Deltaic reservoirs

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Oil Field Production Geology
Series	Memoirs
Part	The Production Geologist and the Reservoir
Chapter	Deltaic reservoirs
Author	Mike Shepherd
Link	Web page
PDF	PDF file (requires access)
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Figure 1 Photograph of the Lena delta, Russia. Courtesy of the NASA Web site. The delta is about 200 km (124 mi) across in this view. The photograph has been rotated such that north faces down the page. The lower diagram is a lithofacies map of the basal Ivishak Formation, Prudhoe Bay field, Alaska (from Tye et al.).

Deltas are major sites of sand and mud deposition. They contain significant volumes of hydrocarbons worldwide (Figure 1). Major petroleum provinces include the Niger Delta in west Africa, the Mahakam Delta in Borneo, the Caspian Sea, and the Maracaibo Basin in Venezuela.

Deltaic sediments often form complex reservoirs

Delta systems make heterogeneous reservoirs, typically with a jigsaw-puzzle to labyrinthine sediment geometry (Table 1). There may also be considerable structural complexity. Many rivers, particularly the larger ones, dump very large quantities of sediment into deltas on top of an unstable substrate containing mobile salt and/or shale. Salt deformation features and growth faulting are common in deltaic sediments, and these can result in numerous segmented reservoirs, such as in the Tertiary Niger Delta in west Africa.

Table 1 Factors influencing connectivity and reservoir development in deltaic reservoirs.

Characteristic	Favorable for reservoir development	Unfavorable for reservoir development
Growth faults common	-	Numerous sealing fault compartments
Shingled geometry	-	Results in bypassed oil in individual shingles
Increasing marine reworking of delta front	Creates increasing lateral connectivity in the delta-front sediments	-
Wave-dominated delta	More continuous, may have an aquifer	-
Fluvial-dominated delta	-	Can show low recoveries caused by labyrinthine geometry
Tidal-dominated delta	-	Low recoveries caused by complex geometry and numerous mud and silt baffles
Distributary channels form narrow sand bodies	-	May be missed by wells in fields with a large well spacing; difficult to locate injection wells
Distributary channel sands commonly the highest permeability facies association in deltas	Can be the most productive intervals in a delta	-
Stacked distributary channels	Larger sand bodies with good vertical connectivity and sweep	-
Mouth bars contain extensive mudstone laminae	-	Mouth bars may have poor vertical connectivity and sweep
Stacked mouth bars	Larger sand bodies with good vertical connectivity and sweep	-
Mouth bars separated vertically by shales	Individual mouth bars can be targeted by horizontal wells; shale prevents water influx from swept units above and below	Poor to no vertical connectivity between mouth bars
Coarser grained distributary channels cutting finer-grained delta-front sandstones	-	Can result in preferential water ingress into the delta-front area, bypassing oil in the delta-front sediments
Peripheral strand-plain complexes with high frequency marine shales	-	Poor vertical sweep with potential for bypassed oil

Deltas are often gas reservoirs

Many deltaic reservoirs, particularly long-lived Tertiary to present-day delta areas, contain more gas than oil. This is because they can be particularly rich in coals and woody kerogen, which form gas-prone humic source material. Gas fields are found in the Mackenzie, Nile, and Irrawady deltas, for instance. Deltas can contain oil or mixed oil and gas where sandstones interfinger with a marine source rock.

Figure 2 Three categories of delta can be defined according to the dominant sedimentary process. These are wave-dominated, tide-dominated, and fluvial-dominated deltas. Courtesy of the NASA Web site.

Figure 3  A gross sandstone thickness map can give an idea of the depositional dip and strike of the sedimentary system. In the Budare field of Venezuela, north–south strike elements correspond to distributary channels in the bottom part of the map. An east–west arcuate depositional element in the north of the map corresponds to a wave-dominated delta front (from Hamilton et al.).

Types of delta

Deltas have been categorized into three classes in terms of sedimentary process: wave dominated, tidal dominated, and fluvial dominated (Figure 2). Coarse-grained deltas include fan deltas and braid deltas. Each specific environment has its own geometries and typical reservoir characteristics. The geometrical patterns shown by the various types of delta can often be recognized on isochore, net-sand, and log-facies maps. For example, a wave-dominated delta will show a T motif on these maps as a result of fluvial lineaments converging at a high angle to a shoreline trend (see Figure 3). The lobate shape of the delta front may also be recognized.

