Error Estimation in Decompacted Subsidence Curves

Error Estimation in Decompacted Subsidence Curves

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Error Estimation in Decompacted Subsidence Curves1

D. Waltham,2 C. Taberner,3 and C. Docherty4

ABSTRACT

Subsidence curves, based on decompacted sediment

thicknesses, are generally displayed without

indicating depth or age errors. This makes their

interpretation ambiguous, leading to incorrect

identification of subsidence pulses and incorrect

identification of periods of steady subsidence.

Error analysis should also form part of thermal

maturation studies based on such subsidence

curves. Formal error analysis of subsidence curves

is a complex problem because of the iterative and

nonlinear nature of the decompaction calculations.

This paper presents an alternative

approach, based on Monte Carlo simulation, that

allows subsidence curves to be calculated along

with error estimates. We also show that identification

of subsidence pulses and identification of

periods of steady subsidence cannot be made on

the basis of such subsidence curves, even when

errors are shown, because errors on successive

points on the subsidence curves are correlated.

Instead, an ensemble of subsidence rate curves

must be calculated at the same time as the ensemble

of subsidence curves so that a mean subsidence

rate curve can be calculated along with

associated error estimates.

INTRODUCTION

Subsidence curves, based on decompacted sediment

thicknesses (Sclater and Christie, 1980), are

important aids to understanding the evolution of

sedimentary basins; however, such curves cannot

be meaningfully interpreted without error estimates.

Consider Figure 1, which shows a subsidence

curve based on decompacted sediment

thicknesses in the San Sebastia section of the easternmost

south Pyrenean foreland combined with

bathymetry estimates derived from facies analysis

(Taberner et al., 1999). With the curve as plotted, it

is impossible to say whether the apparent increase

in subsidence rate near point A on Figure 1 or the

apparent uplift near point B are genuine or simply

the result of inaccurately determined depths, poorly

constrained ages, and unrecognized natural variations

in decompaction parameters. Note that burial

depths are inputs into thermal maturation studies

(Lopatin, 1971; Waples, 1980), and so error estimates

for these depths should form an important

part of risk assessment when investigating the

prospectivity of a proposed new hydrocarbon play.

In this paper, we show how to analyze the data

from Figure 1 in such a way that meaningful statements

can be made concerning whether subsidence

rates change through time. As such, it is a

methods paper that happens to use the Taberner et

al. (1999) data. For a fuller discussion of the geological

setting and an interpretation of the data, interested

readers should consult the original reference.

Note that these data are taken from a field-derived

cross section, but that the methods we discuss in

this paper are equally applicable to data taken from

wells or seismic sections.

Gallagher (1989) discussed subsidence curve

errors associated with uncertainties in compaction

parameters and, for the full backstripping problem,

uncertainties in sediment densities. He found,

AAPG Bulletin, V. 84, No. 8 (August 2000), P. 1087–1094. 1087

©Copyright 2000. The American Association of Petroleum Geologists. All

rights reserved.

1Manuscript received October 9, 1998; revised manuscript received

September 27, 1999; final acceptance January 15, 2000.

2Department of Geology, Royal Holloway University of London, Egham,

Surrey TW20 0EX, United Kingdom; e-mail: d.waltham@gl.rhbnc.ac.uk

3Institut de Cièncias de la Terra, CSIC, C/ Lluís solé I Sugrañes s/n,

Barcelona 08028, Spain.

4Institut de Cièncias de la Terra, CSIC, C/ Lluís solé I Sugrañes s/n,

Barcelona 08028, Spain. Present address: GX Technology, Lawrence House,

45 High Street, Egham, Surrey TW20 9DP, United Kingdom.

This work was funded by the EU Joule-Thermie project (project