Migration (Pratsch, 1983)

7/28/2019 Migration (Pratsch, 1983)

http://slidepdf.com/reader/full/migration-pratsch-1983 1/16

GASFIELDS, NW GERMAN BASIN: SECONDARY

GAS MIGRATION AS A MAJOR

GEOLOGIC PARAMETER

J.-C. Pratsch*

The observable concentration of the major deep gas accumulation areas in the NW German Basin

is the result of focused secondary migration. Gas generated in several effective depocenters (regional

structural lows) from Late Carboniferous source beds migrated into adjacent regional structural

highs. The preferred migration paths are qualitatively predictable on the basis of present basin

geometry. Optimal conditions for gas accumulations exist where the presently trapping regional

highs have also been sites of favorable reservoir development. On-going and future deep-gas

exploration efforts in the basin can be regarded as quite hopeful.

INTRODUCTION

Of the 200 to possibly 300 TCF of natural gas reserves discovered so far or to be discovered in thefuture in NW Europe, more than 100 TCF are located in several fields in the NW German Basin;included here are the NE Netherlands (Figs 1, 2). Basic geologic parameters that satisfactorily explainthese accumulations (their location and size) certainly will apply to the geologic conditions in entireNW Europe and elsewhere. Especially interesting is the question to what degree such geologicexplanations also have predictive value for future gas exploration in NW Germany and thus in NWEurope (Pratsch, 1981, c, d, e).

The gasfields in the NW German Basin are not distributed in a randomly statistical order. Rather,they occur in several distinct geographic clusters (Fig. 2). This concentration of gas accumulations iseven more pronounced when the quantitative distribution of gas reserves per cluster is considered(Fig. 2). The Groningen and Oldenburg field areas alone contain the vast majority of gas reserves inthe NW German Basin (99 TCF or 95% of a basin-wide total of 104 TCF). Inside the Germanborders, the Oldenburg field area contains by far the largest gas reserves.

The lack of gasfields in the onshore portion of the basin north of a line between the cities of Bremen and Hamburg is striking. On a first impression, geologic factors such as the effect of thenorthward deepening of the post-Permian basin appear to be the reason. However, the northwardincreasing lack of reliable geological and geophysical data from deep Permian and pre-Permian beds

is a more likely explanation; it offers several positive possibilities for future gas exploration efforts inthe basin.

Journal of Petroleum Geology, 5, 3, pp. 229-244, 1983 229

*6822 Charlmont Circle, Dallas, TX 75248, USA.





231

GENERAL CONCEPT

The numerous complexities involved in the formation and preservation of any hydrocarbon

accumulation prevent quick and easy solutions for presence or absence of fields in a basin. In the case

of the NW German Basin, too, there are many opinions and data that at first are difficult to place intoa coherent explanation scheme. Some explorationists will seek explanations for the observed field

concentrations in the presence of favorable structural traps but do not mention the many dry holes

drilled in the same basin on presumably equally valid structural anomalies. Others will point to the

widespread (yet possibly locally concentrated) mature gas-source beds in Late Carboniferous coal and

carbonaceous shales (Fig. 3), or at the local concentrations of suitable reservoir beds from

Carboniferous and Permian sandstones to Late Permian carbonates and Triassic sandstones. Still

others will consider the aspects of preservation of trapped gas as most important, related to the

positive influence of capping evaporites of Permian, Triassic, and Jurassic ages. The “right” timing of

geologic events may also be seen as controlling the existence and even the location of gasfields.

All of these parameters, no doubt, contribute to the observable concentrations of gas reserves in a

few small areas in the NW German Basin, but none of them alone appears to be more prominent than

others. It is the integrated efficiency of many geologic processes that controls the gas distribution.

The main problem with a more detailed analysis is the fact that a satisfactory explanation may be

found only after all the data involved are obtained. This will be late in the development of this or any

other basin, and if the parameters cited above indeed were the only ones available, they would have a

low predictive value for hydrocarbon exploration. Hydrocarbon exploration would then be controlled

to a large degree by statistical approaches (Menard, 1981).

