the economic importance of pre-cambrian rocks

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    salheen
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    تاريخ التسجيل : 12/07/2007

    the economic importance of pre-cambrian rocks

    مُساهمة من طرف salheen في 2007-08-07, 9:08 pm

    Pre-Cambrian Basement rocks
    The Pre-Cambrian basement rocks are one of the economically important type of rocks; that many metallic & non metallic ores are mined from them, I the last 20 Pre-Cambrian basement had a special importance that they found to be a reservoirs for oil production.
    Exploitation of Pre-Cambrian basement rocks as mineral deposits source rocks in Egypt
    At the beginning of century Hume worked on the grouping of minerals associated with the Pre-Cambrian crystalline rocks which are[gold, silver, copper, zinc, molybdenum, tungsten, iron, chromium, nickel, lead, tin, platinum& graphite], also these crystalline rocks could be used as source of ornamental stones.
    Many authors tried to classify the minerals according to their associated rocks, one of the most important classifications of mineral deposits in Egypt is that one made by(Helmy&Hussein 1978) as follow:

    1- Ores of mafic association:

    Chromium nickel platinoid association.
    Iron nickel copper sulfide association.

    2- Ores of felsic association:

    Carbonatite- alkaline complexes association.
    Anorthosite iron-titanium oxide association.
    Quartz monozonite-granodiorite copper-molybdenum sulfide association.


    3- Stratiform sulfides of marine-volcanic association.
    4- Stratabound ores of sedimentary association.


    5- Ores of vein and disseminated association.


    6- Ores of sedimentary affiliation:

    Iron deposits.
    Manganese deposits.
    True sedimentary deposits:
    1. biogenic-chemical deposits.
    2. clastic & placer deposits.
    3. evaporates.
    7- Ore deposits of metamorphic affiliation:
    1. Ores of mafic-ultramafic association:
    · Alpine type of chromites.
    · Ilmenite-magnetite(Abu ghalaga).
    · Massive magnetite lenses in gabbroic intrusions.
    2. Syngenetic stratiform deposits :
    · Banded iron oxide ores(um Nar).
    · Magnesite of evaporitic origin(gabal el Rukham).
    3. Volcanic deposits associated with island arc volcanism:
    · Proximal massive sulfide deposits enclosed with acid volcanics and there pyroclastic equivalenlts.
    · Quartz veins with Au-Ag mineralization associated with acid volcanic intrusions.
    · Distal lenticular massive Zn-Cu-Pb sulfide deposits.


    4. Mineralization related to plutonic rocks:

    · Albit-muscovite granites of calc-alkaline to peralkaline nature containing disseminated Ta-Nb-sn (Abu Dabbab).

    · Quartz veins with Au,Sn,W,Mo,Be&F related to granite(Igla area).

    · Pegmatites genetically related to the late granites containing Sn,Be,Mo,U,Th,Nb……

    From the above classification we can see clearly that most of the economic ores in Egypt are associated with Pre-Cambrian crystalline rocks. Also these rocks considered as source of ornamental stones as granites, baslt,diorite, granodiorite…………..etc.
    Exploitation of Pre-Cambrian crystalline rocks as oil reservoirs


    Introduction

    One of the more unusual aspects of the oil exploration business is that hydrocarbon plays can be hosted in fractured granitic basements (eg Yemen, Vietnam, North Sea , USA, Egypt……). Successful exploitation of these reservoirs is usually a challenging task but has been commercially successful, Because of their high degree of geological heterogeneity& fracturing. A key factor in determining site suitability was the nature of groundwater flow(Oil-Water contact movement) in the host rock. In the host rock, flow takes place predominantly through a limited subset of discontinuities, mainly fractures, parts of which form networks of connected channels The degree of fracture network development and interconnectivity play an important role in fluid transport in the crust. different igneous lithologies will respond differently to applied strain, and that bulk petrophysical properties, including porosity and permeability, will differ accordingly. Fractures in crystalline basement are predominantly made up of two types: primary ("cooling") fractures formed during crystallisation of magma; and secondary fractures caused by deformation processes.

