A MODERN LOOK AT THE PETROLEUM GEOLOGY OF THE MARACAIBO BASIN, VENEZUELA

Karl W. Stauffer, Gregory D. Croft
Pantera Petroleum
San Leandro, Calif.

The Maracaibo basin of western Venezuela is one of the world's most important oil producing basins, with a cumulative production of more than 35 billion bbl (Fig. 1)(42038 bytes). The reasons for this great wealth of hydrocarbons are a combination of source beds of excellent quality, thick reservoirs with high porosity and permeability, and a series of sealing shales, faults, and unconformities, which provide large and numerous traps.

Recent discoveries combined with Venezuela's opening to international investment suggest that the story of this basin is far from over.

Surprisingly little exploration has taken place in large parts of the basin, especially southwest of Lake Maracaibo and in the southern part of the lake. This is in a large part because new exploration concessions were not allowed after 1958 and because investment since nationalization has been concentrated in the established producing areas.

HISTORY

The large oil seepages around Lake Maracaibo were noted in the 16th century by the Spanish, who used the tar to caulk their ships and treat skin problems on livestock. The U.S. based General Asphalt Co. conducted the first geological investigations on the east shore of Lake Maracaibo but sold its concession to Shell in 1912. Shell drilled the discovery well at Mene Grande in 1914 and the famous Los Barrosos 2 gusher at Cabimas in 1922. Production grew rapidly and this became Shell's most important producing property worldwide. Edward Doheny's Pan American Oil then took the unusual step of obtaining concessions in the lake itself. These concessions were purchased by Indiana Standard (now Amoco) in 1925, but development was minimal until they were sold to Standard Oil of New Jersey (now Exxon) in 1931. Development in Lake Maracaibo proceeded rapidly after the end of the Second World War and this became Exxon's most important producing property worldwide throughout the 1950s and 1960s.

Nationalism then played a role in the oil industry; no new exploration concessions were offered after 1958, and the industry was nationalized at the end of 1975. The nationalized entity, Petroleos de Venezuela SA, is now one of the world's largest integrated oil companies.

The award of marginal field reactivation blocks to Occidental and Shell in 1994 marked the beginning of a new phase of international participation in the Maracaibo basin. Another block was subsequently awarded to a consortium of Tecpetrol, Nomeco, and Wascana, and Chevron has agreed in principal with Maraven, a subsidiary of Petroleos de Venezuela, on a venture that would include the giant Boscan heavy oil field and Chevron's asphalt division in the U.S. Two blocks in the basin are to be offered in the 1995 exploration round.

In summary, the Maracaibo basin oil fields played a major role in the growth of three of the world's largest oil companies; the Royal Dutch/Shell group, Exxon, and Petroleos de Venezuela. Much early development of the technologies of offshore production and steam injection took place there.

Sir Henri Deterding once described Shell's purchase of the General Asphalt properties around Lake Maracaibo as his best business deal. That is a strong statement from someone whose business deals included the merger of Royal Dutch Petroleum with Shell Transport and Trading.

STRATIGRAPHY

The oil-productive rock units in the Maracaibo basin are the Cretaceous and Tertiary sediments (Fig. 2)(98894 bytes). These sediments fill a basin whose pre-Cretaceous "floor" consists of a wide variety of Paleozoic metamorphic rocks, granitic intrusions, and Triassic to Jurassic graben fill consisting largely of red beds.

The basal Cretaceous sand and conglomerates of the Rio Negro formation filled in the irregular topography and set the stage for the marine incursion to follow. The Rio Negro is in places an excellent reservoir rock, but mostly it is a poorly sorted mixture of clastics with moderate reservoir capabilities. Conformably overlying these clastics are the thick limestones of the Cogollo group. These limestones are largely shallow water, high energy deposits, with some intervening shales and sands. The Cogollo is an excellent reservoir where it has sufficient primary porosity and has also been fractured, as in Mara and La Paz fields.

On top of the Cogollo group lies what is perhaps the world's most prolific oil source rock-the La Luna formation. It consists of black, organic-rich limestones that are characteristically thin bedded and fracture readily because of a high content of chert. This formation was recognized many years ago as a source rock because vugs and ammonites in it are commonly filled with oil. The La Luna is also a productive reservoir where fractured.

The Colon shale conformably overlies the La Luna formation and forms a regional sea] over the whole Maracaibo basin. It is this shale that seals most of the Cretaceous traps. In the southwest part of the basin sand lobes in the upper part of the Colon are called the Mito Juan formation and are often good oil reservoirs. The Socuy limestone in the basal part of the Colon is a prominent seismic marker.

Over most of the Maracaibo basin the transition from the Cretaceous to the Tertiary is conformable. The Paleocene consists of sandstones and shales of the Guasare formation, whose coals supply both the Venezuelan and the Colombian coal export projects. It is an oil and gas reservoir in the southwest part of the basin and may also be a minor source rock.

The overlying Eocene sandstone and shale sequence of the Misoa formation is by far the most important reservoir unit in the Maracaibo basin. It consists of multiple sands, often very clean and unconsolidated, deposited as a deltaic complex that prograded from the south and west. Much of the Eocene sedimentary section was eroded during a late Eocene uplift and redeposited as the Icotea sandstone, another important reservoir.