J. -C. Pratsch



Fortunately, however, there is another geologic parameter that does possess a considerable

predictive value in basin evaluation for hydrocarbon exploration and that at the same time allows one

to collect and to integrate all available geological data into one coherent synthesis early in an

exploration program: this is the geologic parameter of secondary hydrocarbon migration (Fig. 4). In

this approach, the common parameters in hydrocarbon exploration—trap, reservoir, cap, and

preservation—gain in individual value by their integration into one common process.

Secondary hydrocarbon migration is the process of physical movement of hydrocarbons—oil and

gas—from the area of hydrocarbon generation through carrier beds to the area of final entrapment

(Fig. 5). In contrast, primary migration is commonly defined as the process of movement of oil and

gas from the generating source bed to the carrier bed.

Secondary migration in NW German gasfields232



As yet there is no common agreement on all facets of hydrocarbon migration: the physico-

chemical processes involved, the necessary physical or geological requirements, the changes and

variations in the process (Tissot and Welte, 1978); Hunt, 1979). Yet, innumerable geologic

observations and the ever-increasing laboratory data and concepts already permit the use of secondary

migration as a decisive geologic parameter in basin evaluations (Lawrence and Pratsch, 1980;

Pratsch, 1981 a,c). In such an evaluation three major steps are required. These steps are normally

undertaken in any basin evaluation study but are only too often overlooked in the final evaluation

phases (Fig. 6).

1. Definition of the effective depocenter(s);

2. Definition of preferred migration paths;

3. Definition of length of migration paths.

These three steps can be used not only to explain existing hydrocarbon accumulations, but also to

identify areas of higher-than-normal prospectivity of a particular basin portion, and thus of higher-

than-normal hydrocarbon concentrations in such preferred areas. The number of traps or fields is not

the answer, but rather the amount of trapped gas or oil per basin area and thus the distribution of

migrated hydrocarbons in a given prospective basin. The results obtained from migration analysis are

firstly qualitative. However, when quantitative data are added (such as source richness, and the

effectiveness of thermal maturation, secondary migration, trapping, re-migration or preservation), this

approach will furnish semi-quantitative or even quantitative data on hydrocarbon distribution in a

given basin.

DEFINITION OF THE EFFECTIVE DEPOCENTER

A depocenter, as part of a sedimentary basin, contains the maximum thickness of sedimentary

section above basement (Fig. 5). An effective depocenter, in petroleum geology, contains the

maximum thickness of generative sediments and hence the maximum thickness of thermally mature

or over-mature hydrocarbon source beds. Thermal maturity of source beds in a depocenter guarantees

the presence of hydrocarbons that can migrate. Over-maturity of source beds adds the geological-

historical question of where the hydrocarbons are that may have migrated away from the area before

the onset of over-maturity.

A large number of geological, geochemical and geophysical techniques are available today to

define the presence or absence of mature or over/under-mature source beds. Geochemical data not

only explain a portion of the geological history of a basin, they even permit predictions about theoccurrence or absence of source beds by age, type and maturity level (Tissot and Welte, 1978).

Computer-assisted predictive maturation analysis is now routine in this field while quantitative

predictions still depend much on the quality and quantity of input data (Welte and Yükler, 1980).

J. -C. Pratsch 233



The geometry (shape) of a depocenter can be routinely obtained through definition of regional

structure using gravity, magnetics, refraction or reflection seismic, magneto-tellurics, surface or

subsurface geology, or remote data interpretation (Fig. 7). Detailed mapping of the present basin

geometry (structure) at the level of the mature hydrocarbon source bed or regional carrier bed will be

of the greatest importance. Where data are available only from levels above the mature source beds orthe regional carrier bed, such data define at least the minimum subsidence history of the source bed,

thus its thermal maturation history and structural history. Quite clearly, all phases of hydrocarbon

exploration are involved in the step of depocenter definition. It is from here that the input data for

migration prediction are obtained.

