OIL MIGRATION - Journal of the Institute of Petroleum, Vol. 31, No. 264

B y Ja m e s Fr o s t (Student).

T h is p a p e r w as a w a rd e d a S tu d e n ts ’ P rize, 1945.

In t r o d u c t i o n.

This pap er aims a t giving a general account of th e phenom ena of oil m igration an d of th e hypotheses which have been p u t forw ard to explain these fluid m ovem ents in th e rocks.

Oil m igration m ay be divided into th ree phases. These are :—

(a) P rim ary M igration— i.e., m ig ratio n o f oil from th e source rock (usually a clay, b u t som etim es a lim estone) in to th e reservoir rock.

T he reservoir rocks being those of high porosity an d high perm eability, such as sands, sandstones, dolom ites, e t c .;

(b) Secondary M igration— i.e., th e segregation of oil in to pools w ithin th e reservoir r o c k ;

(c) Tertiary M igration or th e dispersal of oil.

T he subject of m igration of oil necessitates some reference to th e origin of oil, since th e tim es in th e sedim entary cycle a t w hich m igration can occur will depend on th e tim e in th a t cycle a t w hich th e oil is generated.

I t is generally accepted to -d ay t h a t th e form ation of oil is b y no m eans as rare, an d does n o t require th e special conditions for its fo rm atio n th a t were once th o u g h t necessary. Oil form ation is believed to be a process common in th e sedim entary cycle, although com m ercial accum ulations of th e fluid rem ain in th e n a tu re of freaks. M ost oil geologists subscribe to th e organic origin theory, a n d believe th a t oil has been generated from sedim ents sim ilar to those now being laid dow n on th e C ontinental Shelf in w aters deeper th a n 600 feet. Sedim ents in th is area are alm ost com

pletely free from interference due to w ave action an d w ater circulation.

In these circum stances sta g n a n t [i.e., anaerobic) conditions easily arise, an d organic m a tte r deposited on th e sea-bed is capable of being entom bed and preserved. R educing conditions are essential for preservation, b u t high organic content does n o t seem to be necessary.

Sedim ents ty p ical of th is environm ent are th e blue a n d black m uds.

These m uds are very common, an d th e d a rk colour, w hich is due to th e presence of sulphides, n o ta b ly iron, testifies to th e reducing n a tu re of th e environm ent.

F rom sedim ents of th is ty p e oil is believed to be generated, b u t th e mode of generation is a problem which is th e cause of m uch discussion.

T he difficulty lies in th e fa c t th a t while blue an d black m uds have been observed an d sam pled, and th e ir organic co n ten t analysed, an d while beds of exceptionally high organic content have been recorded, there is, as yet, no record of a n observation of petroleum being form ed from organic m a tte r. T he form ation of m ethane is very common in n a tu re , b u t the form ation of th e higher paraffin homologues has n o t been recorded on a

F R O S T : U llj ivIIG R A T IO N . 487 (ii) T he high-tem perature or th erm al distillation hypothesis.

T he first state s th a t oil is generated fairly early after th e entom bm ent accepted, chiefly because it gives a b e tte r explanation of p rim ary m igration th an does th e h igh-tem perature hypothesis. consolidated and rendered im pervious to fluids.

This m eans th a t oil m u st be generated fairly early in th e geological

488 F R O S T : O IL M IG R A T IO N .

pore in th e reservoir rock is m uch larger th a n th a t in th e source rock, a n d th e in terfacial tension forces will te n d to m ake th e oil pass from th e fine to th e coarse capillaries. (The surface tension of w ate r/a ir is 72 dynes/cm ., com pared w ith 20-25 dynes/cm . for an oil/air surface.) This effect will u n d o u b ted ly occur a t th e ju n c tio n of th e coarse an d fine pores in th e tw o rocks, b u t th e re seems no reason to suppose t h a t an oil globule several feet aw ay from th is ju n ctio n should be affected especially w hen th e globule w ould have to tra v e l along a very to rtu o u s p a th w ith in th e clay before reaching th e sand. I n travelling along th is p a th th e shape of th e globule w ould have to be d isto rte d m an y tim es, an d th is w ould m ean t h a t w ork w ould have to be done on th e globule. C apillarity cannot provide th e energy required, since capillarity ten d s to prom ote sta tic conditions ra th e r th a n to in itiate flow. F u rth e r, it has been show n experim entally t h a t oil globules under conditions ap p roxim ating to those described above show no desire to move to th e finer p o res.1

