Indications are that there could be oil below the granite...

Indications are that there could be oil below the granite basement with the probability of gas

http://www.geoscience.co.uk/assets/file/Reservoirs%20in%20Fractured%20Basement%20Ver%209_JCG.pdf


GeoScience Limited
Version 9 Aug 2010
HYDROCARBON PRODUCTION FROM FRACTURED
BASEMENT FORMATIONS
INTRODUCTION
This is a compilation of public-domain information about commercial hydrocarbon reservoirs
in fractured crystalline basement formations from approximately 30 different countries.
Although basement reservoir plays have been exploited for decades, since the mid-90’s
there has been increasing interest and exploitation due to a number of factors working
together. These include the impetus derived from major discoveries in Vietnam and Yemen;
the advent of new downhole tools (especially borehole image logs and full waveform sonic
logging), seismic techniques (eg shear-wave attributes) and sophisticated drilling methods;
and the arrival of elevated oil prices to allow re-appraisal of basement projects previously
discounted as difficult or ‘uneconomic’.
Perhaps the best known basement reservoir examples are offshore Vietnam, where the Cuu
Long Basin comprises 95% of the country’s hydrocarbon production and 85% of this value
comes from the fractured granitic basement, and in the Yemen (e.g. DNO in Block 43,
Nexen in Block 14, Total in Block 10, and OMV in Block S2). Other significant discoveries
have been made in Argentina at the Cuyo Field and the Neuquen Field, both producing from
fractured volcanics.
We also know of one company (Hurricane Exploration) specifically set up recently to
appraise and develop basement plays, and they are having success on the UK Continental
shelf West of Shetland.
By definition (see below), this review concentrates only on those reservoirs found in igneous,
metamorphic and volcanic rocks. Although largely a historical review we have tried to make
new information available on a regular basis, and we are currently at Version 9 of an
evolving document started in the mid-90’s. 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the information, for example on production, will be out of date since it is based on historical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. We know from our own
work that there are several fields that are not included here because no information
has been released in the public domain. If you can provide information or examples
that we can use in the compilation we will be delighted to continue developing the
resource.
Compiled by Jon Gutmanis, Tony Batchelor and colleagues at GeoScience Limited. GeoScience Limited
Version 9 Aug 2010
FRACTURED BASEMENT RESERVOIRS - DEFINITION AND
HISTORICAL 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). Nelson (2001) listed some 370 fields where natural fractures are important for
production, a significant proportion being in basement settings, and also stated 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)".
Basement reservoirs are a subset of naturally fractured reservoirs, and various definitions of
'basement rocks' exist (see for example Landes et al 1960, P'An 1982, Koning & Darmono
1984, Aguilera 1995c and North 1990). The definition that we think is most appropriate in
the context of hydrocarbon exploration is that of Landes et al (1960):
any metamorphic or igneous rock (regardless of age) which is unconformably
overlain by a sedimentary sequence
North (1990) however took a different view, considering basement rocks to include those of
sedimentary origin if they have little or no matrix porosity. This definition would be quite wide
including fields hosted, for example, in the Cambro-Ordovician quartzitic sandstones of
Algeria.
Basement reservoirs have been known about for decades but were often passed over as 'of
no economic potential'. Yet they are commonly distributed in hydrocarbon regions of the
world. As early as 1948, Eggleston (1948) reviewed oil production from basement rocks in
California and found that 15,000 bpd were in production, representing about 1.5% of total
Californian production at the time. Soon afterwards Hubbert and Willis (1955) produced a
comprehensive list of fractured reservoirs in the United States.
Landes (1959) speculated that many oil discoveries had been missed because of
inadequate exploration of the barely scratched basement by unsuccessful wildcats.
According to Landes et al (1960), about 100 million bbl had been produced by that time
from various basement formations worldwide, with initial productions being as high as
17,000 bpd. In their view basement hydrocarbon accumulations were not freaks to be found
solely by chance but normal concentrations of hydrocarbons obeying the rules of origin,
migration and entrapment and should be explored for with the same professional skill and
zeal as accumulations in the overlying sediments. They suggested that the potential offered
by fractured basement plays was of sufficient magnitude to justify attempts to discover them
by design rather than by the previous situation of discovery by ‘accident’ (drilling too deep).
Kenney (1996) noted that all the oil fields in the West that produce from crystalline
basements were discovered by accident whereas in Russia and some of the other countries
of the Former Soviet Union (FSU) drilling into crystalline basements has been carried out
intentionally (although a literature search reveals that citations of producing fields in
basement are actually few and far between). Until more recent years perhaps, many oil
companies stopped drilling as soon as basement rocks were intersected. Aguilera (1995a)
suggested drilling at least 300m into basement especially if the cover rocks contain oil and GeoScience Limited
Version 9 Aug 2010
believed that fractured reservoirs contain significant volumes of undiscovered hydrocarbons
which may have been missed by a failure to intersect the mainly vertical to sub-vertical
fracture system (Aguilera 1996).