position of planetary objects. Information was furnished in advance, based on an al-manac. It would be interesting to know the basis for their predictive power. This is, as stated by the author of the book under review, a 'fertile field of research' .
A glimpse of the variety of subj ects which have been discussed in this book can be had by a perusal of the contents. Evolution of Time, Fourth Dimension, Indian Chronol-ogy, Hindu calendar, Astrology and Gems, Earthquake Prediction. These are some of the subjects which are of iinterest to earth scientists. It
will
be seen that the book covers a wide field. What is now important is to disentangle genuine observations from the surrounding myth and establish the scientific basis behind these predictions. This inves-tigation into our early heritage is a necessary part of our education. The book under reveiw will help in drawing out a plan for such future studies.B. P. RADHAKRISHNA
NOTES
c EXPLORATION STRATEGY FOR PETROLEUM
IntrodJl!:tion
Exploration strategy can have both a niacro- and a micro- aspect. Depending upon whether the area under exploration belongs to a geological province where petroleum has or has not already been found, the approach can also be different.
Underlying the formulation of
all
strategies and planning in this work lie the basic concepts of where petroleumin
commercial quantities is most likely to be found. Certain broad ideas as regards this have grown over the ages, but there is still no unanimity of views as to where, in any given area, such accumulations are most likely to be present. Itwould therefore be appropriate if I start with a little historical recapitulation of ideas concerning where to look for oil. Unless otherwise specified, the word oil will henceforth include natural gas as well.
Initial Approaches
The frrst conscious exploration for oil started with drilling or digging wells near surface seepages. It was found that by tapping a seepage zone a little below the surface enables one to obtain a larger flow of oil. This was because the oil there tended to be more fluid and the rocks less clogged with drying up petroleum residues.
One could say that the fIrst exploration strategy was therefore to dig or drill near known seepages - either at the surface or in shallow wells. The Burmese have used this approach for several centuries - so have the people in a number of other places. The modern oil industry in USA was also born through this approach in 1859. Together with drilling near seepages, venturesome exploration groups also used many other exotic tools like the Divining Rod, th{~ Doodle Bug, etc. Inspite of their lack of any scientific basis, some of these were successful in finding commercial accumulations.
Within 2 years, i.e. by 1861, Andrews noticed that many of the successful wells were aligned along the summit of anticilinal structures, but little attention was initially paid to
this observation. The anticlinal theory had to be revived again 25 years later (Le. in 1886) by
I.e.
White. Even then, an oil field (Mannington, USA) had to be discovered on this basis (in 1889), before the theory came to be fully accepted. Alongside this, was developed concepts concerning the role of gravity in the formation of petroleum ac-cumulations. First propounded by Sterry Hunt in 1863, this concept again took some years to get integrated with a number of other rationales on where to look for oil. These illustrate how resistant the industry was initiallyin
accepting even valid new ideas concerning the occurrence of oil. <J,The concept of stratigraphic association, i.e. that of porous or reservoir zone, both underlain and overlain by relatively impervious rocks (usually clays), as a requirement also grew alongside the above concepts. In its next stage therefore, the exploration strategy focussed attention to a large extent on robust anticlinal folds with a closed summit area and the right kind of stratigraphic association.
Margins of folded geosynclinal belts, diapiric structures (both salt & plastic clay controlled), and a host of other basin· centre robust· folds, received a good deal of attention during this phase. The dynamics of folding, fold geometry, and how to locate the underground summit area on the target stratigraphic zones received a lot of attention. Such rudimentary geophysical tools as were then available, and various ways of section construction to depict underground fold geometry, were used as aids in this work.
Refinement of Concepts
Since then, many refmements have been added to the concepts regarding the packages of conditions required for the formation of petroleum accumulations. It was realised that not only gravity, but capillary forces, movement of water, and time, all play important roles in their formation. Neither an anticline nor a thick cover clay is essential to stop the movement of oil, and the same sand body can have oil below and water above, across very minor reversed faults. It is also Dot essential for an accumulation to be in complete equilibrium, and there are many which are essentially transient in nature.
Furthermore, not all petroleum accumulations have oiVwater margins which are horizontal. Relatively small movements of formation water can displace an oil accumula-tion off-centre from the structural high and give a tilt to the oil water interface (King Hubbert, 1953). Transient phenomenon and capillary changes may also lead to such inclined oil water interfaces.
