CHARACTERIZATION AND PREDICTION OF RESERVOIR …

CHARACTERIZATION AND PREDICTION OF RESERVOIR ARCHITECTURE

CT&F - Ciencia, Tecnología y Futuro - Vol. 1 Núm. 3 Dic. 1997

CHARACTERIZATION ANDPREDICTION OF RESERVOIR

ARCHITECTURE ANDPETROPHYSICAL PROPERTIES IN

FLUVIAL CHANNELSANDSTONES, MIDDLE

MAGDALENA BASIN, COLOMBIA

*Colorado School of Mines, Geology Department, Golden CD 80403e-mail: [email protected]

Ecopetrol, Instituto Colombiano del Petróleo, A.A. 4185 Bucaramanga, Santander, Colombia

J. C. RAMÓN* and T. CROSS

* To whom correspondence should be sent

About 1.600 m of core from two major oil fields, and 600 m of outcrop sections near to one of the fields,integrated with thousands of core, side-wall core and outcrop porosity and permeability measurementsshow that petrophysical properties and reservoir continuity change consistently with the Accommodation

to Sediment Supply (A/S) ratio. Channel sandstones exhibit regular, recurring motifs that are associated withchanges in A/S conditions. Manifesting the lowest accommodation conditions are amalgamated channelsandstones, up to 5-m-thick, with intraclast-rich bases capped by up to 1-m-thick floodplain mudstones. AThigher A/S conditions, slightly amalgamated channel sandstones have lateral accretion surfaces and are cappedby a thicker unit of floodplain mudstones. Channel sandstones in the highest A/S conditions are single-storied,possess conspicuous lateral accretion surfaces with thick mud drapes, and have a thick cover of overbank andfloodplain deposits. Detailed well log correlations, oil production and pressure data support that the former aremore lateral continuous while the latter form laterally discontinuous bodies embedded in floodplain mudstones.Petrophysical properties are closely associated with subtle variations in facies, particularly degree of preservationof original bedforms, and both are stratigraphically sensitive. Petrophysical properties of identical sedimentologicalfacies change regularly as a function of its stratigraphic position because of variations in the rates of accumulationand degree of preservation of the sediments. In the case of fluvial strata deposited during an increase in A/S,porosity and permeability are highest in trough cross-stratified sandstones immediately above channel scourbases, and decrease upward to the next scour base. Successive channel sandstones within the same stratigraphicsequence and channel sandstones from one sequence to the next have progressively lower porosity andpermeability values in an overall increase in the A/S. An inverse trend is observed during a decrease in A/S.

Keywords: Colombia, fluvial, reservoir characterization, porosity, permeability.

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J. C. RAMÓN and T. CROSS

Este estudio muestra que las propiedades petrofísicas y la arquitectura de los yacimientos en canalesfluviales y depósitos de crevasse dependen de las condiciones de acomodación y suministro desedimentos - A/S - durante la sedimentación. Estos resultados se basan en la integración del análisis

sedimentológico de aproximadamente 1.600 m de núcleos de perforación de dos campos petroleros y unos600 m de afloramientos en el Valle Medio del Magdalena con medidas de porosidad y permeabilidad denúcleos y afloramientos. La arquitectura y sucesiones de facies en areniscas fluviales se presentan en patronesrepetitivos asociados a cambios de A/S. En condiciones de A/S bajas, las arenas de canal tienden a seramalgamadas, formando cinturones de canal hasta de 5 m de espesor y asociados a delgadas unidades lodosasde llanura de inundación. Bajo condiciones de A/S mayores, el amalgamamiento se reduce y se preservansuperficies de acreción lateral y mayores espesores de depósitos de llanura de inundación. Las arenas de canalde máxima A/S, son más lodosos, poseen claras superficies de acreción lateral y ocurren como cuerposaislados enbebidos en lodos de llanura de inundación. La correlación detallada de pozos cercanos, datos deproducción y presiones indican que la extensión lateral de los cuerpos de arena es proporcional al grado decanibalismo. Canales amalgamados son más continuos que canales individuales. Las propiedades petrofísicasestán íntimamente ligadas al grado de preservación de las facies y cuerpos originales. Como la preservaciónrelativa depende de A/S, esta relación controla los valores de porosidad y permeabilidad incluso en faciesidénticas. En arenas fluviales, los valores de porosidad y permeabilidad son mayores en bajas condiciones deA/S, donde el mayor canibalismo entre las arenas elimina los componentes finos y mejora la selección. Amayores condiciones de A/S, la mayor preservación de los componentes originales resulta en una mayorproporción de finos y peor selección.

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INTRODUCTION

Reservoir geometry, compartmentalization, andporosity and permeability values and distributionsinfluence fluid flow, sweep and recovery efficiencies.An understanding of the geologic controls on theseattributes is beneficial for reservoir characterization,fluid-flow simulation, optimal field development andenhanced oil recovery.

Geometry, continuity, heterogeneity and petro-physical properties of a reservoir depend on a varietyof phenomena including the depositional environment,the accommodation conditions during the time of sedi-ment accumulation, tectonic history, provenance anddiagenesis. Most basic lithological, petrophysical,geometrical and reservoir continuity attributes of strataoriginate during sediment accumulation. Sedimentologicand stratigraphic studies attempt to represent andpredict these rock properties in a four-dimensional(time-space) context. Exceptions are rocks which havebeen extensively altered by diagenesis or fracturing.

