Hydrocarbon Production From Fractured Basement Reservoirs - Version 7
February 2002
Introduction
This compilation presents brief details of the occurrences of commercial hydrocarbon reservoirs in fractured basement rocks from approximately 30 different countries. By definition (see below), the review concentrates only on those reservoirs found in igneous, metamorphic and volcanic rocks.
The document has been assembled primarily from published literature and is therefore, to a large degree, a historical review. However, we have also incorporated responses to the sci.geo.petroleum newsgroup and endeavoured to make new information available on a regular basis. It is made available for personal interest and education only and should not be republished or distributed in any way. Data has not been cross-checked in detail against multiple references so use with care. In addition, some of the information, for example on production, will be out of date since it is based on historical sources.
Information updates, corrections and comments are welcome. Has your Company found an example that we can use in the public domain to validate these occurrences? We would appreciate the data, please help support Version 8. We know from our own work that there are several fields in various countries that are not included here because no information has been released in the public domain. We will make it available as it is released.
This update was released on 11th February 2002 and was compiled by Tony Batchelor, Jon Gutmanis and Colleagues of GeoScience Limited.
If you would like to read it off-line, it is available to download as a PDF file from our Downloads page.
Go directly to directory of Basement Reservoirs
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Background
A large proportion of the world's proven oil reserves have been found in reservoir rocks that are naturally fractured (Waldren & Corrigan, 1985; Nelson, 1985; Aguilera, 1995; Nelson, 2001). In his updated book, Nelson (2001) gives a list of some 370 fields where natural fractures are important for production and a significant proportion are in basement settings. Nelson (2001) also states that "…….in BP Amoco alone, current and future fields in various types of fractured reservoirs are estimated to account for some 21 billion barrels of oil equivalent (BBOE)".
The occurrence of naturally fractured basement reservoirs has been known within the hydrocarbon industry for many years but generally regarded as non-productive, they have failed to draw the attention of the explorationist. Often passed over as 'of no economic potential', their investigation by exploratory drilling has been left to chance. Yet, they are commonly distributed in various petroliferous regions throughout the world.
As early as 1948, Eggleston (1948) carried out a comprehensive survey of oil production from fractured basement rocks in California and found that 15,000 barrels per day were being produced from such rocks. This represented about 1.5% (918,000 barrels per day) of the total California production at that time. Hubbert and Willis (1995) produced a comprehensive list of fractured reservoirs in the United States.
According to Landes et al (1960), about 100 million barrels of oil had been produced by that time from various basement rock sources worldwide with initial productions being as high as 17,000 barrels per day. He goes on to suggest that with accumulation of oil in such quantity, from a source often as not found by accident, the probable reserve in fractured basement rocks is of such a magnitude that discovery by design should become the rule.
Reservoirs in fractured basements, where the oil and gas in place may be held within an extensive fracture network on a variety of different scales rather than within the matrix porosity of the formation, present challenging problems to the petrophysicist and reservoir engineer. Fractured reservoirs are much more difficult and expensive to evaluate than the more conventional reservoirs (Nelson, 1982, 1985, 2001).
A greater understanding of the fracture distribution and connectivity within basement reservoirs may prove to be the key tool for improved exploration and production management of this hidden resource.
Commercial, naturally fractured basement oil deposits have been found largely by accident, whilst looking for other types of reservoir (Aguilera, 1980; Landes et al, 1960). Landes et al, (1960) postulated that basement rock oil accumulations are not freaks to be found solely by chance but are normal concentrations of hydrocarbons obeying the rules of origin, migration and entrapment. Therefore, in areas of not too deep basement, oil deposits should be sought with the same professional skill and zeal as accumulations in the overlying sediments.
