GEOCHEMISTRY OF OILS PROVIDES OPTIMISM FOR DEEPER EXPLORATION IN ATLANTIC OFF TRINIDAD

Suhas C. Talukdar, Wallace G. Dow
DGSI
The Woodlands, Tex.

Krishna M. Persad
Petroleum Consultants Trinidad Ltd.
San Fernando, Trinidad

Since petroleum activity in Trinidad began in 1866, three oil provinces have been discovered. They are the Land (Southern) province, the Gulf (of Paria) province and the East Coast (Atlantic) province (Fig. 1).

The discovery of each led to a new peak of oil production. Production in the country varied between 50-80 million bbl/year during the last two decades. The East Coast province alone currently produces around 70,000 b/d or roughly half of the country's production.

Within the last 18 years, a series of large gas fields was found in the East Coast province in deeper offshore areas where no major oil fields are located.

In addition, extensive drilling off the north coast of Trinidad in the early to mid 1970s also made significant additional gas discoveries.

Exploration interest in Trinidad has increased greatly in recent years because of the discovery of giant oil fields in the Maturin subbasin of Eastern Venezuela, as well as the growing recognition of the huge petroleum potential of the Eastern Venezuelan ba sin as a whole, of which Trinidad is a part.

INTEREST GROWING

The government of Trinidad and Tobago offered four offshore blocks off eastern Trinidad for competitive bidding that closed Jan. 31, 1990. It appears that the government will continue to pursue an active leasing program, aimed at offering all available offshore acreage.

Petroleum Consultants Trinidad Ltd. and a group of consultants that included DGSI recently prepared a non-proprietary report entitled "Treatise of Trinidad Petroleum-Atlas of Stratigraphy."1

It details the stratigraphy, tectonics, reservoir quality, exploration well data, and petroleum geochemistry of Trinidad.

The geochemistry section includes discussions on source rocks, oils, and basin modeling, and provides a broad understanding of the generation and migration of oil and gas and their distribution in Trinidad.

The geochemistry of oils is discussed in this paper as it relates to alteration processes. The results provide optimism for deep exploration in the offshore East Coast province of Trinidad.

PREVIOUS GEOCHEMISTRY WORK

Most of the earlier geochemical work suggested that the crude oils of Trinidad belong to a single genetic type.2 3 4 5 6

Leonard considered late Miocene Lower Cruse shales to be the source of oils in the Columbus subbasin. Other workers showed the predominantly marine organic matter source for Trinidad oils and correlated them with the organic rich shales of the Upper Cretaceous Naparima Hill and Gautier formations.2 6

The gas-condensates in the Columbus subbasin, according to Leonard, were primarily derived from high thermal maturation of Lower Cruse. Barker, on the other hand, suggested that the distribution of gas-condensates and oils in the Columbus subbasin are the result of more terrestrial and more marine organic facies of the Miocene Cruse formation, respectively.7

Among the alteration processes affecting the oils of Trinidad, biodegradation is widely recognized.2 4 6 Other authors demonstrated another alteration process caused by vertical migration that affected the molecular composition and API gravity of Columbus subbasin OilS.4 5

GEOLOGIC HISTORY

Trinidad and its offshore areas occupy the eastern extension of the Eastern Venezuelan basin and can be considered as parts of a polyhistory basin.

Its geologic history during Jurassic to Lower Cretaceous times, involves deposition of thick sediments in response to crustal attenuation and cooling during rifting of the North American and South American continents.

The Upper Cretaceous to Eocene period follows with uninterrupted sedimentation on the passive continental margin of northern South America.

During the Upper Cretaceous time upwelling conditions prevailed and prolific oil source rocks were deposited in outer shelf-upper slope environments along the entire passive margin of northern and northwestern South America in what are now Trinidad, Venezuela, Colombia, Ecuador, and Peru.

Paleocene-Eocene sediments of Trinidad and Eastern Venezuela were deposited in marine environments along the passive margin and show progressive northward regression of the sea.

The tectonic setting changed dramatically in the region since Late Eocene/Oligocene times. An oblique collision of the Caribbean Plate with the South American Plate proceeded diachronously eastward along the northern margin of South America, becoming progressively younger eastward.

As a result, foreland basin conditions became prevalent. The sediments deposited in this setting during the Oligocene-Middle Miocene period were tectonically emplaced as an east-northeast trending fold-thrust belt by the end of Middle Miocene time.

The interior mountain belt of Eastern Venezuela and the Central Range and its subsurface extension to the north (up to El Pilar fault zone) in Trinidad (Fig. 2) are the eroded remnants of this deformed belt.

The foreland basin axis and the deformation migrated southward during the late Miocene.

During Pliocene-Pleistocene times, as a result of the diachronous nature of the oblique collision, the depocenters and deformation moved eastward. The infillings of the Maturin subbasin of Eastern Venezuela and the Caroni and Southern sub-basins of Trinidad all became increasingly continental.