Traditionally, rock attributes important to reservoircharacterization and production are studied andassessed from the perspectives of depositional environ-ments and facies models (e.g., Amaefule, et al., 1994;Weber and Van Geuss, 1990; Jennings, 1987; Pryor,1973; Shelton, 1973; le Blanc, 1972). Facies modelsare considered representative of sediment accumulatedwithin their respective depositional environmentsindependent of time or place. They are constructedfrom facies assemblages observed in examples ofstrata considered equivalent in terms of their environ-ment of accumulation (Walker, 1979; Miall, 1996).Facies models assume that deposits of a geomorphicenvironment are preserved more or less intact; thefacies components of strata are products only of theprocesses operating in the geomorphic environment,not subject to modification or influence by stratigraphiccontrols.

By contrast, a few previous studies have demons-trated that physical characteristics of facies, faciesassemblages and successions, and stratigraphic archi-tecture of identical facies tracts vary with stratigraphicposition (Cross et al., 1991; Gardner, 1993; Sonnenfeldand Cross, 1993; Fajardo, 1995). There are direct rela-tions between dynamic changes in accommodation/sediment supply [A/S] ratio, the numbers and propor-tions of geomorphic elements preserved in strata, and

the character and assemblage of facies preserved. Thestratigraphic record produced from identical environ-ments varies as A/S conditions change. Stratigraphiccontrols contribute to reservoir geometry, facies diver-sity and petrophysical properties at least as much asdo the geomorphic processes of the environment inwhich the sediments accumulated. Facies models donot account for these dynamic changes in A/S condi-tions, the degree of preservation of geomorphicelements, and, therefore, for stratigraphi-cally sensitivechanges in facies attributes, components and propor-tions for identical facies tracts.

This chapter illustrates that large- and small-scalereservoir rock properties and production history arebetter understood and predicted from a stratigraphicperspective than from the perspective of facies models,only For strata of identical environmental origins, chan-ges in sandstone heterogeneity, geometry and lateralcontinuity, and values and ranges of petrophysicalproperties are associated with changes in A/S condi-tions within stratigraphic cycles.

Channel sandstones deposited in low A/S conditionsare amalgamated and areally continuous. The finer sandand muddy sediments composing the upper parts ofchannel bars and most floodplain sediment are notpreserved. The resulting reservoir sandstone is morehomogeneous, porosity is good, porosity ranges arenarrow, and reduced clay content enhances permeabi-lity. Higher A/S conditions result in a lower degree ofamalgamation and cannibalism of channel deposits andincreased preservation of floodplain sediment. Channelsandstones tend to be single storey, isolated bodiesembedded in floodplain deposits.The geomorphicelements composing the individual channels are morefully preserved and more mud is preserved. Porosityand permeability profiles decrease strongly upward.Where several channel sandstones are interconnected,major changes in rock properties and internal permea-bility barriers may exist between channel sandstoneelements. The increased proportion of fine sand andmud reduces permeability and increases capillarypressure.

These unidirectional changes in alternately increa-sing and decreasing A/S conditions can be interpretedfrom one-dimensional cores and well logs. Stratigraphicand sedimentologic responses to these changes maybe used to predict reservoir properties and architecture.This study of subsurface and outcropping strata from

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the Middle Magdalena basin shows that sedimentologicand stratigraphic attributes important for reservoircharacterization, production and management changewith A/S conditions, and can be predicted fromstratigraphic analysis of core and well logs.

STUDY AREA AND DATA BASE

This study pertains to an area of about 180 km2 inthe central part of the intermontane Middle Magdalenabasin, Colombia. Most hydrocarbon production comesfrom fluvial channel and crevasse splay sandstones ofthe Lower Tertiary La Paz, Mugrosa and ColoradoFormations. More than 6.000 ft of cores and 4.117porosity and permeability measurements from these

formations in the Lisama, Tesoro, Casabe and La Ciraoil fields were examined. Scintillometer and permea-bility measurements were taken from outcrops nearthese oil fields (< 20 km away) in strata of the sameages and facies as in producing intervals. Figure 1summarizes the sources of data.

TRADITIONAL FOCUS ON RESERVOIRPROPERTIES AS A FUNCTION OF ENVI-RONMENTS, FACIES AND FACIES MODELS

Traditional evaluations of small- and large-scalesedimentologic and stratigraphic attributes importantto reservoir characterization and production areconducted from the perspectives of depositional

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environment and facies models (Amaefule, et al.,1994; Weber and Van Geuss, 1990; Jennings, 1987;Pryor, 1973; Shelton, 1973; Le Blanc, 1972). Theseattributes include geometry, volume and continuity oflithosomes; degree of heterogeneity; type and degreeof compartmentalization; and facies components,diversity, and petrophysical attributes. Extent andcontinuity of shales that might act as barriers to fluidflow also are evaluated from these perspectives(Mattax and Dalton, 1990, their table 4.3). Porosityand permeability values and distributions are plottedwith respect to facies but not with respect to strati-graphic position (Warren and Price, 1961; Harris, 1975;Mattax and Dalton, 1990; Weber and Van Geuss, 1990).This is an extension of the assumption that petrophysicalproperties are dependent only on environmentallydetermined facies. Little or no attention is given to strati-graphically controlled variability of rock attributes thatare important to reservoir production and management.