M cCoy's Adhesion H ypothesis.3

This adhesion hypothesis is b ased on th e fa c t t h a t oil has a superior adhesion for q u artz th a n for am orphous clay particles, a n d t h a t w ater has a superior adhesion for clay th a n for q u a rtz grains. O n th is basis, say th e theorists, p rim ary m igration is simple. T he oil in th e clay moves into th e sand a n d is replaced in th e clay b y th e w ater w hich leaves th e sand. T hus th ere is a continuous exchange of fluids across th e coarse- fine junction. This th eo ry is subject to th e sam e objections as were offered against th e capillarity hypothesis. F u rth e r, if th e clay a n d sand are initially w ater satu ra te d , as th e y will be, being m arine sedim ents, only lim ited fluid tran sfer will be possible unless th e re is som e initial pressure difference betw een th e tw o beds to cause th e m ovem ent. Large volum es of gas will un d o u b ted ly be generated w ith th e oil, a n d th is would cause an expulsion of fluids from th e clay ra th e r th a n a n exchange.

Effect o f Compaction.

A th y 5 has shown in his studies on com paction t h a t th e process of com

p action begins w ith th e b irth of a sedim ent a n d is continuous th ro u g h o u t its life in varying degrees of in ten sity . H is results m ay be sum m arized briefly as follows : In an y sedim ent th e percentage of w ater p resen t will be a logarithm ic function of th e d e p th of b u rial to w hich t h a t sedim ent has been subjected. F o r exam ple, for one shale analyzed th e sedim ent h a d been com pacted b y m ore th a n 20 p er cent, of th e original volum e a t 1000 feet d ep th of burial, an d th e corresponding figures for 2000 feet an d 3000 feet were 35 an d 40 per cent, respectively. 50 per cent, of th e original fluids h a d been expressed a t 1000 feet, 70 per cent, a t 2000 feet, 85 per cent, a t 3000 feet, a n d 90 p e r cent, a t 6000 feet. T h a t is, com paction a t 1000 feet is only one h a lf of w h at it is a t 6000 feet. I t is a p p a re n t from these figures t h a t as a sedim ent becomes buried deeper an d deeper it is being continuously robbed of its fluids, a n d t h a t th ere is a continuous flow o f fluids upw ards.

I n th e com paction of th e clays these m ovem ents will be alm ost en tirely v ertical. I n all cases th e fluids will find th e lines of least resistance to flow,

and these lines ■will, in general, be upw ards to th e less com pacted sedim ents, w here th e pore spaces will be larger.

T he pores in th e reservoir rocks will n o t suffer so sharp a reduction in size as those in th e clayey rocks. The sands, which will rap id ly ad o p t th e ir m ost economical packing, will act as collectors of fluids, by reason of th eir high porosity, and th e fluids will te n d to rem ain in them , especially if th ere is lateral egress along th e sand for th e excess fluids given u p by th e clays. F u rth e r, a t depth, com paction will reach a stage such th a t fluids entering a sand-bed will he unable to p en etrate th e n e x t clay bed above, owing to th e low perm iability of th e clay bed a t th a t depth. The fluids will th e n be forced to rem ain in th e porous bed an d be subject to th e horizontal flow-lines in th a t bed.

A pplying th is process to th e m igration of oil an d gas, it is ap p aren t th a t th e oil and gas will be expressed from th e source clays to g eth er w ith much connate w ater. W hile th e w ater has an excellent scavenging effect in stripping th e source rock of its oil, it m ust he stressed th a t m ost of th e oil an d gas will be expressed as a result of th e change in volum e dem anded by th e increasing weight of sedim ents above th e source rock. A th y ’s figures seem to suggest th a t m ost of th e oil will have been expressed from th e source rock by th e tim e th e sedim ent has been buried 2000 feet, since 70 per cent, of th e original fluids will have been expressed from it b y th a t tim e.

The Effect o f Gas.

Johnson has a tte m p te d to explain th e whole process of prim ary m igration b y tb e vehicular action of gas. H e suggests th a t th e gas separates out b y reason of its very low density com pared w ith th a t of w ater, an d th a t in its upw ard m ovem ent it carries th e oil w ith it, an oil globule being a ttach ed to each gas bubble. This th eo ry should really be considered u nder th e Effect of G ravity, since it is subject to th e same criticism —- nam ely, t h a t th e pores in th e source rock will be too sm all to p erm it of g rav ity separation, even th e separation of gas.

