DELTAIC PROVINCES A MAJOR FOCUS OF WORLDWIDE EXPLORATION EFFORTS

William A. Wescott
Amoco Production Co.
Houston

Deltas have proven to be important worldwide oil and gas provinces for two main reasons.

First, they are point sources from which sediment is introduced onto continental shelves and into marginal basins, where the interaction of fluvial and basinal processes frequently results in favorable associations of reservoir, source, and seal facies.

Second, the progradation of deltas forms a geomorphic bulge that projects seaward and allows land and shallow offshore drilling rigs to reach deep targets that could only be reached by more expensive rigs in deeper water areas adjacent to the delta.

Significant hydrocarbons have been discovered in at least 17 deltaic provinces worldwide.

Almost 30 billion bbl of oil and more than 100 tcf of gas have been produced from these deltaic provinces, which contain estimated ultimate reserves of more than 50 billion bbl of oil and 320 tcf of gas.

Fields with reserves greater than 500 million bbl of oil occur in five deltaic basins, and fields with more than 100 million bbl of oil have been found in 13 deltas.

Future exploration will focus on deepwater delta-front environments and smaller deltas in more remote areas with hostile drilling environments, such as those in the circum-arctic region.

INTRODUCTION

Ever since Herodotus described the land at the mouth of the Nile River as similar in shape to the Greek letter delta around 400 B.C., deltas have become one of the most studied of all depositional environments. 1 2

Deltas form where fluvial systems enter a body of water and have been geologically defined as:

"The low, nearly flat, alluvial tract of land at or near the mouth of a river, commonly forming a triangular or fan-shaped plain of considerable area, crossed by many distributaries of the main river, perhaps extending beyond the general trend of the coast, and resulting from the accumulation of sediment supplied by the river in such quantities that are not removed by tide, waves, and currents. Most deltas are partly subaerial and partly below water."2

Deltas are significant components of many coastlines, and their deposits make up a substantial volume of the stratigraphic record. 1 3 4 5 6 7 Modern deltas are diverse in form and van, greatly in size and geomorphic setting.

Deltaic morphology and three dimensional distribution of sediments are the result of a complex interplay between fluvial discharge, sediment supply, and the wave and tidal energy of the receiving basin. They occur at almost all latitudes and climates from the arctic to the tropics . 3

DELTAIC RESOURCES

Much of the world's energy reserves are associated with deltas and deltaic deposits.

The vast majority of the world's coal is found in deltaic deposits. 8 Also, deltaic processes frequently result in a favorable association of potential reservoir source and seal facies. 1 Consequently, major oil and gas reserves are commonly found in modern and ancient delta settings.

Modern deltas have also played a significant role in offshore petroleum exploration and production. The introduction of large amounts of sediment into the nearshore environment frequently forms a subaerial bulge that projects further seaward than the adjacent coast and a large, shallow submarine, delta front apron.

These environments provide a convenient platform from which land based and shallow offshore drilling rigs can reach deeper targets that otherwise could only be drilled by more expensive rigs in the deeper water areas adjacent to the delta.

DELTA CLASSIFICATION

Various classification schemes for deltas have been proposed based upon planview geometry, 10 variations in river mouth flow, 11 and the interaction between energy conditions where fluvial and marine processes meet. 9 11 12

The latter approach is the most useful and has become the most widely applied classification system.

Three major classes are recognized: 12 river dominated deltas, wave dominated deltas, and tide dominated deltas.

The Mississippi bird's foot delta is the archetypical example of a fluvially dominated delta; the Rhone and Nile deltas are examples of wave dominated deltas; and the Bengal and Mekong deltas are examples of tidally dominated deltas.

Two major developments have occurred in delta research in recent years. The first is the increased recognition of the importance of instability and slumping on delta fronts. 1

Slumps, slides, and growth faults are very common in many modern and ancient deltas and play a major role in the distributions of lithofacies. 13 14 15

Delta front instability is also a significant factor in locating offshore drill sites and production facilities. The second development is the increased research on and recognition of the significance of coarse-grained and fan deltas in the stratigraphic record. 16 17 18

IMPORTANCE OF DELTAS

The interest in deltaic sediments since the 1920s has been in large measure stimulated by the fact that they frequently contain large volumes of oil and gas. 1

Deltaic facies are attractive exploration targets because of three major attributes. 9

Deltas frequently comprise large volumes of reservoir quality sands. They are also the point source for sediment that is redistributed within the depositional basin by marine processes and therefore have an influence on exploration trends associated with offshore and strandline systems.
High organic productivity is frequently associated with deltaic deposition. On the delta plain large accumulations of terrestrial organics can be deposited in swamps and marshes. With burial these are converted to peats, lignites, and coals and may be significant gas sources. The subaqueous delta front can also be a site of high organic productivity where nutrient-rich fresh water mixes with marine water resulting in a proliferation of plankton.
High sedimentation and subsidence rates at deltaic sites frequently result in structural traps associated with growth faults and load-induced flowage of mobile substrates. Compactional subsidence and juxtaposition of facies can also form potential stratigraphic traps.

