EXPLORATION COSTA RICA GEOLOGIC BASINS LURE WILDCATTERS

Cynthia A. Sheehan, Glen T. Penfield
Aero Service Division
Western Atlas International Inc.
Houston

Emilia Morales
Recope S.A.
San Jose, Costa Rica

The west-northwest trending volcanic arc which forms the axis of southern Central America is flanked by basins on both the Caribbean and Pacific coasts (Fig. 1). Within Costa Rica, these sedimentary basins cover an area, both onshore and offshore, of more than 41,000 sq. kilo. Most explorationists do not view island arc systems as good locations for petroleum exploration. However, Costa Rica has several factors present that are highly encouraging to explorationists.

Organic-rich Cretaceous marine shales seen in the Morote and Manzanillo stratigraphic test wells (location map, Fig. 2).
Seeps, shows and/or oil shales in both the Caribbean and Pacific basins.
Continental clastic sediment sources in the granitic Talamanca Batholith in southern Costa Rica and the Chortis Block in neighboring Nicaragua to improve reservoir potential.
Thick sedimentary sections in both the forearc and backarc basins containing favorable sedimentary environments of deposition.
A modern geological/geographical database, including seismic acquisitions, stratigraphic test wells, and reports on several hydrocarbon evaluation projects, now available to industry.
Geochemical analysis of oils from recent drilling indicate several sources and suites of oils different from those which were produced by the Cocoles and Campo Diablo wells in the 1950's and 1980's. Typical API gravities are 42 to 46.

Previous operations in this area include Sinclair, Union Oil, Gulf, Elf, and Pemex. Although, nineteen (19) exploration wells were drilled in this area between 1920 and 1985, no commercial deposits of hydrocarbons were found. Most of these wells were drilled between 1954 and 1963 without benefit of the better quality seismic data available currently.

A new round of exploration is currently underway which will utilize a nationwide database resulting, in part, from twenty-five million dollars ($25,000,000) of recent work by RECOPE, S.A. (the Costa Rican national oil company), the World Bank, and Petro Canada International Assistance Corporation (PCIAC). Some of the recent studies in this database included a nationwide aeromagnetic acquisition and interpretation program conducted by Aero Service Division, Western Atlas International, Inc. (WAI), extensive geochemical analysis of oil samples, a shallow stratigraphic drilling program and 14,000 m of deep stratigraphic tests.

In 1987, which funding from the World Bank, RECOPE commissioned WAI to perform an integrated exploration study to synthesize the new and existing data into a comprehensive basin analysis. Interpretation of this nationwide data base, including results from several regional and local studies performed as part of the synthesis effort, sheds dramatic new light on Costa Rica's hydrocarbon potential. This recently completed study (WAI, 1988) acts as a summary and overview of the database and provides new concepts for exploration in the basins of Costa Rica. Promising source rock units and the fresh look at Costa Rica's petroleum potential by several leading consultants has inspired renewed interest in the hydrocarbon potential of Costa Rica culminating in a recent bid round.

In 1989, PCIAC, in conjunction with RECOPE, completed a three year program including the acquisition of more than 1500 km of onshore seismic data and a five (5) well deep stratigraphic drilling campaign. Results of this program are now included in the nationwide database.

The new stratigraphic wells (seen in Fig. 2) include the Matina 1 well drilled to 3324 m, the Pataste 1 well drilled to 1980 m and the Tonjibe 1 well, a basement test, drilled to 2,168 m in the San Carlos-North Limon basin. The Curime 1, completed in 1988, was the first deep well (TD 2,800 m) on the Pacific coast of Costa Rica. Results from this well suggest support for the conclusions of aeromagnetic and gravity studies, completed in 1987-1988, which indicated more than 4,500 m of sedimentary section beneath portions of the Gulf of Nicoya. The last of the five PCIAC stratigraphic test wells, the San Clemente 1 well, was drilled in the South Limon basin to a depth of 4,200 m.

TECTONIC SETTING OF THE COSTA RICAN BASINS

Costa Rica is located on the trailing edge of the Caribbean Plate (Fig. 3). This oceanic plate was probably formed as part of the Pacific Farallon Plate (the modern Cocos and Nazca plates) during the Late Jurassic or Early Cretaceous Period at the spreading center located in the eastern Pacific. This plate was later altered by basaltic flows and sills to the extent that the resulting caribbean (B') ocean floor is approximately one kilometer shallower than normal Pacific Ocean floor of Cretaceous age. On at least the trailing edge of the Caribbean seafloor, organic-rich Late Cretaceous marine shales and radiolarian cherts accumulated. Convergence of the unaltered part of the Pacific Farallon Plate with the altered Caribbean part, beginning at the end of the Santonian, resulted in the separation of the Caribbean Plate from the Farallon Plate. This compression produced telescoping of the trailing margin of the Caribbean Plate with detachment of nappes during the early to middle Campanian and the initiation of subduction at the new leading edge of the Farallon Plate. Shallowing of the oceanic floor caused by the emerging edges of the nappes permitted the growth of rudistid biostromes currently seen in outcrop in the Nicoya Peninsula.

