Geology of marine evaporites favorable for oil, gas exploration

Saleh M. Billo
King Saud University
Riyadh

The conditions necessary for evaporite deposition are also important with respect to genesis of source beds for petroleum.

In a restricted basin marked by large-scale salt successions it is presumed that the basin proper is separated from the open sea either by structural or physiographic barriers. These barriers may elevate the effective wave base so that much of the basin is in the stagnant zone or in a reducing environment, where sediments rich in organic matter may be deposited. Such shallow barriers increase the conditions favorable for the generation of petroleum.

Since marine evaporitic basins are not ideally closed systems but are subject to influxes and perhaps refluxes of sea water or brine, much petroleum associated with evaporites is generated from dissolved and particulate organic matter swept from the normal marine into the evaporitic environment.

Only carbonates precipitate in the mesosaline part (4-12% salinity) of such evaporitic environments. They are of great significance in source-rock origin.

The huge reserves of petroleum in the Mesozoic of the Middle East, and many other areas including the Michigan, Paradox, and Delaware basins owe their origin to the thick sequences of carbonates and evaporites of the mesosaline environments.

Repeated cycles of oil and gas formation in the stratigraphic record are related to mild tectonic, climatic or eustatic events or both, and to increasing sedimentary overburden.

Introduction

Significant petroleum production related to carbonate-evaporite sequences has been found in many areas such as the Delaware (Fig. 1 [67509 bytes]), Paradox, and Michigan basins of North America and the Phanerozoic oil-bearing sediments of the Middle East and the North Sea.

The regular association of petroleum and evaporites long has been recognized in almost all major oil-producing chemical and biochemical reservoir rocks of the world. In finding oil, the conventional opinion is that dark and black shales of marine or estuarine origin are believed to be the source beds for petroleum. However, in barred basins, marked by the absence of the standard dark shales rich in organic matter, certain evaporitic environments are of great significance in source-rock origin.

A geologic approach to an exploration strategy may illustrate the facies relationships and hydrocarbon occurrences that provide a model for discovering the physical and chemical aspects of petroleum generation, migration, and accumulation, as well as the stratigraphic-tectonic relations necessary for entrapping oil and gas.

Since the majority of evaporite deposits displays a marked cyclical repetition of members (Fig. 2 [58588 bytes]) representing stages in the restriction of a seaway and the concentration of soluble salts, evaporite associations in carbonate reservoir rocks containing an estimated one-half of the worlds reserves of petroleum may help in predicting stratigraphic traps in similar cyclostratigraphic successions.

This article endeavors to review exploration to date on the productive potential of evaporite basins, to appraise basic requirements for evaporitic environments to contain potential hydrocarbons, and to assess the significance of marine evaporites in petroleum geology.

Evaporites, petroleum

The largest evaporite deposits in the world are concentrated in the sediments of the platforms and basins of the stable interior regions of the continents that grade into the major geosynclinal facies of the continental margins.

The oilfields in the carbonate basins and platforms of the southwest U.S. (Fig. 1 [67509 bytes]) are, similar to those of the Middle East, situated near an old continental margin in a region liable to ultimate subsidence over a long time span. There, however, the comparison ends.

Evaporites are associated with reservoirs or form the trap in 53% of the fields that have a net recovery of 3.5 tcf or more. Entrapping hydrocarbons requires the availability of nearly mature source rocks, porous and permeable reservoir rocks, and impermeable caprocks, and a suitable structure on the reservoir to form a closure.

Many oil pools have reservoirs that are capped by impervious beds of anhydrite. Many reef reservoirs are flanked by evaporite units, and much petroleum has been produced from traps associated with salt domes.

The region of western Texas and southeastern New Mexico, also known as the Permian Basin, is one of the richest producing areas in North America, accounting for about 17% of U.S. production.

Basins

The basic requirements for the formation of a marine evaporite deposit are in arid or semiarid climate and partial or complete isolation of the evaporating body from the open sea. An arid or semiarid climate is necessary in order to have an excess of evaporation over rainfall, runoff, and influx from the open sea.

In major evaporite basins distinguished by large-scale cyclical successions (Fig. 2 [58588 bytes]) it is assumed that marine incursions were restricted in some way, either by structural or physiographic barriers. Tectonic, climatic, or eustatic events may create marine openings in these barriers which are vital to provoke the cyclicity (Fig. 3 [41812 bytes]).

The reflux model was proposed to account for areal distribution patterns in the Late Permian of the West Texas-New Mexico area (Fig. 4 [45738 bytes]). This dynamic model calls for a continuous flow of undersaturated water across the surface of the evaporite basin, matched by a counterflow of dense brines at depth across the sill and out of the basin.

This process may give account for the vast volumes of anhydrite deposited in relation to the amount of halite precipitated. Perpetuation of the dynamic equilibrium state through a considerable period of geologic time would result in the side-by-side accumulation of thickness of salt, anhydrite, and carbonates (Fig. 4 [45738 bytes]).