    The majority of primary porosity and permeability development in igneous rocks, regardless of their emplacement level, develops as a result of physical and chemical processes that accompany the cooling of magma.Different permeability& porosity types might be produced by different controlling agents on different scales(mesoscale or microscale), (e.g In schistose metamorphic rocks, the foliation can provide primary transmissibilty and exert a control on the subsequent development of tectonic fractures that influence porosity and permeability. In granitoid rocks, primary controls on small-scale permeability and porosity evolution include rock composition, texture, tectonic fracturing together with uplift and weathering, While the secondary porosity is mainly related to the type of deformation as most effective factor, that Under brittle conditions in the upper crust, fracturing is the main process accommodating deformation (strain) in crystalline basement, on the other hand in extensional settings the main deformation processes that cause fracturing of rock are: a) sub-seismic hangingwall deformation due to movement of the hangingwall fault block over the fault; b) sub-seismic footwall deformation; c)faulting creating fault damage zones, The variation in finite strain orientations is used to control the orientation of the fractures. details of the site characterisation studies which were used to identify the location, orientation and mineralogical characteristics of the particular discontinuities referred to as Potential Flowing Features (PFFs) which presented by An extensive dataset of 1-D and 2-D fracture characteristics from thin section, outcrop, air photo and Landsatä scales When all four data scales are amalgamated, the fracture length data may be interpreted as being scale invariant over eight orders of magnitude. When plotted against each other, parameters from outcrop photos or thin sections (all normalised for photo/section area) such as

    · total number of fracture intersections (nodes) corresponding to fracture connectivity,


    • total number of fractures
    • total fracture length fracture spacing, could be established. Other source of data are the logging characterization ( e.g, Resistivity response, Neutron-Density response, Gamma ray response….etc)for subsurface data recognition.


    Combinations of these parameters potentially allow 2-D characterisation of fracture networks and further investigation of other fracture patterns is in progress.



    There are many sites of oil production from Pre-Cambrian fractured crystalline rock reservoirs, As example:

    UNITED STATES: In Kansas, oil is produced from Precambrian basement rocks in the Central Kansas uplift. The Precambrian rocks include quartzite, schist, gneiss and granite, however fractured quartzite is the reservoir rock,

    VENEZUELA: Within Venezuela’s Maracaibo basin, oil is produced from fractured granitic and metamorphic basement rocks in the La Paz and Mara fields, which are located 50 km northwest of Lake Maracaibo.

    Egypt:Oil production from Pre-Cambrian fractured crystalline rocksThe oil basement reservoirs were discovered in Egypt in the last 20 years(1981), now there are many sites for production.



    Oil production in ZeitBay

    A- Reservoir modeling in Zeit Bay:The reservoir in Zeit Bay is hydraulically communicated sequence of Pre-Cambrian igneous & metamorphic rocks as well as sedimentary reservoir. The oil accumulation with an 830 ft thick initial oil column cover the total reservoir sequence. The reservoir modeling is complicated by the on lapping & hydraulic communication of the sandstone & carbonate reservoirs on to basement. The basement reservoir was found to be formed of fractured dykes &brecciated granitic zone at the upper part of tight granites. The dykes & brecciated granites act as passage way for oil production while other less fractured inhibit the reservoir performance.


    B-Analysis of fractured zones

    1.Log analysis:By using the logging methods we can know the composition of fractured zones:
    · Density log : high density response.
    · Neutron log: high response.
    · Resistivity log: dykes have lower response than host tight granite.
    · Gamma Ray log: low response if compared to acidic igneous rock.
    From the above data we can see that the fractured reservoirs are basic rocks composed of ferromagnesian minerals & hydrated aluminum silicates with high(OH) content &lower concentration of Uranium in dykes than brecciated zones due to the leaching to meteoric water as Uranium oxides.
    It was found that there are four main types of dykes intruded in granite which are:[Aplite, Microsyanite, Diabase& Lamprophyre], most of these dykes are subjected to alteration; Fe-Mg minerals altered to chlorite, Fds are altered into kaolinite,sericite& later of calcitized.