DEFINITION OF THE PREFERRED MIGRATION PATHS

Oil and gas migrate from the areas of their generation to the areas of their final entrapment. Such

migration occurs under the influence of existing pressure differentials. The natural buoyancy of

hydrocarbon particles in a water environment and effective pressure gradients appear to be most

important (Welte, 1979). At this time there is no general agreement in our profession about all the

geological and physico-chemical parameters that enter into the migration process. There is, however,

general agreement on some basic observations that can serve as a basis for migration predictions (Fig. 5):

1. Hydrocarbons migrate up-dip unless extreme pressure differentials prevent this; fresh-water

influx from basin flanks or local vertical pressure gradients from high-pressure under-

compacted clays to underlying normal-pressured porous layers are two examples of such

limiting pressure differentials.

2. Hydrocarbons migrate laterally or vertically depending on geologic conditions. The

effectiveness of either one of the two modes is much debated in literature, which is a sign of

the complexities and variations from basin to basin (Pratsch, 1981, c, d; Price, 1981).

3. Fracturing of rocks critically enhances permeabilities that are required for early, late, lateral and

vertical hydrocarbon migration. Fracturing on local and regional scales is much more

common and more effective than is commonly assumed as the entire globe is under constant

major stress (Pratsch, 1981, b).

Ideally, migrating hydrocarbons will seek the shortest possible migration path. In practice,

variations in porosities, permeabilities and effective driving forces will cause deviations and

variations of migration paths. As a consequence, there will always be a statistical component in

migration direction definitions related to the quality of data available.

In some basins, re-migration of once accumulated hydrocarbons is known; here basin geometries

changed through time. The present migration paths and hence the present preferred distribution of

migrated hydrocarbons are still related to present basin geometry. All evidence indicates thathydrocarbon migration, while lasting over a long period of time where maturation and generation

continue, can be rapid in geologic terms (a few million years at the most for basin-wide migration

patterns).




The most critical parameters derived from basin or depocenter geometry are the planar map-view

and the symmetry of the basin (Fig. 8). Hydrocarbons normally will not migrate across the basin axis;

each flank of a basin can be considered as a separate unit. Basin symmetry controls the amount of

generated hydrocarbons available for migration in each basin portion.

Basin geometries range from circular to elongate and from straight to curved. Simple basins

containing only one depocenter will have migration patterns different from complex basins containing

more that one depocenter. These distinctions can be used to develop a theoretical system of basin

geometries and of resulting preferred migration paths, assuming uniform permeability, uniform

pressure distribution, uniform source bed richness and uniform kerogen-hydrocarbon conversion rate.

When we add data on the variations of these parameters to the primary qualitative/semi-quantitative

migration definition, we will obtain an increasingly improved quantitative migration pattern, hence an

improved predictive quantitative basin evaluation.

The following basin geometry types can be distinguished:

1. Simple Basins, containing one depocenter

(a) Circular symmetrical (Fig. 9)

No preferred migration direction exists.

Examples: Michigan Basin, USA (parts); single lobes of complicated depocenter patterns.

(b) Circular asymmetrical (Fig. 10)

Crowding of migration rays on the narrow concave side of this geometric form indicates

the potential for migration focusing. Dispersion of migration rays on the wide concave side

of the “basin” indicates dispersion of migrating hydrocarbons, hence less migration

focusing. The degree of basin asymmetry will control the amount of hydrocarbons

available for migration in each basin portion.

Examples: No example is known, except possibly again (as in case (a)) lobes of

complicated depocenters. This form is important, however, for developing

the concept of hydrocarbon migration focusing and basin symmetry.

(c) Elongate symmetrical (Fig. 11)

Migration focusing along the long flanks and migration diffusion

along the short flanks occur “simultaneously”. This and the volumetric differences

J. -C. Pratsch 235



between long and short flanks result in an accumulation preference along flanks of such

basins rather than short flanks.

Examples: Depocenters of Rheingraben, Germany, and of Vienna Basin,

Austria (Pratsch, 1981, d).

(d) Elongate asymmetrical (Fig. 12)

Migration focusing is identical to the previous case (c). Here the long gentle flank with the

larger volume is further preferred due to the increased volume of hydrocarbons generated

in a thicker sedimentary section. The degree of asymmetry will determine where the larger

volume will be found.

Examples: Great Valley Basin, California; Mid-Magdalena Basin, Colombia

(Pratsch, 1981, a).

