sedimentology and stratigraphy

8/19/2019 Sedimentology and Stratigraphy .............................................................................................................…

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Modiv Teje: Hydrocarbon Generation & Migration 1

D. Subsurface Mapping

1. Lateral Correlation of Logged Data

D.1. Using Data Points to Define Surfaces

Regional groups of well logs display distinctive thin sequences, key beds, & similar curveconfigurations or kicks which are commonly called correlation points. Examination of cuttings &

wireline curves at the same depths may or may not satisfactorily account for the genesis of all these

markers. Some of them cover only a few hundred acres, while others can be recognized over

hundreds or thousands of square miles.

Some markers are confined to linear trends, some are disposed in roundish patches, & some

are in sinuous belts. As more wells are drilled in a province, the areal patterns of these markers become better known & inferences about their origins are better justified.

The most significant markers for the subsurface geologist are those recognizable over large

portions of a petroleum province. In the United States Gulf Coast, for example, marine transgress-sions are typically abrupt. Sharp vertical stratigraphic changes are excellent regional markers & are

situated immediately above extensive transgressive sands, which usually have good reservoir

potential (Fig.1). A specific example of these widespread thin neritic shale markers is the Bolivina perca marker (Fig.2), which can be recognized extensively in the Gulf Coast. Some geologists claim

it can be recognized from South Texas to South Louisiana.

Fig 1

Fig 2


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Each vertical change displayed on logs may be conceived from either an empirical or

theoretical viewpoint. The logged change is seen either as a boundary of a mass of strata containing

some property in common or as a tangible trace of an abstract geological event, such as the end ofdeposition of a genetic increment. In either case the subsurface geologist takes the logged point as

belonging to a set of points that generate a surface. He calls it a rock-stratigraphic, or more simply a

lithologic, surface if he is thinking of a lithologic boundary. If he is thinking of a basin-wide surfaceseparating older from younger beds, he calls it a timestratigraphic, time-lithologic, or most simply a

time-rock surface.

It follows from basic principles Fig.3), that a given time-rock surface is most unlikely to be

marked by identical logged markers throughout a depositional basin. Lithofacies & biofacies shiftlaterally in space through time. As a consequence of overlap, facies surfaces tend to step-up or step-

down across a province with respect to time-rock surfaces. For example, in the case of transgressive

overlap (onlap) the paralic facies is said to climb the section landward, & in regressive overlap(offlap) the paralic facies climbs seaward, as in Fig.4 (Schematic & idealized diagram showingvertical & lateral relationships between three principal depositional environments in Gulf Coast

geosyncline).

The obliquity of facies & time-rock surfaces assures that basin-wide correlation of time-rocksurfaces in general results in the connection of markers in one portion of a basin to dissimilar ones

in another portion. In some provinces extending a time-rock surface all over the province involvesmaintaining parallel relations among many overlapping local depositional events, or time-rocksurfaces.

Using Surfaces to Delimit Units

It is clear to the subsurface geologist that operationally, log markers come first, that themarker points generate surfaces, and that the surfaces, in turn, delimit or enclose units. These units

may be of three categories: rock units such as members, formations, or groups; biostratigraphic

zones; or time-rock units such as stages and series; or any arbitrarily chosen subdivision of theabove. The point here is that in subsurface practice, a stratigraphic unit is a second-order entity

defined by two measured and named first-order stratigraphic surfaces. On the other hand, according

to traditional stratigraphic usage the unit is the first-order concept, and it receives the stratigraphicname. Consequently, surfaces separating contiguous units always have two designations: each is at

once the top of one unit and the bottom of another.

FiFig 3

Fig 4


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The subsurface geologist feels no compulsion always to

follow tradition by employing names such as "top of the lower

middle X Stage." He or she may name a mapping surface by asingle letter or numeral. Fig 5 illustrates the use of marker-defined

units named for paleontological species. Reservoirs in multi-pay

fields are usually given alphabetic or numeric designations,although usage is not consistent; in some cases the symbols are

applied to thickness units, in other cases to the surfaces which

delimit the thickness units.

