickness (2-10km) and is typically of shallow marine, paralic and continental sediments deposited over long periods. Subsidence curves for these basins are poorly constrained before the Mesozoic, but generally show slow and continuous subsidence for several hundred million years.

Image: Cratonic basins: petroleum resources and exploration maturity. Note the immaturity of the South American and African basins compared to those of North America and Russia. For comparison with above.

More globally, the poor age-constraints on the initiation of cratonic basins allow their origins to be tied to the breakup of the Late Precambrian supercontinent. The linkage with a large continental mass, its thermal consequences and its subsequent break-up, remains highly speculative, but nonetheless potentially a part of this complex and subtle puzzle.

OIL & GAS

From the perspective of resources, there appears to be great variation in the oil and gas deposits developed in cratonic basins. In North America, the Williston and Michigan basins can boast thousands of wells and several billion barrels of oil discovered. Recent developments in horizontal drilling and hydraulic fracturing have highlighted the oil potential of the Late Devonian Bakken Shale Formation of the Williston basin.

In contrast, the cratonic basins of South America and sub-Saharan Africa generally have far fewer wells and much less oil discovered. Does this difference simply reflect their exploration maturity, or something fundamentally missing in their geology, such as a prolific source rock or insufficient heat flow for maturation? The question remains open.

Image: An integrated gravity, magnetic and structural basement image, highlighting the main cratonic basins of Brazil.

A number of driving mechanisms have been proposed for the formation of these large basins. Hartley and Allen¹ identified nine postulated driving mechanisms in the literature, pointing out the profusion of hypotheses and the lack of clear understanding of these basins. Hypotheses range from modifications of the lithospheric stretching and thermal contraction model to sub-aerial erosion due to thermal uplift followed by sediment load driven subsidence. In line with this history, recent publications focussed on the formation of the Congo basin, have reached markedly different conclusions. Crosby et al. ² argue for a protracted thermal subsidence history based on a Cambrian rift event; in contrast Downey & Gurnis³ argue for subsidence driven by a high-density object deep within the lithosphere.

The driving mechanism controversy is fuelled by the poor seismic imaging of cratonic basins and a consequent lack of understanding of their deep crustal structure. Such imaging could illuminate as yet unknown rift structures, or show conclusively that such rifting is absent. It would also improve our understanding of the depth, stratigraphy and structural history of these basins and their resource potential.

Gravity and lithospheric thickness in South America. The Parnaiba basin is characterised by a negative free air gravity anomaly and relatively thick lithosphere. (Data care of Professor D. McKenzie, Cambridge University)

Image: Gravity and lithospheric thickness in South America. The Parnaiba basin is characterised by a negative free air gravity anomaly and relatively thick lithosphere. COmpare with Fig 3B below (Data courtesy of Professor D. McKenzie, Cambridge University)

The Parnaiba basin analysis project will comprise three elements:

Acquisition of a ~800km seismic reflection and refraction profile with the intent to image the basin’s stratigraphic architecture and geometry, basement topography and deep crustal structure. Authorisation to acquire the line must be granted by the Brazilian ANP (Agência Nacional do Petroleo);
Examination of deep crustal and mantle structure through seismology and igneous petrology;
Creation of an integrated geophysical and geological understanding of the dynamic structural and thermal evolution of the Parnaiba basin.

SUBSIDENCE

The Parnaiba basin was chosen for a number of reasons. First, the basin has many of the features believed to be typical of a cratonic basin. It is sub-circular in present-day outline; formed on relatively thick lithosphere with a negative free air gravity anomaly; apparently has its origin in the Late Precambrian to Early Paleozoic, with long and slow subsidence through to the Tertiary (interspersed with periods of uplift and erosion); and is poorly understood in terms of formation and resource potential⁴.

Image: Gravity and lithospheric thickness in South America. The Parnaiba basin is characterised by a negative free air gravity anomaly and relatively thick lithosphere. (Data courtesy of Professor D. McKenzie, Cambridge University)

Second, it is relatively accessible both for seismic acquisition and geological fieldwork. Outcrops around the margin of the basin are excellent and offer a good chance to tie the regional seismic data to outcropping chronostratigraphic equivalents. Third, the basin has only been lightly explored with fewer than 30 exploration wells and 25,000 line-km of 2D reflection seismic. Recent gas discoveries have been announced, and both oil and gas shows have been recorded in the past. However, the basin remains far from an established petroleum province.

Regionally, the Parnaiba basin has potential linkages with the other cratonic basins of Brazil and, in the context of Gondwana, the major African cratonic basins of North Africa and the Congo. As such, we believe that it represents an excellent laboratory to examine the fundamental driving mechanism of cratonic basins in general and to deepen the understanding of the basin-forming process and the controls on petroleum resource potential.

The project, expected to commence in early 2012, will conclude in 2015. It will involve collaboration with a number of universities from Brazil, Britain and the USA. The underlying philosophy will be to drive a truly integrated geological view of this accessible basin and use that to further our understanding of its resource potential and the fundamental driving mechanism of cratonic basins generally.

A Lower Paleozoic South. America/Africa reconstruction, showing a potential Devonian (Frasnian) coastline and associated depositional environments. Note the implied widespread nature of the marine basin beyond present day basin outlines. (Palegeog

Image: A Lower Paleozoic South. America/Africa reconstruction, showing a potential Devonian (Frasnian) coastline and associated depositional environments. Note the implied widespread nature of the marine basin beyond present day basin outlines. (Palegeographic reconstruction care of BP Exploration Plc).

References

Hartley, R & Allen P A 1994: Interior cratonic basins of Africa: Relation to continental break-up and role of mantle convection, Basin Res., 6, 95-113
Crosby, A C et al. 2010: Structure and evolution of the intracratonic Congo Basin, Geochem Geophys Geosyst , 11, Q06010, 20pp
Downey, N J , & Gurnis M 2009: Instantaneous dynamics of the cratonic Congo Basin, J Geophys Res , 114, B06401, 29pp
Milani, E J & Milani P V 1999: An outline of the geology and petroleum systems of the Paleozoic interior basins of South America, Episodes, 22.03

* Mike Daly is the Executive Vice President for Exploration in BP.

Geoscientist Online

Bruce Yardley appointed Chief Geologist

Chartership news

Climate Change Statement Addendum

Cracking up in Lincolnshire

Critical metals

Déja vu all over again

Done proud

Earth Science Week 2014

Fookes celebrated

Geology - poor relation?

Nancy Tupholme

Power, splendour and high camp

The Sir Archibald Geikie Archive at Haslemere Educational Museum

Top bananas

Cratonic Basins - the missing data