Sedimentary Structures, Facies Analysis and Graphic Sedimentary Logs

Understanding how sedimentary structures form is a powerful tool for interpretation of the environment of deposition.

Grain size and sedimentary structure  Lithofacies

Facies Analysis enables the depositional environment to be interpreted.

Process:

1) Describe grain size and sedimentary structures in a vertical succession (e.g. core) in a Graphic Sedimentary Log

2) Name the lithofacies

3) Analyse the facies by looking for facies associations and facies successions

Cross‐bedded sandstone lithofacies

Facies Associations = several facies that occur in combination. Typically these represent one depositional environment. But, few individual facies are diagnostic of an environment.

Facies Successions = facies associations with a characteristic vertical order.

Meandering River Faciesassociation

C b dd d S d Channel Fill Facies

Floodplain FaciesSuccession

Cross‐bedded Sand Lithofacies

Channel Fill FaciesSuccession

Graphic Sedimentary Log

Walther’s Law (1894) states that two different facies found superimposed on one another and not separated by an unconformity, must have been deposited adjacent to each other at a given point in time

The aim of this practical exercise is to familiarise yourselves with graphicalsedimentary logs. These are a fundamental tool in sedimentary and petroleumgeology, and are an effective way of enabling a sequence of sedimentary deposits tobe described and the depositional environment to be interpreted. They are thestandard way of communicating sedimentologic observations to other people, andare often presented alongside geophysical log data from wells in the petroleumindustry, such as neutron‐density and gamma logs. We will be using sedimentologicy, y g g g glogs through the remainder of this course, so make sure you know how to read them!

Practical Exercise – Part 1: Drawing a Sedimentary Log

1) From the information provided, draw a graphic sedimentary log for the section describeddescribed.

2) Classify according to lithofacies(devise your own lithofacies classification, appropriate for this logged section)

3) Interpret the depositional processes involved• We will focus on depositional environment interpretations tomorrow

h d h f h h h d h b k dFurther Reading – The two papers from which these data have been taken provide a good example of lithofacies description and depositional environment interpretation (Ghazi and Mountney, 2009) and subsurface sedimentary facies description (Ghazi and Mountney, 2010).

Excerpt from a logged section; thickness measured from base of section up (base = 0 m; this excerpt is from 68 – 100 m):

Unit 1: 68 – 73 m‐ See Plate 1. Grain size 1

Unit 2: 73 – 77 m‐ Grain size 2, Structure 1, erosive base

Unit 3: 77 – 80 m‐ Grain size 3, Structure 2

Unit 4: 80 – 86 mUnit 4: 80 – 86 m‐ Grain size 4, Structures 1, 2, 3 (bottom to top)

Unit 5: 86 – 88 m‐Grain size 5, Plate 2

Unit 6: 88 – 95 mG i i 6 Pl t 3‐Grain size 6, Plate 3 

Unit 7: 95 – 100 m‐ Plate 1, Grain size 1

Plate 1

1 cm

1 cm1 cm

Plate 2

11 m

Plate 3

1 cm

Examples of structure 1

Examples of structure 2

Examples of structure 3

Pl iPlan view

Cross‐section view

LITHOFACIESSTRUCTURES PROCESSINTERPRETATION

Examples of patterns and symbols used on graphic sedimentary logs