The practice of storing digitized subsurface data, however, has notably standardized andimproved the internal logic of stratigraphic nomenclature, and the trend seems to be towards

acknowledging the primacy of the surface in stratigraphy. Thus a great variety of stratigraphic units

can be made to order for various purposes, each unambiguously defined and precisely measured. Itshould be noted, however, that subsurface stratigraphy is practiced in each province separately. The

network of well log correlations is internal to a province, and inter-provincial temporal correlations

must still be made with the aid of traditional paleontological inferences.

Contour Mapping

The time-consuming task of gathering subsurface information and interpreting it leads to thecontour map, the subsurface geologist's most valuable tool. It is more than a device for displaying

and relating borehole information in plan; it is a vehicle by which knowledge is extended beyond

and below control wells. Subsurface contour mapping is exploring.

The map view of the subsurface complements the profile view given by a geological crosssection. The cross section can display a full range of vertical relationships but cannot deal with

lateral relationships apart from a single line of profile. A contour map, however, can tie together

many cross sections and can impart strikes or trends to a large and varied body of borehole data. Theessence of planned exploration is recognizing promising trends and following them.

A contour line is the locus of holes or points in which a certain quantity is observed to-or is

presumed to-have the same value. It is an isoline or isopleth, a line of equal value. An astonishingnumber of different kinds of subsurface quantities can be contoured. The most important are: eleva-

tion of a surface (topographic, lithologic, timelithologic, faunal, pressure, temperature, potential,

etc.); thickness of an interval bounded by surfaces; porosity or permeability of an interval;

concentration gradients of ground waters; gravity of hydrocarbon fluids; fluid production rates; &lithofacies ratios or percentages.

The problem in drawing contour lines within arrays of boreholes (called control points) ishow to space them: that is, how to interpolate & extrapolate between, beyond, & below control.Only within oil or gas fields do control points provide more or less uniform samples of data, by area,

elsewhere wells are scattered irregularly, & lateral rates of change of the contoured quantity must be

inferred, perhaps from knowledge of surface topography, dips, or drainage patterns, perhaps from

geophysical patterns, or perhaps from depositional models. Obviously where there is an average ofonly two or three boreholes per township, right & wrong can have no meaning with respect to

contour patterns.

Mechanical & Interpretive ContouringComputer programs are available for contouring well information stored in digital form. For

the most part, these schemes interpolate contours uniformly between control points. Someexploration geologists believe that this style of contouring has much to recommend it. Of course,

nobody supposes that strictly linear slopes between control wells are correct anywhere. In the first

Fig 5


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place, each of the control wells was itself drilled at a place where a petroleum field was suspected;

& petroleum accumulations are anomalous features marked by, indeed generated by, nonlinear

conditions. The rationale for mechanical spacing of contours is not that it is realistic but that it isdone without prejudice. The equal-spacing-between-control-points school desires primarily to

discover valid prospects for drilling. A prospect revealed by an unbiased mechanical contouring

method is probably a real anomaly, and all the more so if coincident anomalies are displayed oncontour maps of several different subsurface quantities.

Mechanical contouring has many pitfalls, to be sure. It is costly to set up & to operate. Input

data must be carefully checked by subsurface geologists. Mechanical contouring tends to suppress

small structural features which could be prospective, & it may result in unrealistically large features.These pitfalls can be partly overcome by using

computer programs that instruct the plotter to contour the

trend surfaces & the small features (residuals) separately.The same subsurface data may be contoured in different

styles. In Fig 6 a data set is interpreted as a large anticlinal

feature, while in Fig 7 it is interpreted in terms of severaltransverse high-angle faults & fairly uniform south dip.

Assume that a good reservoir sand blankets the area at the

marker level and that some of the wells produce oil & gasfrom this sand.

The hydrocarbon saturations permit a judgement as

to which structural picture is true to more facts. The large

mechanically contoured feature seems on that evidence to be unlikely because (a) in the northern part of the area oil

is being produced at the same level as a dry hole to the

northwest and higher than a gas well to the southwest, &(b) in the southern part of the area a dry hole is level with a

gas well. The picture of several separate fields on different

fault blocks is more realistic.Interpretive contouring is preferable to mechanical

contouring for display, for recommendations to lease or to

drill, & for any presentations to management. The

interpretation should not conflict with the geological modelthat has been adopted for the mapping project. It must

honor all data, which will have been evaluated carefully foraccuracy & weighted accordingly. Generally the simplestinterpretation that fits the data is the best.