Unconformably above the Icotea is a Miocene to recent sand-shale sequence including the Lagunillas and La Rosa formations, which are important reservoirs in the Bolivar Coastal Complex and adjacent areas. When not deeply buried, these Miocene reservoirs are known for their excellent quality with porosities around 30% and permeabilities; in the darcy range.

STRUCTURE

The structural history of the Maracaibo basin may be divided into two parts-a Cretaceous to Middle Eocene stage and an Eocene to present stage (Fig. 3)(63183 bytes). During the first stage the Cretaceous sediments were deposited in a gradually deepening marine environment with little tectonic activity. Through the middle Eocene coarser clastic sediments encroached on the basin, especially from the south and west.

At about the end of the middle Eocene the northeastern portion of the Maracaibo basin was uplifted, resulting in the erosion of a very thick wedge of mostly Eocene sediments. At the same time transverse faulting occurred, producing a major high in the center of Lake Maracaibo that is bounded on the west by the Icotea fault. Transverse faults, mostly with left-lateral displacement, seal many of the major oil accumulations in the basin.

The post Eocene structural phase was dominated by the uplift of the Sierra de Perija in the west and the Merida Andes in the south. The formation of these two mountain ranges also was responsible for numerous compressional faults parallel to the two ranges, and for the deposition of very thick Miocene and younger sediments eroded from the emerging ranges. Compressional structures associated with the Sierra Perija are productive of oil and gas in the Tarra area of Venezuela and the Catatumbo area of Colombia.

OIL FIELDS

The Bolivar Coastal Complex, composed of the Lagunillas, Bachaquero, Tia Juana, and Cabimas fields, is by far the most important producing area in the Maracaibo basin. The complex is a single contiguous producing area which, if considered a single field, is second in cumulative oil production only to Saudi Arabia's Ghawar field. Considering them separately leaves Lagunillas as merely the largest oil field in the Western Hemisphere. The complex is bounded to the northwest by the left-lateral Icotea fault. On the northeast it is bounded by erosional truncation at the Eocene level and by onlap at the Miocene level. Large scale production comes from both Eocene and Miocene sands and is enhanced by cyclic steam injection.

Lama and Lamar fields are located along the Icotea fault south of the Bolivar Coastal Complex. Each of these fields contains many normal faults which separate reservoirs in the multiple productive Eocene sands. Cumulative production from these fields is over 3 billion bbl.

Centro, Lago, Ceuta, Mene Grande, and Motatan fields are located along approximately north-south trending faults that are east of and roughly parallel to the Icotea fault. These fields produce from Eocene and Miocene sandstone reservoirs. Mene Grande, as its name implies, is the site of an enormous seepage and was the first major oil discovery in the Maracaibo basin.

La Paz, Mara, and La Concepcion fields are located northwest of the city of Maracaibo. Most of these fields' production has come from fractured Cretaceous lime-stone reservoirs in structural

traps, although Tertiary sands and fractured granitic basement also produce. Many wells in these fields had very high initial flow rates and the oil is light by Venezuelan standards.

Boscan field is a very large heavy oil accumulation west of Lake Maracaibo. The Eocene sandstone reservoir is bounded on the east side by the Boscan fault and on the west by truncation of the reservoir against the late Eocene unconformity.

Tarra, West Tarra, Tibu, and Rio Zulia fields are located north of the Colombian city of Cucuta in Venezuela and Colombia. These fields produce from Cretaceous, Paleocene, and Eocene reservoirs in recent, high-relief, anticlinal structures. The Cretaceous in this area includes considerable amounts of sandstone.

In addition to the fields described above, numerous smaller fields contribute to the Maracaibo basin's oil production.

BRIGHT FUTURE

The Maracaibo basin has a long history as a major oil producing basin, but many areas remain poorly explored; the large East Ceuta discovery of 1987 was less than 20 miles from the original Mene Grande discovery of 1914.

The large exploration potential combined with the enormous amount of remaining oil in place in known reservoirs guarantees that the Maracaibo basin will have a long future as a major oil producing basin.

BIBLIOGRAPHY

1. Gonzales de Juana, C., Iturralde, J.M., and Picard, X. Geologia de Venezuela y de sus Cuencas Petroliferas, Caracas, Ediciones Foninves, 1980.

2. Martinez, A., Cronologia del Petroleo Venezolano, Caracas, Ediciones Foninves, 1976.

3. Ramirez, E., and Marcano, F., Ceuta-Tomoporo Field, Venezuela, Giant Oil and Gas Fields of the Decade 1978 to 1988, AAPG Memoir 54, 1989.

4. Staff of the Caribbean Petroleum Co., 1948, Oil Fields of the Royal Dutch/Shell Group in western Venezuela, AAPG Bull., Vol. 32, No. 4, 1948, pp. 517-628

5. Venezuela, Direccion de Geologia, Lexico Estratigrafico de Venezuela, 2nd. Ed. Bol. Geol., Caracas, 1970.