A combination of all the above possible modes of control leads to myriads of places where petroleum can be sought but indiscriminate drilling in all these places may result in the cost of exploration exceeding the value of the oil discovered. Attention was consequently given byvarious research groups to the geotectonic and other environments
with which oil is most often associated. .
During the 19308 & 194Os, it had been noted that the geotectonic environment of a basin and its component segments exercised a profound influence on the nature of the rock types deposited. It was 'now found that the frequency and size of the petroleum accumulations formed also had a strong correlation with their geotectonic environment. Knebel and Rodriguez - Eraso (1956), for example, found that about 71% of the oil discovered upto that time was from the shelf environment, 23% from the hinge zone, 2% from the basin centre and 4% from the basins steeper side or the mobile rim.
This pattern is by and large true for geosynclinal type of basins which have suffered subsequent orogenic deformation. For intra-cratonic basins, where gravity is the
ling tectonic force, structures near the basin centres can also have important accumula-tions.
Further contributions concerning the 'habitat of oil' were made by a number of other workers - notably by Weeks in 1958. To these, can be added the concept of differential entrapment of oil and gas, through which Gussow (1954) tried to explain the distribution pattern of oil and gas fields in a number of petroliferous regions. Others added concepts on where to look for giant gas/oil fields. The Shelf or Platform areas of basins came to be accepted as important for such search, after the discovery of Bahrain in 1932 and Damman in 1938.
The above concepts relate primarily to the trapping and accumulation of petroleum. During the last few decades, a number of others have been added related to. the genesis of oil. For convenience, the more important of all these concepts are summarised below:
a) A marine environment is not essential for the formation of petroleum. This can take place in widely ranging conditions, from fresh water to marine and from a fairly wide range of organic materials. Most of the paraffinic crude oil, for example, are believed to be derived from terrigenous material.
b) The organically-rich fmer clastics and biologically produced carbonates are regarded as the more common petroleum source rocks. Purely inorganic sources are also not ruled out.
c) To be converted to petroleum hydrocarbon liquids, the organic material entrapped in the above has to be subjected to a range of time cum tempera-ture conditions, often referred to now-a-days as the 'oil window'. Maturation and palaeo-temperature studies of organic-rich sediments can sometimes provide a clue to whether, where and when such petroleum forming 'Kitchens' are 1ikely to have been developed. .
d) In any large basin area, the local depressions which have silled conditions at the bottom (where little replenishment of oxygen takes place) can be better locations for the generation of hydrocarbons. This is not however an essential pre-requisite and, with rapid burial, petroleum can also form in basins with otherwise highly oxidising conditions.
e) For petroleum accumulations to occur,' some of the reservoir rocks available should have had access to areas favourable for petroleum formation at the appropriate geological times concerned, and adequate porosity/permeability must be retained through the subsequent phases of diagenesis and compac-tion.
f) There is a tenden.cy for petroleum accumulations to get concentrated in certain specific types of geotectonic regions (platform areas, hinge belts, central regions of basins within cratons where gravity is the controlling tectonic force, etc.).
g) Within a petroliferous basin, the accumulations have often a tendency to occur in progressively younger horizons towards the basin centre. Gulf of Mexico and NE Assam are examples.
h) Hydro-dynamics and differential entrapment may be important in the precise location of accumulations.
i) Petroleum does not always need an anticline in which to accumulate, even minor changes in the slope of beds and/or permeability can lead to the formation of an accumulation.
j) Petroleum accumulations formed through the controlling mechanism of
a:
given set of geological conditions, in any basin, often tend to occur in clusters with the individuals ranging in size from large to small. With sound geological approach to exploration, the largest accumulation would often be the first or the second to be discovered. The subsequent ones would tend to be progres-sively smaller.k) In any single basin, there may be more than one such cluster, each needing to be discovered separately and each controlled by a different set of conditions. In their geographic distribution, the individual clusters may be totally separate or may partially overlap each other.
t) Certain classes of heavy crude occur close to land surfaces both past and present. Termed by some as 'unconformity oil', their presence can often indicate the existence of better oil away from the unconformity surface or the outcrop concerned.
m) Age-wise, petroleum accumulations have been found in sediments ranging from the Cambrian to the Pleistocene. At times, petroleum formed in a basin has also found reservoir conditions and have accumulated in the metamor-phic basements and in igneous sills. So far, only minor attention has been paid to the Proterozoic, but now that evidence of life is well established from formations of this age, these may also need more attention in any search for oil.
n) Maturation of coal beyond the stage where it has 83% carbon, leads to the release of methane which can then either remain there trapped within the coal seam itself, or move to a nearby reservoir rock. Conceptually, it is also possible to have accumulations of inorganically derived methane.
