RESERVOIR PROPERTIES OF TRINIDAD FORMATIONS

RESERVOIR PROPERTIES OF TRINIDAD FORMATIONS

Allan Russell, Division Reservoir Engineer, Trintoc

This paper deals with the reservoir properties of geological formations found in Trinidad's land and marine areas. Data has been obtained from previously published geological and engineering sources This data would serve as a guide to the evaluation of these formations, since reservoir data should be as representative as possible in order to typify, both the range of values to be found, as well as provide a measure of average conditions.
Reservoir data required for the evaluation of geological strata fall into three major categories viz:
rock data, fluid data and production data.
In this presentation, reservoir data of Trinidad formations would be presented for major geological provinces in accordance with the generalized stratigraphic chart.

MAJOR GEOLOGICAL PROVINCES
Geologically, Trinidad can be sub-divided into the following major geological provinces:
(1) Southern Basin
(2) Central Range
(3) Northern Basin
(4) North Coast Marine Area
(5) Columbus Basin

Southern Basin Formations
Producing reservoirs of the Southern Basin occur between depths as shallow as 300 feet to 12,000 feet drilled depth, and are from shallowest to deepest Morne L'Enfer, Forest, Cruse, Deep Cruse, Karamat, Shallow and Deep Herreras.
Porosity decreases with depth from 30-33% for shallow Morne L'Enfer Formations, to 25-27% for Forest and 30 to l8% for Cruse and Deep Cruse respectively. Porosity is generally of a primary, nature with clays providing the cementation. Unconsolidated reservoirs are typical.
Water saturations vary from 20% to 35% with an average of 30%. For land areas water salinity increases with depth being fresh near the surface, increasing to 20-30,000 ppm chlorides with depth,
Permeabilities vary from 10 to 1495 md depending on shaliness of the rock. Shallow formations exhibit high permeability's of 0.5 to 1.5 Darcys. Permeability decreases with depth generally and at Deep Cruse level average 50 md.
Hydrocarbons usually found in these sediments are oil with gravities varying from <10 0API tars/bitumens to medium gravity crude oils of l4-28 0API. Small gas reservoirs exist, with occasional small gas caps underlain by oil or water,
Production mechanism is generally of the solution gas drive type with some gravity drainage for high relief high permeability reservoirs. Crude oil recoveries range from <l% for the shallow heavy oil to 15 to 25% for the medium gravity crudes.
Herrera turbidites occur at shallow levels, less than 6000 feet, at intermediate levels between 600' and 10,000' and at deep level below 10,000'. The shallow Herreras exhibit 20 to 28% porosity, 30-30% water saturation and permeability's ranging between 10 to 200 md. These sands are usually unconsolidated. At Intermediate levels, an average porosity is reduced to 20.7% average, with water saturation of 38%. Permeability to air average 190 Id. Fluid types vary from heavy black crude oil, 180 API, to light oils to gas condensates. The reservoir sands are well cemented.
At Deep Herrera levels, porosity is reduced to 6-180, water saturation increased to 35-40%, and permeability to fluid to <1 md. Oil gravity varies from l8to 280 API. Reservoir sands are well consolidated. Producing mechanism is via solution gas drive.
The Catshill formation occurs at depths between 1000' to 3500' and has porosity of 30-35%, permeability of 30-40 md and oil gravities of 250 to 450API.

The Gros Morne formation is a consolidated sand reservoir with porosity of 24-26%, water saturation of 36%, permeability of 185-200 md and produces a crude oil of gravity 320 API.

Central Range
Producing formations in this province are Manzanilla, Durham, Sum Sum and Narivas. Nariva sands are found between 1800' to 8500' exhibit porosities between 19% to 29% with higher porosities occurring in the clean basal (blanket) Nariva sands. Pemeabilities to air range from 20 to 1750 mds, averaging 280 mds. Oil gravity ranges between 300 to 360 API.
Manzanilla formations occur between 4000' to 9500' depth and exhibit porosity of 30%. Oil gravities vary from 18 to 210 API.
Durham' and Sum Sum sands have been reported as productive, usually gas bearing.

Northern Basin
Producing formations are in the Springvale and Manzanilla formations. No published data is available. Fields found in this area are Couva Marine, South Domoil, Mahaica and Freeport.

North Coast Marine Area
No published data is available. However, gas discoveries have been reported from in Miocene sediments.

Columbus Basin
Formations producing in this province are of Miocene and Lower Pliocene ages. Porosity ranges between 16% to 30%, decreasing with depth. Water saturations range from 15% to 49% and average 30%. Hydrocarbons found are light oil >300 API, gas condensate and wet gas fields.
Petrophysical analyses of cores in the Samaan Field show that the rock is a quartzitic sandstone of 80% quartz and 4% feldspars with predominant kaolinite clays. Porosity decreases with depth from 24% to 19% and water saturation increases from 21% to 33%. Permeability to air ranges from 160 to 250 md.
For the Teak Field porosity ranges from 20 to 30% in the Upper Pliocene and 14 to 20% in the Lower Pliocene. Permeability to air decreases from 300 to 500 Rd for the Upper Pliocene to 150 to 200 md for the Lower Pliocene. Oil gravity is 24.50 to 320 API, having no wax nor sulphur. Reservoir pressure gradients average 0.46 psi/ft and temperature gradient of 1.250F/100 feet.