In the late Campanian and Maastrichtian, the continued compression along the subduction zone, bordering the trailing edge of the Caribbean Plate, initiated the formation of an emergent andesitic island arc located east of this zone. Reefal carbonates, platforms, and volcaniclastic sediments were deposited along the slopes of the emerging volcanic islands. The andesidic volcanic island arc has been active periodically since the beginning of the Tertiary. Collision of the arc with the South American Plate and northward migration of the Nazca Plate has produced a foreland type fold-thrust belt in northern Panama which extends into southeastern Costa Rica.

The west-northwest trending volcanic arc which forms the axis of Costa Rica is flanked by basins on both the Caribbean and Pacific coasts. A structural cross section traversing southern Costa Rica (Fig. 4) displays the typical volcanic island arc assemblage from west to east: trench, forearc ridge, forearc basin, magmatic arc, the backarc basin. The distribution of these tectonic zones in Costa Rica are shown in Fig. 1. Ophiolites in the forearc ridge are exposed along the Pacific coast in the Santa Elena, Nicoya and Osa peninsulas and the Quepos arch. The Terraba basin and the Tempisque basin are deep elongate grabens parallel to the island axis. These forearc basins are bounded to the west by the ophiolites and oceanic crust of the Nicoya and Osa peninsulas and to the east by the volcanic arc and uplifted plutonic bodies of the Cordillera. Smaller downdropped basins including the Mal Pais-Nosara, Nicoya, Quepos, Dominical, and Golfo Dulce cut across the forearc trend. In the Valle Central basin, the Cordillera of the island arc is breached by a similar transverse graben. South of this basin, the Talamanca Batholith is exposed along the crest of the arc. Smaller intrusive bodies are exposed in the northern Cordillera. The Caribbean backarc basins form broader extensional basins than the Pacific forearc basins. The Limon Sur basin, along the southern Caribbean coast, appears to have been extensively modified from its original backarc configuration by Neogene compression and thrust faulting. North and west of the Moin High (Fig. 1), in the San Carlos Limon Norte-Moin basin, the effects of the thrust faults die out, and broader basement structures dominate.

SEDIMENTARY BASINS

The San Carlos-Limon Norte is the largest basin, occupying a broad plain on the Caribbean side, north and west of the Moin High. It includes approximately 12,000 sq km onshore and 1,500 sq km offshore within the 200 m bathymetric contour. A large Neogene deltaic complex adjoins the basin to the northeast and extends into deeper water. South of the Moin High, the structurally more complex Limon Sur Basin includes over 2,500 sq km onshore and 1,200 sq km offshore within the 200 m bathymetric contour. The Tempisque Basin encompasses approximately 7,000 sq km onshore and offshore between the Nicoya Peninsula and the volcanic arc. The Terraba Basin includes over 5,000 sq km. The smaller, mainly marine, transverse basins of the Pacific coast account for over 12,000 additional sq km. The 10 perspective Costa Rican basins (Fig. 1) have been divided into four main basinal areas, according to their location and structural style. The generalized stratigraphic column for these basins are shown in Fig. 5.

NORTHERN CARIBBEAN BASINS

North of the Moin high, the broad Tertiary backarc basin contains three depocenters: the San Carlos near the magmatic arc, the Limon Norte along the coast, and the Moin basin further offshore. The San Carlos and Limon Norte form a common basin with depocenters divided by a north-trending high. Located in an area where water depth exceeds 200 m, the Moin Basin is separated from the Limon Norte depocenter by a change in basement slope. The Moin basin is considered to be outside the economic water depth range, given current market conditions. However, this deep basin is of interest since it may have acted as a source for hydrocarbons migrating to structures in the shallower Limon Norte area.

Unconformities at the top of the Eocene, middle Oligocene, and early Miocene record breaks in the deposition of the Paleocene (or older) to Miocene neritic limestones and clastics which filled the broad San Carlos-Limon North depocenters. The Moin basin contains deeper marine shales and turbidites. Except for isolated exposures of Miocene sediments, Neogene volcaniclastics blanket most of the onshore portions of the basin. Fig. 6 is a paleogeographic diagram showing deposition of sand and reef facies in the backarc basin during the Miocene. Along the northeastern boundary of Costa Rica, sediments from the Rio San Juan River have constructed a major Miocene to Recent delta complex. According to aeromagnetic studies, the total sediment thicknesses in the southern portions of the basin range from 5,000 to 6,000 m and locally may exceed 7,000 m.