The best way to learn about the variety of possible

types of subsurface maps is a case method: examining

selected published maps to see what subsurface problemsthey were devised to solve & how well they succeeded.

B.2. Structural Contour Maps

Structural Contour MapsThe great bulk of subsurface mapping is structural contouring, in which the configuration of

a marker surface (which may or may not be a time-rock surface) is expressed in terms of elevationwith respect to a horizontal surface, normally sea level. First the marker is identified in each well

log. Next the effect of topographic altitude is removed by subtracting the elevation of the derrick

Figure 1.Fi 6

Figure 2.Fig 7


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floor (D.F.) or the Kelly bushing (K.B.)-the usual zero points of well logs-from the logged depth of

the marker (Fig 8). Except in High Plains and Rocky Mountains provinces (where altitudes are

several thousands of feet), significant marker elevations in the United States are minus values (i.e., below sea level) after reduction. These are called "subsea depths."

Adequacy of Contours for Depicting StructureStructural contour maps alone are sufficient to depict

the structure of many petroleum provinces. Except for its

salt domes & diapiric shale plugs, the Gulf coastal plain

has relatively simple structure, with low-relief foldsassociated with oblique normal faults.

The successive stages of folding in the Tom O'Connor

field & their influence on oil accumulation are adequatelydepicted without resorting to cross sections (Fig 9, 10, 11,

& 12). A rule of thumb is that geological regions in which

subsidence is the predominant motion of the basement canusually be described by contour mapping alone.

On the other hand, re-

gions in which basement blockshave been differentially uplifted

may contain structure so intense

or complex that profiles together

with maps are necessary to depict it. fig 13 alone is not a suf-

ficient description of the trap-

ping of petroleum in the Brent-wood field, California. It must be

supplemented by a x-sect, which

illuminates the high-angle fault-ing & the truncation & sealing of

the Martinez producing sand by

a channel subsequently filled

with Meganos Shale. In belts oflow-angle overthrusts, structural

contours are wholly inadequateto describe the structure of a marker, except in local spots such as

the Painter Reservoir field pro-

per (fig 14, as of March‘79),

southwestern Wyoming.fig 15 indicates in profile the impossibility of expressing the regional configuration of the

Nugget Sandstone by structural contours, where the Nugget is repeated two or more times as thrust

slices are stacked vertically. The study of systems of thrusts is at the forefront of research in struc-tural geology. Boyer et al. (‘82) have classified thrust systems into two general groups: imbricate

fans & duplexes (fig 16). Major reserves in the western overthrust belt of North America have

spurred seismic surveying & drilling, & the use of the dipmeter has helped to tie data from these twosources together.

Figure 1.Fi 8

Fi 9 Fi 10

Fi 11 Fi 12


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In overthrust belts, each major slice requires its own contour map. Because of steeply plung-

ing anticlines, abrupt changes in axial direction, minor thrusting, & recumbent folding, sets of

isometric block diagrams are probably the best way to illustrate the structural complex as a whole.

Detecting FaultsMost computer programs for structural contouring are not able to deal satisfactorily with

faulting without special handling by the geologist. Faulting is indicated on a structural contour mapat prospect scale by changes in both dip & strike of a marker surface together with abrupt changes indepth to the marker.

In the case of faulting contemporaneous with deposition, there may be additional differences

in thicknesses of stratigraphic intervals, either due to additional deposition on downthrown blocks,

erosion of upthrown blocks, or both. If faulted & unfaulted well logs are both available, comparisonmay reveal sections missing by normal faulting (Fig 17) & sections repeated by reverse faulting (

Fig 18). But as a suitable unfaulted log may be unavailable & as correlations often are uncertain

near faults, examination of borehole-scale changes in dip are helpful in identifying or confirming thefault surface.

Abrupt increases & decreases in dip angle (fig 19, 20 & 21) commonly mark the place where

a borehole crosses an oblique fault. The type of fault may be inferred from the dip direction asindicated by the azimuths or bearings of the "tails" of the dipmeter symbols. For example, fig 19,

shows that a contemporaneous normal or growth fault has many dips towards the fault surface in the

Figure 6.