Facies models are considered typical of sandstonebody morphologies and component facies for theirrespective environments without regard to time, placeor stratigraphic position. For example, progradation ofa barrier island or shoreface forms a �tabular� sandsto-ne body (Figure 2a), whereas a fluvial channel tendsto form narrow, elongated sandstone bodies (Figures2b and 3a). Sandstone geometry facies models, suchas those illustrated in Figures 2 and 3, are used to descri-be reservoir geometry early in field development sothat the direction and probable extent of specifichydrocarbon-bearing units could be extrapolated fromlimited data.

Such facies models are abundant for continentalstrata. Friend (1983) suggested two dominant modesof occurrence of fluvial sandstones; the areally exten-sive �sheet� and the more laterally restricted �ribbon�sandstone bodies. Schumm (1977) proposed a four-fold geomorphic classification of fluvial sandstones.Other authors recognized a greater variety of channelsandstone morphology and proposed up to 15 faciesmodels (Miall, 1996).

There is a general opinion that strata of fluvial sys-tems are unorganized because they are heterogeneousand compartmentalized, facies occur in a patchworkmosaic, and lateral transitions are abrupt. Consequently,prediction of reservoir continuity and geometry in conti-nental strata has been considered difficult or impossible(Weber and Van Geuss, 1990). Although Bridge andLeeder (1979) showed theoretical transitional changesin large-scale geometry and architecture of channelsandstones with respect to stratigraphic position, onlya few studies have described transitional changes ingeometry, morphology, degree of amalgamation, arealcontinuity and architecture of channel sandstones froma stratigraphic perspective (Sanchez-Moya et al.,1996; Olsen et al., 1995; Cross et al., 1993; Davies etal., 1993; Gardner, 1993; Melvin, 1993; Willis, 1993;Miller, et al., 1990; Puigdefabregas and VanVliet,1978).

Theoretical and observed regular transitions inlarge-scale geometric and architectural attributesshould suggest similar regularity in small-scale attribu-tes of strata as a function of stratigraphic position.

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Traditionally, however, reservoirs are divided intodifferent facies representing different geomorphic ele-ments or environments. Petrophysical measurementsare recorded and evaluated in these facies irrespectiveof stratigraphic position. Permeability and porosityaveraging normally is done with conventional statistics(arithmetic, geometric or harmonic means) or withgeostatistical techniques. The means and ranges ofpetrophysical properties are applied to that facies atall stratigraphic positions. Again, there is little recog-nition that petrophysical properties of identical facieschange parallel with changes in degree of preservationof original geomorphic elements. Analyzing and grou-ping petrophysical properties by facies alone, irrespec-tive of their stratigraphic position, increases the scatterin the values of the measured attribute.

The following discussions review some large- andsmall-scale attributes of strata representing identicalenvironments that are stratigraphically sensitive.Stratigraphic variations in geometry, architecture andcontinuity of facies tracts are related to degree ofcannibalization, amalgamation and preservation of origi-nal geomorphic elements. Small-scale, stratigraphicallycontrolled variations in identical sedimentologic facies,in turn, are related to stratigraphic variations in porosityand permeability. The combination of stratigra-phicallysensitive small-and large-scale attributes stronglyinfluence reservoir performance and production history.

RESERVOIR GEOMETRY ANDCOMPARTMENTALIZATION

Origins and types of reservoir compartmentsIn the absence of structural overprinting that influen-

ces fluid flow, the most common reservoir compartmentboundaries are coincident with the stratigraphic bounda-ries of genetic sequences (Cross et al., 1993). Hanson(1989) and Little (1994) showed that the limits ofproducing shoreface reservoirs are defined by deposi-tional limits of the shoreface facies tract within eachgenetic sequence of the Eagle Sandstone in the BearPaw and Tiger Ridge gas fields, Montana (Figure 4).Marine shelf mudstones which bound the seawardportions of the genetic units are seals between super-posed reservoirs.

Other common stratigraphically controlled fluid-flow compartments are the smaller-scale, offlappingstratigraphic packages arranged as shingles withingenetic sequences. These form by episodic progra-dation of environments with depositional topography.For example, when a prograding delta lobe switchesto another position or temporarily ceases to prograde,muds may drape the surface of the lobe. When anotherlobe returns to the same geographic location, or whenprogradation resumes, the resulting deposit is an overallcoarsening and shallowing-up facies succession

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interrupted by low-energy mudstone drapes.In both cases, high-resolution stratigraphic correla-

tion permits accurate representation of reservoir com-partmentalization in a spatial and temporal framework.High-resolution correlation is the best way to defineheterogeneities in the reservoir that form differentfluid-flow compartments and that control fluid-flowpathways and rates.

In alluvial systems the boundaries of the mainreservoir compartments also are coincident with strati-graphic boundaries of genetic sequences and faciestract offsets associated with these boundaries. Channelsandstones and in some cases crevasse complexes arethe reservoir units; overbank mudstones are seals andfluid flow barriers. The size and geometry of alluvialsandstone reservoirs are defined by the degree of amal-

gamation and connectivity of fluvial channel sandstones,the concomitant elimination of mudstone by canniba-lization, and by the extent of progradation in crevassecomplexes.

