1.Evaluation of Austin and Buda Formations From Core and Fractured Anaylisis

7/27/2019 1.Evaluation of Austin and Buda Formations From Core and Fractured Anaylisis

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EVALUATION OF AUSTIN AND BUDA FORMATIONS FROM

CORE AND FRACTURE ANALYSIS

Richard H. Snyder and Milton Craft1

ABSTRACT

The Austin and Buda formations have been the target of active exploration due to the increased oil prices. These reservoirsare almost totally dependent on natural fracturing for productivity. Matrix permeability is normally less than 0.5 md. Thenatural fracturing is present throughout the vertical interval with fractured density ranging from one per foot to more than 25fractures per foot. These micro-fractures have widths ranging from 0.1mm to 0.4 mm. Oil saturations measured from routinecore analysis vary in the Austin and Buda formations from zero to 60 percent and are erratically distributed throughout theformation. Procedures for measuring fracture density, dip angle, and dip direction and criteria to distinguish between naturaland induced fractures have been formulated for over 7,000 ft of recovered and analyzed core from these formations. Resultsof these core and fracture analyses indicate the following criteria must be present to successfully complete an oilwell: (1) thefracture density must be in excess of one per foot; (2) residual oil saturation in the matrix must be in excess of10 percent; (3)there must be some indication of matrix permeability, normally 0.01 md. It is apparent that to have sustained production fromthe Austin and Buda, oil saturation must be present in the matrix as the fracture volume is extremely small and is rapidlydepleted. Relationship between fracture width, fracture block height, porosity, and permeability have been developed fromtheoretical calculations and appear to be confirmed by well performance. These calculations indicate that a fracture porosityof 0.1 to 0.25 percent is common throughout the Austin Chalk trend. These data also have been used to calibratefracture-finding logs and to assist in the development of completion programs.

INTRODUCTION

The increase in the price of oil combined with improved techniques for formation permeability stimulationhave led to the current interest in the Austin and Budatrend. Production is from very low matrix permeability limestone combined with a natural fracturesystem that varies in extensiveness and intensity. Experience established in the early days of exploration indicatednatural production usually declined at a very rapid rate,sometimes to a few barrels within a few hours. Currentactivity began with a successful completion in the AustinChalk by Southland Royalty near Pearsall in 1974. Thissuccess was followed by increased drilling activity in Frio

County. Exploration spread along the trend into Zavala,Dimmit, and Maverick Counties to the south and intoWilson County to the north. Recent favorable results inGonzales and Lee Counties have focused attention to thenorthern portion of the trend, and considerable activity isexpected in that area. An index map of the producing t rendis shown on figure 1.

STRATIGRAPHY

The Austin Chalk in the South Texas area consists oflight gray to buff, hard micritic limestone with an abundance of shale, both dispersed and in streaks and laminations. There are also occasional soft marls with limestonestreaks. Local abundance of pyrite, glauconite, and fossilfragments are present.

Insoluble residues often account for 30 percent of total

rock volume. The updip portion frequently has porosityfrom 25 to 28 percent, and oil-saturated intervals throughout the vertical chalk section are often continuous.

In the downdip chalk, the porosity range is from 3 to 9percent. The oil-saturated matrix zones in the deeperchalks occur in streaks, ranging in thickness up to 15 ft.

'Core Laboratories, Inc., Dallas, Texas

The matrix permeability in both intervals averages lessthan 0.1 md, but occasionally, samples with 1.0 md arenoted. The total thickness of the interval ranges from 300to 1,100 ft; the formation contains intervals of naturalfractures of variable frequency and fracture quality. Thefracture planes in most cases are vertical, and their heightapparently is controlled by streaks and laminations ofshale, stylolites, and soft limestone. The fractures have apreferential strike direction in individual well-bores. Bothopen and partially mineralized fractures are present; however, many of the fractures are tightly closed or completely mineralized. Fracture widths are normally small,less than 0.1 mm. Occasionally, fracture widths up to 20mm, partially to completely mineralized with calcite, have

been observed in association with fault zones. Photographs of typical fractured Austin formation are shown onfigures 2 through 5.

The Buda limestone consists of a light grey, very finetextured, very hard, dense, micritic limestone with manystylolites. The porosity is less than in the Austin Chalk,and there is less insoluble material than in the AustinChalk. The fracture system is normally more intense andshows less apparent order than does the Austin Chalkfracture system. Examples of this fractured reservoir areshown on figures 6 and 7.

Coring in the Austin Chalk and Buda has provided several opportunities to observe the Eagle Ford section. Theupper portion of the Eagle Ford "shale" often is 5 percent, or greater, limestone and siltstone and is often highly

fractured. An example of a fractured Eagle Ford core isshown on figure 8. The potential reservoir carbonates andsandstones also often have a highly oil-saturated matrix.The reservoir potential of this section should beevaluated. The Eagle Ford shale is a very petroliferousshale and probably is a source bed for much of the Budaand Austin Chalk hydrocarbon; however, the AustinChalk section contains sufficient petroliferous shale to bethe source for most, if not all, the hydrocarbon foundwithin it.