In this post-sutural stage, construction of a continental embankment off the east coast of Trinidad took place. It is characterized with the deposition of very thick suites of deltaic and associated sediments that prograded over the continental shelf edge onto the oceanic crust.

With the building of the continental embankment, syndepositional detached listric and related growth faults and associated rollovers were developed. Oblique collision in Plio-Pleistocene times resulted in wrenching and associated structures in these sediments.

OIL, GAS GENERATION

The oil and gas accumulations in Trinidad, with some minor exceptions, occur in both onshore and offshore areas to the south of the Central Range.

Oil source rock correlation studies not discussed here indicated that the Upper Cretaceous shales and argillaceous limestones of the Naparima Hill and Gautier formations are the probable source of the majority of these oils.

Oil generation and expulsion from the source rocks in this region took place mainly during the Upper Miocene-Pliocene times.

Excellent reservoir sands occur within the Oligo-Miocene through the Pleistocene formations and structural traps were formed during the Pliocene-Pleistocene wrench tectonics. The oils migrated into Upper Tertiary reservoirs along faults.

OILS, ALTERATION

Alteration processes of oils in Trinidad reservoirs are related to the geologic history of the basin.

These alteration processes actively participated in changing the oil compositions. These processes include thermal maturation, microbial degradation, water washing, and evaporative fractionation.

Thermal maturation, biodegradation, and water washing are widely recognized alteration processes of reservoired oil.

Thermal alteration can upgrade the commercial value of an oil accumulation by increasing the quantity of light hydrocarbons and paraffinicity while reducing the percentage of asphaltic compounds high in nitrogen, sulfur, and oxygen.

In certain geologic conditions, the oil in the reservoir might suffer extreme maturation and be destroyed and converted to gas. On the other hand, biodegradation can reduce the economic value of an oil by destroying paraffins, removing light ends, and oxidizing the remaining fraction of the oil.

In extreme cases, biodegradation can destroy an oil accumulation. Thermal maturation increases the API gravity of an oil, and biodegradation lowers the gravity.

Evaporative fractionation, a term first coined and defined by ThompsonB and subsequently described by the same author in later publications, is a less recognized alteration process.

It is a secondary alteration process and leads to abnormally high concentrations of light aromatic and naphthenic hydrocarbons relative to paraffins in oil is suggested as the causal mechanism for the fractionation of light ends.

Progressive gas loss from gas-saturated oil is suggested as the causal mechanism for the fractionation of light ends.

OIL CHARACTERISTICS

Geochemical data on Trinidad oils include whole oil gas chromatograms obtained from 24 oils from both land and offshore areas.

Land oils are produced from Miocene, Lower Pliocene, and Middle Pliocene reservoirs in several fields from the Southern subbasin, and the South Range of the Land Province. The depths of reservoirs range between 800-8,400 ft, and API gravities vary between 20-49.

Offshore oils are produced from Lower, Middle, and Upper Pliocene reservoirs of the Columbus subbasin. The reservoir depths vary between 4,000-12,000 ft. The API gravities of oil range between 27-43.

Based on geochemical data not discussed here, the oils studied are found to be genetically related and appeared to have originated from Upper Cretaceous source rocks. Thermal maturity of the oils is also within a limited range and relatively low.

A gas chromatogram of an unaltered, original oil is characterized by the usual exponential decrease in straight chain paraffins or n-alkanes (Fig. 3A). Figure 3B is a computer enhanced portion of the oil's light ends.

Biodegradation has affected many of the Trinidad oils from depths of 800-7,500 ft. It is recognized by extensive removal of n-alkanes and isoprenoids as shown in the chromatogram in Fig. 4A.

An anomalous feature in many of the biodegraded oils is an abundance of light hydrocarbons which biodegradation and water washing should have removed.

The distribution can be explained by introduction of natural gas/condensate that was introduced into already biodegraded heavy oils.

Oils that are neither original nor biodegraded show a characteristic deltoid profile in the N-alkane distribution where the dominant n-alkane is commonly between nC6 to nCl5 (Fig. 4B).

When the distributions of Fig. 4B are compared with those of original oils (Fig. 3), the deltoid pattern appears to indicate partial light end loss.

A display of the ratios of tolune/n-heptane (termed B value) versus n-heptane/methylcyclohexane (termed F value) originally defined by Thompson (1987), is given in Fig. 5 for the Trinidad oils.

The B value is associated with the property of aromaticity and is related to evaporative fractionation. The F value is related to paraffinicity which increases with increasing maturity.

This diagram is used by Thompson to classify oils and gas-condensates according to four types of alteration processes: evaporative fractionation, maturation, water washing, and biodegradation.8

The light end hydrocarbon ratios (B and F values) of only two oils plot within the field of original oils (Fig. 5). These oils have the typical exponentially decreasing nalkane distribution.

An oil with an exponentially decreasing n-alkane distribution lies outside the field of original oil because of thermal maturation. Many oils, however, have the B and F values plotting along the trend of evaporative fractionation.

