seismic geomorphology of a salt-controlled deltaic...

Figure 1. Map of Texas with inset of southeast Texas with survey area in red polygon. The stippled area indicates the extend of Houston in 1951. Adapted from Halbouty (1951).

Seismic Geomorphology of a salt-controlled Deltaic Depositional System Oyetayo M. Akintokunbo* and Kurt J. Marfurt, University of Houston

Summary

Interpretation of a “megamerge” 3D seismic volume coupled with volumetric seismic attributes over a region riddled by multiple salt diapirs reveals deposystems that can shift exploratory focus from dome flanks to withdrawal basins.

Introduction

For the past decade, volumetric seismic attributes have been used to help define depositional systems (Nissen et al., 1999, Posamentier, 2003). Seismic attributes provide the geometry of paleo-environments, particularly in settings where there is a sharp contrast in the geometry of adjacent systems, for instance in channel belts and associated systems. Most published studies have been applied to very large, high quality marine surveys that provide good temporal and spatial resolution of the features of interest. In this study, we apply the same methodology to a large mega-merge land survey of moderate quality acquired over east Texas. This survey is large enough to illuminate significant parts of the depositional system but suffers from the lower frequency content and signal to noise ratio of land surveys, further exacerbated by the merging of at least seven separate (heterogeneous) data acquisition programs taking place over the data. In this study we apply stratal and horizon slice interpretation techniques to several volumetric attributes to describe the depositional systems within a depositional time interval that is strongly influenced by salt tectonics.

Study Area

The study area is in the southern part of Liberty County, east Texas (Figure 1). It lies well within the Houston diapir province and contains nine (9) whole or partial (Figure 2) salt stalks at different stages of growth, which has to a large extend defined the structural framework of the area. The area has extensively been explored for hydrocarbon since the early 1900’s, targeting particularly the flanks of the salt domes. From Galloway’s (1986, 1989b, 2000), pioneering work of the Cenozoic depositional environment of the Texas Gulf coast using a suite of well logs and 2D seismic data sets, the depositional systems within the study area can be described as transiting from slope and delta systems in the Paleocene to siliciclastic shelf/retrograde apron and delta systems in the Eocene, to bedload dominant fluvial and deltaic systems in the Oligocene followed by periods of non deposition from the Miocene to the Pleistocene.

This study however uses a “megamerge” of not less than seven (7) 3D surveys that have been reprocessed in a consistent manner, such that the amplitudes and phases are reasonably consistent across the approximate 600 sq. miles (1,554 km sq) of the megamerge survey. The overall quality of the data is adjudged to be of moderate with a frequency range of between 10 – 85 Hz, which by implication may lead to the amalgamation of non-coeval adjacent depositional systems.

Interpretations

Based on conventional seismic interpretations we identified three major depositional units; Syn-withdrawal deposits, late withdrawal deposits and post withdrawal deposit (Figure 3). Analysis of the syn-withdrawal and late withdrawal deposits is limited to the northeastern part of the study area; we are unable to map this feature across adjacent salt diapirs and withdrawal basins. This paper focuses on the late withdrawal deposits in the NNE part of the survey.

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917SEG/San Antonio 2007 Annual Meeting

Seismic Geomorphology of a salt-controlled Deltaic Depositional System

Syn-withdrawal deposits – As the name betrays, the deposition of this unit was coeval and at pace with salt growth. This interpretation is reached due to the lack of bidirectional downlapping surfaces that is expected from prograding deposition into an existing basin. However, reflector surfaces are observed onlapping on basin margins. Time slices through the unit also reveal that sedimentation was controlled by the adjacent salt diapir that was fed by the withdrawing salt. Late withdrawal deposits – With a retardation of salt growth (and contiguous withdrawal basin) due either to increased rate of sedimentation or near complete depletion of underlying salt source, clinoforming to oblique sedimentation occurs at the upper part of the unit. The lower portion show sub-parallel strong reflections overlain by transparent reflections. This unit unconformably overlies the unit below and is riddled with normal growth faults in the southeast section. Post withdrawal deposit – The reflectors in this unit are parallel and continuous, with an apparent truncation of underlying units

Depositional Elements:

The depositional elements that constitute the stratigraphy of the late withdrawal deposit include the mini-basin and “duct channel” system, and the overlying prograding sequences.

Mini-basin/ Duct channel – The base of this unit appears to be erosive updip, cutting into underlying deposits. In planform, the geometry of the basin deposit (Figures 4 and 5) looks like a reversed submarine fan lobe with wide proximal section and a narrowing distal section that connects to the duct channel.

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Figure 2. Time slice at t =2244 ms through the seismic data indicating salt domes, radial faults (in red) and salt withdrawal basins in (magenta) within the study area.

Figure 3. Line AA’ through the seismic data volume a) without and b) with interpretation of depositional units. Map of green horizon is shown in Figure 4.