(e) Elongate symmetrical curved (Fig. 13)

The concave long flank A is clearly favored for migration focusing. The width/length ratio

of such a basin will determine to what degree the short flanks or the long convex flank B

have the largest accumulation (migration focusing potential).

Examples: Szeged Basin, Hungary (Pratsch, 1981, d).

(f) Elongate asymmetrical curved (Fig. 14)

The distribution of migration focusing is similar to that of basin form (e)—elongate,

symmetrical, curved. However, the additional differences in sediment volume between thetwo flanks of such asymmetrical basins further accentuate the potential of the long concave

basin flank A. Another complication can arise when the basin axis lies nearer to the long

concave flank A. In such a case, the migration focusing effect of the concave long flank

can be offset by the volumetric effects of asymmetry.

Examples: Los Angeles Basin, California; Wind River Basin, Wyoming; Bighorn Basin,

Wyoming; Powder River Basin, Wyoming; Reforma Region, Mexico;

Eastern depocenter of Po Valley Basin, Italy.

2. Composite basins, containing two or more depocenters

In such basins, hydrocarbons migrating from a pair of adjacent depocenters can accumulate eitherin joint or in separate trapping areas. A third or fourth depocenter will merely form more pairs of

adjacent depocenters. In composite basins only one such pair may be analyzed at a time.

Conceptually, composite basins develop from classes (e) and (f), elongate curved.




(a) Composite linear (Fig. 15)

Migration focusing will be most pronounced in the center of common flanks; less focusing

will occur in individual long flanks, and on short flanks.

Examples: Great Valley Basin, California; Baltimore Canyon Area, USA; Lower Magdalena

Basin, Colombia; Mid-Magdalena Basin, Colombia; Reconcavo Basin, Brazil;

Gulf of Suez, Egypt; Sirte Basin, Libya; Po Basin, Italy.

(b) Composite parallel (Fig. 16)

Migration focusing is identical to the previous case.

Examples: Mackenzie Delta, Canada; Gippsland Basin, Australia; Pre-Salt Plays, Gabon;

Mahakam Delta, Indonesia; Hassi Messaoud Region, Algeria.

(c) Composite asymmetrical

In these cases specific hydrocarbon migration patterns and specific hydrocarbon distribution

patterns will develop, depending on the degree of symmetry.

Example: NW German Basin.

J. -C. Pratsch 237



There is no known dependence of migration focusing potential on specific geologic basin types or

classes. In other words, basin classifications do not enter into migration prediction. To what degree,

quantitatively, hydrocarbons will be generated and can migrate in a given basin depends on specificgeologic factors. Of all the regional-geological parameters that have been proposed in attempts to

correctly predict hydrocarbon distribution, migration focusing, or preferred migration on the basis of

existing basin geometry, appears to be the most powerful predictive concept.




DEFINITION OF THE LENGTH OF MIGRATION PATHS

Quite commonly, the length of a migration path cannot be predicted. Therefore, it is usually not

possible to say where in the preferred migration path oil or gas accumulations will be found (Fig. 5).

Detailed exploration techniques such as subsurface geology, seismic reflection data or comparative

geology will aid in this determination. In most cases potential traps will first be tested in regionally

high positions; subsequent tests will be further down-dip until the region of the effective depocenter

itself is tested. While inter-particle porosity and permeability will decrease with depth, fracturing of

originally tight rocks can add new prospects at greater depth.

These limitations of predictive exploration appear negative at first. However, we must realize that

in most productive basins 75 % or more of hydrocarbon reserves are found in 25 % or less of thebasinal areas. Even the difficulty of defining the length of migration paths cannot be a reason against

the extreme usefulness of the concept as an exploratory and predictive hydrocarbon exploration

approach.