The continuity and geometry of coarse membersand their proportion with respect to the floodplain andlake mudstones depend on a variety of processesincluding subsidence rate, sediment supply, and avulsionrate, among many others. Allen (1978, 1979), Bridgeand Leeder (1979) and Blakey and Gubitosa (1984)suggested that fluvial channelbelt sands depositedduring high frequency of avulsion relative to subsidencerate form vertically and laterally interconnected,blanket-like sandstone bodies that could function assingle compartment reservoirs. By contrast, channelbeltsandstones of identical depositional systems may occur

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as isolated, stringer-like reservoir sandstones if depo-sited during low frequency of avulsion relative tosubsidence rate. In this case, each sandstone bodywould function as a separate fluid-flow compartment.

Vertical transition between these two end situationsare observed in the Middle Magdalena basin. Figure 5shows a correlation between wells in the AguasBlancas field. Fluvial channels toward the base of thecycles tend to be interconected and associated withfew overback mudstones, while toward the top of thecycle floodplain mudstone proportion increase andchannels become thinner and laterally discontinuous.

Origin of Changes in Facies Composition, De-gree of Amalgamation, and Stratigraphic Archi-tecture

Channel sandstones in the Middle Magdalena basin

range transitionally from isolated sandstone bodiesembedded in floodplain mudstones to amalgamatedmulti-storied, areally continuous sheet sandstones(Figure 5). Sandbody width/depth ratios range fromabout 100 to more than 500. Transitional changes inchannelbelt sandstone morphology, geometry, faciescomponents and successions, and diversity of faciestracts are stratigraphically sensitive, and these arerelated to changes in the A/S conditions through time.

During high A/S conditions most of the originalgeomorphic elements of continental environments arepreserved in strata. The high aggradation rate of theentire alluvial plain produces isolated channel sands-tones separated by floodplain, lake or crevasse-splaydeposits (Figure 6a). These channel sandstones arecomposed of fully preserved geomorphic elements ofthe river, from the base to the top of the channel, resul-ting in typical pointbar facies successions (Figure 6b).

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Geomorphic elements of the floodplain also arepreserved more or less intact, as they aggrade intamdem with river bed aggradation. After a series ofavulsion events to low areas of the floodplain, achannelbelt might reoccupy the same geographicposition it previously occupied. The channel will erodeto its base level. If the previously deposited channelsandstones are below this erosion level, they areincorporated into the stratigraphic record withoutfurther modification (Figure 6c).

As A/S conditions decrease, geomorphic elementsof channels and floodplain are increasingly cannibali-zed during channel migration and avulsion. As A/Sconditions decrease, progressively more of the leasteasily preserved geomorphic elements are cannibalizedand the stratigraphic record is enriched in those portionsof geomorphic elements which have the highestpreservation potential.

Under low A/S conditions, channel erosion removesthe upper parts of previously accumulated sediments(Figure 6d) since they are not carried beneath newbase levels through time. The migration of channelsand relatively high rate of avulsion with respect to therate of accommodation space increase produces late-rally amalgamated, interconnected channel sandstonebelts (Figure 6f). The deposits of a meandering channelconsist predominantly of cut-and-fill macroforms com-posed of amalgamated trough-cross bedded to massivesandstones, because only the geomorphic elements atthe base of the channel are preserved (Figure 6e).

Figure 7 illustrates the changes in channel architec-ture with variations in the A/S conditions trough time.This 220-feet-thick section of the Gala-7 well showstransitional change in channel sandstone facies andarchitecture in a series of small-scale base-level cycles

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within longer-term cycles. Note a parallel change inthe degree of amalgamation and relative proportion ofchannel sandstones from bottom to top. Also, faciesdiversity in channel sandstones increases upward. Thechannel sandstones toward the base of the cycle(Figure 7c) consist of scour and fill macroforms. Noevident trend in grain size is apparent and no floodplainmudstones are preserved. By contrast, the channel

sandstone toward the top of the cycle (Figure 7a) havevarious facies and grain sizes and it is associated withthick floodplain mudstones.

The parallel changes of facies successions anddiversity and channel sandstone architecture can beused to predict sandstone geometry and architectureby using stacking pattern analysis in core-calibratedwell logs. This is called stacking pattern analysis.

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Predicting position, continuity and areal extentof reservoir units within stratigraphic cycles

Stacking pattern analysis is a method for predictingsandstone geometry and position, facies tract continuityand stratigraphic architecture in core-calibrated welllogs. Genetic sequences containing shoreface and conti-nental strata provide an easily understood illustrationof stacking pattern analysis. In a series of geneticsequences containing shallowing-up shoreface faciessuccessions, the proportions of shallower and deeperfacies within one genetic sequence are compared withthe proportions of the same facies within the overlyingsequence. If the younger sequence has a greater pro-portion of shallower facies and a lesser proportion ofdeeper facies, the younger shoreface is displaced sea-ward with respect to the underlying unit (Figure 8 top).Conversely, if the proportion of deeper facies is greaterin the younger sequence than in the older sequence,then the younger shoreface is displaced landward com-pared with the underlying sequence (Figure 8 bottom).

The ability to predict the geographic position of thereservoir facies by stacking pattern analysis has directimpact on development strategy. In this example, ifthe upper-shoreface sandstone is oil saturated, the nextdevelopment well will be located seaward if the stac-king pattern is seaward stepping (Figure 8b). It will belocated landward if the stacking pattern is landwardstepping (Figure 8e).