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378 TRANSACTIONSGULFCOAST ASSOCIATIONOFGEOLOGICAL SOCIETIES VolumeXXVII, 1977

Fractured reservoirs can be classified in many ways.Two broad systems of classification have become apparent in our studies of these types of reservoirs in many oilproducing provinces of the world. The first classificationsystem is based on the size of the fractures: mega-fractures and micro-fractures. Mega-fractured reservoirs

have predominant fracture widths of 6 mm; often thiswidth is enhanced by dissolution of the fracture planes.Reservoirs in this category include the giant Iranian foldedanticlines, such as Agha Jari, Gachsaran, and BibiHakimeh. Producing rates of individual wells are limitedonly by tubing size, which often is 7-in. casing. Rates inexcess of 100,000 BPD through 1 ft of perforated intervalhave been reported. Fracture solution in the Yates Field ofWest Texas has also resulted in spectacular producingrates. Many fields in Mexico, especially in the Reformatrend, have very high productivities due to numerous widefractures.

Micro-fractured reservoirs are defined as reservoirscontaining a vast majority of fractures 1.0 mm to 0.1 mm,

FIGURE 3. Fractured Austin Chalk core showing partiallymineralized fracture plane which acts as a propping agent.

the so-called hairline fractures often described in cores.The Spraberry Field in West Texas is a classic example ofthis type of fracturing (Hubbard and Willis 1955, pp 71).Other fractured reservoirs that can be included in thisclassification are the Danian/Maestrichten Chalks in theNorth Sea, M.I.S. Field in Iran, and the Monterrey

"shale" fields of California. The Rhourde El Baguel Fieldin the Algerian Sahara, a quartzite reservoir has fracturingof this type that significantly contributes to the productivity of the reservoir. In the measurements of fractures inthe Austin and Buda, the vast majority of the fractureshave been found to be 0.1 mm wide with an occasional 0.4mm wide fracture.

A second classification of fractured reservoirs can bemade based on the fluids contained within the matrixfracture block. Again, a two part classification can bemade: those fractured reservoirs with no oil saturationwithin the fracture block, and those with gas or oil saturation.

Examples of fields with no matrix saturation include the

FIGURE 4. Intensely fractured Austin Chalk core with numerous parallel hairline fractures



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SNYDER, CRAFT 379

FIGURE 5. Intensely fractured Austin Chalk core showing fractures terminating at horizontal stylolite

fractured quartzitic basement fields of Kansas andCalifornia, Jatibarang Field in Indonesia, and the AinZaleh Field in Iraq. These fields have produced or areproducing at significant rates indicating an adjacentsource of hydrocarbon sufficient to replenish the fracturevolume.

However, the majority of fractured reservoirs exhibitmatrix oil saturations, including the Austin Chalk andBuda fields. If a reservoir has a combination of bothmega-fracturing and oil-saturated matrix, the field is oftenin the giant class.

FRACTURE ANALYSIS

During the last two years, over 15,000 ft of fracturedAustin and Buda cores from 36 wells throughout the trendhave been examined. During this time over 7,000 ft of

Austin and Buda formations have been analyzed forroutine porosity, permeability, and fluid saturation in conjunction with analysis of the natural fracture system.

A successful fracture study must begin at the well site.Precautions must be taken to insure that the core is properly fitted together and marked with scribe lines so that thecore may be accurately laid out in the laboratory for fracture analysis.

The coring process and core recovery are critical factors in obtaining meaningful da ta for a fracture study.

Core recovery less than 100 percent always createsuncertainties. If less than 100 percent recovery is attained,

what is lost can often be the highly fractured section; andany fracture analysis would not include the most significant fractures. With new coring techniques and tools, utilizing orientation lugs, plastic liners, and rubber sleeves,recovery of fractured reservoir rock has improved.

If fracture orientation is required, then oriented coresare mandatory. Although costs are higher, in our experience, recovery is normally better. In several cases, ifshale, anhydrite, or other thin beds can be identified, bothon the dipmeter logs and cores, the orientation of thefractures can be related back to the orientation of thesebeds, and true azimuth of the fractures can be calculated.In the Austin Chalk and Buda formations, this technique,unfortunately, is not too reliable as the shales, especiallyin the Austin formation, exhibit sedimentary dip ratherthan structural dip.