Landes (1959) stated that once the basement rock had been reached during drilling, it was thought that there was little or no chance for oil production. Many oil companies still stop drilling operations as soon as basement rocks are intersected. Aguilera (1995a) suggests that drilling should be continued into the basement rocks for at least 300 m, especially if the basement is overlain by an oil yielding formation. Kenney (1996) states that in the western countries, all of the oil fields that produce from crystalline basements were discovered by accident. Aguilera (1995b) and Russell (1995) continue, stating that most naturally fractured reservoirs (sandstones, carbonates, cherts, shales and not just basement reservoirs) were discovered by accident. In Russia and some of the other countries of the Former Soviet Union (FSU) however, drilling into crystalline basements has been carried out intentionally (Kenney, 1996), although a literature search reveals that citations of producing fields in basement are actually few and far between. Landes (1959) wonders how many oil discoveries have been missed because of inadequate exploration of the barely scratched basement by unsuccessful wildcats.
Exploration specifically for naturally fractured basement reservoirs has been known to encounter serious problems. Most natural fractures of commercial importance are vertical or sub-vertical (Aguilera, 1996). Consequently, vertical wells are unlikely to be as successful as directional or horizontal wells in target location. Aguilera (1996) believes that significant volumes of undiscovered, profitable hydrocarbons exist worldwide and may have been missed by a failure to intersect the natural fractures.
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What is a basement rock?
Many definitions of 'basement rocks' exist. These have been discussed by Landes et al (1960), P'An (1982), Koning & Darmono (1984), Aguilera (1995a) and North (1990).
The definition of a 'basement rock' used for this work follows that of Landes et al (1960). Here, basement rocks are considered as any metamorphic or igneous rocks (regardless of age) which are unconformably overlain by a sedimentary sequence.
P'An (1982) in a major study of petroleum in basement rocks, considers two definitions. The first, where metamorphic and igneous rocks (regardless of age) are unconformably overlain by a younger oil-generating formation (source rock). The oil, which is generated from the overlying sediments, is stored in the older metamorphic and igneous rocks. The second case considers any rocks that unconformably underlie oil-generating or oil-bearing formations as basement.
Aguilera (1995c) does not consider sandstones and carbonates as basement rock, even if they conformably underlie oil bearing or oil generating formations.
North (1990), however, has a different view towards defining basement rocks. Unlike Aguilera (1995c), North considers basement rocks to include those of sedimentary origin if they have essentially little or no matrix porosity. He states that 'basement' should not be compared with 'PreCambrian', and that basement rock may have considerable fracture porosity due to deformation, weathering or both (North, 1990). This definition would be quite wide and would include fields hosted, for example, in the Cambro-Ordovician quartzitic sandstones of Algeria
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What favourable conditions are required in basement rock reservoirs?
All basement reservoirs underlie a regional unconformity and almost all lie on an uplift or high. This uplift or high was generally continuously uplifted for long periods of geologic time and was subject to a long period of weathering and erosion. Younger sediments, which act as hydrocarbon sources, either flank or directly overlie basement providing the opportunity for entrapment of oil in the basement rock.
Structural highs in the basement are created by fault tectonics or by the submergence and subsequent covering with sediments of hills sculptured in the basement rock during its emergence. It is important that the reservoir is overlain by a seal and that oil from adjacent source rocks is able to migrate into this trap.
Unconformities can play an important role in basement reservoirs as they can be the pathway for oil migration. The unconformity surface often provides evidence that the basement rocks have undergone weathering, erosion, solution and leaching for so long a time that porosity and permeability have increased greatly and, hence, the accumulation of petroleum facilitated, P'An (1982).
The usual 'cap rock' for basement accumulations is relatively tight (low permeability) sedimentary rock. However, a tight zone in the basement rock at the Mara field in Venezuela is barren and may act as the seal. At the other extreme is the situation in several California fields where a thick oil column extends from an oil-water interface within the basement rock upward through a continuous reservoir, which includes such permeable material as wash and basal sandstone, until a tight rock is reached somewhere above the base of the sedimentary section, Landes et al (1960).
Most basement rocks are hard and brittle with very low matrix porosity and permeability, consequently reservoir quality depends on the development of secondary porosity.
