sedimentary facies variation and hydrocarbon reservoirs in continental sediments--a predictive model

Journal of Petroleum Geology, 7,1, pp. 67-76, 1984 67

SEDIMENTARY FACIES VARIATION AND HYDROCARBON RESERVOIRS IN CONTINENTAL

SEDIMENTS - A PREDICTIVE MODEL

Graham A. Blackbourn*

A simple model is outlined which links continental sedimentary environments and facies with their relativeposition within a basin, and with theprevailing climate. The modelmay be usedfor predicting vertical and lateralfacies changes within a succession of continental sediments. It is a useful tool both in the search for hydrocarbon reservoirs, and in the understanding of reservoir properties.

INTRODUCTION

Continental sediments are deposited in a wide range of environments, ranging from coarse alluvial fans, river channels and alluvial plains, through arid aeolian deserts and salt flats, to lakes and delta plains. These deposits are of considerable value as hydrocarbon reservoirs, and host many of the large oilfields of the world. However, such sediment bodies are often very complex, with rapid changes in thickness and facies, and hence in reservoir quality. This paper presents a facies variation model which provides a conceptual framework for the consideration of various features of these rocks, and for predicting facies changes through time and space. This model has been found useful for predicting facies changes in continental successions encountered during oil exploration on the UK continental shelf. It is, however, considered to be applicable worldwide, and may also be applied to sedimentological and palaeogeographical studies of rocks in outcrop.

OUTLINE OF MODEL

Van Houten (1973) pointed out that continental “red bed” successions usually range between two rather distinctive end members. The first end member includes extensive formations of quartz-rich sandstones and siltstones, often with tongues of marine limestone, and with evaporites and aeolian sandstones as evidence of aridity. The other end member consists of thick sequences of immature sandstone and mudstone with laterally equivalent conglomerates. Deposition of these sediments usually took place in a range of fluvial channel and associated overbank environments.

* Department of Applied Geology, University of Strathclyde, 75 Montrose Street, Glasgow GI IXJ. Present address: Britoil plc., Stratigraphic Laboratory, 1 SO St. Vincent Street, Glasgow G 2 5LJ.

68 Sedimentary facies variation - a predictive model

Flood basin/ alluvial plain

4

Desert sands or pediment

- - - c- - 0 0 proximal Fan -

wid Marginal breccio-conglomerate Hut

Fig. 1. Basic continental facies variation model, showing environments in which the different facies develop.

Although the sedimentological characteristics of the two end members are rather different, the conditions for the deposition and preservation of each, other than rainfall or humidity, are similar. Both tend to be preserved in fault-bounded continental or marginal marine basins, which have been filled by sediments derived from surrounding uplands. Continental sediments of aeolian and fluvial aspect sometimes alternate in a single vertical sequence. In basins of either type, the depositional environments usually grade systematically from the margins to the interior, and fluvially-dominated basin margins may encompass an area of aeolian sand.

Fig. 1 illustrates the basic facies variation diagram which has been developed. The association of facies present at any position within a basin is taken to be a function of the environment of deposition. The diagram is based in two important variables: (1) that described by Van Houten (1973), here denoted as arid to humid; and (2) the variation in distance from the basin margin, i.e. proximal to distal. This diagram can be used to plot the relative positions of continental environments, and the facies associations typical of them. It should be stressed that the terms “proximal” and “distal” do not refer to actual positions within a basin, but to the relative types of sediment being deposited. An abrupt uplift of the source area may lead to more “proximal” deposits being laid down throughout the basin. If a basin fills up without source area rejuvenation, the sediments deposited at any position are likely to become more “distal” upwards. A vertical succession can often be drawn as a line with a proximal to distal trend (Fig. 2).

SEDIMENT TYPES Marginal breccio-conglomerate

Breccias and conglomerates frequently form piled-up against marginal fault scarps. They usually form simply from material falling down the scarp face and accumulating at the base.

Graham A. Blackbourn 69

Dolomitic playa mu

(1) Depositional framework in Playa Complex model. After Eugster and Hardie (1975).

Variscan

Plava

Facies

(2) Environmental interpretation of ORS of Anglesey, Wales. After Allen (1967).

Desert

(3) Conceptual cross section through Rotliegen in Southern North Sea. After Glennie 1972.

Fig. 2. Three examples of continental successions described in the literature, to illustrate how they may be plotted on the facies variation diagram. The lines plotted give the general position and trend,

although the total range of environments lies in an area around them.


They are rarely more than several tens of metres thick, thinning rapidly basinward. Their nature is little affected by the climate, and they are characterised by a lack of internal stratification. Platey clasts accumulating on the depositional slope may take on an imbricate structure, dipping downslope. These should not be mistaken for current-imbricated clasts, which dip upstream. The deposits are clast-supported, and commonly contain a poorly-sorted mechanically-infiltrated matrix. Where the basin margin is not fault-bounded a basal breccia or scree accumulation may nonetheless have resulted from local weathering processes.