Depositional environments

The influence of river, wave, and tide on deltaic sediments produces a complex mix of macroforms. Fluvial processes dominate in the upper delta plain, although this area can also be swampy with marshes and lakes present. The lower delta plain is subjected to marine influence, acting to modify the fluvial-derived sediments. Delta fronts comprise nested complexes of distributary channels, mouth bars, tidal bars, and reworked delta-front sediments (Figure 1).

Distributary channels

Distributary channels are so called because of the way in which they branch off from the main feeder river and distribute water and sediment across the delta. Where the distributary channels split off from the main feeder river, the volume of water carried by individual channels will be a fraction of that in the main river. By comparison to fluvial channels, distributary channels tend to be narrower and shallower. Gibling noted that distributary channels show a common width range of 10–300 m (33–984 ft) (see Table 2). Distributary channels tend to be straight where they incise a mud substrate and more sinuous within a sand substrate.

Table 2 Width and thickness relationships of fluvial sediments in various settings.¹

Depositional environment	Thickness	Width	Width/thickness ratio
Braided and low sinuousity rivers	1-1200 m (3-3937 ft); most < 60 m (197 ft); common range 5-60 m (16-197 ft)	50 m-1300+ km (164 ft-808+ mi); many > 1 km (0.62 mi); common range 0.5-10 km (0.3-6 mi)	15-15,000+; some > 1000; common range 50-1000
Meandering rivers	1-38 m (3-125 ft); common range 4-20 m (13-65 ft)	30 m-15 km (98 ft-9 mi); most < 3 km (1.8 mi); common range 0.3-3 km (0.1-1.8 mi)	7-940; most < 250; many < 100; common range 30-250
Delta distributaries	1-35 m (3-115 ft); most < 20 m (65 ft); common range 3-20 m (10-65 ft)	3 m-1 km (10 ft-0.6 mi); most < 500 m (1640 ft); common range 10-300 m (33-984 ft)	2-245; most < 50; many < 15; common range 5-30
Channels in eolian settings	1-19 m (3-62 ft)	2.5-1500 m (8.2-4921 ft); most < 150 m (492 ft)	1-90; most < 15
Valley fills on bedrock unconformities	12-1400 m (39-4593 ft); most < 500 m (1640 ft)	75 m-52 km (246 ft-32 mi); most < 10 km (6 mi)	2-870; highly variable; mainly 2-100
Valley fills within alluvial and marine strata	2-210 m (6-689 ft); most < 60 m (197 ft)	0.1-105 km (0.06-65 mi); common range 0.2-25 km (0.1-15 mi)	4.6-3640; highly variable; common range 10-1000; many from 100 to 1000
1From Gibling, Journal of Sedimentary Research. Reprinted with permission from, and © by, the SEPM (Society for Sedimentary Geologists).

Sand is deposited within linear distributary channels as side bars. In the modern-day Mahakam Delta, Borneo, side bars alternate on both sides of the distributary channels. These form elliptical sand pods, 5–8 km (3–5 mi) or more long and up to 1 km (0.6 mi) wide. Channel fills typically show an upward-fining sediment profile and an upward-decreasing permeability profile. From the base upward, a distributary channel comprises the active channel fill, showing decimeter-scale trough cross-bedded sets; a partial abandonment fill with mainly centimeter-scale cross-beds; and sometimes an abandonment channel fill of thinly interbedded fine sand, silt, and shale.

Figure 4 A schematic delta showing a range of sand body types at their average dimensions, together with several oil and gas fields at the same scale. The delta front is divided into three segments that are storm-, fluvial-, and tidal-dominated, respectively. The delta and its divisions are not to scale (from Reynolds).

Figure 5 Fluvial-dominated delta environment, Mississippi Delta. Photograph courtesy of the NASA Web site. The inset box on the photograph measures 34 times 42 km (21 times 26 mi). The lower diagram is a box diagram showing the sedimentological relationships within the inset box (after Fisk).