Secondary porosity may be divided into two main kinds by origin;
tectonic porosity (joints, faults, fractures, etc at a range of scales from microfractures to seismic scale faults and their damage zones), and
dissolution porosity (ranging from solution effects in weathering zones or fault zones to effects associated with hydrothermal circulation (Smitt, 1998)).
McNaughton & Garb (1975) and Aguilera (1995b) characterise fractured reservoirs based on porosity distribution between the matrix and the fracture system. In basement reservoirs matrix porosity is effectively close to zero and most of the storage capacity and permeability is due to fractures. Reservoirs of this type (Type C of Aguilera, 1995b) can be characterised by initially high production rates that decline to uneconomic limits in a short period of time. However, the exceptions to this have been worthy of their discovery and include the Edison and Mountain View Fields in the San Joaquin Valley of California; the El Segundo, Wilmington and Playa Del Rey Fields in the Los Angeles Basin; the La Paz-Mara Fields in Venzuela; and the Amal Field in Libya.
Methods for exploring and evaluating fractured basements are well described by North (1990), Aguilera (1995b), Nelson (1979 & 1985) and Stearns & Friedman (1972). A review of the engineering and geological problems encountered in naturally fractured reservoirs is presented by Waldren & Corrigan (1985).
There is now a growing wealth of papers published on fractured basement reservoirs. Examples include Younes et al (1998), Areshev et al (1992) and Salah & Alsharhan (1998). Recently the SPE held a week long meeting on the topic of fractured reservoirs in general (St. Maxime, 2000) which included considerable discussion of basement reservoirs. In early 2001 there was a Petroleum Group Conference on 'Hydrocarbon Production in Crystalline Basements' at the Geological Society London. GeoScience gave a keynote presentation on granitic hydrocarbon reservoirs.
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Source of oil in basement reservoirs
There are many possible sources for the oil accumulations in basement reservoirs, however, three sources are referenced most commonly:
Overlying organic rock from which the oil was expelled downward during compaction.
Lateral, off-the-basement but topographically lower, organic rock from which oil was squeezed into an underlying carrier bed through which it migrated updip into the basement rock.
Lower, lateral reservoirs from which earlier trapped oil was spilled due to tilting or overfilling (Landes et al, 1960).
Mechanisms have been identified that could allow the downward migration of oil into fractured basement when fracture dilation is caused during shearing in an anisotropic stress field (Pine & Batchelor, 1984). Dilatancy in the underlying reservoir rock reduces hydrostatic pressures in local areas of deformation. Pressure gradients are thereby established between the potential basement reservoir rocks and the overlying source and carrier beds containing oil, gas and water. Thus, a tendency to 'suck in' fluids into the basement rocks will be created; this view is supported by direct observation, McNaughton (1953) and McNaughton & Garb (1975).
Recent work by Kitchka (1998), supports the theory of an inorganic mantle origin of petroleum. His paper introduces the concept that petroleum represents a complex derivative of the fluid inclusions saturated with hydrocarbons in crustal and mantle minerals. He concludes that the multi-stage segregation and migration of deep petroleum are realised by fracturing and faulting. He cites a total of 370 oil and gas fields with commercial productivity from crystalline basement. Other hypotheses by Kropotkin (1986), Krishna (1988), Szatmari (1989), Porfir'ev (1974), Hunt (1998), and Gold (1980 & 1985) also consider the abiogenic/ mineral origin of petroleum.
A review of these more exotic prospects of oil accumulation in deep basement is given by Harrelson (1989). He considers that drilling for deep igneous and metamorphic prospects, which are considered at or below economic basement or worse, should become increasingly commercial as deep drilling technology progresses, the current oil glut is eliminated and more exotic gas prospects become accepted.
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BASEMENT RESERVOIRS
Countries With Hydrocarbon Finds In Basement Reservoirs
Examples of producing basement reservoirs covering many countries throughout the world have been documented within the public domain. This compilation has attempted to refer to hydrocarbon fields where production figures can be cross-referenced to published literature or traceable sources.
The reservoirs are organised by continent
Europe
North America
South America
Asia
Africa
CIS and Russia
Middle East
Oceania |