J. -C. Pratsch 239



APPLICATION OF MIGRATION FOCUSING IN THE NW GERMAN

GAS REGION BASIN TYPE

The NW German Basin north of the outcropping Paleozoics (Variscan mobile type) possesses all

of the characteristics of a long-lived passive continental margin (Fig. 1). The effects of the Variscan

orogeny in the mobile belt to the south are visible in the form of deformation style and sedimentary

records. Certain additional characteristics reminiscent of back-arc basins are also found. These

include shear and tension deformation, post-orogenetic volcanic activity, and molasse sediments. This

is not surprising because continental passive margins and back-arc basins indeed are products of very

similar geologic processes (Pratsch, 1978, a). The development of the NW German Basin began with

an Early Paleozoic passive stage, that, during Middle and Late Paleozoic time, changed to an activemargin in the south (Figs 17,18) (Pratsch, 1978, b). It is difficult to see how in this case of dynamic

progressive geologic development an evaluation of hydrocarbon potentials could be based on a basin

classification scheme, as proposed by many, which is always static at best.




The gas-productive section in the NW German Basin, as known today, is largely the result of Late

Paleozoic post-orogenic events in a quasi-back-arc setting. Northwards, towards the Sveco-

Scandian/Russian craton and beneath Late Carboniferous sediments, prospects related to the originalpassive continental margin of Early Paleozoic age should be present (Fig. 3).

No detailed regional structure maps of gas productive zones or of mature gas source beds across

the NW German Basin exist in the public domain. The map that comes closest to the required goal is

a regional basement structure map obtained from integrated magnetic, gravity and subsurface data

(Pratsch, 1978, b) (Fig. 20). Comparisons with published regional thickness and facies maps have

proven that this basement map is reliable for regional basement geometry. Further verification comes

from a basin-wide pre-Permian paleographic map based on well results (Bartenstein, 1979;

Pratsch, 1978, b) (Fig. 19). The major gasfield areas in the basin superimposed upon

regional basement structure and upon pre-Permian paleogeology are shown on Figs 19 to 22.

The NW German Basin is a complex basin with several synsedimentary highs and lowspresent in l inear and paral le l arrangements (Fig. 20). The mature gas source bed

in te rva ls l ie in La te Carbonife rous Westpha l ian coa l beds and carbonaceous

shales occurring at least across the southern part of the basin (Plein, 1979). If the present

J. -C. Pratsch 241



regional lows were already synsedimentary lows during Late Carboniferous time, these lows will be

effective depocenters. And even if these present regional lows do not contain thicker or richer gas

source beds than adjacent regional highs, the regional lows will still be preferred sites for gas

generation because of their pronounced subsidence history since at least Late Carboniferous time. The

regional lows in the NW German Basin are thus seen as effective depocenters from which gas was

able to migrate into adjoining regional highs for long geologic periods since the onset of major gas

generation.

Preferred Gas Migration Paths

The regional low areas in the NW German basin appear here as elongate symmetrical depocenters

trending in a general N-S direction. Preferred gas migration will be along their individual western and

eastern long flanks. In addition, northward plunging regional highs will act as migration focal areaswhere they plunge into a depocenter at their northern end. A somewhat more simplified view with the

same result is to classify the NW German Basin as complex linear and/or complex parallel. In each

case, regional highs will be areas of preferred gas migration from the depocenters.




LENGTH OF MIGRATION PATHS

The longest possible migration path for gas in the NW German Basin is less than 100km and thusnot at all extreme. The general problem of length of migration paths, therefore, is here reduced to that

of optimum trapping conditions for migrated gas inside the preferred migration paths.A pronounced special character of synsedimentary regional highs is important: they act not onlyas migration focal areas but also as optima for reservoir development (carbonates and clastics) and fortrap formation, both stratigraphic and structural (Fig. 5). In fact, in the NW German basin all majorgas accumulations possess major stratigraphic trap parameters: in Groningen, Rotliegend clastics liebetween northern shale-out and southern pinch-out zones. The majority of Oldenburg gasfieldsproduce from Zechstein carbonates which shale-out northwards and pinch-out southward; Rotliegendsandstone reservoirs in the Dethlingen region appear to undergo diagenetic alterations following aregional pattern possibly related to internal trends of the Dethlingen regional high (Drong, 1980). Andgas reservoirs in Early Permian and Late Carboniferous sandstones across the basin depend much onpre-existing sedimentation conditions, and thus on Carboniferous and Permian subsidence, uplift anderosion, in addition to secondary diagenesis.

