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PSFractured Basement: New Exploratory Target in La Concepcion Field, Western Venezuela*
By
Jesus S. Porras1, Carlos E. Ferro2, Carla Castillo3, Vanessa Machado3, Luis Ochoa3, Nelson Chirinos3, and Fernan Perez3
Search and Discovery Article #10140 (2007)
Posted November 20, 2007
*Adapted from poster presentation at AAPG Annual Convention, Long Beach, California, April 1-4, 2007
1Petrobras Energía Venezuela, Maracaibo ([email protected])
2Petrobras Energía Venezuela, Caracas ([email protected])
3Petrowayuu Filial de PDVSA, Maracaibo ([email protected])
In Western Venezuela, production from naturally
fractured basement has been known for decades. Since El Totumo, Limon and giant
Mara-La Paz oilfields discoveries in the early twentieth century, new commercial
oil production from basement had not been reported in years. In the 1960-decade,
unsatisfying results were obtained searching for oil from basement in El Mojan,
Sibucara, Los Lanudos, and La Concepcion fields. In La Concepcion field,
evidence in some wells suggested the existence of basement as a producing
reservoir
.
However, a combination of factors including: operational problems that impeded production testing, the experience of having dry basement in some former wells and the insignificant production obtained in the only well where basement was proved--when compared to its Cretaceous production--prevented further pursuit of basement as a commercial target.
Nevertheless, it was not until 2004 when commercial production from basement was obtained through well C310, initially producing at 4600 bopd. Further drilling continued basement development; well C313 was completed in July 2004, producing 3915 bopd, and well C314 was completed in November 2004, producing 3714 bopd. Recently, in 2006, well C325 also confirmed basement production.
This poster aims to describe the main characteristics
of fractured basement in La Concepcion field. Also, it is intended to evaluate
the reservoir
under a new perspective, in order to determine its potential,
after exploration, and further studies and analyses.
As a result, additional opportunities could be obtained, and as a consequence, hydrocarbons reserves would be increased.
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La Concepcion field is located approximately 20 km west of the city of Maracaibo, in the northwestern Maracaibo Basin, Venezuela.
Discovered in 1924, the field produces
from two main reservoirs: a siliciclastic
An interesting volume of oil has been
produced from basement in the last three years. Nearly five million
barrels of produced oil are accredited to basement Production of Cretaceous reservoirs is attributed to massive fracturing and assuring that increase close to faults. Basement producing wells are located instructural highs (pop-up structures), on central and southern areas of the field and are related to secondary minor faults linked to major bordering faults.
La Concepcion Basement Facts
The stratigraphic column encompasses passive margin carbonate sediments (Cretaceous) and a thick foredeep mixed siliciclastic-carbonate sequence (Paleocene-Eocene) overlying the igneous-metamorphic basement, object of this study.
La Concepcion Basement is composed by igneous (granitic) to metamorphic rocks (gneiss) and is mainly composed by crystalline or milky quartz, pink or orange feldspar, biotite and secondary mafic minerals. Oil is stored in fractures.
Figures 3-1 – 3-2
All data presented in this study was
obtained from files that contain drilling, mudlogging and operations
information from wells. Sample cuttings as well as fragments of
cores were used for petrographical and visual description of rocks.