DELTAIC OIL, GAS AREAS

Consequently, many of the world's modern deltas have been sites for oil and gas exploration.

Significant hydrocarbons have been discovered in 17 deltaic provinces (Table 1).

Five deltaic basins contain fields with resources of greater than 500 million bbl of oil.

Fields with estimated reserves greater than 100 million bbl of oil equivalent have been discovered in 12 deltas. Resource data for giant fields in deltaic provinces are presented in Table 2.

The five most prolific deltaic provinces are:

The Mississippi delta in the southwestern U.S.
The Mackenzie delta in Northwest Canada.
The Niger delta in Nigeria, West Africa.
The Baram delta in Malaysia and Brunei.
The Mahakam delta in Kalimantan, Indonesia.

The Mississippi delta has produced more than 7.4 billion bbl of oil and 82.5 tcf of gas, predominantly from Tertiary reservoirs associated with structural traps formed by down to the basin growth faults and salt tectonics.

To date about 1.4 billion bbl of oil and 12.3 tcf of gas have been discovered in the Mackenzie delta from reservoir intervals in late Paleozoic, Cretaceous, and Paleogene strata. 19

The Niger delta has produced 13.3 billion bbl of oil and more than 6 tcf of gas mainly from Miocene deltaic sands. Traps are predominantly formed on the downthrown side of growth faults. 20

Similarly, gravity induced growth faults, coupled with northeast-southwest trending compressional folds, form the major hydrocarbon traps in the Baram delta province off Sarawak and Brunei. 21 More than 3.1 billion bbl of oil and 7.4 tcf of gas have been produced from Miocene reservoirs beneath this delta.

The fifth major producing delta is the Mahakam, which has produced 1.8 billion bbl of oil and 3.5 tcf of gas from Miocene-Pliocene reservoirs. In this deltaic province the oil and gas are believed to have been generated from Type III terrestrial kerogens that occur dispersed in claystones or concentrated in humic coal beds. 22

RECENT ACTIVITY

Deltaic provinces have continued to remain active oil and gas exploration areas throughout the late 1980s and the beginning of this decade (Table 3).

During this period various exploration activities including licensing, acquisition of gravity and magnetics, seismic, and drilling were conducted in the following areas:

In the Niger, Nile, Senegambia, and Ogooue (Gabon) deltas in Africa.
In Pakistan, India, and Bangladesh activity centered on the Indus, Krishna-Godavari, and Bengal deltas, respectively.
The far East saw activity on the Irrawaddy, Mekong, Baram, Mahakam, and Sabah deltas.
The Orinoco (Columbus basin, Trinidad) and Amazon deltas were sites of interest in South America.
North American activity was predominantly associated with the offshore Mississippi delta province.
In Europe, the Po River delta remained an active exploration province.

Moreover, licensing and bid rounds were recently conducted or planned for the near future for blocks in the Bengal, Mackenzie, Mekong, and Nile deltas. 23 24 25 26

FUTURE EXPLORATION

The geological attributes of deltas and deltaic deposits assure that then, will continue to be active oil and gas exploration provinces throughout the 1990s and into the 21st century.

Deepwater delta-front facies in front of the Mississippi delta will continue to attract considerable attention in the U.S.

In Canada, the building of the Mackenzie Valley pipeline will open up the Mackenzie delta for additional exploration, and settlement of "native land" claims will result in increased activity in northern Canada, some of it focused on the smaller deltas feeding the Beaufort Sea and Amundson Gulf.

The trend toward international exploration will result in continued exploration and development around mature, developing, and emerging deltaic basins.

In more mature provinces activities will be concentrated on deeper horizons and deeper water delta front facies. Basins associated with smaller deltas may also be explored.

Also, as developing nations become more industrialized, deltaic provinces that have previously been recognized as uneconomic or gas prone will see a resurgence in activity. The Rufiji delta off Tanzania and the Zambezi delta in Mozambique are two examples that fit this category.

Finally, deltas in remote areas with hostile operating environments, such as those in the circum-arctic region, will also see an increase in exploration.

ACKNOWLEDGMENTS

Production figures and reserve estimates were compiled from the Nehring Oil & Gas Fields of the U.S. database, Petroconsultants, and the American Association of Petroleum Geologists. 27 Steve Waller, Larry Park, Sandy Rushworth, and Joe Maglowicz assisted in compiling data. Amoco Production Co. authorized publication.

