Hydrocarbon exploration began in Costa Rica with Sinclair Corporation's five well ... tions, 600 Ian of dynamite, multifold reflection and 50+ km of refraction ...
Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, Vol. 16 R. L. Miller, G. Escalante, J. A. Reinemund, and M. J. Bergin (Eds.) Energy and Mineral Potential of the Central American-Caribbean Regions Springer-Verlag, Berlin Heidelberg 1995
AN INTEGRATED GEOLOGICAL AND GEOPHYSICAL INTERPRETATION OF THE SAN CARLOS BASIN, COSTA RICA Lesmes E. Ballestero C., Mihaela M. Dobrinescu, and Gerardo Jager C. Refmadora Costarricense de Petr6leo, S. A. (RECOPE), Apartado 4351, San Jose, Costa Rica Ian R. Mayers Petro-Canada International Assistance Corporation (pcIAC), P. O. Box 2844, Calgary, Alberta T2P 3E3 Canada
ABSTRACT Hydrocarbon exploration began in Costa Rica with Sinclair Corporation's five well program of 1916-1923. Since then geological, geochemical, potential field, land, and marine seismic surveys have been made with 49 stratigraphic and exploration wells drilled, the majority ofthese in the Limon Sur basin. Some intriguing non-commercial oil and gas shows were encountered. The San Carlos basin in northern Costa Rica was considered prospective and made an objective of the 1986-1989 RECOPEPCIAC exploration/technical assistance program. Building on sparse geological information and new potential field interpretations, 600 Ian of dynamite, multifold reflection and 50+ km of refraction seismic surveys were conducted there. These data were interpreted, along with older information, and drill hole locations were selected. These stratigraphic wells at Pataste No.1 and Tonjibe No.1 (Figure 1) were finished in November, 1987, and January, 1988, respectively. They significantly increased the understanding of the basin. INTRODUCTION The San Carlos basin, with an area of 8,000+ 1an2 , lies northeast of the Guanacaste Cordillera (inset map, Figure I, northwest part of volcanic arc), which is continuing to grow by active volcanism. The underlying Cocos Plate's subduction zone here dips to the northeast at > 25°. The basin forms part ofthe Central American back arc region, and is separated from the Limon Norte basin to the east by a basement ridge. From Early Cretaceous it has been intermittently filled with 6 Ian of primarily fluvial and marine rocks of flysch and molasse facies. Potential reservoirs include dolomitized limestones and also turbidites. Basin geochemical analyses of drill samples identified a predominantly terrestrial kerogen within the Tertiary sediments. This type of kerogen is prone to gas and condensate generation, but oil and gas seeps do occur in the region. These indicate the presence of a richer, unidentified source rock, perhaps within the underlying Cretaceous strata. The high geothermal gradient (4.3°C/lOO m)
suggests hydrocarbon generation beginning at a shallow depth of 2.0-2.5 km. Structurally, the San Carlos basin is bounded by northwest-aligned normal faults which began forming in the Cretaceous as a result of northeast-south west-directed back arc extension. During the Neogene, east-trending structures were formed by north-south compression of the Tertiary strata. Future exploration should concentrate along the resulting east-west structural trends or on the basin's flanks, where migration pathways and traps are focused. The presence of "bright spot" anomalies also indicate stratigraphic prospects may exist elsewhere in the basin. TECTONIC FRAMEWORK AND REGIONAL GEOLOGY The Middle American Trench subduction zone parallels the southwest coast of Costa Rica. At this convergent
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areas. Also shows locations of wells drilled for straligraphic infonnation and interpreted faults.