0) Some accumulations can be very unorthodox. In Lake Kivu (Africa), for example, the bottom water has dissolved in it enough methane to provide a theoretical reserve of the order of 57 trillion cubic metres. Together with this, there is also around 190 trillion cubic metres of C02 and 4 trillion cubic metres of nitrogen.
p) An important matter to remember in any search for oil is that not all the oil discovered so far has been through well planned exploration. A substantial quantity has been found accidentally by people following at that time no acceptable guidelines whatsoever. In fact, in many instances, the develop-ment of concepts on where to look for oil has followed rather than preceded the discoveries concerned. One therefore needs to make an allowance for one's 'ignorance factor' in formulating any scheme for exploration.
As already implied, the above does not constitute an exhaustive list. Many other associations have been noted, but these cannot always be extrapolated and used as exploration guides in other areas. For example, it has been stated that the shear patterns produced by deep-seated wrench-faults have influenced the linear distribution of petroleum traps in many areas extending from Alaska, through California, to Colombia-Venezuela. A similar association has also been noted from the Gulf of Guinea, but it is not clear whether this is a special feature pertaining to some local areas only or it has a much more general application. Likewise, most of the oil-rich belt in Indonesia and its
JOURGEOL.SOC.INDIA,VOL.44,JUL Y.1994
adjoining region, occurs on the inner side of their island arc systems, behind the frontal mountain belt of the Indonesian Plate. Whether a similar association can be expected in an places where two plates have collided is not clear.
Then again, there is the growth fault controlled roll-over anticline regime which has been found to contain a lot of oil in certain delta environments like the off ~shorc
Nigeria, but we do not seem to be clear about the key combination that has produced this. May be, a combined geological cum statistical analysis will produce the clues that are required to make these relations more widely useable.
In any case, even the guidelines formulated on the above concepts can only lead to the delimitation of the types of environment where oil has been found so far. In addition, there must be other categories of areas, also favourable, in which no discoveries have been made, and which may continue to remain exCluded if guidelines are only sought from the p·ast.
Technology Developments
Furthermore, all these concepts call for a reconstruction of the geotectonic frame·· work of each basin - its environment, mode of formation and subsequent sedimentation--cum-tectonic history - ranging over a time period of many las or laOs of million years. Of the enormous mass of diverse data required for such reconstruction, only a minute proportion is normally dir,ectly available. This has spurred the petroleum and its suppor·· tive industries world over to develop a multitude of geophysical and geochemical concepts, equipment, instruments and interpretative techniques -
all
aimed at obtaining indirectly the information required on the underground nature, disposition and sequence of rock layers, and their fluid contents (including oil and gas).Starting from the initial Torsion Balance gravity surveys, which were very slow, the industry has now available gravimetric, magnetic, electrical and seismic surveys of various types backed by sophisticated equipment and computer aided interpretation methods. There are also a variety of similar highly sophisticated well bore logging and analytical techniques. In theory, some of these should be able to provide even a molecular analysis of the underground formations and their fluids, provided the depth of invasion by the mud filtrate is very low. 1b add to these, there are also a large number of gyochemical techniques and greatly improved support systems involving palaeontological, palynologi-cal and petrologipalynologi-cal study of surface samples, drill cuttings and cores. On the macro-scale, air-photographs, combined with the more recently developed multispectral im-ageries, have the capability of bringing out much more clearly the regional tectonic and stratigraphic picture and their inter-relationships.
Direct Indications of oil
Under certain favourable circumstances, petroleum accumulations may also produce some directly observable surface manifestations, like macro-& micro- seepages, and recognisable relation to geophysical measurements from the surface.