Evaporative fractionation selectively removed the normal paraffins such as n-heptane while enriching the oil in light aromatics such as toluene. The oils showing light hydrocarbon fractionation also exhibit deltoid n-alkane distribution pattern.

Some oils show effects of evaporative fractionation in light ends but lack the deltoid n-alkane distribution. These are biodegraded oils that lost their n-alkanes in an earlier intense alteration phase through biodegradation.

Light hydrocarbons were introduced into earlier heavy biodegraded oil pools through later gas migration and mixing. A part of the light hydrocarbons were subsequently lost with accompanying evaporative fractionation.

The sequence of processes were: early oil migration into reservoir; intense biodegradation; natural gas mixing through introduction of migrating gas; and light hydrocarbons loss and accompanying evaporative fractionation.

Three oil samples plot in the area of biodegradation in the B-F diagram (Fig. 5) showing alteration of light hydrocarbons. These oils also extensively lost their n-alkanes and isoprenoids, as exhibited in their whole oil gas chromatograms.

The sequential processes affecting these oils could be envisioned as: early oil migration into reservoir; intense biodegradation; gas mixing; and biodegradation of light ends.

It can be concluded from above that the history of genetically related oils in Trinidad was quite complex and variable since the time of the initial migration and accumulation into the reservoirs.

A few oils retain their original composition, but most are affected by a combination of processes including biodegradation, mixing, and evaporative fractionation.

Two phases of biodegradation can be identified in some oils.

The earlier (main) phase removed most of the n-alkanes and isoprenoids from original oils. The later phase affected the light hydrocarbons that mixed with earlier biodegraded oils.

Evaporative fractionation in light ends with accompanying light hydrocarbon loss changed many of the oils.

The evaporative fractionation was caused by continuing migration of gas into oil pools and subsequent loss of selected hydrocarbons in gaseous solution from these oil pools.

MANY OILS RESIDUAL

It appears that many Trinidad oils in reservoirs are residual in the sense that they lost their light hydrocarbons through evaporative fractionation.

Where did the light hydrocarbons that escaped from these oils accumulate and what would their composition be like?

Gas-condensates are known off Trinidad and are predominant in the eastern half of the Columbus subbasin. They are found in reservoirs at relatively shallow depths and within younger stratigraphic and higher structural levels compared to that of the crude oils which show loss of light hydrocarbons.

The authors interpret these gas-condensates to be derived through the process of evaporative fractionation during tertiary migration.

They are neither the mature gas-condensates that Leonard (1983) believed, nor were they derived from the more terrestrial organic facies of Miocene Cruse formation at lower thermal maturity as Barker suggested.7

IMPACT ON DEEP EXPLORATION

The following scenario is what might be expected in many offshore areas south and east of Trinidad including the Columbus subbasin.

The area shows very high rates of subsidence and heating during hydrocarbon generation. As a result, Upper Cretaceous source rocks generated oil during Upper Miocene-Pliocene times and went through the gas generation phase very rapidly during the Pleistocene.

Active tectonic conditions prevailed throughout Pliocene-Pleistocene times. Faults developed during this period and provided avenues for vertical migration of deep thermal gas into overlying oil reservoirs.

The gas selectively removed light hydrocarbons from the oils and led to tertiary migration and accumulation of evaporative gas-condensates in shallow reservoir.

The implication of evaporative gas-condensates in the region is very significant. In the areas where only evaporative gas-condensates are known at present, one would expect to find deeper residual oils and possibly thermal gas-condensates and gas deposits at even greater depths.

Exploration of deeper targets, therefore, looks very promising.

REFERENCES

Petroleum Consultants Trinidad Ltd., Treatise of Trinidad Petroleum-Atlas of Stratigraphy, Vol. I through IV, 1989.
Persad, K.M., et al., Review of the hydrocarbon prospects of the Trinmar area. GPE Report 323, 1980, unpublished.
Leonard, R., Geology and hydrocarbon accumulations, Columbus Basin, Offshore Trinidad, AAPG Bull., Vol. 67, 1983, pp. 1,081-1,093.
Ames, R.L., and L.M. Ross, Petroleum geochemistry applied to oil field development, Offshore Trinidad. Trans. First Geological Conference of the Geological Society of Trinidad and Tobago, 1986, pp. 227-236.
Ross, L.M., and R.L. Ames, Stratification of oils in Columbus basin off Trinidad. OGJ, Sept. 26, 1988, pp. 72-76.
Rodrigues, K., Oil source bed recognition and crude oil correlation, Trinidad, West Indies. In Advances in Organic Geochemistry 1987, Org. Geochem. 13, Nos. 13, 1988, pp. 365-371.
Barker, C., Organic geochemistry in petroleum exploration. AAPG Continuing Education Course Note Series No. 10, 1979, 159 p.
Thompson, K.F.M., Fractionated Aromatic Petroleums and the Generation of Gas-condensates. Org. Geochem. Vol. II, No. 6, 1987, pp. 573-590.