GEOTHERMAL HOT SPOTS AND

OIL OCCURENCES OVER TRINIDAD

K. Rodrigues
(TRINTOC)

INTRODUCTION

For more than 50 years individual geologists (e.g. Van Orstrand, 1934) have observed an association between hydrocarbon occurrences and positive temperature anomalies or 'hot spots' as reflected in higher local geothermal gradients relative to the regional gradient.

Geothermal anomalies have been mapped over oilfields in several basins of the world and reported by Ball (1981), Gatenby (1981), Handique and Bharali (1981), Meyer and McGee (1985), Majorowicz et al. (1988), McGee et al. (1989). Structurally as well as stratigraphically controlled fields, and both oil and gas accumulations, show positive geothermal anomalies at producing levels.

The correlation between positive geothermal anomalies and the occurrence of hydrocarbons is based on the premise that water is the major agent of both hydrocarbon migration and heat transfer in sedimentary basins, and suggests a genetic association between geothermal gradients, hydrodynamics and hydrocarbon occurrences. This association has generated growing interest in geothermal studies in relation to petroleum deposits. Indeed, according to Ovnatanov and Tamrazyan (1970), "Although many methods are used to explore for petroleum, the geothermal method definitely deserves to take its rightful place among the standard exploration tools."

The major objectives of this study are as follows:
(i) To determine whether any positive geothermal anomalies or 'hot spots' identified correlate with the presence of oilfields as reported in the literature in other parts of the world.
(ii) To test the hypothesis that these 'hot spots' over oilfields originate in the upward and lateral movement of subsurface formation fluids, including oil, into traps, bringing higher temperatures from depth.
(iii) To explain local and regional variations in the geothermal gradients over Trinidad in relation to lithological variations, basement configurations, structure and fluid dynamics.

METHODOLOGY
The temperature underground normally increases with depth below the surface, and the geothermal gradient is a measure of this rate of temperature increase with depth. The geothermal gradient is obtained by dividing the difference between the temperature of the formation and the mean annual surface temperature (ambient) by the depth of the formation, and is usually reported as ⁰F/100' or ⁰C/km.
Ambient temperatures for onshore and offshore wells have been considered uniform in this study and a value of 75 ⁰F (24 ⁰C) was used.
Most temperatures obtained from oil and gas wells are measured when the hole is under less than stable thermal conditions. Formation temperature is disturbed by the circulation of drilling fluid, which often results in the lower part of the hole being cooled and the upper part heated.
The primary sources of temperatures from wells are:
(i) Electrical logging runs.
(i i) Static bottom hole pressure (BHP) tests or temperature surveys.
(iii) Drill stems tests (DST’s).

Static BHP's and DST’s are the most reliable and provide the most accurate temperature data but are generally not available in large quantities locally. Although being the least reliable, due to the relatively brief time required to run logs, not allowing for subsurface temperatures to stabilize, maximum mud temperature readings recorded on the headings of electric well logs are the most abundant and are the only practical source of enough temperature data in Trinidad.

Due to the non-availability of such data as circulation time, time logger on bottom and time since cessation of circulation, and insufficient logging runs in most wells it was not possible to correct measured bottom hole temperatures (BHTS) for true formation temperature using the Horner technique of Fertl and Wichmann (1977).

Wells were selected on the basis of depth of penetration, both in terms of straitgraphy and footage, and location so as to provide adequate stratigraphic and geographic coverage. More than 1000 wells were examined in the first instance for reliability and consistency of temperature data, and eventually data from 300 wells were used to compile the geothermal gradient map (Figure 1).

RESULTS AND DISCUSSION

Regional present day geothermal gradients over Trinidad vary from 0.8 ⁰F /100 (15 ⁰C/km) to 1.8 ⁰F/100 (32 ⁰ C/km), and rarely exceed 1.5 ⁰F/100' (28 ⁰C/km). They are comparable to the worldwide average of 1.4 ⁰F/100' (25 ⁰C /km) and to the 1.3 - 1.4 ⁰F/100' reported by Talukdar et al. (1988) for the Maturin Subbasin of Eastern Venezuela.

The lowest gradients (0.8 - 1.O ⁰F/ 100') are recorded in the Southeast Coast and North Coast Marine areas, whereas everywhere onshore and in the Gulf of Paria gradients are >1.1 ⁰F/100'. The highest geothermal gradients (1.5 ⁰ F/100') are recorded as 10 anomalies or 'hot spots' and are listed as follows (Figure 1).

GULF OF PARIA (1) Posa-48; (2) Manicou/South Domoil; (3) Couva Marine (4) Main Soldado/North Soldado; (5) East Soldado;

SOUTHERN BASIN - (6) Rock Dome/Moruga West: (7) lnniss .(8) Trinity/Goudron; (9) Balata;

EAST COAST MARINE AREA - Emerald - 1 ;

The principal sources of heat to the upper few miles of the earth's crust are in the outward flow of heat from the central core of the earth, in the presence of igneous magmas that are cooling, in the disintegration of radioactive elements and in the heat of subscrustal thermal convection currents (Levorsen, 1967).