As in shoreface strata, changes in vertical faciessuccessions and proportions can be used to predictposition, continuity and areal extent of fluvial reservoirsandstones. Stratigraphic attributes, such as degree ofsandstone connectivity, that influence fluid flow inreservoirs may be interpolated between and extra-polated beyond control points through analysis of large-scale stratal architecture and geometry and small-scalesedimentologic attributes. The direct relationship bet-ween small-scale sedimentologic (e.g. facies composi-tions and proportions) and large-scale stratigraphicattributes (e.g. areal extent and continuity) is very po-werful for prediction purposes.

In low A/S conditions the greater degree of canni-balism and amalgamation among individual storeys tendto form laterally amalgamated, interconnected, sheet-like sandstone bodies (Figure 9 middle section). Thesesandstones have low facies diversity, homoge-neousvertical grain-size profiles, and are composed predomi-

nantly of amalgamated trough-cross stratified sands-tone within large-scale scour and fill macroforms(Figures 6e, 6f).

In higher A/S regimes, individual storeys are preser-ved as isolated sandstone bodies which have reducedareal continuity and connectivity (Figure 9 upper andlower sections). These single-storey channel sandsto-nes are characterized by high facies diversity, completefining-up profiles of channel sandbodies, and usuallyoccur with floodplain deposits (Figures 6b, 6c).

Summarizing, a vertical trend of increasing faciesdiversity, grain size heterogeneity and channel sandsto-ne proportion indicate a decrease in the lateral continuityof channel sandstones. This allows prediction of reser-voir continuity and compartmentalization from analysisof well logs.

PETROPHYSICAL PROPERTIES

Previous workPetrophysical properties of sandstones have been

studied for years from sedimentologic and engineeringperspectives (Darcy, 1856; Muskat, 1937; Levorsen,1966; among others). These studies have concentratedon identifying relations among petrophysical properties,textural and fabric attributes, and facies.

Several studies have established relationshipsamong porosity, permeability and texture. Grain size,shape and sorting have a strong influence on porosityand permeability (Krumbein and Monk, 1942; Gaithor,1953; Rogers and Head, 1961; and Pryor, 1973).Theoretically, porosity is independent of grain size foruniformly packed spheres of uniform size (Rogers andHead, 1961). In practice, however, coarser sands tendto have higher porosity than finer sands (Sneider etal., 1977), probably reflecting variations in sorting andnonspherical grain shape. Permeability declines withdecreasing grain size because pore and throat diametersdecrease and capillary pressure increases (Krumbeinand Monk, 1942; Dodge et al., 1971). Porosity andpermeability increase with improved sorting (Fraser,1935; Rogers and Head, 1961; Dodge et al., 1971;Beard and Weyl, 1973; Pryor, 1973). Better sortedsands have less fine matrix which tends to fill and clogpores and pore throats, thus reducing porosity andpermeability. Data on the effect of grain shape (roun-dness and sphericity) are sparse. Fraser (1935) inferred

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that porosity might decrease with sphericity becausespherical grains may pack more closely than irregularlyshaped particles.

It is widely recognized that grain size and sortingvariations are closely associated with facies changes.Several studies showed regular variations of porosity

and permeability within successions of different facies(Chapin, 1991; Hurst, 1991; Jackson, et al., 1991; Ja-cobsen and Rendall, 1991). In a fining-up meanderingfacies succession, trough-cross stratified sandstoneshave the highest porosity and permeability values.Overlying small-scale trough-cross-stratified sands-

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tones, and ripple- and wispy-laminated sandstones haveprogressively reduced porosity and permeability values(Figure 10). Also, identical facies from different geo-morphic elements - for example ripple-laminated ortrough-cross-stratified sandstone facies from channelor crevasse splay sandstones - may have differentporosity and permeability. Generally, crevasse splaysandstones have lower porosity and permeability thanfluvial channel sandstones of the same facies.

Besides these known changes in porosity and per-meability values among different facies, porosity andpermeability values change regularly within identicalfacies as a function of stratigraphic position. Kusuma-negara (1994) and Fajardo (1995) showed that at thereservoir scale petrophysical attributes of fluvialchannel sandstones are stratigraphically sensitive, asare most other attributes of sedimentary facies.

Porosity and permeability variations of facies asa function of stratigraphic position

Changes in petrophysical properties with respectto stratigraphic position are parallel to changes indegree of preservation of identical facies depositedunder variable A/S conditions. Regular, stratigraphi-cally controlled changes in petrophysical properties andother physical sedimentologic attributes occur inconstant facies within a genetic sequence and fromone genetic sequence to the next.

In fluvial trough cross-stratified sandstones, porosityand permeability changes are exactly coincident withsubtle variations in sedimentary facies attributes whichchange as a function of stratigraphic position. In thecase of fluvial strata deposited during an increase inA/S conditions, porosity and permeability are highestin trough cross-stratified sandstones immediately above

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lism of previously deposited channel and floodplaindeposits, as well as decreased amalgamation of sedi-ments accumulating in the active channelbelt, result inincreased facies diversity, decreased channel sands-tone continuity, and better preservation of finer graincomponents (worst sorting). Because the upper partsof channels are better preserved, a higher proportionof lower energy ripple- and wispy-laminated sandstonesare preserved and thus the porosity and permeabilityprofiles would strongly decrease upward. This, in turn,results in more heterogeneous reservoirs. Also, in highA/S conditions, mud drapes are more common andmore continuous and have a higher chance offunctioning as fluid-flow barriers.

