Subsidence How to analyze it with Loadcap
Latterly, an interesting research paper, entitled “Alternative methods for calculating compaction in sedimentary basins” whose autors are Martiìn-Martìn M. and Robles-Marìn P., was published on Marine and Petroleum Geology.
In this paper, the authors propose alternative methods for calculating compaction in sedimentary basin and, in particular, suggest the use of Loadcap software.
On that basis, Geostru company wanted to tell more about the concept of subsidence and the usefulness of Loadcap for calculating compaction in sedimentary basin relating to subsidence itself.
What is Subsidence?
The term subsidence refers to a surface point sinking to a lower level applied to the Earth’s surface. Therefore, the subsidence is considered like a process of downward displacement of the ground generally due to insufficient support from beneath and/or superimposed load (Whittaker and Reddish, 1989; Marker, 2013) and it can arise from natural or artifical causes. Sediment compaction and the resultant subsidence are fundamental processes in sedimentary basins.
The importance of sedimentary basins
Sedimentary basin are regions formed because of tectonically induced long-term subsidence of the Earth’s surface in wich sediments accumulate to considerable thickness and be preserved for relatively long geological time periods (Einsele 2000; Miall 2000; Leeder 2011; Allen and Allen, 2013; Lee et al., 2019). These basins are of great importance not only, from the point of view of academic research but also for their resource potential (Allen and Allen, 2013; Bjorlykke, 2014; Lee et al., 2019).
To understand the internal framework of a basin, analysis and interpretetion of many different kinds of geoscientific dataset are required and studied (e.g. sedimentary petrology, lithostratigraphy, depositional facies, structural deformation, geophysics, etc.). It results mainly from interplay between volume and grain size of sediment supply (depending on erosion rate and source rock size) and the generation rate and spatial distribution of accomodation (Lee et al,2019; Fig.1). The generation of accomodation is controlled by tectonic subsidence and up-lift, global sea-level change and compaction (Allen and Allen, 2013).
Fig.1 – Sketch of a sedimentary basin; red arrows show the subsidence due to sedimentary loading (Lee et al.2019, modified).
How to perform subsidence analysis
The present-day stratigraphic framework (e.g. age, thickness, lithology) of a sedimentary basin fill does not show the primary thickness of each sedimentary layer because of the compaction effect (Van Hinte, 1978). Therefore, restoring thicknesses of the compacted layer using the techinque for decompacting sedimentary strata is important to start subsidence analysis.
The technique of Decompaction aims at reversing the progressive effects of sediments compaction by restoring of thickness of a sedimentary layer at any time and depth in the past (Allen and Allen, 1990; Fig.2).
Fig. 2 – Time-depth diagram. A) The time-depth model refers to the present framework without considering the compaction effect of underlying layers thicknesses (dotted red line depicts the sediment accumulation; red arrow shows backward path without compaction); B) Decompaction of each stratigraphic unit: restoring of thickness of the compacted layers through time (the red line represents the total subsidence; the direction path of decompaction is shown by the red arrow); C) comparison between A e B curves trend; D) 3D rapresentation of scheme B.
This technique is based on the equation (1) for porosity-depth relation trend (Rubey and Hubbert, 1959) wich shows exponential porosity decrease with depth (e.g. Van Hinte, 1978; Schmoker and Halley 1982; Bond and Kominz 1984).
Equation (1): ϕ_N =present-day porosity; ϕ_0= original porosity; c=costant for each lihology; z=present depth of burial.
Pertanto, per valutare la subsidenza totale, è necessario ripristinare lo spessore originale correlato allo spessore attuale come mostrato nell’equazione (2).
Equation (2): T0=original thickness; ϕ_N=present-day porosity; ϕ_0= original porosity; T_N=present-day thickness.
Taking into account the aforementioned, Martiìn-Martìn and Robles-Marìn (2020) use Loadcap for calculating compaction relative to subsidence analysis.
Why Loadcap
References
Allen P.A. and Allen J.R., 2013 – Basin analysis: principles and application to petroleum play assessment. Wiley-Blackwell, Oxford.
Bjorlykke, K., 2014 – Relationships between depositional environments, burial history and rock properties. Some principal aspects of diagenetic processes in sedimentary basins. Sediment. Geol. 301, 1–14.
Einsele G, 2000 – Sedimentary basins. Evolution, facies, and sediment budget. Springer, Berlin.
Lee E.Y., Novotny J. and Wagreich M., 2019 – Subsidence Analysis and Visualization: For Sedimentary Basin Analysis and Modelling. Springer Publishing Company, Incorporated, 1st edition.
Leeder M, 2011 – Sedimentology and sedimentary basins: from turbulence to tectonics. Wiley, Chichester.
Marker B.R., 2013 – Land Subsidence. In Bobrowsky P.T. (eds) Encyclopedia of Natural Hazards. Ecncyclopedia of Earth Sciences Series. Springer, Dordrecth.
Martìn-Martìn M. and Robles-Marìn, 2020 – Alternative methods for calculating compaction in sedimentary basins. Marine and Petroleum Geology 113. Miall A.D., 2000 – Principles of sedimentary basin analysis. Springer, Berlin.
Van Hinte, J.E., 1978 – Geohistory analysis: application of micropaleontology in exploration geology. AAPG (Am. Assoc. Pet. Geol.) Bull. 62, 201–222.
Whittaker B.N and Reddish D.J., 1989 – Subsidence: Occurrence, Prediction and Control. In Developments in Geotechnical Engineering, 56, ElsevierScience.