Fig 13

Fig 14

Fig 15 Fig 16


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downthrown block, while dip in the upthrown block is uniformly away from the fault. fig 20 shows

that dip directions in the downthrown block of a normal fault in a consolidated sequence exhibiting

drag are generally away from the fault surface. In the case of a reverse fault with drag ( fig 21) dipdirection is the same throughout, while the dip angle increases sharply, then decreases sharply as the

borehole passes through the fault zone.

Fault zones in brittle rocks commonly display symptoms of increased shear strain as the borehole approaches the fault surface. The shear zone enclosing a fault surface may be marked by

increases in fracture porosity and permeability, as indicated by sonic logs, density logs, or dipmeter

logs; oil or gas saturation, detectable by mud logs; concentrations of uranium-bearing minerals,shown by alpha and gamma radiation; mud gas kicks or loss of drilling mud, registered on mud logs;

and other symptoms. Shear zones of currently active faults may be marked by abrupt changes in

temperature gradient, including inversions of temperature, resulting from dilatant expression ofdeeper fluids upward in fault zones, as at Rhourde el Baguel ( Figure 6 , Figure 7 and Figure 8 ), a

horst field in Algeria.

i gure 3 .Fig 19i gu re 4 .Fi 20

Fig 17 Fig 18

i gure 5 .Fi 21 Fi 22

Fig 21 Fig 22


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Figure 8. Figure 7.

Vertical Faults

Nomograms Used in Mapping Oblique Normal FaultsTypically, in the Paleozoic provinces of interior North America and in other cratonic regions, the

presence of faults in the subsurface is not directly evident, but must be inferred from such evidence

as steep dips, abrupt changes in strike, or geophysical indications of faulting. As a rule, these fault

surfaces are not intersected by boreholes because they are vertical or nearly so. Vertical faultsappear on structural contour maps as linear traces which do not shift laterally with depth, and a 100

ft fault trace looks the same as a 1000 ft fault trace. Strictly vertical faults, of course, cannot be

placed in a normal or a reverse category, and it is often a question of whether the high block went upor the low block down.

igure 1.

Figure 2.

Provinces in which the basement subsides continually (such as the northwestern Gulf of Mexicoregion) contain oblique normal faults. These faults present no difficulty in deciding which block

went down but present two routine problems in subsurface mapping because of their obliquity:

depiction of fault breadth or cut-out, and depiction of lateral step-out. Please refer to Figure 2 (thekey for Figure 1). This diagram illustrates an ideal oblique normal fault in cross section. The marker

surface being mapped is displaced from A to C along the fault; it is "cut out" or "faulted out." In


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plan or map view the breadth of the cut-out belt (BC) is a function of the dip of the fault plane and

the displacement of the fault. Cutout belts are depicted as blank strips containing no structural

contours. What the subsurface geologist calls "fault displacement" or "vertical throw" or "cut-out" isessentially the vertical component of the net slip (AB in Figure 1 ). This quantity is readily

measured with well logs. Correlate a log suspected to be faulted with a nearby unfaulted log. The

measured locus of shortening is where the borehole intersected the fault, and the quantity ofshortening is AB, vertical cut-out. Two possibilities that could give rise to a less-than-true value of

vertical cut-out are: stratigraphic expansion (i.e., depositional thickening) in the hanging block, and

steep dips (i.e., rollover) in the hanging block.

On Nomogram I find the vertical cut-out (AB) on the vertical axis. From that point, a line parallel tothe fault dip intercepts the horizontal axis at a value which is the horizontal component of net slip, or

BC, or breadth of fault cut-out.