Figures 11 and 12 illustrate regular changes inporosity and permeability in several channel trough-cross-stratified sandstone reservoirs though 270� (82

channel scour bases, and decrease upward to the topof the sandstone. In channel sandstones composed ofcut-and-fill macroforms, porosity and permeabilitydecrease from the base to the top of each macroform,and generally from one macroform to the next in strati-graphic succession. Successive channel sandstoneswithin the same stratigraphic sequence and channelsandstones from one sequence to the next one haveprogressively lower porosity and permeability valuesin a long-term increase in A/S conditions (Figures 11and 12). These stratigraphically sensitive changes inporosity and permeability are coincident with reducedamalga-mation and cannibalism of the original dunesthat formed the trough cross-stratification, as reflectedby increasing set thickness of cross-stratification atthe macroform and channel belt scales.

At even higher A/S conditions, decreased canniba-

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m) of continuous cored section. There is a gradualincrease in abundance of channel sandstones from3.070' to 2.980'. This increase in abundance is accom-panied by a decrease in spacing between channelsandstones and a decrease in abundance of wetfloodplain and lake facies associations. From 2.980' to2800�, a reversal of this tendency is apparent from theconcomitant increase in crevasse splay and soil faciesassociations and the decreased thickness and frequen-cy of channel sandstone facies associations. The gra-dual changes in the facies associations and successionswithin successive small-scale cycles are paralleled bychanges in porosity and permeability values. In theinterval 3.070� to 2980� there is a reduced dispersionof porosity values within each channel unit. Whereasthe trough-cross stratified sandstones at 3.060�-3.052�show decreasing-up porosity values (25% to 13%),the trough-cross stratified sandstones in the 3010�-2960� interval exhibit less variability and no clear trendin porosity change. The mean value for the lowerchannel sandstone is about 20% whereas the meanvalue in the upper sandstone is about 25%. From

3.000�- 2.876� an opposite tendency is apparent. Anincrease in dispersion of porosity values and a decreasein the mean porosity in each channel sandstone isobserved (Figure 11). At lower A/S conditions (3.000�- 2.950�) the higher cannibalism among channel storeysremoved the upper parts of the channel sediments.

Permeabilities of trough-cross-stratified channelsandstones (Figure 12) show similar variations withrespect to stratigraphic position. From 3.060� to 2.970�there is a marked increase in the permeability rangesfor each channel unit, from 1 - 10 md to 36 -500 md.From 2.970' to 2.870�, this tendency is reversed andpermeability ranges decrease to 2 - 20 md range (Fi-gure 12). The greater reworking of sediments in themultistorey channels at lower A/S conditions causesbetter sorting and removal of the finer fractions. Bettersorted sands have larger and more homogeneous poresand pore throats and possess higher permeabilities. AsA/S conditions increase, progressively more of thedifferent facies and grain size populations originallypresent in the geomorphic environment are preservedin strata. This reduces the pore size and increases pore

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other environments also are stratigraphically sensitiveor stratigraphically controlled, and co-vary withphysical sedimentologic attributes. Figure 14 shows thevertical distribution of porosity, permeability and oilsaturation for an approximately 90 ft (30 m) coredsection of crevasse splay complexes from the La Cira-1880 well. This section was deposited during a long-term decrease in A/S, or base-level-fall hemi-cycle,as evidenced by the coarser, thicker, more amalga-mated character of the upper crevasse splay complex.Porosity, permeability and oil saturation of trough-crossand massive sandstone facies increase upward withineach coarsening-up crevasse-splay, as expected fromthe upward increase in grain size and sand content.Besides this �facies controlled� petrophysical proper-ties, there is an overall increase in porosity and permea-bility and oil saturation values toward the top of thecore, parallel to the decrease in A/S ratio.

These unidirectional changes in petrophysicalproperties of constant facies within stratigraphic cyclesare present in all wells analyzed. When identical facieswithin the same stratigraphic cycle are analyzed inseveral geographic locations the absolute values and

size heterogeneity, decreasing porosity and permeabilityvalues.

Figure 13 shows the changes in porosity and per-meability of stacked channel sandstones whichaccumulated in lower A/S conditions than those shownin Figures 11 and 12. This 90 ft (30 m) thick, sandstone-rich section shows parallel changes in preservedchannel sandbodies and petrophysical properties. Inthe lower and upper part of the cored interval channelsandstones are well preserved, exhibit conspicuousfining-up profiles, and are embedded in floodplain andlake mudstones. By contrast, the middle portion of thecored interval contains only amalgamated troughcross-stratified sandstones, probably within cut-and-fillmacroforms. The cored interval is interpreted as asymmetrical cycle, with the fall-to-rise turnaround(minimum A/S) in the middle part of the core. Meanporosity values are about 3% higher in the middle,lowest A/S section; permeability is one order ofmagnitude higher than in the upper and lower channelsandstones which accumulated in higher A/S condi-tions.

Petrophysical attributes of facies tracts representing

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ranges of these parameters vary laterally, but theincreasing or decreasing trend is the same in everylocation. Figure 15 shows histograms of porosityaverages of trough-cross-stratified channel sandstonestoward the base and the top of a long-term base-level-rise hemi-cycle. A1 and A2 sandstones are reservoirzones defined by the field engineers. Porosity valuesshown in Figure 15 are averages of all trough-cross-stratified channel sandstones within each zone.Regardless of the differences in porosity values andranges in every well a reduction in porosity is apparentin this base-level rise cycle (higher A/S conditionstoward the top).