There have been reports concerning the presence of radioactive, soil hydrocarbon and bacterial count anomalies above a number of known oil fields, but not many have tried to use these as serious exploration tools. Basically, all these seem to be ultimately derived from micro-seepages of hydrocarbons from somewhere below. The problem is that, in their upward path, these seepages are liable to get swept laterally by water moving
through the intervening rocks. Any observed anomaly may therefore be miles away from the accumulation which is its source. Nevertheless, anything which suggests the presence of oil somewhere below may itself be a valuable information. The hydrocarbon anomaly mapped by ONGC near their !chapur well in West Bengal, drilled partly on this basis may be an example.
It has been known for some years that substantial gas columns down to a moderate depth (say 6000 - 7000
ft.)
can bring the reservoir's seismic velocities down enough to produce features like hot spots, bright spots, etc. Looking for these can therefore be a very useful exploration tool. There are also other direct indications possible.For example, in very old oil or gas accumulations, mineralisation and diagenetic changes could be much less in the oiVgas filled parts of the reservoir than in its surrounding water-bearing part. In consequence, the latter could have a much lower porosity. Eventually, both can thereby have the same type of resistivity, but widely different seismic velocities. Provided the reservoir and its oiVgas column has an adequate vertical extent, these changes may therefore show up in a combination of resistivity cum seismic study.
Other Developments
Scientific and engineering developments during the World War-II had a major impact in many operational areas in the years that followed. In consonance with this, the entire range of facilities and interpretational techniques available to the petroleum industry has also had an explosive growth. Even so, the amount and quality of information that one can obtain today still falls considerably short of what is required. That is why the petroleum exploration and production industry is still a venture so full of uncertainty and risks. One needs almost to develop a 'third eye' or a sixth sense to be able to perceive more and thereby reduce the risks and uncertainties.
I must mention here in passing that there has been a parallel explosive growth in drilling, engineering and production technology as well, but the details of these would be beyond the scope of the present
talk.
Reservoir Engineering Concepts
There is one more aspect I would like to touch before turning to exploration strategy in general. To understand a petroleum accumulation, one needs to know not only the geological character of its reservoir and surrounding rocks, but also a host of other parameters related to the reservoir, its trapping conditions, fluids and forces present. These latter aspects, which would now be called reservoir engineering, grew into a separate specialised branch of knowledge from around the 1950s, although some of the basic laboratory studies have been carried out as far back as the 1930s. Many of the early geologists did not have an adequately strong background of mathematics and kept away from the growth of reservoir engineering. Many still do so in a large number of organisa-tions. Ideally however, subsurface geology and reservoir engineering should complement each other, since without an integrated view point, a full understanding of the petroleum accumulations is not possible.
Location of large oil pools on top of robust anticlines can be done where present, through conventional geological means. But location of accumulations held in complex stratigraphic and other subtle traps can be helped a good deal if the explorer has
comprehensive understanding of both geological and reservoir engineering factors. In the production domain also, the economic recovery from a given accumulation may be small or large depending upon how it is handled. Here again, an integrated geological cum reservoir engineering approach can end up with a better recovery than a purely reservoir engineering solution. This integration had been started in a small way in Assam, and I hope
will
continue further.Enhancing Recovery Expectations
Even in an advanced country like the USA, the overall average recovery expectation to-day is only about 33%. Notwithstanding this, there are individual oilfields with favourable environment, where, with proper reservoir engineering approach, it has been possible to aim at much higher recoveries. In Alberta (Canada), for example, 53 pools under solvent flood operations (with a total of 868 million Cu. metres of original oil in place) are expected to provide an overall ultimate recovery of 59%. Many of the individual ultimate recoveries are expected to range from 60 to 86%. In one of these pools (WIZard Lake D-3A with 67 million m3 OOIP), 91% of the expected 86% ultimate recovery had already been realised by the end of 1991. In Fenn-Big Valley D-2A pool (80
million m3 OOIP) likewise, 98% of the eJ..'Pected 61 % ultimate recovery had been realised by the end of 1991. Many of the high recovery expectations placed on these solvent flood operations therefore are unlikely to be merely wishful thoughts.