Several factors can produce lateral variations or anomalies in the earth's temperature field, resulting in regional variations in geothermal gradients in sedimentary basins. These are discussed by Meyer and McGee (1985) and some of the more relevant factors are:
(i) Differences in heat flow rate from the source in the basement beneath the basin.
(ii) Lateral and vertical variations in thermal conductivity of the rocks due to a variety of causes (e.g. structural configuration, lithology (Figure 2), cementation, degree of compaction, permeability, fluid content).
(iii) Sources of heat within the rocks as a result of exothermic chemical reactions, mineralogical transformations and radioactive decay.
(iv) Recent intrusive or volcanic activity.
(v) Effects of moving water and other fluids (e.g. petroleum), including connate and juvenile water moving upward and meteoric water moving downward through fractures and pore spaces producing a warming and cooling effect respectively.
(vi) Tectonic friction causing heating.

Not all of these factors are equally important in any sedimentary basin and in some areas most of them can be dismissed as negligible or irrelevant. Factor (ii) is considered one of the primary causes of anomalous geotemperatures in most sedimentary basins. However, factor (v) has recently received considerable attention in the literature as a possible cause for positive geothermal anomalies recognised over oilfields (Meyer and McGee, 1985; Zielinski et al., 1985; McGee et al., 1989).

Anomaly 1 in the western Gulf of Paria is centered around Posa-48 (1.8 ⁰F /100'). This well encountered Lower Cretaceous limestones at 8800', overlain by Miocene/Pliocene clastics.

The Manicou/South Domoil geothermal anomaly 2 is located east of the Posa-48 anomaly and includes the small South Domoil oilfield (442,000 barrels) and the Manicou gas field. South Domoil -- 5 drilled Lower Cretaceous at 7000' overlain by Pliocene/Pleistocene clastics.

Further east, anomaly 3 is centered around the Couva Marine wells (1.6 - 1.8 ⁰F/l0O') and is associated with a small oilfield (227,000 barrels). Miocene and younger shales, sandstones, and conglomerates overlie Lower Cretaceous shales at 6500' and a 3000' - 7000' thick evaporate section at 9000' in Couva Marine-2 and 8000' in Couva Offshore -1. High heat flow in this area, as evidenced by higher than average geothermal gradients, is supported by high vitrinite reflectance values (Ro > 2.0%) measured on Lower Cretaceous shales (Rodrigues, 1988).

As elsewhere (e.g. Handique and Bharali, 1981) the geothermal patterns relationship between oilfields correlate with regional structural and gravity trends. High temperatures recorded as geothermal anomalies mapped in Posa - 48, Manicou/South Domoil, Couva Marine and Emerald –1 correspond to local structural highs which are the expression of basement highs associated with numerous deep-seated faults. Temperatures are highest on the crests of these local structures, decreasing toward the flanks. In addition to the known stratigraphy drilled in these wells, where Lower Cretaceous occurs at depths of 6000'-- 9000', Bouguer anomaly gravity dataover Trinidad show gravity highs termed the Posa - 40, Domoil, Gulf and Emerald Highs corresponding to anomalies 1,2, 3 and 10 respectively. Gravity highs are in many areas associated with anticlines or with horst blocks, both being structures which bring older denser rocks nearer the surface (Griffiths and King, 1974).

Eight of the 10 positive geothermal anomalies mapped are associated with oilfields: Manicou/South Domoil, Couva Marine, North and Main Soldado, East Soldado, Rock Dome/Moruga West, Inniss, Trinity/Goudron and Balata.

The fact that positive anomalies were not recorded in many other oil-fields (e.g. Forest Reserve, Palo Seco, Penal, Brighton Marine etc.) and that 2 positive anomalies (Posa - 48 and Emerald -1) are not related geographically to producing fields suggest that the association between productive oil-fields and positive geothermal anomalies may not be related simply to the presence of hydrocarbons, perse.

Anomaly 5 (East Soldado) is more difficult to explain as this structure is not a simple one. McDougall (1985) reported Pliocene/Pleistocene uplift along the south side of the Los Bajos fault and steep dips (400) in the eastern portion of the field as due to mud tectonism producing an anticline or mudcored uplift. The temperature anomaly may be related to heat influx associated with the upward intrusion of hot over-pressured shales (mud tectonism) in the vicinity of the Los Bajos fault. Both the gravity and oilfield distribution data interpreted with respect to occurrence of temperature anomalies (Figure 1) indicate that the observed temperature highs over some oilfields may reflect the complex relationship between:

(i) lower thermal conductivity of hydrocarbon bearing reservoir rocks, which results from hydrocarbons (lower thermal conductivity, Figure 2) replacing water in these rocks;

(i i) lateral variations in rock conductivity due to lateral changes in structure or lithology (Figure 2);

(iii) the heat carried along with upward and lateral fluid movement, which becomes trapped when the fluids are trapped.