CAPILLARY PRESSURE

Another property normally measured for reservoircharacterization is capillary pressure. Capillary pressu-re is the pressure difference across the interface bet-ween two immiscible fluids, one of which wets thesurface of a solid. Capillary pressure is illustrated bythe common physics experiment of introducing a thintube into water; the water rises in the tube above thewater level. The thinner the tube, the higher the waterrises. This phenomenon is the result of the water

wetting the tube. Water rises until the weight of thewater column balances the wetting force of surfacetension.

Capillary pressure in porous rocks depends onwettability, fluid saturations and pore and throat sizes.Capillary pressure increases as pore and throat diame-ters decrease, other conditions staying constant. Thesmaller the grain size and the higher the mud content,the higher the capillary pressure. Capillary pressuregenerally is proportional to permeability.

Air-water capillary pressure curves are measuredfrom core-plugs to estimate irreducible water saturationand reservoir quality. Jennings (1987) showed that theshape of capillary pressure curves can be used todetermine pore-size distribution and thickness of thetransition zone (between 100% water -to 100% oil-saturated rock) (Figure 16).

As with other reservoir rock properties, capillarypressure measurements normally are analyzed withrespect to facies or environment. An example of suchcomparisons between different facies of different envi-ronments is shown in Figure 17 from the Gala-7 well.Trough cross-stratified channel sandstones and ripplela-minated crevasse splay sandstones have very different

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capillary pressure curves, as expected from the diffe-rent environments and grain size distributions. Al-though crevasse-splay and channel sandstones havethe same source of sediment, crevasse-splay depositscontain more fine sand and mud. The higher mudcontent in crevasse-splay sandstones reduces pore andpore throat sizes, and increases the capillary pressure(Figure 17).

Besides these facies-dependent changes in capillarypressure, A/S conditions also control capillary pressurevalues of identical facies, similar to the changes obser-ved with porosity and permeability. In fluvial systems,more reworking and cannibalism during low A/Sconditions increases grain sorting, eliminates fines, andproduces more homogeneous pore throat and poredistributions, higher permeabilities and lower capillarypressures. At higher A/S conditions more geomorphicelements of channels and floodplains are preserved,increasing facies diversity and grain-size distributions.These cause wider porosity ranges, lower permea-bilities, and higher capillary pressures.

Figures 18 and 19 show capillary pressure curvesfor fluvial channel and crevasse splay sandstones

deposited in high and low A/S conditions. Both faciesassociations show an increase in capillary pressure anda decrease in reservoir quality with increasing A/Sconditions. Decrease in reservoir quality is moderatein the case of channel sandstones (Figure 18). Cre-vasse sandstones have a stronger change in capillarypressure. The drastic decrease in reservoir quality isthe result of a marked increase in the proportion of siltand clay in the crevasse sandstones. In lower A/Sconditions, reworking of crevasse deposits removefines and substantially increase permeability. Reducedreworking during high A/S conditions result in betterpreservation of original geomorphic elements and grainsize populations. Distal portions of crevasse complexesrich in clay and silt have much lower permeabilitiesand higher capillary pressures.

OIL PRODUCTION AND DRIVE MECHANISM

Changes in the areal extent, continuity and petro-physical properties of channel sandstones with respectto stratigraphic position influence oil production declinerate, ultimate oil recovery and efficiency of drive me-chanisms.

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Low accommodation channel sandstones havegreater lateral and vertical interconnection and aremore homogeneous. Detailed correlations within oilfields in the study area (e.g. Galan and Lisama) showthat they also occupy larger areas and have higher netsand content. Composite sandstone bodies can occupyareas 4*2 km. Associated mudstones are thinner anddiscontinuous. In combination, these attributes producelarger reservoir compartments and reduce the pro-duction decline rate (Figure 20). According to Daviesand Bernal (1995) these sandstones in the Lisama fieldcan have high cumulative production (0,5 to > 2MMBO) from individual wells.

High A/S channel sandstones have less lateral andvertical interconnection because they occur as isolatedsandstone bodies embedded in thicker and areallycontinuous floodplain mudstones. These fully preservedchannel sandstones are more heterogeneous and have

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CT&F - Ciencia, Tecnología y Futuro - Vol. 1 Núm. 3 Dic. 1997 39



lower sandstone-to-mudstone ratios. Correlations showthat these sandstones have a wide range of lateral conti-nuity. Composite sandstone bodies can be correlatedbetween 100s of meters apart up to 1,5 km. Reservoircompartments are smaller and oil production ratesdecline faster (Figure 20). Davies and Bernal (1995)reported low total cumulative production (< 0,25MMBO) from individual wells in the Lisama field.

Solution gas and water encroachment are the mostcommon drive mechanisms in the Middle MagdalenaValley fields. The relative importance and efficiencyof each mechanism change with stratigraphic positionand is dependent on reservoir size. Because channelsandstones deposited under low A/S conditions gene-rally are areally extensive, have a higher probability ofbeing connected to regional aquifers. Amalgamated,interconnected sandstone bodies in the northern portionof the Lisama field have water recharge. Some late-

rally-interconnected channel sandstones in the La Cirafield are water bearing and the water chemistry indi-cates meteoric origin (Dickey, 1992). By contrast,isolated, ribbon channel-belt sandstones of low A/Sconditions show that the solution gas is the main drivemechanism (Davies and Bernal, 1995).