    2. Structural analysisThere were found that there are two major types of fractured dykes:
    · Highly fractured dykes;
    Dykes intruded within the Pre-Cambrian rocks & this type is incorporated in production.
    · Less fractured dykes;
    Dykes intruded after Pre-Cambrian, here the dyke boundaries only are fractured.
    On the study of micro scales & mesoscale fracturing & from the particular formation micro scanner results, we can see that there are four main trends of fracturing:
    · Clysmic trend[gulf of suez trend].
    · Aqaba trend.
    · Tythian trend[E-W].
    · Non clysmic trend[N-S].
    The following data were obtained from the above:
    A- Dip trends:
    The dip of fractures in dykes & breccia is less than that of the host tight granite.
    B- Strike trends:
    The fractures have strike trends related to the four main trends of the area[G.O.S, Aqaba, Tythian, Non clysmic].
    C- Morphology of fractures:
    The brecciated zones are encountered in the upper most part of the basement complex which were subjected to continental environment where mechanical weathering increase the surface area of the rock mass together with chemical activity and leaching on the other hand these zones were subjected to extensive tectonics resulted in several dyke intrusions. The leaching is affect the fracture surface by undulation & thus increase the storage capacity & productivity. The fractures are of secondary type.




    C- Factors controlling the production rate

    1. The thickness of the fractured zone:The maximum thickness present at the north-eastern part of Zeit Bay field from which the maximum production rate;i.e as the thickness of the fractured zone increase the production rate increase.


    2. Dykes & brecciated zone dip:We can determine the fracture dips by using the formation micro scanner technique & directed thin sections; we can see that fractures have different dips:
    · Tight granite fractures:
    Has dip ranging from 70to90 degrees, that it has low production rate, high water cut& rapid water move up.
    · Brecciated zone fractures:
    Has dip ranging from 50to80 degrees; that it has higher production rate, slower water move up.
    · Highly fractured dykes fractures:
    Has dip ranging from 30to60 degrees; that they have the highest production rate, slowest water move up& lowest water cut.


    3. Fracture’s orientation:Combination of different fracture trends produce higher fracture rates, thus increase the production rates.


    4. Movement of oil-water contact:The water rise in the fractures by capillary action. Hence in fractured zone the fractures are more wide than those of tight granite host rock, So the move up of water in tight granites will be higher than that in brecciated fractured zones& then in the areas with maximum brecciated zone thickness the water-oil contact move up is slower, the field should be operated under gas injection into a primary gas cap to augment of the partial pressure support from aquifer in flux.


    Conclusion· Development of oil only from fractured basement rocks(not massive).
    The rate of production of oil is related to many factors(fracture orientation, dip, width, water rise).
    · Only highly fractured rocks can be production reservoirs.
    · In the well designing, the well must cut more than one fracture system.
    · Some wells must penetrate the oil-water contact to overcome the rapid water rise up.


    References

    CHARACTERISING FRACTURED GRANITIC RESERVOIRS – CASE HISTORIES, TECHNIQUES AND LIMITATIONS(Jon Gutmanis and Tony Batchelor GeoScience Limited, Falmouth Business Park, Bickland Water Road, Falmouth, Cornwall, TR11 4SZ)

    Reference notes of Suez oil company. Geology of Egypt(Rushdi Said 1990).
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    era11
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    رد: the economic importance of pre-cambrian rocks

    مُساهمة من طرف era11 في 2007-10-27, 4:06 am

    great topic thnx
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    abdovolcano
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    رد: the economic importance of pre-cambrian rocks

    مُساهمة من طرف abdovolcano في 2007-11-04, 3:01 pm

    موضوع جامد شكرا

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