T he effect of gas should, however, n o t be minimized. W ben one con

siders th e v a st am ounts of gas (chiefly m ethane) which are being form ed to-day, th e v a st am ounts of n a tu ra l gas in th e oil an d gas fields of th e world, and, fu rth er, w hen th e facility w ith which gas can escape is ta k e n into account, th e n it is ap p aren t th a t enorm ous quantities of gas m u st have been evolved w ith th e sm aller volumes of oil which it accom panies. I t is contended here t h a t th e effect of th is gas in m igration has been m uch m inim ized in th e literatu re. I t appears possible th a t m ost of th e oil is tran sferred from th e source rock to th e reservoir rock either as gas, or as small films of oil surrounding gas bubbles w ithin th e w ater. Segregation of th e oil from th e gas probably does n o t occur u n til th e fluids are in a coarse m edium , w hen coalescence of gas hubbies causes th e small films of oil to unite, u n til a drop of oil grows sufficient in size to exist in its own right. The effect of gas will be considered fu rth er under secondary m igration. I t is m entioned bere to stress th e im portance of gas.

Conclusions.

I t appears th e n th a t th e m ajor force operative in th e expulsion of oil and gas from th e source rock to th e reservoir rock is th e fluid m ovem ents

I X V U O l . V J L U M IG E A T IO N . 489

490 F R O S T : O IL J ftiliK A T IO S .

from th e source rock caused b y th e compaction, o f t h a t sedim ent, an d t h a t th e transference of th e oil a n d gas is g reatly aided b y th e presence of v a st qu an tities of gas an d b y th e high m obility of th is fluid.

SECONDARY MIGRATION.

Secondary m igration concerns itself -with th e m ovem ents of th e petroleum fluids ■within th e reservoir ro ck a n d w ith th e causes o f th ese m ovem ents.

Consider a globule of oil ju s t expressed or forced in to th e reservoir rock from th e source rock. T he globule w ill still be subjected to th e same com paetive forces a n d w ater drive w hich caused i t to be expelled from th e source rock, a n d th u s it will te n d to continue its u p w ard m ovem ent—- in fa c t, i t will m ove u p w ard w ith g rea ter facility, d u e to th e larg er pores in th e reservoir ro ck com pared w ith th o se in th e source rock. T h e pores in th e san d reservoir rocks w ill reac h a m inim um size early in th e geological h isto ry of th e sedim ent, since a re-a rran g em en t of th e sand-gram s to give th e m o st econom ical packing is all t h a t com paction can achieve. (This does n o t, of course, ta k e in to account th e possibilities o f th e san d being cem ented b y solutions, etc.)

D u e to th e presence o f th ese relativ ely large pores, th e fluids w ill favour flowing w ith in th e reservoir ro ck ra th e r th a n e n te r a b e d contain m g pores o f a sm aller size. T he flow lines will th u s te n d to be d iv erte d from th e v ertical to an up-dip direction, an d th e fluids will seek egress a t th e outcrop of th e reservoir rock (if one occurs) ra th e r th a n force th e ir w ay th ro u g h th e n e x t clay b ed in th e succession.

T he globule of oil u n d er th ese conditions will te n d to follow th e direction of m ovem ent of th e w ater. I ts a b ility to do so w ill d ep en d on several factors. These are :—

(1) T he ra te o f flow o f th e w a te r ;

(2) T he concentration of th e oil (i.e., th e oil w a te r r a tio ) ; (3) T he viscosity of th e o il:

(4) T he presence or absence of gas w ith th e o il;

(5) T he porosity a n d perm eability of th e reservoir rock.

U n d er sta tic conditions th e globule will, fro m surface-tension considera

tions, te n d to assum e a spherical shape. T h a t is, it will te n d to occupy a pore large enough for i t to do th is.

H o w of w a te r th ro u g h a sand containing th e se oil dro p lets w ill hav e no effect on th e oil as long as th e r a te of flow rem ains slow, since th e w ater will be able to by-pass th e oil, b y reason of th e in itial w ater-w et condition of th e sand, a n d due to th e fa c t t h a t each oil globule w ill b e su rro u n d ed b y w ater. As th e ra te of flow increases th e oil will begin to a c t as a g reater a n d g re a te r hindrance to th e flow of w ater. I n o th er w ords, w e m a y sa y t h a t the oil globule experiences a pressure difference across itself w hich te n d s to m ove it in th e direction of flow. To m ove th e oil i t w ill be necessary to change its shape so as to force i t th ro u g h a n y of th e sm aller channels leading from its pore. To produce a given change in shape (i.e., in c u rv atu re of th e oil-w ater face) a finite pressure g rad ien t is necessary, a n d u n til th is is reached th e oil will n o t m ove. T he critical pressure g rad ien t to in itia te oil m ovem ent can b e reached b y increasing th e r a te o f flow of w a te r or

F E O S T : O IL M IG E A T IO N . 491 by increasing th e oil/w ater concentration. W hen th is pressure is reached th e oil will move along w ith th e w ater. The m ovem ent of th e oil will n o t be sm ooth, it will move jerkily, since th e shape of each globule will be changing continuously as th e oil moves through th e m ulti-sized pores in th e rock.