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margin the Cocos Plate is consumed beneath the Caribbean Plate at 9 cm/yr down a low angle (average dip < 25°) Benioff Zone (Mora, 1981). Forces in the subduction zone have created two northwest-southeast-trending 20 to 50-km wide ranges separated by an east-west-striking rift, the Valle Central. The northern range, the Tilaran-Talamanca Range, consists of active and dormant volcanos. During the early Tertiary it was the locus of mafic eruptions. Since the Pliocene the volcanism has become more acidic. This volcanism supplied the magmas that are thought to rise to the surface along a northwest-trending fault zone. The fault zone may also be the southwestern boundary of the San Carlos-Limon Norte (back arc) basin (inset map, Figure 1). The basin is probably cut by a suite of deep vertical faults aligned northeast-southwest (Montero and Dewey, 1982; WAI, 1988). The Costa Rican back arc basin has been the site of relatively continuous volcanism and sedimentation since the Paleocene which resulted in a Tertiary sequence 6-10 km thick immediately to the northeast of the volcanic arc. It is subdivided into the intensely thrust and wrench faulted and folded Limon Sur sub-basin and the less deformed Limon Norte and San Carlos sub-basins located to the northwest of the northeast-trending "Puerto Limon-Moin Ridge." The Limon Sur sub-basin is relatively well understood, having been studied through surface geology mapping, exploration drilling, and considerable seismic coverage. It will not be discussed further except to assist in the general understanding of the back arc region . The Limon Norte and San Carlos sub-basins cover an (onshore) area in Costa Rica of 14,500 km 2. Much of the surface area is covered by young volcanics and alluvium deposits except along the banks of the Rio San Juan and in the San Carlos-Venado area (Calvo and Bolz, 1987). Geophysical studies have shown that the sub-basin area is divided into the San Carlos basin to the northwest and the Limon Norte basin to the southeast by a basement ridge that runs northeast-southwest beneath the San Carlos Plain.
In this paper we will consider only the 8,000 km 2 of the San Carlos basin. THE SAN CARLOS BASIN-EXPLORATION ACTIVITY Potential Field Data Mapping
Gravity and Magnetics The basin was surveyed by RECOPE in 1982 using a La Coste-Romberg meter with an in-line station spacing of 0.3 km and an average distance between polygons of 5.0 km. The Bouguer gravity anomaly map (Figure 1) shows that the basin consists oftwo major troughs. The larger, Plorencia Low (to the south of the east-west-trending PatasteHigh), is estimated to be filled by >5 km of post-Cretaceous sediment. The northern San Jorge Low is estimated to be 3 km deep. The Pataste High is thought to be caused by a hypabyssal andesite intrusion. The major structural trends of these anomalies run either west northwest-east southeast or east northeast-west southwest. The interpreted fault trends are predominantly west and west-southwest. In 1983 Aeroservice surveyed the basin using an optically-pumped magnetometer flown a1762 m ASL (abo~e sea level) in a 5 km grid with control lines 20 km apart. Agam the data indicate that the basin is composed of two troughS, the Plorencia Low (in the south) and the San Jorge Low, separated by the large, upper Miocene-lower Pliocene andesitic Pataste High (Obando, 1986) (Figure 2). The northeasttrending Los Corrales High extending into the San Jo~ge Trough is believed to be produced by the same serpentme body that was encountered at the bottom of the PCIACRECOPE Tonjibe No. 1 well. The Cureila High is a highfrequency sequence of anomalies, paralleling the Nicaraguan border, produced by lava flows of the upper Miocene-lower Pliocene Cureila Formation.
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Figure 3. Interpretation of north-sout seismic section from the San Jorge Trough in the south across the Los Corrales High toward the
northern edgeofthe basin. The lines designated yellow line and blue line are seismic reflectors indentified in the text.
Geological and Geophysical Interpretation of the San Carlos Basin
The Well Data Base The San Carlos basin has been drilled by two deep wells, PCIAC-RECOPE Pataste No.2 (TD 1,979 m) and PCIAC-RECOPE Tonjibe No.1 (TD 2,168 m) (Figure 2). Both of these had a comprehensive set of logs, including velocity surveys. In addition, nine shallower RECOPE core holes have been drilled and logged in the area: Chamorro No. 1 (TD321 m), Mayhew No. 1 (TD 132 m), Medio Queso No. 1 (TD401 m),MedioQuesoNo.2(TD355m),MedioQueso No.3 (TD632m),MedioQuesoNo.4(TD 132 m), Monterrey No.1 (TD 620 m), San Jorge No.1 (TD 550 m), and Pataste No. 1 (TD 320 m).
