Attribute Expression of Mass Transport Deposits in an Intraslope Basin- A Case Study.

Supratik Sarkar*, Kurt J. Marfurt, Belinda Ferrero Hodgson, Roger M. SlattConocoPhillips School of Geology & Geophysics, University of Oklahoma

Mass transport deposits (MTDs) are common features along most continental margins that help us reconstruct the depositional environment in complex deepwater systems. MTDs are associated

with genetically related turbidites and fans through the sequence stratigraphy of the basin. Alternatively called slumps, slides, mass flow units, or debrites, MTDs constitute large volumes of

sediments in deepwater settings. Unlike turbidite sands that form in the same environment, MTDs only rarely form hydrocarbon reservoirs. While MTDs can compartmentalize pre-existing fans

deposits, they can also form a good seal. Near the water bottom, recent MTDs can indicate the risk of future slumping hazards to submarine platform legs, drill stems, pipelines, and communication

cables. MTDs commonly exhibit an overall chaotic seismic pattern; several other associated features help to differentiate MTDs from other kinds of deposits in deep water depositional environments.

MTDs have similar characteristics in intraslope basins (also called salt minibasins) but vary as a function of restricted transport direction for sediment input, limited accommodation space, and

syndepositional salt movement. By coupling principles of geomorphology with seismic attributes and a depositional model, we analyze the characteristics of an MTD within an offshore Gulf of Mexico

study area to determine how it differs from other deepwater architectural elements and how it affects the overlying and underlying sediments.

Summary

Schematic Figure illustrating different facies present In MTD ( After Weimer and Slatt, 2006)

Mass Transport deposits within a sequence stratigraphic framework

Location and Geologic Setting

The area of study is within the tabular salt-minibasin tectono-stratigraphic province, which is structurally characterized by the presence of allochthonous salt tongues or tabular salt within sediment-filled minibasins that are formed by salt withdrawal (Diegel et al.,1995).

Sequences identified on the basis of seismic characters, well logs and Biostratigraphy.

Inline 5155

Depth Slice at 13500 ftDepth Slice at 17000 ft

Demarcating Salts

The final geometric configuration of the minibasin is a function of the interaction of the continuously-deposited sediment load on top of the allochthonous salt, giving rise to temporally-varying lateral changes in subsea topography

Objectives

‘Chaotic’ MTDs have a distinct seismic geomorphological expression that can be easily recognized but not easily picked. The objective of the current study is to analyze the seismic attribute

expression of a MTD in an intraslope basin salt minibasin in the deep water Gulf of Mexico, with the goal understanding the seismic geomorphologic significance of different elements and its

impact on the underlying and overlying sequences and the basin configuration as a whole.

Methodology• Use conventional interpretation pattern recognition skills to identify MTDs on vertical seismic slices.

• Demarcate bounding surfaces for the mass transport deposit using the seismic texture, well logs, and paleontological data (to validate the markers).

• Study the attribute expressions of the mass transport deposit including: RMS amplitudeCoherent Energy (the square of the RMS amplitude of the coherent component of the data)Eigenstructure coherenceGeneralized Sobel filter edge detectorsAmplitude gradientsMost-positive and most-negative curvatureVariance

• Proportionally slice amplitude and attribute volumes between the two bounding horizons, generating a suite of stratal slices. Although attributes such as RMS amplitude, coherence, and curvature are mathematically independent, they are often coupled through the underlying geology. For this reason, some the features can be identified on all the attributes while others are illuminated by only one or two attributes. Using this approach, characterize different components of the mass transport deposit within a seismic geomorphology framework

• Evaluate the control of MTDs by topography and lithology of underlying packages, erosion by MTDs of underlying packages, and control (primarily through changes in accomodation space and differential compaction) on overlying packages.

BA

Seismic lines (a) AA’ and (b) BB’ showing a mass transport deposit (MTD) imaged by our 3D seismic survey. The MTD unit is comprised of two discrete flow events forming two discrete layers having different seismic characteristics. The attribute expression of the two units are also quite different. We denote the two flows as Flow A and Flow B.

The Upper packages (Flow B) exhibits wavy, undulating and chaotic seismic character.

The Lower flow unit (Flow A) is less undulating and chaotic, has lower reflectivity, and exhibits some stratification.

Note how Flow B of the MTD cuts down through and erodes Flow A . Erosion is maximum along the flow axis, where the flow magnitude is maximum and decreases gradually away from the axis.

(Okuma et al., 2000)

B’

A’

SSE

B’

NNW

B

Mass TransportDeposit

Basin Floor fan;Levees

Begin backfill of leveed-channels

Incised valleys backfill;Mud deposited in basin

Transgressive sands;Condensed section; MFSall on shelf.

Deltaic/Interdeltaicdeposits

A singlerelativesea levelcycle

Slatt, 2006

Lowstand systemstract (LST)

Transgressive systemstract (TST)

Highstand systems tract (HST)

Mass TransportDeposit

NW SE

UC1

Salt

UC2

Sequence 1: GephyrocapsaL

Sequence 2: MFS102

Sequence 3: LacunosaVar

Sequence 4: TrimA

Sequence 6: LacunosaA

Sequence 5: Truncatulinoides

Sequence 7: Flexuosa

De

pth

(K

ft)

2.5

10

7.5

5

De

pth

(K

ft)

7

6

5

9

8

1 Km

Seabed

MTD2

Studied Mass Transport Complex Interval

Top of Mass Transport Deposit

Base of Mass Transport Deposit

NNW SSE

Flow B

Flow A

Salt Body in South

Salt Body in West 1.6 Km

A A’

De

pth

(K

ft)

2.5

10

7.5

5

17.5

15

12.5

20

Flow

axi

s of

Flo

wB

Eroded remnant of FlowA

Post Flow

Pre Flow

Flow

axi

s of

Flo

wB

HA

R

HA

R

Top of Mass Transport Deposit

Base of Mass Transport Deposit

Flow B

Flow A

Pre Flow

Debris Flows

Glide blocks

Thrust blocks

Escarpment

Rotated blocks

Outrunner blocks

SeismicMax

Min

0

SeismicMax

Min

0SeismicMax

Min

0

SeismicMax

Min

0