In order to draw a fault cut-out belt in its proper lateral relation to faulted boreholes, use NomogramIl, which depicts for three fault dip values the rate of lateral step-out (YA in Figure 1 ) in relation to

depth difference between fault intersection and marker intersection (XY in Figure 1 ). In a well cut

by a 55 fault, for each 1000 ft of depth difference between the fault and the contoured marker, theedge of the cut-out belt steps out horizontally 700 ft from the well. Determine XY from the log of a

faulted well and apply the appropriate rate of step-out to obtain the value YA for use in drawing the

structural contour map of the

Reverse Faults

Mapping UnconformitiesUnconformities are structural surfaces, and all the beds above an unconformity are

everywhere younger than any of the beds below. Every unconformity, of course, has a conformable

sequence contemporaneous with it somewhere downdip. It is possible to make a set of paleogeomorphic maps showing the erosional and the depositional terranes at successive points in

time.

In some situations a major unconformity may be a desirable surface for contour mapping.The significance of unconformities as avenues of secondary migration of petroleum and as

governing traps and seals for reservoirs is well-known. As well control increases, regional

unconformities are likely to appear more complex than they seemed at first. Careful study of logs

may reveal a converging of several unconformities, with successively younger erosion surfacestruncating older ones in the direction of regional positive areas. To determine where the products of

each erosional pulse were deposited as potential reservoir beds, give each unconformity a sequentialnumber and make separate paleogeomorphic maps of each erosional-depositional surface.


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igure 1.

Figure 2.

Dipmeter logs may reveal angular unconformities. Figure 1 ( Northern Algeria) illustrates two

unconformities not indicated by other data. The lower unconformity separates beds dipping 20¡ to30¡ from an overlying sequence dipping about 10. Immediately above the upper unconformity, beds

dip very steeply, probably due to filling of hollows on the erosional surface by the earliest post-

unconformity sediment. Angular unconformity is evident in Figure 2 ( Louisiana) , where

deterioration of the underlying beds by weathering shows up as a group of dips whose azimuth isapproximately constant, but whose angle decreases with depth below the unconformity.

Exercise 1.Could the subsurface geologist use electric logs in correlating (a) Cenozoic units between coastal

Texas and coastal California?


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Solution 1:

Definitely no. The principle here is that electric log correlations are intraprovincial only; areas in

which deposition was contemporaneous but not in-terconnected can be correlated stratigraphicallyonly in respect to their fossil content. If the two areas were interconnected during deposition,

however, electric-log correlation remains a possibility to be tried.

Exercise 2.What are the subsea depths of Markers X, Y, and Z on Figure 1 ?

Figure 1.

Solution 2.

2873' Log depth Mkr X- 1085' D.F.- 1788' Subsea depth Mkr X

4120' Log depth Mkr Y

- 1085' D.F.- 3035' Subsea depth Mkr Y

6411' log depth Mkr Z

- 1085' D.F.

- 5326' Subsea depth Mkr Z

Exercise 3.Draw a structural contour map of marker surface P, using the subsurface data on subsea depths of

markers, subsea depths of fault intersections, and magnitudes of fault cut-outs. Assume faultsurfaces dip 55 and use a contour interval of 100 ft. ( Figure 1 and Figure 2 ).


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igure 1.

Figure 2.

B.3. Isopach & Other Subsurface Maps

Isopachs of Time-Rock IntervalsThe second most common type of subsurface map, after the structured contour map, records

the vertical distance between two horizontal surfaces. A contour is an isopach line, a line of equal

(Greek iso) thickness (Greek pachys). Isopach maps are highly versatile devices. They must be

distinguished from isochores. Figure 1 shows the difference between these concepts. An isopachvalue, a, is the thickness of a horizontal stratigraphic unit measured in a vertical hole. An iso-chore

value, b, is the thickness of an inclined unit measured in a vertical hole and not corrected for dip

(i.e., deviation of strata from horizontal). Again, an iso-chore value, b', is the thickness of aninclined unit measured in a non-vertical hole and corrected for deviation of the hole from vertical

but not corrected for dip.

Figure 1. Figure 2.

The isopach map shown in Figure 2 , which covers some 1200 square miles (3100 square km) of the

Powder River basin. In this case, the upper surface of the thickness interval is a bentonite marker

and the lower surface is a regional unconformity at the base of the productive Muddy Sandstone.This unconformity is marked by river channels incised into the underlying Skull Creek Shale during

a post-Skull Creek episode of uplift. It is apparent that Figure 2 does display the structure of the


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buried unconformity during the deposition of the bentonite marker; but it is also apparent, from the

relatively slight local deformation of the bentonite ( Figure 3 , structure of bentonite marker bed,

showing general homoclinal westward tilt into Powder River basin. Contour intervals, 100 ft and500 ft ), that the structure of the surface of unconformity is largely topographic rather than tectonic.