Most of the presently known structures in the onshore portions of the basin are broad, basement-related arches and normal faults which trend either northeast to east-northeast across the basin or west-northwest. Several newly defined northeast trending crossarc features appear to be associated with strike-slip faulting. The west-northwest structures are typically associated with either a magmatic arc or are related to the normal faults bounding the Nicaragua Depression to the north. East-west compressional structures in the northern part of the basin may be related to the suturing of the arc to the Chortis Block, possibly beginning as early as the Paleocene.

Several locations of particular interest for hydrocarbon exploration are present in this area. Offshore, in the Rio San Juan delta, seismic data (Fig. 7) document the presence of northerly trending listric faulting in the Miocene and recent deltaic sediments of the Rio San Juan where both structural and stratigraphic traps are expected. The broad, low-relief structures in the San Carlos-Limon Norte basin have the potential to have formed large stratigraphic traps. A seismic line, Fig. 8, from the area of the Pataste oil seeps displays the normal faulting common in this area. Flat spots, possible fluid contacts, can be seen in a gently tilted basement block to the right. Stratigraphic traps are possible in the clastic and reef facies present along the north flank of the Moin High and on other basement highs indicated on aeromagnetic and seismic data.

The geochemical analyses of oil shows the seeps in the San Carlos basin indicate the presence of at least one, and possibly two or three, source rock horizons. Dolomitized limestones, reefs, and offshore sandbars provide potential reservoirs in this area. Fracture porosity is expected to provide reservoir potential in local, highly faulted areas.

SOUTHERN CARIBBEAN BASINS

South of the Moin High, the backarc basin narrows. The basin sediment accumulation may still range from 5,000 to 6,000 m, with local accumulation exceeding 7,000 m. Compared to the northern backarc area, the individual stratigraphic units are relatively thin, with frequent facies changes. The presence of numerous unconformities indicates the sedimentation in this area, which has continued from the Albian age to the present, was interrupted frequently. Deposition appears to have been strongly influenced by volcanics and mountain building, beginning in the late Cretaceous and continuing throughout the Tertiary. The mid to late Eocene and late Oligocene strata indicate periods of marine invasion which produced carbonate and clastic units with only minor volcaniclastics present.

Thrust belt structures dominate the Limon Sur basin. Compression from the south and west produced a series of thrust faults and folds oriented east-west to northwest. Foreland-type structures, including thrust faults and basin deepening, developed as a result of northerly compression associated with the Neogene Panama Deformed Belt. One of these thrust faults can be seen in the seismic line in Fig. 9. This line also shows one of the diapirs commonly associated with these still active faults.

Seventy-two percent of the thirty-six stratigraphic-test and exploration wells used in this study are located in, or adjacent to, the Limon Sur basin. These include several with significant oil shows. However, the seismic data presently available indicate a wide variety of potential petroleum traps that have not yet been tested. These include stratigraphic traps in the reefs and fringing facies flanking the Moin High, as well as more than 30 untested structures along the thrust belt.

Geochemical analyses of the source-rich potential of Tertiary formations in this basin indicate that oil-prone sediments occur in basinal marine sections of the Miocene and in localized sections in the Oligocene and Eocene shales. These units are thermally mature in the deeper portions of the basin.

At least four types of potential reservoirs have been identified in this area. Extensive reef trends have been identified on seismic data in the Oligocene limestones. Dolomitization of the Eocene to Miocene limestones has resulted in locally abundant intercrystalline porosity. Oligocene, Miocene, and Pliocene sandstones with porosities greater than 25% have been reported in the Limon Sur wells. In the thrust belt, fracture porosity provides reservoir potential in a variety of rock types and ages.

NORTHERN PACIFIC BASINS

The northern portion of the forearc basin is referred to as the Tempisque basin. The grabens of the Nicoya, Quepos and Mal Pais-Nosara basins are oriented perpendicular to the forearc trend. The stratigraphic section is composed of three main cycles of deposition: Late Cretaceous to early Paleocene, Eocene, and Neogene sediments deposited on the pre-Santonian igneous-sedimentary Nicoya basement complex. The Late Cretaceous/early Paleocene deposits include deep-water marine shales which are the most organic-rich deposits in the country.

A period of relative uplift occurred in the late Paleocene/early Eocene, as evidenced by the presence of turbidites and boulder avalances in the eastern Tempisque basin and areas of nondeposition and uncomformities. Minor subsidence in the late Eocene permitted the deposition of shallow marine sandstones and limestones. Renewed uplift in the Neogene resulted in the deposition of shallow marine to nonmarine clastics.

The Tempisque basin is a graben initiated during the Late Cretaceous. Although some evidence of thrusting is present in the pre-Santonian Nicoya Complex, structures in the younger sedimentary section are primarily related to normal faults and folds, with some wrench faults present. The grabens which cross the forearc ridges are younger, and most of the sediments were deposited since the Oligocene. An offshore seismic line illustrates the offset along a series of half grabens (Fig. 10).