Standard analyses were performed to these samples. A visit to the
Isla de Toas outcrop was done for a better understanding of the
fracturing
For each well, the acquired data was
incorporated and displayed in a composite chart that contents: depth
of basement, thickness, footage, drilling parameters, hydrocarbon
shows, underbalanced drilling information, testing and production
data, geological information, visual fracturing and fracture
mineralization, and structural position. The chart was used to
select and identify the most significant variables for basement
Oil Migration, Sourcing, and Seal
Vertical migration due to rock dilatancy,
as well as lateral up-dip migration through faults, are the
suggested (1) Short vertical migration from source rock (La (Luna formation), passing the naturally fractured limestone of Cogollo Group into basement, may be explained by dilatancy phenomena. This hypothesis was proved -and well documented-in Mara–La Paz fields through pressure measurements done in both reservoirs. Particular observations about secondary sealing have been made in La Concepcion field. (2) Lateral migration from longer distances is attributed to horizontal movement through faults and fracture conduits. Lateral migration took place in zones where source rock and fractured limestones are directly in contact with basement. A restricted zone with these characteristics has been identified in the field. Producing wells of basement are located within this zone. Faults and fractures are the main pathways for hydrocarbon migration. Like many of the basement reservoirs of the world, an unconformity is identified above the basement in La Concepcion. All producer wells show calcite-filled fractures acting as a cap-rock, above the productive section of basement. In producing wells, calcite-filled fractures are found once the top of basement has been reached. Oil in basement has been produced in the same area known as producer in overlying sediments (Cogollo Group) and has the same composition. La Luna Formation is believed to be the source rock for both reservoirs.
Thickness of calcite-filled fractures interval can vary from 50 to more than 200 feet. Origin of calcite has not been determined yet, even though it is supposed to come from dissolution and precipitation of calcium-carbonate rich waters of Cogollo Group.
Figures 5-1 – 5-2
Banded–gneissic-structures due to moderate metamorphism have been recognized in sample cuttings. Petrographically, basement is a holocrystalline igneous plutonic rock, with granular hypidiomorphic texture. It is s composed of polycrystalline quartz, less monocrystalline, with strong wavy extinction, crystals of moderate to intensely altered plagioclase and potassic feldspars, moderately kaolinized with wavy extinction, represented by orthoclase, and scarce microcline. The igneous basement (granite, granodiorite) has been described as mottled white and black, light gray, and pink, coarsely crystalline, with clear and milky quartz, white and pink,or orange, feldspar, biotite, and dark minerals (hornblende).
Outcrops of basement occur north of La Concepcion Field in the Isla de Toas area. In this area, basement is very sheared and fractured and is intruded by basalt and rhyolite dikes and sills. Fracturing is the result of intense deformation due to right lateral strike-slip motion of the Oca fault and-or-transpression faulting. The granite of Isla de Toas is stratigraphically and chronologically correlateable with outcrops of the Perijá Range and subsurface samples of deep wells of the basin, including La Concepcion wells. This granite consists of two textural varieties: one is rose colored, very fine-grained with orthoclase feldspar, perthite, quartz and phenocrysts of hematite and magnetite. The other type of granite, gray in color, is characterized by a porphyritic texture, which in local areas is pegmatitic. On weathering, the granite changes from pink to gray and the biotite becomes golden brown. The rock becomes soft and crumbly and is easily eroded. Both granites have been identified in the La Concepcion subsurface.
Basement exposure can be used as an
excellent analog for
Three old wells, all located in the southern structure, have been cored in basement. The total cored length is 69 feet with only 24% of recovery (16’8”), demonstrating the high grade of fracturing.
Figures 6-1 – 6-7, Table 6-1
Formation Water Analysis Chemical analyses have been done to four producer wells in order to characterize basement formation water. There are three main components that help to determine the formation water origin.
1) Basement formation water is enriched in Ca+2, ranging from 12000 to 14000 ppm; even higher Ca+2 content can be found as in well C313 (>30000 ppm), comparable to neighbor fields, Mara and La Paz. 2) Cl-1 content is higher in basement than Cretaceous formation water, ranging from 32000 to 38000 ppm. 3) Lower Na+1 content is common in basement formation water (<9000 ppm).
On average, in La Concepcion field, total dissolved solids and salinities are very similar for both basement and Cretaceous waters, there is not a defined pattern; hence to characterize formation water all chemical parameters must be considered.
Figures 7-1 -7-2, Table 7-1
First well with commercial production was C310 completed in natural flow on December 2003, initially producing at a rate of 4600 bopd. Originally drilled to Cretaceous objectives, the well was decided to explore for hydrocarbon accumulations into basement because of the low displayed potential.
Development of basement
Evidence of oil and gas has been observed in several wells that penetrated basement: increase of gas during drilling and some petroleum production once wells were completed.
