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Deltaic Deposits and Linked Downslope Petroleum Systems*

By

Harry H. Roberts1 and Richard H. Fillon2

 

Search and Discovery Article #40149 (2005)

Posted April 3, 2005

 

 *Adapted from extended abstract, prepared by the author for presentation at AAPG International Conference & Exhibition, Cancun, Mexico, October 24-27, 2004. 

 1Coastal Studies Institute, 304 Howe-Russell Geoscience Complex, Louisiana State University, Baton Rouge, LA 70803-7257 ([email protected])

2Earth Studies Associate, 3730 rue Nichole, New Orleans, LA 70131 ([email protected])

 

Introduction 

Deltas in siliciclastic and mixed carbonate – siliciclastic deposystems are key to understanding processes that transfer terrigenous detritus from continental uplands to deep-ocean environments. The Lagniappe Delta deposystem (Figure 1) located on the shelf and slope in the northeastern Gulf of Mexico has characteristics that make it a useful laboratory for developing petroleum system insights.

 

 

uIntroduction

uFigure Captions

uGeologic setting

uLagniappe clinoforms

  uReservoir analogs

  uHydrates

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uIntroduction

uFigure Captions

uGeologic setting

uLagniappe clinoforms

  uReservoir analogs

  uHydrates

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uIntroduction

uFigure Captions

uGeologic setting

uLagniappe clinoforms

  uReservoir analogs

  uHydrates

 

 

 

 

 

 

 

 

 

 

Figure Captions

Figure 1. During periods of falling-to-low sea level, rivers build seaward-thickening multilobed delta complexes at the shelf-edge. These complexes contain thick clinoform sets that drape onto the upper continental slope. This schematic diagram shows the complex internal geometries of the Lagniappe shelf-edge depocenter. The inset map locates the Lagniappe delta along the Mississippi-Alabama shelf-edge just east of the active Mississippi River delta.

Figure 2. High resolution seismic profiles (see inset map for location within Lagniappe delta complex) show clinoform architectures typical of shelf-edge deltas. The high amplitudes in the lower parts of the clinoform packages reflect the presence of bubble-phase gas. These deltas contain reservoir facies ranging from massive distributary-mouth bar and channel sands to delta-front turbidites. The fluvial facies illustrated by X-ray radiographs is composed of gravel-rich sands while clean sand comprises the delta facies. These X-ray radiographed core samples were selected from the MP303 corehole (see inset map).  

Figure 3. The shelf-edge delta illustrated in Figure 2 is seen in the broader context of a shelf-margin system bounded by a major growth fault (at right) and a deeply rooted near-surface salt body (at left). Here, the shallow delta lobe is seen as back-stepping from the more seaward position of an older delta which has been displaced downward along the growth fault. This cross-section provides a hydrocarbon migration model in which vertical migration along the salt body is blocked by the development of gas hydrates. Gas liberated from the hydrate stability zone in response to temperature and pressure fluctuations may laterally migrate through delta-front turbidite beds into more massive updip reservoir facies.

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Geologic Setting 

During the last 100 ka glacio-eustatic cycle, delta lobes fed by southern Appalachian rivers with relatively high sand-to-mud ratios, prograded rapidly across a broad shelf, reaching the shelf-edge only about 1000 years before the maximum lowstand. Offset stacking of delta lobes at the shelf-edge is responsible for facies heterogeneity and is of importance in predicting sediment bypass to deep-water reservoir systems.

 

Lagniappe Clinoforms 

Reservoir Analogs (Figure 2) 

Thick sandy Lagniappe clinoforms constructed at the shelf-edge are excellent analogs for the growth-fault-related hydrocarbon reservoirs in the Gulf of Mexico and other petroleum basins. A strongly laminated prodelta apron, constructed on the upper slope, grades down-slope into hemipelagic drape but is characteristically punctuated by occasionally striking, but often subtle, bypass features related to channelized flow and basinal submarine fan development. It is generally assumed that sediment transport to deep water peaks during maximum lowstands. However, true maximum lowstand deltas are rare and have not been studied in detail in the Gulf of Mexico.  

Delta-front clinoforms often exhibit strong acoustic impedance contrasts suggesting the presence of bubble-phase gas. Because growth faults and salt structures commonly coexist with shelf-edge deltas, they may offer the migration linkage between deep hydrocarbon systems and the lateral migration pathways provided by distal clinoforms that are directly linked to deltaic reservoirs. Thin fine sand, silt, and clay laminae in the prodelta apron create effective capillary seals that inhibit vertical hydrocarbon migration while allowing hydrocarbons to move laterally updip within coarser laminae. Gas presently seeping from truncated clinoform sets and anomalous d18O and d13C values of authigenic carbonates within sediments of the clinoform packages strongly suggest that hydrocarbon migration is an on-going process.

 

Hydrates (Figure 3) 

The hydrate stability zone plays a critical role in the delta-slope system, in regulating the updip migration of hydrocarbons through the delta-front turbidites and in triggering slope failure that may lead to long-term sediment bypass routes to deep-water depositional sites. Slope failures can mobilize large volumes of shelf-edge clinoform and prodelta apron sediments, creating turbidity currents and debris flows that nourish deepsea fan systems. These processes are modulated by sea-level change. Gas hydrates in continental-margin sediments decompose as hydrostatic pressure decreases during a sea-level fall and the upper slope becomes bathed in warm surface waters. Depending on the rate of this decomposition, gas may be slowly released for updip migration into reservoir facies, or more rapid gas production from destabilized hydrates may induce sediment instability and slope failures. 

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