Figure 5. a) Traverse section BB’ in Figure 4b showing the salt weld (green horizon) with remnant salt (yellow arrow) withdrawing to the NW. Horizon slice amplitude map 73ms above salt weld through b) seismic amplitude and c) most negative curvature volumes. Red arrow indicates basin fill. Green arrows indicate growth faults that cut the duct channel.

A vertical section foldout reveals that the basal deposit is made up of interbedded parallel continuous strong and transparent reflection. The thickness of the transparent unit increases up section. The overall sequence resembles that of an interslope basin (Hamiter et al., 1997) which assumes that the filling of the basin is due to a drop in sea level.

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Our interpretation to the whole reversed tear drop geomorphic feature is that the high reflection at the base of the unit represents a salt weld horizon that is overlain by gravity-induced sediments triggered by rising adjacent salt diapirs (Figure 5a). Amplitude extract of a strata slice about 73ms

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Figure 4. Attribute maps of mini-basin base corresponding to green horizon in Figure 3b, showing reverse lobe geometry (white arrow) and duct channel (blue arrow) a) Cross-line gradient energy attribute map, b) Amplitude map with superposed structural contours. Gray arrow show regional dip direction. BB’ is shown in Figure 5.



Figure 6. a) Horizon slice through the seismic amplitude volume along the red horizon shown in Figure 5a; note the textural differences (dark to light shaded green arrows) in the gravity induced sediments ranging from slide blocks to slumps to debris flow. Horizon slices through b) seismic amplitude and c) coherence 36ms above the red horizon display showing the texture of the insitu slide blocks. d) Traverse YY’ through the seismic indicated in c). Red arrows in b), c) and d) indicate scarp from where the slide blocks detach. The yellow arrow in d) indicates an increase in gradient possibly related to the fault below and consequently transforms slide blocks to slumps.

above the salt weld reveal the reverse lobe shape of the deposit (Figure 5b). The duct channel that limits the basinward extent of the lobe was created by topographic highs of rising subjacent salt bodies. It contains similar deposits as the mini basin but is marked by normal faulting as shown on the most negative curvature map (Figure 5c).

Prograding sequences - The transition between the mini-basin transparent unit and the prograding sequences is marked by a conformable, possible sandy unit that exhibits a progressive slide-slump-debris flow texture (Figure 6)

trapped up section.The gravity-induced deposits are unconformably overlain by distributary channel systems (Figure 7) that possibly fed the prograding sequences. This element indicates an increased rate of sedimentation.

Conclusion

The use of strata and horizon slices through volumetric seismic attributes permit detailed description of sediment depositional systems that were induced by salt tectonics.

Sedimentation contemporaneous and at pace with salt movement appear to onlap edges of adjacent withdrawal basins, while deposition during slower salt growth is marked by an increase in sediment supply which erodes the underlying sediment and deposited sediments on unstable slopes, from where they are remobilized as gravity flow deposits. Subjacent salt domes created a funnel shaped channel that caused the retainment of gravity-induced sediments within a reverse lobe-shaped basin.

Regional dip-perpendicular geomorphic ridge-like features that grade into chaotic features are interpreted as gravity flow deposits (Figures 6a, b, c and 7a) that are of possible reservoir quality.

Acknowledgement

Thanks to Seitel for the use of data in research and education

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Figure 7. a) Horizon slice through the inline coherent energy gradient attribute about 120 ms above red horizon shown on b) line XX’ through the seismic data. Channel systems (red arrow) fed the prograding sequence. The yellow polygon shows gravity flow deposits.

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EDITED REFERENCES Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2007 SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web. REFERENCES Galloway, W. E., P. E. Ganey-Curry, X. Li, and R. T. Buffer, 2000, Cenozoic depositional history of the Gulf of Mexico basin:

AAPG Bulletin, 84, 1743–1774. Galloway, W. E., D. K. Hobday, and K. Magara, 1982a, Frio Formation of Texas Gulf Coastal Plain: Depositional systems,

structural framework, and hydrocarbon distribution: AAPG Bulletin, 66, 649–688. ———, 1982b, Frio Formation of the Texas Gulf Coast basin-depositional systems, structural framework, and hydrocarbon

origin, migration, distribution, and exploration potential: University of Texas at Austin, Bureau of Economic Geology Report of Investigations 122, 78.

Halbouty, M. T., and G. C. Hardin Jr., 1951, Types of hydrocarbon accumulation and geology of South Liberty Salt Dome, Liberty County, Texas: AAPG Bulletin, 35, 1939–1977.

Hamiter, R., A. Lowrie, M. MacKenzie, and E. Guderian, 1997, Origin of salt withdrawal basins, proven producers along present and paleo-Louisianna slope: Gulf Coast Association of Geologist Society Transactions, 47, 185–191.

Nissen, S. E., N. L. Haskell, C. T. Steiner, and K. L. Coterill, 1999, Debris flow outrunner blocks, glide tracks, and pressure ridges identified on the Nigerian continental slope using 3D seismic coherency: The Leading Edge, 18, 595–599

Posamentier, H., 2003, Seismic geomorphology — New tricks for an old dog: Reservoir, 30, 18-19 and 28-30.


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