CONSIDERATIONS FOR NEW - RESEVOIRS
Exploration for deeper reservoirs must address the concern for porosity preservation with depth. Cementation changes and mineral overgrowths should be considered in the evaluation of formations prior to exploration drilling since a loss in porosity would drastically reduce rock permeability. Mineralogy of the formations and special core analyses would provide clues to reservoir properties at depth.
If tight formations are expected consideration should be given to hydraulic fracturing. It is emphasized that core and reservoir fluid analyses are essential for the proper evaluation of new reservoir.

REFERENCES

I. Bitterli, P., 1958, Herrera Subsurface Structure of Penal Field, Trinidad.BWI. AAPG Vol 42, p 142-158
2. Ablewhite, K. and Higgins, G.E., A Review of Trinidad West Indies, Oil development and the Accumulations at Soldado, Brighton Marine, Grande Ravine, Barrackpore-Penal and Guayaguayare, Fourth Caribbean Geological Congress, 1968, p 41-73
3, Bower, T.H. 'Geology of Texaco Forest Reserve Field, Trinidad, W.I.. ' Fourth Caribbean Geological Congress POS, p 75-86
4. Jones. H.P., 'The Geology of the Herrera Sands in the Moruga West Oilfield of South Trinidad.' Fourth Caribbean Geological Congress 1968, p 91-100
5. Bane. S.C. and Champong. R.R.. 'Geology and development of Teak Field. Trinidad and Tobago'. Fourth Latin American Geological Congress, p 672-683
6. Beard, J.T., 'The General Geology of the Guapo Field, Trinidad', Fourth Latin American Geological Congress, p 684-689
7. Kiriani. K.U., 'Geology of the Inniss, Catshill and Balata East Oilfields in Trinidad. West Indies'. Fourth Latin American Geological Congress, p 700-713
8. Low. B.M., 'The Geology of the Fyzabad Main Field', Fourth Latin American Geological Congress. p 714-719
9, Radovsky. B. and Iqbal, J., 'Geology of the North Soldado Field', Fourth Latin American Geological Congress, p 759-769
10. De Landro, W.V.C. 'Petrophysical Study of the Samaan Field Shaly Sands, Offshore Trinidad', First Geological Conference of GSTT. p 183-205
11. Maharaj, D.H. and Bissoondath, J.C., 'Cyclic water flooding in the Coora Field'. SPE T & T 198 Conference, p 107-118
12. Bartlett. G.W., 'Considerations of Reservoirs in Trinidad for the In-Situ Combustion Recovery Process'. SPE T & T 1982 Conference, p 119-131
13. Bertrand, W.G., Elliot, C.E. and Chambers, J.C., 'The Geologist's Role in the Development of Trintoc's heavy oil Reserves'. SPE T & T1982 Conference, p 132-139
14. Wattley, G.S., 'Producing Horizons and mechanisms at Forest Reserve', SPE T & T 1976 Conference, p 20-27
15. Simpson, G.J.H., 'Waterf1ooding the Navette '410' Reservoir', SPE T & T 1976 Conference, p 44-58.
16. Alleyne, N.A.B. and Harper, W.N.. 'Distinguishing Between Oil and Gas in Shaly Sands - A Case History', SPE T & T 1976 Conference, p 95-121
17. Ramsaran. A. 'Evaluation of the Production Performance of the Parrylands Pilot Steam Project, SPF T & T 1987 Conference
18. Taylor, C.H.. 'Design Criteria and Early Response to Steam Injection in multiple Sands of the Cruse 'E' Pilot Steam Project in Point Fortin', SPE T & T 1987 Conference
19. Besson B. development of a Gas Field Offshore Trinidad'. SPE T & T 1984 Conference
20. Russell, A. 'Water Influx in the Intermediate Herrera Reservoir of Trintoc's Southwest Penal Field SPE T & T 1984 Conference.
21. ~ Russell, A. and Bertrand, W.G., Development of the 173 'B' Sand Reservoirs'. SPE T&T Conference l980.
22. Russell, A., Taylor. C.H. and Thomas, E.P., 'Review of the Balata East Field'. SPE I & I 198 Conference. p 44-5;'
23. Russell. A. 'Stimulation Alternatives for Low Permeability Deep Herrera Reservoirs in Trintoc's Barrackpore Field'. SPE I & I 1987 Conference
24. Russell, A. and Attai, R.N., 'Pressure Transient Analysis as an Aid to Infill development drilling In Trintoc's Point Fortin Field,' SPE I & T 1985 Conference

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