The most obvious case could be
observed in well C302, which according to a PLT initially taken,
1250 bopd came from basement (33% of the produced petroleum by the
well). On the other hand, the cumulative production of 17 and 29
MMbbl of oil from wells C148 and C227, respectively, neighbors of
the area, only can be explained with production coming from either
basement or Cretaceous
In December 2006, well C325 was drilled on
North Cretaceous structure. This well is located in the northern tip
of the
Crude oil from basement average 36° API
(light oil) and seems to be the same type that is in overlying
Cretaceous Both Cretaceous and Basement reservoirs can be characterized by high decline rates in short periods of time. Production levels become stabilized in values, which in some of the cases, are below economic limit. According to the production history of the wells, evidence of interference between them has not been observed. GOR values have stayed stable from their beginnings, between 500 and 1200 scf/bbl. All wells are actually producing by artificial gas lifting system.
Although production mechanism has
not been fair determined, by analogy with North Cretaceous
According to the behavior of production and pressure, a water-driven mechanism has not been observed. Water production in completed wells in basement is relatively low.
Two pressure measurements (wells C310 and
C313) were taken once the
When comparing initial pressure of
basement as to the obtained on the producing Cretaceous
Figures 8-1 – 8-5, Tables 8-1 – 8-4
The basement is drilled near-balance through deviated wells with directional trajectories perpendicular to the strike of fault planes in order to intercept the higher number of fractures. Average inclination of wells is 22°, and can reach more than 45 degrees. Considering most of the wells had initial cretaceous objectives, wells were completed open hole. Drilling results show that 75% of deep wells reached more than one hundred feet into basement. Ten wells have penetrated 500 to 1000 feet. Maximum thickness reached by a well is 1440 feet. The productive section of basement has not been fairly well defined because the difficulty of run image and/or production logs.
Drilling within the basement has
been a common practice from several years. Among forty deep wells
drilled in the field with cretaceous objectives, twenty-nine reached
basement. Only four of these have proved production from the
basement
Figures 9-1 – 9-2, Table 9-1
Key Factors for Basement Production
La Concepcion basement Statistical analyses have established that for producing from basement, the following factors must simultaneously occur:
La Luna/Cogollo Group in Fault Contact with Basement
Productive wells are located within the
uplifted structures and are related to secondary minor faults
linked to the major bordering faults. Fractures develop around the
faults. The fractures provide both storage capacity and the
fluid-flow pathway in La Concepcion basement
Cap-rock or Seal (Calcite-filled Fractures), Cretaceous Basal Sandstones Width of zones with calcite–filled fractures is variable across the field. Presence of massive limestones within basement have been detected in various wells. They may be interpreted as part of faulted blocks.
Hydrocarbon shows Nearness to a source rock or production zone
Figures 10-1 – 10-7, Table 10-1
Analogy with Other Fields
The basement
Figure 11-1, Table 11-1
1. Although basement
2. Basement production strictly depends on
fractures (Secondary porosity/Type 1
3. A productive well from basement is
obtained when most of the key factors simultaneously occur in the
4. In La Concepcion field, basement
5. Pressure levels at initial conditions
differ significantly from Cogollo Group to basement
6. Improvements in geological, seismic and
Table 12-1
The authors wish to thank Petrobras Energía Venezuela and Petrowayuu Filial de PDVSA for the approval and permission to publish this paper.
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García, J., Gonzalez, G.,
Leandri, P., Lorin, T., and Reymond, A., 2000, La Paz Field study:
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carbonate Nelson, R.A., Moldovanyi, E.P., Matcek, C.C., Azpiritxaga, I., and Bueno, E., 2000, Production characteristics of the fractured reservoirs of the La Paz Field, Maracaibo Basin, Venezuela: AAPG, v. 84, p. 1791-1809. P’an, Ch.-H., 1982, Petroleum in Basement Rocks: AAPG Bulletin, v. 66, p. 1597-1643. Sircar, A., 2004, Hydrocarbon production from fractured basement formations: Current Science, v. 87, 25 July, p. 147-151.
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