CONCLUSIONS

This study does not support the contention that positive geothermal anomalies are uniquely associated with oil or gas accumulations and hence could be used as an exploration tool. In the first place the major thrust faults (Figure 1) considered to be the primary migration avenues are not correlatable with any temperature anomalies mapped. These faults, although deep penetrating and presumably heat conducting, do not produce temperature anomalies. Even the 2 major wrench faults (Los Bajos and Central Range/Warm Springs) show no correlation with temperature anomalies.

Secondly, temperatures used in compiling the geothermal gradient map are not producing level temperatures taken from measurements made during drill stem testing but are bottom hole temperatures.

Thirdly, non-hydrocarbon bearing water strata could conceivably produce temperature anomalies by themselves as long as a trap is present.

Some temperature highs are associated geographically with oilfields but this is not an unique association. The positive anomalies correspond to local structural highs which are the expression of basement highs, Lateral and vertical variations in thermal conductivity of the rocks due to a variety of causes (e.g. structural configuration, lithology) seem to be the main factor in producing thermal anomalies over Trinidad.

The geothermal gradient map has several relevant applications e.g. in defining the depths of the oil window over Trinidad, in delineating areas with bacterially altered oils, in predicting the presence of overpressured shales and the temperature profile of a new well.

REFERENCES

Ball, S.M. 1981, Exploration applications of temperatures recorded on log headings: theory, data analysis and examples: AAPG Bull., v. 65, No. 7, p. 1359 (Abs.)
Fert 1, W.H., and P.A. Wichmann, 1977, How to determine static BHT from well log data: World Oil, Jan. 1977, p. 105-106.
Gatenby, G.M., 1981, Temperature anomalies and Gulf Coast structures: AAPG Bull., v. 65, no. 7, p. 1360 (Abs.)
Gretener, P.E., 1981, Geothermics: Using temperature in hydrocarbon exploration: AAPG Education Course Note Series no. 17, 156pp.
Griffiths, D.H. and R.F. King, 1974, Applied Geophysics for Engineers and Geologists. Pergamon Press, Oxford, 223 pp.
Handique, G.K. and B. Bharali, 1981, Temperature distribution and its relation to hydrocarbon accumulation in Upper Assam Valley, India: AAPG Bull., v. 65, no. 9, p. 1633-1641.
Levorsen, A.I., 1967, Geology of Petroleum. W.H. Freeman and Company, San Francisco, 724 pp.
Majorowicz, J.A., F.W. Jones and K.G. Osadetz, 1988, Heat flow environment of the electrical conductivity anomalies in the Williston Basin, and occurrence of hydrocarbons: Bull. Canadian Petroleum Geology, v. 36, p. 86-90.
McDougall, A.W., 1985. Geology of the East Soldado Field: 4th Latin American Geological Conference, Transactions, Trinidad. 1979, vol. II, p. 720-725.
Mc Gee, H.W., H.J. Meyer and T.R. Pringle, 1989, Shallow geothermal anomalies overlying deeper oil and gas deposits in Rocky Mountain region: AAPG Bull., v. 73, no. 5, p. 576-597,
Meyers, H.J. and H.W. McGee, 1985, Oil and gas fields accompanied by geothermal anomalies in Rocky Mountain region . AAPG Bull., v. 69, p. 933-945.
Ovnatanov, S.T. and G.P. Tamrazyan, 1970, Thermal studies in subsurface structural investigations, Apsheron Peninsula, Azerbaijan, USSR: AAPG Bull., v. 54, no. 9, p. 1677-1685.
Rodrigues, K., 1988, Oil source bed recognition and crude oil correlation, Trinidad, West Indies: Advances in Organic Geochemistry, 1987, Org. Geochem., v. 13, nos. 1-3, p. 365-371.
Talukdar, S., 0. Gallango and A. Ruggiero, 1988, Generation and migration of oil in the Maturin Subbasin, Eastern Venezuelan Basin : Advances in Organic Geochemistry, 1987, Org. Geochem., v. 13, nos. 1-3, p. 537-547.
Van Orstrand, C.E., 1934, Some possible applications of geothermics to geology: AAPG Bull., v. 18, p. 13-38.
Zielinski, G.W., J.A. Drahovzal, G.M. De Coursey and J.R. Ruperto, 1985, Hydrothermics' in the Wyoming Overthrust Belt. AAPG B u II, v. 69, no. 5. p. 699-709.

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