The presence of organically rich, Late Cretaceous marine shales makes this area very interesting for petroleum exploration. Total organic carbon (TOC) locally exceeds 40%, and is typically in the 14% - 26% range. Analyses of the Cretaceous shale outcrops in the Tempisque basin (Fig. 11) indicate that this unit is Type 1 (oil prone) organic material. In the Morote 1 well on the westerly side of the basin, shales with TOC values of up to 26% were identified. The Manzanillo 1 well drilled on the eastern side of the basin encountered approximately 10 meters of the organic-rich shales seen in Morote, suggesting that this potential source rock is present across the basin. Although these shales were immature in the shallow margins of the basin where the wells were located, the center of the basin deepens rapidly to possibly greater than 3,000 m, and may exceed 4,000 m in three local depocenters. Thus, the shales in the central part of the forearc graben are expected to be within the oil window.

Several potential reservoir rocks have been identified in the northern Pacific basins: carbonate platforms in the Late Cretaceous/early Tertiary and Oligocene deposits; fractured Late Cretaceous cherts; and calcareous sandstones of Miocene age.

SOUTHERN PACIFIC BASINS

The Terraba basins, the southernmost of the forearc basins, is separated from the Tempsique basin by the Quepos arc and basin. The Terraba basin is a half-graben with the eastern side downdropped significantly more than the western side. This results in an asymmetric synclinal basin. In addition, compression during the Pliocene to Recent produced a westward-directed thrust belt along the western margin of the trough. As along the northern Pacific coast of Costa Rica, the forearc ridge forming the western boundary of the forearc basin is segmented by two transverse basins: The Golfo Dulce and Dominical.

The main depositional event in the Terraba Basin occurred during the late Eocene to Miocene. Shallow marine sandstones, volcaniclastics, and algal reefs or late Eocene age were overlain by deep marine and turbidite facies as the basin subsided. After a depositional hiatus during the early Miocene (due perhaps to very low depositional rates), turbidite and deep-water marine sedimentation resumed during the mid-Miocene while the Talamanca batholith was being unroofed immediately to the east. Significant uplift produced shallow marine deposits during the Pliocene. Fluvial sands, conglomerates, and volcanics followed in the Pleistocene to Recent.

The Southern Pacific basins of Costa Rica are the least explored basins in the country. To date, no wells have been drilled, and seismic data are limited to 1,250 line km of offshore data and one small localized seismic grid onshore east of the Osa Peninsula and on the border between the Terraba and Gulfo Dulce basins. Although the section (Fig. 5) contains rocks from the Cretaceous/Paleocene to Pleistocene, the few available surface source rock samples that have been analyzed to date are limited to the Oligocene. If the Cretaceous shales of the Tempisque basin extend this far south, they would provide rich potential source rocks for the basins. Clastic facies may provide the best potential reservoirs. However, carbonate reefs have been reported in the basal Eocene deposits.

PLAYS AND PROSPECTS

In the past, most of the exploration wells have been drilled along the crests of surface anticlines in the Limon Sur portion of the Panama Deformed Belt. However, the obvious structures have proved to be problematical targets. Seismic data indicate that flow of the Uscari shales has produced semi diapiric shale-cored folds. The flanks of these folds may provide better drilling locations both to avoid the shale core and to test for possible contemporaneous sand deposits suggested by amplitude anomalies in the seismic data.

In the Caribbean basin north of the thrust belt, several potential plays have been identified. Seismic stratigraphy studies have shown that Oligocene reefal trends and onlapping clastic sequences may flank the Moin High. On the crest of this high, faults can be seen in the strong Eocene limestone reflector. These types of plays may also be present along other untested basement highs to the north and west. Bright reflectors in the seismic data have been interpreted as possible shoreline coal deposits with hummocky reflectors seaward that have been interpreted as bar sands. Structural closures have been identified in rotated blocks due to listric faulting in the San Juan River delta. This large delta complex receives sediments from both the Costa Rican arc and the continental Chortis Block.

In the northern Pacific basins, carbonate platforms overly the rich marine shales providing a stratigraphic play. Repeated fluctuations in sea level associated with tectonic uplift and subsidence have produced a complex series of pinchouts, unconformities and facies changes that may also act as stratigraphic traps. Several structural closures have been noted on the offshore seismic data related to faults along the margin of the forearc and the transverse graben basins. This type of fault play is also expected in the southern Pacific basins. The thrust belt along the western flank of the Terraba graben has not yet been tested by either well or regional seismic acquisition.

Thus, new interest is brewing in the hydrocarbon potential of Costa Rica. The nationwide geological/geophysical/geochemical database provides a concise source of data, often difficult to obtain in developing countries, for the explorationist.