Wadi and alluvial fan deposits Even in the most arid climates, there are normally some water-lain deposits within a

sedimentary basin. Deposition is often concentrated near the basin margins where fault scarps provide an abrupt break in slope, and streams decelerate on emergence from confined gulleys on to a broad plain. In the desert environment, sediments may be introduced during rare flash floods which can transport huge loads of accumulated debris into the basin from adjacent highlands, leaving a wedge of sand or breccio-conglomerate known as a “wadi” fan.

With greater regularity or duration of rainfall, there is more opportunity for sediment sorting, and well-developed alluvial fans may build up against marginal fault scarps. The facies relationships within a fan have been described by Rust (1979). It is common to divide a fan into proximal, middle and distal parts, although definitions vary from author to author. The proximal to distal changes on semi-arid fans may differ to some extent from those on humid fans (Collinson, 1978; Rust, 1979). In the former case, mudflow deposits tend to pass downstream into streamflood and fluvial channel deposits. On humid fans, clast-supported fluvial conglomerates commonly give way downstream to cross-bedded, pebbly sandstones, accompanied by a decrease in maximum particle size and thickness of the cross-bedded sets (Bluck, 1967). No firm definitions for the different parts of a fan are required when using the facies variation diagram (Fig. 1) although, as a very general guide, proximal areas may be dominated by clast supported conglomerates and debris flow deposits, and distal areas by sands and pebbly sands. Although Bluck (1967) shows that debris flows may occur on the distal part of a fan and beyond, braided channels tend to dominate on these lower parts. Meandering channels are more likely to form distal to the fan itself, on a low-gradient alluvial plain (Allen, 1970).

Basin floor clastics These include sediments laid down in many different environments, ranging from all the

varieties of river-channel and flood-plain deposits to purely aeolian sands. The basin floor area is divided into three parts on the facies variation diagram (Fig. 1). Sediments typical of deserts on the arid end of the spectrum are separated from waterlain deposits on the humid end by azig- zag line representing the large area of overlap, or interdigitation, of the two types of deposit. A second line within the field of fluvial deposits marks the distal limit of braided channel deposits, which tend to give way to meandering channels basinward (Collinson, 1978). Changes in fluvial environments and facies associations in response to a changing climate, both of source area and basinal area, are summarised by Miall (1980); Cotter (1978) stresses the lower proportion of channelised stream deposits which were laid down before the spread of terrestrial vegetation during the Late Palaeozoic. At the same time, the field of aeolian deposits was reduced, as unconsolidated sediment was bound by vegetation or covered by organic-rich soils.

The water-table One important variable not yet represented on the diagram is the depth to the water-table.

This is represented by a line running across the diagram, which can be moved in a proximal to distal direction (Fig. 3). The line represents the position in the basin where the water-table reaches the surface, and thus marks a marine or lake margin. Other things being equal, an increase in rainfall or humidity leads to a raising of the water-table, so the line is given a slight gradient. Different environments, such as coastal alluvial fans (“fan-deltas”), can be


‘S A Deeper

Fig. 3. Possible positions of the “water-table line”, where the ancient water-table reached the surface. The area distal of the line represents a sea or lake. Varying lake environments are suggested in (a).

represented by moving the line to a more proximal position (Fig. 3). For the remainder of this discussion the line will be assumed to be in its most distal position so that the full range of aeolian and alluvial deposits may be considered.

OTHER FEATURES

Various other deposits and structures which are common in continental sediments can be plotted on the basic facies-variation diagram. Some of these are discussed below.

Evaporites A comprehensive summary of continental and supratidal evaporite facies has been given by

Kendall(l979). Evaporites are unlikely to be deposited where there is a regular supply of fresh water, so will rarely be associated with channelised fluvial deposits. The most commonly preserved continental evaporites, the playa or continental sabkha deposits, are usually precipitated in the lowest areas of enclosed arid drainage basins, characterised by almost horizontal and largely vegetation-free surfaces of fine-grained sediments (Kendall, 1979). Precipitation is from near-surface saline ground-waters or shallow lakes, so if the water-table is too low no ground-water discharge occurs at the surface and evaporites are normally absent. Supratidal, or coastal sabkha, evaporites are similarly formed by capillary rise or evaporative pumping of sea-water in the coastal sediments (Kendall, 1979). The field of evaporites on the facies variation diagram is thus linked to the water-table line, and will move proximally or distally as the line moves.


0 . 0

Sabkha **. I " . I 0' I * "

f 0 O 0 0

n

Evaporites O0

imits move with 0,

vater table line) o

. v . . . . . . . .