Once oil m ovem ent has been initiated, th e possibilities of an oil-pool resulting will depend on th e ty p e of structures which th e oil encounters on its journey. I f th e fluid m ovem ents are dom inantly upw ard, th e oil will te n d to be filtered off where th e fluids enter a finer rock (i.e., one containing pores sm aller th a n those in th e reservoir rock, since th e critical pressure gradient to in itiate oil flow in th e fine rock will be m uch higher th a n for th e coarser rock).

This filtration effect a t coarse-fine interfaces is of great im portance in oil accum ulation, especially in th e early stages, before com paction has rendered th e cap rock com pletely im pervious to fluids.2

W hen th e fluid m ovem ents are dom inantly up dip, th e oil will, under flow conditions, ten d to move tow ards th e to p of th e reservoir rock, by reason of its lower density com pared w ith th a t of w ater. I t has been dem onstrated th a t buoyancy is effective in producing separation in th e fluids under conditions of flow, b u t th a t buoyancy has no effect under static conditions.2 Oil will th u s m igrate tow ards th e high spots in a structure, where it will rem ain unless th e flow of w ater becomes fast enough to scour it from its position or some destructive interference disturbs th e accum ulation. Oil will be continuously added to th e accum ulation as long as th e w ater flow carries oil from th e source rock an d as long as the oil does n o t ta k e up too m uch room so as to hinder th e flow of w ater.

E quilibrium will be set up when there is ju st enough room for th e w ater to flow p a st th e oil. A t this stage no fu rth e r addition will be m ade to th e accum ulation, and an y oil in th e w ater will be carried on to th e n ex t structure.

The advantage of this hydraulic th eo ry is th a t it explains alm ost every ty p e of accum ulation of oil, from th e stru ctu ral accum ulations of the faulted and folded regions, such as th e R ocky M ountain Region, California, B urm a, etc., to th e porosity trap s, such as those in th e A ppalachian area, th e “ wedge ” fields like E a st Texas, Glenn, etc., and to th e freak shoestring fields of K ansas.

B u t th ere are several objections to be offered against th e hydraulic-flow theory. The th eo ry assumes th a t continuous expression of fluids is taking place from th e rocks during th e ir burial, a n d th a t even when com paction has ceased, w ater circulation continues. These assum ptions are by no m eans universally accepted. H ow is it th a t some sands are d ry ?, and how do sands become sandstones if th e y are always wet ? and how do clays become in d u rated an d h ard if they, too, are always wet ? I f these w ater m ovem ents do, in fact, occur in th e e a rth ’s crust, do th e w aters move a t a ra te sufficient to cause th e oil m ovem ents outlined above ? Opponents of large-scale m igration have objected to th e hydraulic theory by pointing o u t th a t if w ater m ovem ents were so active, th ere would n o t be such large divergencies as are found in th e chemical properties of underground waters from bed to bed, and w ithin th e same form ation. A t very least these divergencies testify to th e slow rate a t which th e w aters move.

M ost recorded m easurem ents of th e rate of flow of underground waters

492 F R O S T : O IL M IG R A T IO N .

show t h a t it is extrem ely slow, of th e order of 1 to 2 miles per year, a n d th is seems h ard ly adequate to accom plish th e secondary m ig ratio n p ro cesses outlined above. R ich ,6,7 himself, one of th e leading exponents of th e theory, expresses d o u b t as to th e ab ility of th e know n ground w a te r cu rren ts to overcome th e resistance offered b y th e globules of oil, a n d suggests th a t some facto r has been overlooked in th e problem of oil accum ulation.

Some of these criticism s can be answ ered b y th e fa c t th a t, while oil generation a n d p rim ary m igration pro b ab ly occur fairly early in th e sedim entary cycle, secondary m igration is often a m uch la te r phase,, an d m a y be in itiated by fluid m ovem ents which have some cause o th e r th a n

W dokumencie Journal of the Institute of Petroleum, Vol. 31, No. 264 (Stron 34-41)