A number of natural water-drive reservoirs in Assam and Gujarat seem to be heading in for higher than normally expected recoveries. Even so, introduction of the most appropriate reservoir engineering techniques, to supplement whatever natural advantage there is, may be able to help improve upon these recoveries. In Duliajan, an experimental polymer flood introduced in the 1970s through reservoir engineering approach seems to have been able to produce substantially more oil than originally expected. A lot should be possible through a combined geology cum reservoir engineer-ing approach, and the industry may need to break down any mental barriers it may have in achieving such higher recovery goals.
Since any improvement in recoveries from say 30% to 60% would be equivalent to doubling the discoveries, all exploration in the technological field on how to achieve this should be able to rank equally in its claim on funds with exploration for equivalent sized new oiIfie1ds.
Exploration Strategy
Most of you must have studied petroleum geology and are presumably well aware of many of the existing concepts of how petroleum accumulations form and where these generally tend to occur. Notwithstanding this, I hope my talk so far has helped to give you an integrated thumb nail sketch of the total picture. We can now therefore pass on to the next stage - i.e. that of discussing various possible strategies for its exploration.
In any exploration activity, it is necessary first to take stock of the viability of the area or areas available for exploration from their likely economic, legal, environmental and scientific angles. All the four are very important, but, in this talk, I am going to confine myself primarily to the scientific aspect. This of course has built into it its own economic aspects as well.
A scientific evaluation needs an understanding of the tectonic setting, the 3-dimen-sional stratigraphy and the geological history of the entire basin of which the area
available may happen to be only a part. In the old days, much of this had to be worked out from the scratch. Now-a-days, there are very few completely virgin areas and some initial information is usually available.
Once the above information has been assembled, interpreted and evaluated, it should be possible to say whether the basin as a whole has favourable characters and whether that part of it which is available for consideration is located in a reasonably attractive geotectonic situation. The answer can be either 'no' or 'yes' on all counts, or an entire range of 'intermediates' . Depending upon this, one can either give up the option or proceed with an initial show of interest and some additional selective planned data gathering to enable a better evaluation to be made.
If
this indicates that the scientific cum techno-econonllc considerations justify proceeding further, and that the possible rewards give it reasonable interse priority vis-a-vis other available areas, one would have to prepare a more detailed exploration programme, which maintains a reasonable balance in its fund requirements with what can possibly be expected as the discovery rewards. Since the prospective users of gas in some countries are prepared to pay much less for gas than for oil, their areas with primarily gas prospects tend to be at a disadvantage in this regard.Much also would depend upon the size and geo-diversity of the available area. For example, if the area includes one or two fairly robust and clearly mappable folds under anyone of the possible favourable geo-tectonic conditions, one could decide to drill a well on the most attractive looking summit, without waiting for the completion of the detailing work over the rest of the area. In fact, one could even go back to the old policy of drilling close to oi1Jgas shows, if their distribution in relation to the local geology so warrants.
Then again, if commercial accumulations are already known to exist in the neigh-bouring areas, there would be available information about their mode of occurrence and geotectonic association. These would include the kind of stratigraphic sequence and reservoir rocks in which the oil has accumulated, the possible source rocks, the source areas from which this oil may have come, whether the discoveries are in the folded geosynclinal belt or in the shelf zone, etc. The frrst step can in this case be one of attempting to locate rapidly some more accumulations of the same type in the hope that they would thereby be able to generate production and income in a relatively short period of time and thus support the overall effort of the venture.
Three kind of steps may be involved:
a) that of stepping out from the existing discoveries to see if other areas in the vicinity with roughly similar conditions also contain any commercially producible accumulations,
b) jumping out from the existing discovery area with a view to locating a new accumulations cluster, and
c) seeing what the other geotectonic environments in the area have to offer.
In all these, it would be useful to remember that there is always a law of diminishing return. In stepping out from the neighbouring area's accumulations cluster, one may end up with finds that are smaller and smaller (103 page (k)). In trying to step ou~ and locate a new set of accumulations cluster, one mayfmd that the anticipated controlling geologi-cal faCtors have unfortunately not been operative in the specific area.
If
one does not know when to stop, one can then go bankrupt drilling one dry hole after another. Thesame would apply to the pursuit of exploration in other geotectonic environments as well. Every so often in a chain of failures, it would be useful to pause and see if there are any new directions.