REFERENCES

Miall, A.D., Deltas, in Walker, R.G. led.), Facies Models: Geo Series 1, Second Edition, 1984, pp. 105-118.
Bates, R.L., and Jackson, J.A., Glossary of Geology: American Geological Institute, Alexandria, Va., 1987, 788 p.
Shirley, M.L., and Ragsdale, J.A. (eds.), Deltas in their Geologic Framework, Houston Geological Society, 1966, 251 p.
Morgan, J.P., and Shaver, R.H. (eds.) Deltaic Sedimentation: SEPM Special Publication No. 15, 1970, 312 p.
Broussard, M.L. (ed.), Deltas, Models for Exploration, Houston Geological Society, 1975, 555 p.
LeBlanc, R.J. (ed.), Modern Deltas, AAPG Reprint Series No. 18, 1976, 205 p.
LeBlanc, R.J. led.), Ancient Deltas, AAPG Reprint Series No. 19, 1976, 226 p.
Fairbridge, R.W., Deltaic sediments, in Fairbridge, R.W., and Bourgeois, J. (eds.), The Encyclopedia of Sedimentology, Dowden, Hutchinson and Ross Inc., Stroudsburg, Pa., 1978, pp. 235-240.
Fisher, W.L., Brown Jr., L.F., Scott, A.J., and McGowen, J.H., Delta Systems in the Exploration for Oil & Gas: A Research Colloquium, Bureau of Economic Geology, University of Texas at Austin, 1969, 78 p.
Bernard, H. A., A resume of river delta types (abs), AAPG Bull., Vol. 49, 1965, pp. 334-335.
Coleman, J.M., and Wright, L.D., Modern river deltas: variability of processes and sand bodies, in Broussard, M.L. (ed.), Deltas: Models for Exploration, Houston Geological Society, 1975, pp. 99-150.
Galloway, W.E., Process framework for describing the morphologic and stratigraphic evolution of deltaic depositional systems, in Broussard, M.L. (ed.), Deltas: Models for Exploration, Houston Geological Society, 1975, pp. 87-98.
Coleman, J.M., Prior, D.B., and Lindsay, J.F., Deltaic influences of shelf edge instability processes, in Stanley, D.J., and Moore, G.T. (eds.), the Shelfbreak: Critical Interface on Continental Margins, SEPM Special Publication No. 33, 1983, pp. 121-137.
Winker, C.D., and Edwards, M.B., Unstable progradational clastic shelf margins, in Stanley, D.J., and Moore, G.T. (eds.), The Shelfbreak: Critical Interface on Continental Margins, SEPM Special Publication No. 33, 1983, pp. 139-157.
Nemec, W., Aspects of sediment movement on steep delta slopes, in Colella, A., and Prior, D.B. (eds.), Coarse-Grained Deltas: International Association of Sedimentologists Special Publication No. 10, 1990, pp. 29-73.
Koster, E.H., and Steel, R.J. (eds.), the Sedimentology of Gravels and Conglomerates, CSPG Memoir 10, 441 p.
Nemec, W., and Steel, R.J. (eds.), Fan-Deltas: Sedimentology and Tectonic Settings, Blackie, Glasgow-London, P. 444.
Collela, A., and Prior, D.B. (eds.), Coarse-grained Deltas, International Association of Sedimentologists Special Publication No. 10, 1990, 368 p.
Campbell, G.R., Exploration update: Mackenzie/Beaufort Sea Region, Arctic Canada, Proceedings, 22nd annual OTC, OTC 67?1, Vol. 2, 1990, pp. 9-22.
Evamy, B.D., Haremboure, J., Kamerling, P., Knaap, W.A., Molloy, F.A., and Rowlands, P.H., Hydrocarbon habitat of Tertiary Niger delta, AAPG Bull., Vol. 62, 1978, pp. 1-39.
Rijks, E.J.H., Baram delta geology and hydrocarbon occurrence, Geological Society of Malaysia Bull., Vol. 14, 1991, pp. 1-18.
Durand, B., Bessereau, G., Doligez, B., and Oudin, J.L., Generation and migration of petroleum in the Mahakam delta, Indonesia (abs.), AAPG Bull., Vol. 72, 1988, P. 999.
Bleckner, N.H., Oil and gas development in Far East in 1989, AAPG Bull., Vol. 74, No. 108, 1990, pp. 257-280.
Creed, R.M., Hewitt, M.D., Hogy, J.R., and Yeo, C.J., Oil and gas developments in Western Canada in 1989, AAPG Bull., Vol. 74, No. 10B, 1990, pp. 1641119.
Foreign Scouting Service, Vietnam, Semiannual resume first half 1991, Petroconsultants SA, Geneva, 1991, 8 p.
Popescu, B.M., Oil and gas development in North Africa in 1989, AAPG Bull., Vol. 74, No. 10B, 1990, pp. 315-335.
Carmalt, S.W., and St. John, B., Giant oil and gas fields, in Halbouty, M.T. led.), Future Petroleum Provinces of the World, proceedings of the Wallace E. Pratt Memorial Conference, AAPG Memoir 40, 1986, pp. 11-53.