SUMMARY AND IMPLICATIONS FORRESERVOIR CHARACTERIZATION ANDFLOW SIMULATION

Current engineering and geological approaches tocharacterize reservoirs and define fluid flow units(Amaefule, et al., 1994; Weber and Van Geuss, 1990;Jennings, 1987) utilize the perspective of facies andenvironments. Stratigraphic controls on variations infacies attributes, components and proportions, andtherefore reservoir properties, are not recognized.

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Reservoir heterogeneity is a very important factorin production. Of the seven types of reservoir hetero-geneities described by Weber (1986; Figure 21), threeor four change with A/S conditions. These are thegeometry and size of genetic units, permeability zona-tion and continuity of bafles within genetic units, anddiversity of sedimentary structures. Well data showthat within stratigraphic cycles tens to hundreds ofmeters thick, sandstone connectivity and compartmen-talization change dramatically as a funciton of A/Sconditions (Figure 22). Channel sandstones depositedin high A/S conditions tend to be isolated, more disconti-nuous and associated with thick floodplain and lakemudstones (Figure 22a, top and bottom). At lower A/S

conditions, amalgamation increases and interconnectionbetween channelbelt sandstones increases substantially(Figure 22a, middle). Such changes in stra-tigraphicarchitecture and reservoir properties should be utilizedin the design of simulation grids (Figure 22b). At higherA/S positions, cells must be smaller and thinner toaccount for the strong compartmentalization and abun-dant fluid-flow barriers. By contrast, at lower A/Sconditions cells may be larger due to the more homoge-neous character of the reservoir and the lower degreeof compartmentalization.

Smaller scale, internal heterogeneities such as per-meability zonation and baffle geometry and continuity(Figure 21 cand 21d) also change with changing A/S

41



conditions. Increased amalgamation of low A/Schannel sandstones reduces facies diversity and per-meability baffles, and generates more homogeneous

porosity and permeability distributions (Figure 23a).This causes less tortuous fluid-flow paths. High A/Schannel sandstones contain a larger proportion of origi-

42



nal geomorphic elements resulting in increased faciesdiversity and internal heterogeneity, poorer sorting, andincreased range in porosity and permeability (Figure23b). Fluid flow follows more tortuous paths, and oilsweep is less efficient due to high permeability contrastsand more abundant permeability baffles.

In summary, many attributes of strata important forreservoir production and management are strati-graphically sensitive. Variations in stratigraphic andsedimentologic attributes as a function of stratigraphicposition should be incorporated in reservoir assessmentand set up for flow simulations. When petrophysicaldata are plotted in a stratigraphic context, and not justwith respect to facies, porosity and permeability valueshave significantly reduced scatter. Regular changes inpetrophysical attributes with respect to stratigraphicposition can be incorporated into statistical analyses,such as stratigraphically changing variograms. Since

changes in petrophysical attributes are regular andpredictable, the cost of laboratory analysis can be redu-ced by a priori definition of the best places to take andmeasure samples. Prediction of laterally continuousversus isolated reservoirs and of reservoir heterogeneitywill allow to better design waterflood programs andreservoir simulation models.

CONCLUSIONS

· Development of oil and gas fields requires anaccurate geological description of reservoir charac-teristics, especially geometry, size, heterogeneity,and porosity and permeability distribution. This isachieved through an understanding of the depo-sitional environments and the A/S conditions duringsediment accumulation. These two factors com-bined control the shape, size, trend, lateral continuity

1

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and quality of sandstone bodies. The resultingreservoir geological model provides the optimumframework for reservoir subdivision, correlation andmapping, estimating reserves and defining simulationmodels and enhanced recovery programs.· Sedimentological and stratigraphical studies of

outcrop and more than 6.000 ft of core from threebig oil fields in the Middle Magdalena basindemonstrate parallel, unidirectional changes ingeometry, compartmentalization, and petrophysicalproperties (porosity, permeability and capillarypressure) of channel and crevasse sandstones.Channelbelt sandstones deposited under low A/Sconditions are laterally more continuous andinterconnected, forming bigger reservoir com-partments that have higher and more homogeneousporosity and permeability. They exhibit lowerproduction decline rates and their dominant drivemechanism is water encroachment. Isolated,laterally discontinuous channelbelt sandstonesdeposited under high A/S conditions form smallerreservoir compartments that have lower and widerrange of porosity and permeability. They exhibithigher decline oil production rates and lowercumulative hydrocarbon production. Solution gas isthe dominant drive mechanism.· These changes in reservoir properties within long-

term changes in A/S conditions are parallel tochanges in sedimentological and stratigraphicattributes that can be interpreted from cores andcalibrated well logs. Thus, prediction of lateralcontinuity and compartmentalization of channelsandstones and trends in petrophysical propertiescan be achieved from stratigraphic analysis of welllogs and cores.

ACKNOWLEDGMENTS

This research is part of Ramón�s Ph.D. project andwas supported by grants from Colfuturo, Ecopetrol,and CSM foundation. We received helpful suggestionsfrom Ramona Graves (CSM) and the editorialcommittee of CT&F. Johan Kusumanegara andAndres Fajardo�s results inspired this research. Thispaper benefited from their input. Special thanks toMaria Helena Mogollón and Claudia Malagón for their

help with petrophysical data. We are grateful toEcopetrol for access to data and permission to publish.

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