The Seismic (Refraction and Reflection) Data Base The seismic data base used in this study consists primarily of 520+ km of 1,200-1,500 percent dynamite reflection data of variable (poor to very good) qUality. In addition, 50+ km of refraction data were interpreted and used.
Interpretation Potential Field Depth to Basement Map (Figure 2) This map is derived from both sets of data using the slope method and the simple and weighed average (in the space domain) method. Bodies estimated to be at depths equal to or less than 1 km are inferred to be intrusives and lava flows; those at >2 km are believed to be basement. Two major structural trends show up in the data: (1) northwest-southeast trends related to the late Mesozoic-early Tertiary opening of the basin, under east-west extension, and (2) northeastsouthwest trends produced later, in the upper Tertiary, by northwest-southeast compression. The three major troughs are the Florencia-Aguas Zarcas Low, the San Jorge Low, and the Volcan Tenorio-Medio Queso Low. The major basement highs are the Pataste and Los Corrales Highs. It is estimated, from this map, that the San Carlos basin covers some 7,850 km 2 , of which about 5,000 km 2 has the Mesozoic basement covered by more than 2 km of sediment.
Seismic Section No. 44 (Figure 3) This north-south interpreted seismic section (s. p. 221366 only) passes 1.8 km to the east of PCIAC-RECOPE Tonjibe No.1 well, from the San Jorge Trough in the south, across the Los Corrales High (crest at s. p. 259) toward the northern edge of the basin. It shows the complex reverse and normal faulting that occurred as a result of wrench movements under north-south compressional forces. The Yellow (Y) Reflector is an early Pliocene alluvial shale and the Blue (B) Reflector, a late Miocene sandstone/volcanic contact in the Tonjibe No.1 well. At s. p. 350, at reflection times of 0.8
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sec and 1.5 sec there are two "bright spots" in small anticlines on the upthrust side of a reverse fault which may represent gas accumulations. The tectonism that deformed the strata was probably recurrent during the late Tertiary, with the most recent episode occurring in the late Pliocene. The Tonjibe No. 1 well drilled into the Los Corrales High (Figure 2). It encountered a shallowing upward sequence of Holocene, Pliocene, and Miocene marine to nonmarine sediments down to 1,700 m. Below that, primarily non-marine, Lower Cretaceous sediments were drilled. Below 1,819 m the well entered serpentinized and chloritized tuffs and basalts, representing economic basement. It appears that the Los Corrales basement high acted as local focusing ramp, over which the Tertiary strata were upwarped and faulted during the late Tertiary north-south compression.
Contour Map ofDix Interval Velocities (Figure 4) (At a 2.0 sec time slice. below the +200 mASL datum. or-2S00m) Throughout the back arc basins a generally good correlation has been noted between (carefully edited) Dix Interval Velocities (DIV) (calculated velocities between two parallel seismic reflectors) and the check shot/VSP (vertical shot point) interval velocities. Smoothed DIV contour plots in the plane of the seismic sections and in map form frequently improved the structural interpretation in areas where reflector patterns were not easily distinguished or correlated. The smoothed DIV plots also facilitated reasonably accurate depth mapping and regional interpretation. For example, a DIV contour map at a 2.0 sec time slice shows the higher velocities and fault densities associated with thePataste High igneous rocks and the lower velocity zones delineate the clastic depocenters.
Blue Reflector Isochron Structure Map The Blue Reflector surface of Figure 5 is one ofthe few reflectors that can be fairly continuously mapped across most of the basin. It represents a time transgressive event, which at Pataste No. 2 was correlated with the contact between a middle Miocene (Venado Formation) limestone overlying a lava. At Tonjibe No. 1 it corresponds to a sandstone over volcanics contact dated as being within the late Miocene. The Venado Formation limestone was absent at Tonjibe No.1. The maximum mapped time on this reflector was 2.3 sec in the Florencia Low and the minimum time was 0.6 sec over the Pataste High. Structurally, the map shows the dominant east-westtrending, reverse faulting and folding (with probably a wrench component) which was produced by north-south-directed, late Tertiary compressional forces. The major structural elements are the Los Corrales High (drilled by Tonjibe No. 1), Pataste High (drilled by Pataste No.2), Florencia Low, and the San Jorge Low.
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