Petroleum source beds

Barred or silled depositional basins promote the conditions favorable for the generation of petroleum.

Evaporites were of significance because they originated in such closed basins with restricted circulation, an environment in which the preservation of organic matter was favored. New sea water was introduced by seasonal breaching and sealing of these barriers.

Much of the petroleum associated with evaporites is reproduced from organic matter swept from the normal marine into the evaporitic environment. Marine embayments in which gypsum or gypsum and halite (Fig. 4 [45738 bytes]) were being deposited frequently featured a mesosaline environment in which only carbonates precipitate in the mesosaline part (4-12% salinity) of such an evaporitic environment (Fig. 4 [45738 bytes]) and no great dilution of organic matter by clastic or biogenic sediments occurs.

These primary fine-grained basinal carbonates are chemical sediments deposited under mesosaline conditions and are considered evaporites. Thus, an evaporite cap rock may be both a cap rock and a supply of the oil.

Delaware basin

The Permian Basin (Fig. 5 [68463 bytes]) contains thick carbonate and evaporite sequences deposited during Permian time.

Large-scale cyclic repetitions within the Ochoa evaporites mirror deepening and shallowing of water in the Delaware basin during transgression and regression of seas. The basin was encircled by a carbonate depositional environment or reef zone prior to evaporite deposition. It is separated from the Midland and Marfa basins respectively by the Central Basin platform and Diablo platform, and apparently connected with the ocean to the south.

Submarine fans of anyhdrite or gypsum accumulated during Castile time (Fig. 2 [58588 bytes]) along the eastern and northern basin margin due to influx of sediment from back-reef areas through passes in the surrounding reef.

The environmental conditions favored repeated accumulation of large amounts of organic materials in the deep marine basin and subsequent conversion of these substances to kerogen and later to liquid hydrocarbons that migrated into traps within the basin and also formed the reservoirs associated with the broad shelves of carbonate rocks surrounding the basin. The oil and gas formation was enhanced through mild tectonism. The deposition of Ochoa evaporites helped to seal the traps and preserve the hydrocarbons.

Patterns of salt dissolution in the Delaware basin are related to the bedrock geometry and hydrology that developed following uplift, tilting, and erosion in the Late Cenozoic. Removal of Permian Ochoa evaporites and overburden collapse were caused by deepwater-discharge augmented by faulting and high-permeability shelf aquifers (Fig. 5 [68463 bytes]).

Another example of the removal and thinning of salts in the cover by solution, and the proximity of important surface drainage (Pecos River), is shown in Yates field in West Texas (Fig. 6 [72999 bytes]). The salt removal trend on a thickness map of Salado (Ochoa) salt series shows that salt removal trend closely parallels a highly productive trend of oil and gas deposits in those permeable reservoir facies referred to earlier (Fig. 5 [68463 bytes]).

Other basins

In many provinces, evaporites are related spatially and temporally to known source rocks. In the Middle East, the accumulations of thick sequences of carbonates and evaporites are known to have occurred many times from the Triassic to Cretaceous. These cyclical successions depict a basin (or basins) marked by prolific production from limestones associated with evaporites (Fig. 7 [156618 bytes]).

Evaporitic conditions may have also been responsible for the large accumulations of petroleum in reefs around the Michigan basin and many other areas, including the Paradox and Delaware basins (Fig. 7 [156618 bytes]).

Conclusions

Barred basins, which probably have contributed the greatest volume of marine evaporites, not only concentrate oceanic salts but also enhance the conditions favorable for the generation and accumulation of petroleum.

Petroleum resources are associated with evaporites in many areas of the world including the Delaware, Michigan, Paradox, and Middle East basins. The problem of identification of source rock in these areas has revived the interest in the search for oil in evaporitic sedimentary basins and the reconstruction of their geologic history.

There is increasing recognition that barred basins form an effective trap for organic matter. The oil in these basins may have originated in dense black carbonates deposited within the huge basin center. The mesosaline conditions re- peatedly gave rise to source rocks over a period of millions of years.

Acknowledgment

King Saud University supported this work and sponsored the author to participate in conference activities under royal approval number 5-B-3974.

Bibliography

Anderson, R.Y., et al., Permian Castile varved evaporite sequence, West Texas and New Mexico, GSA Bull., Vol. 83, 1972, pp. 59-86.

Hiss, W.L., et al., Saline water in southeast New Mexico, Chem. Geology, Vol. 44, 1969, pp. 341-360.

Kirkland, D.W., and Evans, R., Source-rock potential of evaporitic environment, AAPG Bull., 1981, pp. 181-190.

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Roberts, W.H., III, Personal communication, AAPG research conference, Temperature environment of oil and gas, Santa Fe, N.M., Sept. 13-17, 1981.

This article is based on a poster prepared for the AAPG Eastern Section meeting at Michigan State University, East Lansing, Mich., Sept. 18-20, 1994.

The Author