No matter what is the approach, it would require a good deal of back up work. This can include a lot more of geology and geophysics to generate more details and a more clear picture. Locally, one could check on surface manifestations of hydrocarbons as well. This can be in terms of distribution of both macro-seepages of oil and micro-seepages of hydrocarbon gases. Since, some of the hydrocarbon gases produce a green house effect, it is just conceivable that, apart from ground survey, multi-spectral Remote Sensing Imageries may also be able to locate the consequential temperature disturbance. In a relatively virgin area, one would of course have to build up the geological picture from a much more 'hazy' beginning. Information may have to be sought to start with on a wide grid, to be filled up progressively with more and more details. A comprehensive understanding has to be developed of the area's geotectonic frame work and history, and on any perceivable pattern in the distribution of the area's major depositional centres, potential source and reservoir rocks, oiVgas shows and geochemical . indicators. The process may have to include a close examination of the seismic profiles, to detect any possible development of the right kind of stratigraphic cum structural association and features like the hotspots, bright spots, etc. believed to be capable of directly indicating the presence of gas bearing zones.
Where shallow zones are also prospective, various kinds of resistivity surveys may also be of help. There can be areas with tight reservoir formations which call for specialised surveys to see if any resistivity/velocity anomalies of the type mentioned in page 104 are present. After all this preliminary work is completed, one should be able to locate one or two of the more attractive areas in each geotectonic environment (like the marginal folds, the hinge :zone, the outer and the inner shelves, the basin centre of intra-cratonic basins, etc.,) for testing.
After the above preliminaries, one would have to follow step by step a planned sequence of data gathering, review and test-well drilling with a clear idea of why any work is being undertaken and what informations is being precisely sought. As each well is drilled and more information comes to light, there would be need for further geological, geophysical and geochemical work to resolve new questions. There would, in conse-quence, have to be periodic:: synthesis and review leading to the determinations of fresh priorities and fresh locations.
In many cases, the same data may be amenable to more than one valid interpreta-tion, and the identification of the favourable locations may accordingly differ. The right mental attitude of being alive to this can be very important.
Since, there is a limit to what one can spend on exploration, its magnitude must always be kept in tune with the reward expectations, which may be widely different between oil and gas. Where the operations are spread over more than one basin, assessments should also be concerned with determining inter-basinal priorities. Together with any other operational imperatives, these should also be taken into account in deciding upon any exploration programme.
It would be necessary to remember that the very act of drilling through an oil zone does not ring a bell at the surface. It may take a good deal of reliable data collecting, monitoring and scientific perception to recognise its presence. There are many instances of wells drilled either on or near oiVgas accumulations, where the operator has been unable to recognise their presence. Some year's back, the Oil and Gas Journal had
brought out a pamphlet on "Oh. Why Was my Well Dry? Or was it?", which can be relevant even to-day.
Concluding Remarks
Summarising, if the circumstances so warrant, one can still start exploration programmes to-day by drilling on valid geological targets near oil shows and/or on anticlinal summits, if only a few of these are available within the area. These could conceivably make some discoveries and permit an early revenue generation provided favourable conditions are present.
Then again, if one'is operating in an area which is known to have a geologically controlled cluster of accumulations and the object is to fmd more individual elements of the cluster, one could statistically analyse the situation, determine any spatial pattern in their distribution and conceivably use this as a supportive logic in deciding upon where to drill. In cases where the nature of geological control i.,s not very clear, and one has only an existing pattern, one could even consider drilling on a properly oriented and spaced grid, provided this had potentialities of being more cost effective.
In most other cases, there would be very little alternative to that of going through an objective oriented exploration programme using every possible geological, geophysi-cal and geochemigeophysi-cal technology or tool that has been devised.
In spite of
all
the complex and sophisticated modalities that are available today, it is ultimately the man, who decides what tools should be used, pieces together the small bits of information as these come forward, integrates these into an overall picture and then draws the right conclusions, that becomes the most critical factor in any exploration. Estimates concerning the total quantity of oil likely to be ultimately discovered 'inthe world used to be initially quite low. With development of concepts on how oil forms and the wide variety of environments where its accumulations can be found, these assessments jumped from about 600 billion barrels in 1947 to about 1500 billion barrels in 1949 (Levorsen). The rise in these expectations continued with fluctuations till 1965, when these were placed at about 2000 billion barrels by weeks and have remained within 500 billion barrels of this figure ever since. What we need today are new concepts on where to look for oil, so that these estimates can be raised substantially further.