0 . 0 .

0 . O O O oo{o g o Cbo . . .

o + I :t I ;+ I

I k I 4 : ; I

+ I I 0"

I ooo + 7ooo + I

I I + I

o + I

, " + oo + I

o +

w+ ' + Coal $ 1 + I

I I i

I I I I

I Slumped 1 bedding

abundant

I : Cornstones 2 I . I Cornstones . I I + b +

~~

Fig. 4. Positions of common terrestrial sedimentary features on the facies variation diagram.

Within the total field of evaporites a number of sub-divisions could be distinguished on the basis of the succession of salts precipitated with changes in salinity, or the positioiwithin a playa lake (Kendall, 1979).

Caliche Leeder (1975) has noted that cornstones (fossil caliche) and evaporites are generally

mutually exclusive in a sediment. Deposition of pedogenic carbonate occurs from the combined effects of evaporation and changes in the partial pressure of carbon dioxide in the soil zone (Blatt ef al., 1980). In the latter case, increase of CO, released by plant roots in the A soil horizon accompanied by sufficient rainfall causes HC0,- ions to be carried down to the B horizon, where calcium carbonate is deposited. In more arid regions with less organic material, the evaporation of vadose water is more important (Blatt et al., 1980). Soil water rises by capillary action and, with the evaporation of water and loss of carbon dioxide gas, calcium carbonate precipitates. The depth of the caliche zone is thus related to rainfall, and in temperate regions it rarely forms with rainfall less than 40cm, or greater than 100cm/year (Jenny and Leonard, 1934).

Cornstones commonly occur in fine-grained sediments, and are frequently encountered within overbank deposits of ancient meandering rivers (e.g. Allen and Williams, 1979; Steel, 1974). Nonetheless, they are not restricted to these environments. Large masses of calcite, approaching 0.5m across, occur in the coarse alluvial fan conglomerates of the Triassic Stornaway Formation, Scotland. The carbonate, with a typical cornstone texture (Steel, 1974) partially replaces the sandstone matrix, and encloses pebbles of gneiss exceeding 10cm in diameter.


Cornstones thus plot on the facies variation diagram in an area covering the complete range of proximal to distal environments, and in an intermediate position on the humidity axis which has little or no overlap with the evaporite field (Fig. 4).

Soft sediment deformation/dewatering Numerous varieties of soft sediment deformation structures occur, which include folding

caused by downslope movement, sand volcanoes and irregular dewatering structures. Okada and Whitaker (1979) cite references to observations of sand volcanoes and related deformational structures being formed over the last two centuries, and they all involved thoroughly waterlogged sediments responding to earthquake shocks. It appears from numerous Scottish continental successions known to the author that widespread soft sediment deformation in sandstones does not occur where cornstones are common. The Pre-Cambrian Torridon Group in NW Scotland (Stewart, 1959) contains abundant evidence of soft sediment deformation, but no cornstones have been recorded. Other successions with abundant deformation include the Foula Group and the Eday Beds in northern Scotland (Blackbourn, 1981), in both of which cornstones are rare or absent. The Foula Group includes a number of red mudstones similar to those described by Allen (1974) from South Wales where cornstone development is common.

However, despite evidence of surface dessication, no cornstones are developed on the Isle of Foula. In the very similar Late Devonian facies of the Chiroilfield to the NW, cornstones found in situ and as clasts in intraformational conglomerates are common (Blackbourn, 1981). It is difficult to estimate the frequency of slumped bedding from the Devonian Cfuir cores. Although deformation is present, it is clearly not abundant. It may be concluded that the level of rainfall or ground water supply required for the mobilisation of sediments is generally too high for caliche formation.

Because of the low cohesiveness of dry sands, and the steep gradient of avalanche deposits, synsedimentary deformation is also quite common within aeolian sands. This has been discussed by McKee (1979).

Some process such as the creation of an “oversteep” slope, an earthquake, or herds of stampeding dinosaurs (Selley, 1978) is required to initiate movement in soft sediments. For this reason, deformation is common near boundary faults and in higher gradient areas such as alluvial fans or braidplains. Nonetheless, it is not restricted to these environments, so no definite boundary can be set for the field of widespread soft sediment deformation. It is however, most likely to be encountered in humid, proximal environments and aeolian sand dunes.

Miscellaneous Numerous other features can be plotted on the diagram. Examples include aeolian dune

bedding, vegetation, coal, and different faunal groups. Suggested fields for some of these are shown on Fig. 4. Although the positions of lines on the diagram are only relative, it would be possible to plot an approximate scale of rainfall by, for example, using the maximum and minimum values noted above for the formation of caliche deposits. The diagram could also be used for determining the climate prevailing during deposition, and where changes in climate are detected, they could be valuable for regional stratigraphic correlation.

