Petroleum Systems Models: visual representations of data that tell us something about petroleum potential and/or occurrences in a sedimentary basin.

Key aspects: burial history, thermal history, maturation predictions, migration pathways and much more.

Basin Modeling Week 2: Subsidence and Geohistory Modeling (part one)

1. ReviewIsostacy (Airy/Pratt). Local compensation Principle of isostatic compensation – applied to basinsSubsidence mechanisms

2. Geohistory Analysis: what is it and why is it useful?

3. Early approaches to geohistory analysis

4. How-To: Geohistory analysis made easy (1D/backstripping)

Readings – all are online:Van Hinte, 1978Angevine et al., 1990. AAPG Continuing Ed. Course Notes, available online at:

http://faculty.gg.uwyo.edu/heller/shortcourse(90).htmAllen and Allen textbook Ch. 2, 9

Homework (due next week):Create your own 1 page ‘cheat sheet’ or personal reference guide for geohistory

modeling.

(Angevine et al., 1990)

Fundamental cause of subsidence = isostacy.

Isostatic compensation: below a certain depth, asthenosphere of equal density underlies both columns (amount displaced is related to load). Above this depth, both columns must have equal weights.

Mass column 1 = Mass column 2

So, crustal topography is controlled by density and thickness variations of the crust.

1) Review

(Angevine et al., 1990)

Air (p=0 g/cc): Z=0.9 kmWater (p=1 g/cc): Z= 1.3 kmSediment (p=2.3 g/cc): Z=3 km

Isostatic models (local compensation): density vs thickness variations.

Geohistory modeling assumes local Airy isostatic balance.

Review cont’d

Regional isostacy and flexural loads (subsidence)

Review cont’d

Review: subsidence mechanisms

2) Intro to Geohistory Analysis

What is a geohistory diagram?

A graphical representation of the vertical movement of a stratigraphic horizon in a sedimentary basin as an indicator of subsidence and uplift history in the basin since the horizon was deposited (Angevine et al., 1990).

3) Early attempts

Van Hinte, 1978

Van Hinte diagram

Van Hinte diagram

Example: generating a geohistory diagramA) : What data do you need to collect?

• Formation Information: thickness, lithology, age

• Pet Sys Components

• Heat flow – modern vsancient, paleogeography

Mean geothermal gradient in the upper 1 to 4 km of the crust in the U.S. (Blackwell et al., 1996).

Geohistory modeling with missing time

??

Paleobathymetry

A) Gather (good) data

B) Create uncorrected sediment accumulation curve

B) Create uncorrected sediment accumulation curve

B) Create uncorrected sediment accumulation curve, then

C) Make incremental decompaction corrections to get true ‘original’ thicknesses of each section

What controls compaction?

Porosity assumptions

C) The ‘correct’ method of decompaction.

(use iteration to solve)

Assumptions: porosity (phi) decreases exponentially with depth, and volume of grains (1-phi) does not change

C) The ‘correct’ method of decompaction.

(use iteration to solve – 23% difference in decompacted thickness)

Step To (solved) To (approx) difference1 928.68 800 128.682 970.4502 928.68 41.770223 983.4414 970.4502 12.99124 987.4268 983.4414 3.9854645 988.6443 987.4268 1.2174886 989.0158 988.6443 0.3714377 989.129 989.0158 0.1132758 989.1636 989.129 0.0345419 989.1741 989.1636 0.010532

10 989.1773 989.1741 0.003211

Solving for Unit 6 (shale, 800 meters present thickness) after removing unit 7

To=-1000*EXP(-5*10^-4*To)+1599

Approximation Method for compaction correctionNow (‘new’)

Decompacted(‘old’ or original)

ΦN = Φo e (-c z)

C) Decompaction equations: Approximation method (Van Hinte).

To = [(1-ΦN)TN]/[1-Φo]

Phi - Calculate or est from chart:ΦN = Φo e (-c z)

* Assumptions and equations…

Decompactionexercise

(approximation method)

(or use the chart ->)

To = [(1-ΦN)TN]/[1-Φo]

Phi - Calculate or est from chart:ΦN = Φo e (-c z)

C) Now we have a decompacted sediment accumulation curve (sort of)What about compaction due to water load?

Corrected cumulative subsidence curve by accounting for paleobathymetry as well as decompaction

Can predict basin depths (Z) filled with materials of variable density (air, water, sediment). Z Air (0.9), Z Water (1.3), Z sediment (3) so Zw = 1.5 Za; Zs = 2.3 Zw. Ratios between Za, Zw, and Zs remain the same…

D) Corrections for subsidence due to sediment + water load = backstripping

The backstripping equation

How do we determine variable density of sediment load through time?

Following Steckler and Watts (1978)

Example of backstripping exercise

Residual subsidence curve

Decompaction exercise (approximation

method)