In the past, periods of major development in basic concepts on where to look for oil have alternated with periods of major development in technology and equipment. We seem to have passed a major concept development period in the 1950s & 1960s and a major technology cum equipment development period in the 1970s & 1980s. Perhaps, we are approaching a new phase of development in basic concepts. If so, it would be nice to see some of you being associated with such advancements.
In your efforts to achieve this, you would find it useful to remember three points. Firstly, whilst scientific concepts have helped discover and produce lot of oil and gas, too rigid adherence to the known concepts only has also delayed the discovery of many oilJgas fields. In fact, many of the concepts have initially come into being following (rather than preceeding) accidentally made discoveries. Too rigid a pursuit of existing concepts can therefore be counter productive.
Secondly, improvements in instrumentation and data processing, have now reached a stage, where much of the first level interpretations are being obtained via standardised computational modes, directly from the machine. Ordinarily this is very useful and helps reduce computational time and work load considerably. However, once in a while, you would be liable to face a situation, where the standardised computational mode is not
applicable, and the 'press button' solution may be misleading. You may need to develop an ~tinctive alertness against such possibilities.
Lastly, human mind's capacity to perceive much more than what the normal five senses can directly see or feel is very high. As mentioned in page, you will find it very useful, if you can set about deliberately to improve your perceptive capabilities.
State Planning Board Goot. of West Bengal
6, Carmac Street, CALCUTTA 700 017
AB. DAS GUPTA
CRUSTAL SECTION OF PENINSULAR GNEISS - GONDWANA - EASTERN GHATS, INDIA. (IGCP - 288, FIELD WORKSHOP 1994)
One of the main objectives of IGCP-288 is the correlation of Proterozoic mobile belts including fold belts, sutures, shear zones and major faults and determination of their continuity on reconstituted Gondwana Supercontinent. The 1994 Field Workshop was conducted in South India during January 15-20, 1994 along east-northeasterly crustal section of 800km from Hyderabad in interior Andhra Pradesh to Visakhapatnam on the east coast. The traverse coverd five lithotectonic belts - (i) Peninsular Gneiss/Granite,
(ii)
Nellore Schist belt (both of Archaean),(iii)
Pakhal metasediments, (iv) Eastern Ghats belt (both of Proto:rozoic) and (v) Gondwana belt (Mesozoic/Cenozoic). There were nineteen participants from eight countries - Australia, Brhil, Germany, India, Japan, Nigeria, United Kingdom and U.S.A. A well prepared guide book was provided to each participant.Along the traverse are Hyderabad granites near Pochampalli village, granodiorite gneiss at Nakrekal village and typical tonalitic Peninsular gneiss at Paleru lake. A 'transitional zone' of foliated granitic rocks occurs at Nakrekal village. The Peninsular Gneiss contains mafic enclaves which exhibit at least two phases of folding and shearing. Near Kothagudem town can be seen the relationship between Archaean Schist belt, Proterozoic Pakhal sedim.ents and Mesozoic/Cenozoic Gondwana sediments. Structures between the frrst two seem to be conformable. The Gondwana sediments near Yellandu road are considered to have been affected by low' angle thrusting (sliding?) during post-depositional cratoni(; movements.
Archaean garnetiferous amphibolites and Proterozoic nepheline syenite gneiss occur near Tallada and Ashwaraopet respectively. The volcanics near Rajhamundry are comparable to those of Rajrnahal and Deccan Traps. Acid charnockites at Elleswaram and the basic variety at N arsipatnam were the next to be visited. At Visakhapatnam three types of khondalites which grade into one another, were clearly identified. The domal structure at Madhuravada with leptynite core and khondalite rim, and the acid charnock-ite of Aganampudi were areas of considerable interest in Visakhapatnam district.
A regional lithotectonic map with detailed structure along with radiometric dating in critical areas is badly needed and it is hoped that in the near future efforts will be directed towards filling gaps in our knowledge.
Dept. of Geology Bangalore University
Bangalore - 560 056 B. MAHABALESWAR