Hence, Figure 2 is to be taken as a paleotopographic map and is a useful indicator of the

depositional trends of Muddy Sandstone.

Figure 3. igure 4.

igure 5. igure 6.

In the case of Pennsylvanian Cherokee sands in Kansas, the base of the isopach interval is an

unconformity ( Figure 5 , Map, attitude of the top of the Mississippian formation. Datum sea level.

Contour interval is 25 ft. Depicts the bedrock paleotopography of a former land surface now buried4400 ft deep, as modified by tilting and by later folding including post-Cretaceous folding ) and the

top is the Fort Scott Limestone ( Figure 4 ), deposited horizontally but subsequently locally folded

and regionally tilted. By contouring the interval between these surfaces, the effect of post-

Pennsylvanian deformation is eliminated, and the isopach map ( Figure 6 , Note the association ofoil wells producing from Cherokee sandstones (large filled circles) with the buried paleo valley

system delineated by the 125 ft thickness contour lines) depicts a buried paleo valley system. Figure

7 displays three Cherokee sand reservoirs deposited by rivers flowing in this ancient valley.


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Figure 7.

Isopachs of Rock (Lithologic) Units

In addition to representing time-rock units, thickness maps also lend themselves to the depiction ofthe volume of productive reservoirs ( Figure 1 ) or of simply-shaped masses of relatively uniform

lithology. Figure 2 and Figure 3 illustrate a productive deltaic sand body in the Marchand trend of

the Anadarko basin of western Oklahoma. Here the delta front sand body depicted by the isopach

map is virtually coexistent with the oil reservoir. Figure 4 shows the thickness of the "reef"limestone facies of the Permian Horseshoe atoll in west Texas. Thickest areas mark the crest of the

atoll whose pinnacles are now producing petroleum.Figure 1.

Figure 2.


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igure 3. Figure 4.

Isolith MapsCommonly important stratigraphic traps consist of multiple thin bodies of a particular rock type, and

it is desirable to illustrate, in map form, the trends and areas of such composite fields. Figure 1 shows typical logged sequences of potentially productive granite wash beds in the Palo Duro basin,

Texas Panhandle. Figure 2 (Source area was Precambrian basement exposed in Matador arch fault

block ) shows the thickness and extent of a single productive granite wash lobe. It is a true isopach

map. Figure 3 is an isolith map, which depicts the net thickness of all granite wash beds in the lowerPennsylvanian section in the Palo Duro basin. These isolith patterns indicate thickness of

megafacies units, not time-rock units, and may be useful in planning exploration strategies

Figure 1. Figure 2.


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Figure 3.

Compound MapsFigure 1 illustrates, by contours and areal patterns, the changes in thickness and in the ratio of sand

to shale across the Rocky Mountains region during the deposition of Upper Cretaceous rocks. It is amatter of opinion whether such summary maps of large regions and thick sequences are useful in

practice. One school of thought foresees possibly serendipitous consequences from the melding of

many sets of stratigraphic measurements on one map. On the other hand, others point out that since

facts are unable to reveal their own meanings, geologists should impose clear meanings on sets ofmeasurements deemed worthy of publication.

igure 1.

Subcrop Maps


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Subcrop maps are important in depicting large fields associated with unconformities. Figure 1 shows

the giant Oklahoma City field with its pattern of truncated productive pre-Pennsylvanian units

visible directly beneath the basal Pennsylvanian unconformity.

Figure 1.

igure 2.

In Algeria, there is long-range migration of oil from subcropping Silurian source shales to Cambro-Ordovician fields beneath the unconformity, such as the super-giant, Hassi Messaoud ( Figure 2 ). If

your intention is to emphasize the pattern of lateral variations of stratigraphic units beneath an

unconformity, construct a subcrop map ( Figure 3 ). If emphasis is to be placed on lateral variationsof stratigraphic units lying upon a surface of unconformity, draw a worm's eye map, in which a

lithofacies pattern is seen from below.