Mouth bars

Mouth bars form where a distributary channel enters a standing body of water and sediment drops out. A shoaling to emergent sand body grows at the channel mouth. The resulting obstruction can cause the channel to bifurcate at the upstream head of the mouth bar. Mouth bars show an arcuate fan shape in plan view and a wedge-shaped profile in cross section. Reynolds gave average dimensions for mouth bars of about 3 km (1.8 mi) wide and about 6.5 km (4 mi) long (see Figure 4; Table 3). Relatively straight distributary channels building out into deep water will form more linear deposits known as bar fingers (Figure 5).

Table 3 Statistics of dimensional data for deltaic sandstone bodies in meters.*

Sand body type	Width			Length			Thickness
	Mean	Max.	Min.	Mean	Max.	Min.	Mean	Max.	Min.	N
Incised valleys	9843	63,000	500	-	-	-	30.3	152	2	91
Fluvial channels	755	1400	57	-	-	-	9	24	2.5	6
Distributary channels	518	5900	20	-	-	-	7.8	40	1	268
All types of systems tracts	25,365	106,000	1600	93,166	190,000	47,000	19.1	49	2.7	67
Highstand systems tract	16,425	43,000	16,000	-	-	-	-	-	-	36
Transgressive systems tract	7150	20,000	3300	-	-	-	-	-	-	5
Distributary mouth bars	2868	14,000	1100	6477	9600	2400	9.7	42	1.2	26
Flood tidal delta complex	6201	13,700	1700	12,300	25,700	2900	6.7	23	1.8	13
Crevasse channels	58	400	5	-	-	-	2.4	17	0.2	44
Crevasse splays	787	7700	18	5577	11,700	160	1.4	12	0.3	84
Lower tidal flat	994	1550	400	-	-	-	4.6	9	2	14
Tidal creeks	813	1550	161	-	-	-	5.2	18	1	15
Tidal inlet	1850	2550	700	4300	4300	-	4.8	7	3	3
Estuary mouth shoal	2400	2900	1700	3750	4700	2200	10	35	10	4
Chenier	3650	6400	900	21,758	38,600	49,000	5.8	7	4.6	2
All sands	5094	106,000	5	35,313	190,000	160	-	-	-	671
*From Reynolds. N = number.

Figure 6 Idealized log and permeability profiles for deltaic sand bodies (from Sneider et al.). Reprinted with permission from, and © by, the Society of Petroleum Engineers.

An upward-increasing grain size profile is characteristic for mouth bars. The lower parts are finer grained, more poorly sorted, and with common shale intercalations. Upward, the texture is coarser although there may be many laminations of clays and organic material. Permeability typically increases upward (Figure 6).

Mouth bars usually show lower overall permeabilities than distributary channel fills. For example, Tye et al. gave average rock property values for the various lithofacies associations within the Ivishak Formation of the Prudhoe Bay field in Alaska. The mouth bars have a mean permeability of 151 md compared to 315 md for the distributary channel fills.

The coarsest and best sorted sediments in the mouth bars form near the stream mouth and along the bar margins adjacent to the distributary channels. Tye and Hickey found an order of magnitude higher permeability in this part of the point bars in Prudhoe Bay field, Alaska. Outward and down slope, the sediment becomes finer grained. Downstream, along the outer edge of the mouth bar, fine sand and silts interfinger with prodelta muds.

Delta front

The delta front area comprises a jigsaw-puzzle to labyrinthine complex of channel sandstones, mouth bars, and sediments formed by marine reworking. Wave, tidal, and fluvial processes act to rework the sediments on the delta front.

Wave reworking tends to produce relatively smooth lobate delta fronts. As the degree of wave reworking of the isolated mouth bars increases, the sediments become more continuous, coalescing to form laterally extensive beach-ridge and coastal-barrier sand bodies. The outlines of individual mouth bar forms start to become indistinct as they are reworked. Sometimes the sites of mouth bar deposition may only be recognizable by local thickening of the delta front.

Delta-front sandstones tend to be finer grained, although better sorted, than distributary channel-fill sandstones. Later stage channels may cut into or overlie mouth bars and delta-front sandstones, creating jigsaw-puzzle geometrical complexity. Laterally and offshore, the sandstones become interbedded with background prodelta mudstones and will pinch out into them. The sandstone quality deteriorates laterally toward the margins.

See also

• Eolian reservoirs
• Meandering fluvial reservoirs
• Braided fluvial reservoirs
• Siliciclastic shorelines and barrier island reservoirs
• Deep-water marine reservoirs
• Carbonate reservoirs

External links

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