CONCLUSION

Secondary migration of gas and oil follows simple laws: the preferred direction of hydrocarbonsmigrating from the generating low effective depocenter to trapping higher basin flanks or adjacentregional highs is qualitatively predictable where meaningful present basin geometry is known. Themajority of trapped hydrocarbons will be found along these predictable preferred regional migrationpaths. This approach allows us to improve regional exploration concepts and programs and helps todefine additional exploration plays, even in basins that are at a relatively high degree of explorationdevelopment. Where exploration data of all types can be integrated into a coherent basin evaluation,the definition of secondary migration directions can well be utilized as basis for forward-explorationplans.

ACKNOWLEDGEMENT

I thank the Management of Mobil Oil Corporation for the permission to publish this paper; it wasfirst given during the “Seminar on the Exploration for Gas Fields in the ECE Region (Geology andGeophysics)”, May 18-23, 1981, Hannover, Germany. I also thank P. L. Lawrence for his constantadvice and contributions during our work on the concept of focused secondary migration of oil and

gas. Finally, I appreciate the agreement from the editorial board of “Erdöl und Kohle” to publish thisslightly altered version of a paper planned for their future publication.

SELECTED REFERENCES

BARTENSTEIN, H., 1979. Essay on the coalification and hydrocarbon potential of the NW European Paleozoic.Geol. en Mijnb., 59 , 2, 155-168.

DRONG, H. J., 1980. Diagenetische Veränderungen in den Rotliegend Sandsteinen im NW-Deutschen Becken.Geol. Rdsch., 68 , 1172-1183.

HUNT, J. M., 1979. Petroleum Geochemistry and Geology. Freeman, San Franc., 678 p.LAWRENCE, P. L. and PRATSCH, J.-C., 1980. Regional analysis of hydrocarbon migration using geophysics:

Gippsland Basin, SE Australia. Abstr., 15 Ann. Mtg. NE Sect. GSA, Philadelphia, 68.

J. -C. Pratsch 243



MENARD, H. W., 1981. Toward a rational strategy for oil exploration. Sci Amer., 244, 1, 57-64.PLEIN, E., 1979. Das Deutsche Erdöl and Erdgas. Jh. Ges. Naturk. Württ., 134 , 5-33.PRATSCH, J.-C., 1978, a. Future hydrocarbon exploration on continental margins and plate tectonics. J our .

Petrol. Geol., 1, 2, 95-105._________ ,1978, b. Regional structural elements in NW Germany. Jour. Petrol. Geology., 2, 2, 159-180.________ ,1982, a. Hydrocarbon concentration through preferred migration—Middle Magdalena Basin,

Colombia, South America. Erdöl-Erdgas Zeitschrft., in press.

________ ,1981, b. Wedge tectonics along continental margins. 1981 Hedberg AAPG Research Conference,

Galveston, AAPG Mem.

________ , 1981, c.Focused migration of gas and oil. Abstr., 1981 Annual Mtg., AAPG Bull., 65, 5, 974.________ , 1981, d. Basin evaluations and concentrations of oil and gas accumulations. Proc. Symp. Complex

Oil-Geol. Aspects Offsh. Coastal Adriatic Areas, Split, Yugoslavia, 53-66.________ , 1981, e. Regional structural elements and major gas accumulations in NW Europe. Proc. Seminar

Expl. Gas Fields ECE Region, Hannover, ECE, 6p.________ , 1982. Focused gas migration and concentration of deep-gas accumulations, NW German Basin.

Erdöl und Kohle, Erdgas, Petrochemie, Feb., 59-65.PRICE, L. C. , 1981. Mobilization and documentation of vertical oil migration in deep basins. Jour. Petrol.

Geol., 2, 4, 353-387.TISSOT, B. P. and WELTE, J. H., 1978. Petroleum formation and occurrence, a new approach to oil and gas

exploration. Springer, Berlin-New York, 538 p.WELTE, D. H., 1979. Neuere Überlegungen und Erkenntnisse über die Erdolmigration. E r d ö l - E r d g a s -

Zeitschrft., 95 , 207-208.________ , and Yükler, 1980. Evolution of sedimentary basins from the standpoint of petroleum origin and

accumulation—an approach for a quantitative basin study. Organic Geochem., 2 , 1-8.


Migration (Pratsch, 1983)

Documents