QUANTITATIVE BASIN ANALYSIS
Definition and Objectives

. . . In a nutshell: To determine the tectonic significance of the preserved stratigraphy within a basin or margin. . . .


Figure explanation: The figure above compares and contrasts the chronostratigraphy of the Congo and Cabinda margins illustrating the link between extensional phases and stratigraphic development of the margin. By quantitatively modeling the distribution, thickness, and age of the preserved sediment packages that comprise the margin, we can define the tectonics controlling where accommodation is created and destroyed and thus the tectonic significance of the preserved stratigraphy. Bold lines mark the onset of rifting for the three main episodes of tectonic deformation. Sandstones are shown in yellow, shales in dark green, light green represents a composite of sandstones and shales, evaporites are shown in pink, and finally, limestones are shown in blue. Stratigraphic variability between the sand- prone Congo and shale-prone Cabinda margins is consistent with the Congo margin being part of a low-relief accommodation zone while the Cabinda margin is within a major border fault segment.

QUANTITATIVE BASIN ANALYSIS: Definition

The combination of quantitative and isostatic basin modeling with the principles of seismic sequence stratigraphy.

- Quantitative basin modeling defines the vertical motion of the lithosphere in response to extension and compression, sedimentation, eustatic sea-level variations, in-plane force variations, erosion, and magmatic underplating. In other words, we can predict the creation and destruction of accommodation as a function of space and time;

- Seismic sequence stratigraphy uses the stratal patterns and lithofacies preserved in basins to define where and when accommodation space is created and destroyed and how that accommodation is filled by sediments as a function of time. Consequently, seismic sequence stratigraphy tells us exactly how to fill the accommodation created by the deformation of the lithosphere. For example, the infilling sediments might onlap earlier basement structures, thereby diagnostic of along axis sediment transport, or downlap as part of a progradation system, or drape form a pelagic or hemi-pelagic drape over the pre-existing structures. In addition, the stratal patterns help to define how much of the available accommodation is being infilled as a function of space and time within the basin. Quantitative modeling of the lithospheric deformation using the observed stratal patterns and well data allow us to constrain in a rather rigorous way the tectonics responsible for the development of the basin.

QUANTITATIVE BASIN ANALYSIS: General Objectives

1) Reconstruct the multi-phase tectonic history responsible for the development of a margin from the preserved stratigraphy (in particular, the stratal patterns),

2) Determine the distribution, magnitude, and timing of the tectonic events responsible for the margin (extension, compression, and inversion) in addition to the relief, timing, and interaction of hinge zone and topography development across or adjacent to the basin;

3) Determine the tectonic significance of the preserved stratigraphy within the basin using a knowledge of the first-order processes responsible for both the deformation of the lithosphere and the transport and deposition of sediments,

and 4) calculate the basal heat flow, source maturation, and regional migration pathways consistent with the reconstructed tectonic history.

QUANTITATIVE BASIN ANALYSIS: Exploration Objectives

The scientific and exploration objectives of QBA are to determine the regional, three- dimensional geometry of rift basins by generating a 3-D structural map of the rift-onset unconformity using a kinematic and flexural model for lithospheric deformation. The stratal patterns are critical in constraining the timing and distribution of extension. The modeled 3-D structural map are used to define:

1) drainage networks and thus sites of sediment input into the basin (i.e., identify sand-prone and shale-prone regions within the rift section),

2) a tectonic rationale for the regional, three-dimensional migration pathways within the rift system,

and 3) the flexural interaction between the footwall and hangingwall blocks of the various sub-basins and thus the degree of reworking of pre-rift and earlier syn-rift sediments.

QUANTITATIVE BASIN ANALYSIS: How does it work?

Given structure maps indicating the general position of the main basin bounding faults and estimates of syn-rift stratigraphic thickness, the model can determine if these observables are consistent with the potential field data. Perhaps more importantly, iterating between the observed gravity and the fault geometries leads to an estimate of the three-dimensional rift basin architecture. Sedimentation is assumed to fill the available accommodation as constrained by the biostratigraphy and observed stratal patterns. The model calculates the isostatic response to lithospheric unloading due to extension and subsequent sediment loading and erosional unloading. The gravity effect of the resulting basin and lithospheric structure is calculated and compared with the observed. Observed stratal relationships are used to help constrain the generation and destruction of accommodation due to faulting and rift block interactions.