PREDICTION OF RESERVOIR QUALITY

Continental sediments form extremely important hydrocarbon reservoirs throughout the world, but exploration and development of such reservoirs is hampered by the rapid changes in facies, and consequently in reservoir quality, which occur in these rocks. The facies variation diagram has proved to be a valuable tool in the prediction of facies changes in sub-surface reservoirs, where direct evidence of lateral variations is usually poor. By plotting the position or range of depositional environments responsible for a sedimentary succession in one well, the


diagram will show the environments and corresponding facies types which are likely to be represented elsewhere in the same basin. The diagram will give an indication of the relative proximity of the basin margin, and will show the most likely associations of facies in more proximal and distal positions. Facies which occur far removed on the arid-humid axis are unlikely to be represented.

Fig. 5 is an attempt to “contour” the facies variation diagram in terms of reservoir quality. The contours are not intended to be quantitative, but show the general variation expected across the diagram. The three main factors considered to affect primary porosity and permeability are the degree of sorting, grain size, and the presence of syndepositional cements (including evaporites). Well-sorted sediments tend to have relatively high primary porosities, because there are fewer small grains to choke the pores between the larger grains (Beard and Weyl, 1973). The degree of sorting increases in a general way from the basin margins to its centre, although this is counterbalanced by the lower permeability and porosity of the fine- grained sediments with abundant clay minerals which are concentrated in more distal positions. The best reservoir sections in continental sediments thus tend to lie in an intermediate position on the proximal-distal axis, and are concentrated towards the more arid environments, in which aeolian processes may have produced very well-sorted sands. In fluvial sediments, the best sections are usually more proximal than the mudstone-rich floodplain sediments, although channel sandstone bodies and related sands laid down, for example, as crevasse plays and levees, may form good-quality reservoirs within finer silts and muds. Very proximal fluvial deposits also tend to form poor reservoirs because of their generally very poorly sorted nature.


Evaporites in a sequence greatly reduce porosity, and while they can seriously impair reservoir quality, a persistent horizon may form an excellent seal. The closeness on the facies variation diagram of the aeolian sands to the evaporites points to an excellent hydrocarbon trap. This possibility is realised below parts of the Southern North Sea, where aeolian Rotliegendes sands were transgressed by the Zechstein sea. The Zechstein evaporites now form the seals to huge accumulations of gas. On the more humid end of the scale, a flood-plain mudstone can form a seal over a permeable sandstone, although such a mudstone is rarely completely continuous. A few shoestring sand bodies in it may be sufficient to release hydrocarbons. Deposits of a more permanent lake, or of a marine transgression could, however, form an excellent cap rock.

Apart from organic-rich lacustrine deposits, continental sediments do not contain significant hydrocarbon source rocks. Oil or gas in these reservoirs has usually been introduced following migration from a different stratigraphic level. Because continental sediments are frequently preserved as the oldest sediments within post-orogenic basins, they often directly overlie economic basement. In these cases, significant tectonic uplift relative to an organic-rich source is usually required for the potential reservoir to accumulate hydrocarbons.

CONCLUSIONS The facies variation diagram is a simple method of representing, in a generalised way, a range

of environmental and lithological features common in continental sedimentary basins, and of visualising their relationships. Although few continental successions will fit the model perfectly, the model attempts to “distil” the wealth of information in these successions, “boiling away’’ the local details, in the manner described by Walker (1979) in the construction of facies models. Like a facies model, the variation diagram aims to be not only a summary, but also “a norm for the purposes of comparison, a framework and guide for future observations, a predictor in new geological situations, and the basis for hydrodynamic interpretation of the environment or system it represents” (Walker, 1979). The reservoir quality diagram clearly shows where good reservoir sands may be found in a wide range of continental basin types, and has been found useful by the author in predicting facies changes in successions known from limited core and structural data. In addition, it helps in the understanding of the processes and controls operating during deposition of continental successions. The diagram is by no means exhaustive, but provides scope for refinement and addition by its users.

A similar facies variation diagram could be devised for other environments. For example, the variation on submarine fans might be represented by plotting the proximal to distal variation from fan head to abyssal plain versus the sand/mud ratio of the sediment input, or clastic shoreline deposits could be shown in terms of proximal/distal changes versus tidal range. A variety of similar diagrams for different environments could be valuable for the explorationist looking for predictive models, as well as the sedimentologist seeking to understand processes and the student requiring a simple summary of complex sedimentological systems.

ACKNOWLEDGEMENTS I thank Roger Anderton for useful discussions, and Stuart Haszeldine for reading the

manuscript. The work was carried out during the tenure of a British National Oil Corporation research studentship.

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sedimentary facies variation and hydrocarbon reservoirs in continental sediments--a predictive model

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