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Figure 3.

Miscellaneous MapsFigure 1 , Figure 2 , and Figure 3 illustrate two less common types of subsurface maps. Figure 1 is a

profile of wells across Rhourde el Baguel field, Algeria. Temperatures of fluids recovered in

successions of open-hole drillstem tests show that the Cambrian reservoir contains hot streaksresulting from water and petroleum fluids actively invading the reservoir from Silurian source

shales, which are being thermally dehydrated currently in contiguous down-thrown blocks. The

complex configuration of the 100¡C isotherm cannot be depicted satisfactorily by contour lines because of local inversions in the sequence of isotherms. Accordingly, the warm and cool spots must

be generalized in plan by colored or patterned areas ( Figure 2 , Dashed line indicates line of section

in Figure 1 ).

Figure 1. Figure 2.


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Figure 3.

The cool spots appear to be genetically connected with areas of high Productivity index ( Figure 3 ).P.I. is the ratio of the volume of petroleum produced during an interval of time to the accompanying

drop in reservoir pressure. It is likely that the cool spots on Figure 2 are associated with downward

legs of convective cells within the reservoir. Production from wells in cool spots benefits fromgravity flow, while production from wells in warm spots dissipates some of the heat which drives

convective circulation within the reservoir. A cool spot is analogous to a floor-level duct in a hot-air

heating system, which takes in heavier cooler air and returns it to the furnace for re-heating and

subsequent dissipation of this added heat through the building. Convective systems would not beactive at all today if fresh heat were not continually being fed into the reservoir along its flanks by

the operation of a non-convective process of fluid mass transport. The result of producing

convecting fluids is greater drops in pressure, and hence lower P.I. values, in warm areas than incool ones.

Figure 4. Figure 5.

There are many other types of maps which can usefully summarize or relate special categories ofsubsurface data. Sometimes informal units are of real value in planning the most profitable and

efficient exploitation of a field. For example, Figure 4 and Figure 5 show contoured quantities called

"hydrocarbon-feet" and "permeability-feet." The former is the summation of porosity timeshydrocarbon-saturated values computed at 6-inch intervals. The contours in Figure 4 represent feet

of 100% hydrocarbon saturation. Contours on the "permeability-feet" map ( Figure 5 ) represent the


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sum of permeabilities computed at 6-inch intervals. Such composite maps are said (Holland et al.,

1976) to permit a degree of predictability of production rates for each well which would not be

possible using only the conventional net gas isopach map ( Figure 6 ). This type of work is mostlydone by production geologists.

Figure 6.

Exercise 1.A.) Figure 1 and Figure 2 are a subcrop map of Algerian Paleozoic units beneath a sub-Mesozoic

unconformity. Does this subcrop pattern indicate Pre-Mesozoic uplift? What is the evidence?

igure 1.


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Figure 2.

B.) Given that the principal period of structural trap formation in eastern Algeria was Cretaceous in

age (Austrian orogeny), and given that the source of petroleum is Silurian shale, are the following

statements true or false: Most of the petroleum in the eastern Algerian Sahara did not migrate fromits source rock until Cretaceous or later time. True or False?

Long-distance secondary migration is evident in this province. True or False?

Solution 1:

A.) Yes. The Pre-Mesozoic uplift is proved by the fact that older strata are exposed in the cores oflocal high features, which accordingly must have been uplifted before Mesozoic strata were

deposited on the regional unconformity.B.) Both statements are true.

Exercise 2.How can an isopach map be construed as a structure map?

Solution 2:

Assuming initial horizontality ( Figure 1 ), a thickness map can be seen as depicting the buried

structure of the older or lower surface of a unit at the time the younger or upper surface was beingdeposited.


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Figure 1.

Exercise 3.

Using the following rough areal values, compute the approximate volume of oil-saturated sandsketched in Figure 1 .

Mean areas of oil saturation

From OWC to 10' above OWC-1000 acresFrom 10' to 20' above OWC-850 acres

From 20' to 30' above OWC-700 acres

From 30' to 40' above OWC-550 acres

Figure 1.

Solution 3:


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31,000 acre-feet.

sedimentology and stratigraphy

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