In describing a fracture system, the first problem encountered is to separate the in situ, or natural fractures,from those induced by the coring process and the handlingof the core. The criteria shown in Table I have beendeveloped in our studies of fracturing. All of these criteriaare not applicable to the Austin and Buda fractures, and

all criteria are not necessarily noted in a given fracture.The criteria are listed in decreasing certainty, although notnecessarily equally weighted. Item 8 is a questionablecriterion in that the mud and lost circulation material can

FIGURE 6. Buda limestone core with numerous stylolites



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380 TRANSACTIONSGULF COAST ASSOC IATIONOFGEOLOGICAL SOCIETIES Volume XXVII, 1977

fill fractures caused by fracturing of the cores after theyare cut as overburden pressure is removed.

The induced fracture criteria, especially the "fresh"appearance, is, to a certain degree, intuitive, and thisfeeling is acquired after describing much fractured core ina given formation. The identification and tabulation of

induced fractures is important. The fact that a fracture didoccur in a particular plane is meaningful and can be ofinterest when designing stimulation programs.

Employing a goniometer (Fig. 10) dip angle and dipdirection on all measurable fractures are tabulated.Measurable fractures are defined as those fractures withsufficient length to pass through the core so that an accurate dip angle and direction can be measured. The lengthof each fracture is measured and a visual qualitative fracture analysis of each fracture is made. Special attention isplaced on mineralization on the fracture plane as this may

FIGURE 7. Buda limestone core, heavily fractured, with twofracture orientations

provide a propping agent in the reservoir. Semi-qualitativeestimates of permeability are made and related to fracturequality. Relationships of the fractures and fracture blocksto the lithology and rock hardness are noted along with thepresence of hydrocarbon in the fractures in the form of oilstain or bleeding. Stylolite amplitude and frequency are

measured.Routine conventional core analysis provides informa

tion on the matrix portion of the reservoir. Zones withmatrix oil saturation, porosity, and permeability are pinpointed. Whole core analysis can be useful in providinginformation on the total porosity of the sample and permeability along fracture planes. Typical matrix porositiesare in the range of4 to 10 percent in the Austin Chalk withresidual oil saturation ranging from 10 percent to 60 percent of pore space. Matrix permeabilities average lessthan 0.1 md, but in isolated cases may be as high as 1.0 md.Typical routine core analysis data are illustrated on figure11.

The Buda matrix porosities are normally less than 5percent with oil saturation comparable to the AustinChalk. Matrix permeabilities average less than 0.1 md,rarely exceeding 0.2 md. Whole core permeability alongfracture planes in excess of 2,000 md have been measuredin both formations. Figure 11 is an example of typicalAustin Chalk core analysis data. These data indicate thepresence of hydrocarbons and show potential zones ofmatrix storage capacity; however porosity data wouldlead to an interpretation of a non-productive zone. Tosuccessfully interpret these data , fracture information

FIGURE 8. Widely spaced, open fractured shale-limestone sequence in the Eagle Ford Formation



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SNYDER, CRAFT 381

TRATIGRAPHIC COLUMN

AUSTIN I BUDA TRENDS

SOUTH TEXAS

LITHOLOQY FRACTURING TYPICALPRODUCING FIELDS

SAN MIGUEL

ANACACHO

AUSTIN

EAGLE FORD

BUDA

DEL RIO

GEORGETOWN

EDWARDS

SOME

MODERATETO

INTENSE

INTENSE

MODERATE

RBARtALL

MLLIV A M *WVOT ARIA L IMDM>

COUNTYFRIO

FRIOFRIO

MA RICKOOMZALtlLf f

TABLE I

FIGURE 9. Stratigraphic column, Austin and Buda trend, southTexas, with typical producing fields

must be combined with these data to make a realisticinterpretation of the interval's potential productivity.

An example of the Core Fracture Log (Fig. 12) showsthe graphical presentation of data collected from the fracture study. The fracture density column tabulates all thenatural fractures noted inagiveninterval;i.e., measurableplus non-measurable fractures. Measurable fractures aredefined as those fractures with sufficient length to passthrough the core so that the dip angle and direction can bemeasured. This log combines saturation and porosity datain conjunction with fracture occurrence and density. Intervals of maximum completion potential are readily apparent, and the logs can be used to correlate other down-hole fracture logs.

Measurable fracture orientation is presented in twoformats: as frequency occurrence, and as fracture densityrose diagrams (Figs. 13 and 14). The purpose of these twodiagrams is to show the frequency at which a given dipdirection of fracturing occurs for all fractures and theorientation of the intensity of fracturing. The followingexample can clarify the difference. If three measurablefractures have a dip direction of N80E, then the frequency of occurrence for this direction would be 1,

CRITERIA FOR IDENTIFICATIONOF

NATURAL AND INDUCED FRACTURES

Natural

1. Crystal growth and mineralization on fractureplane

2. Slickensides3. Oil staining on fracture4. Asphaltic material5. Closed fracture grading

to open6. Stylolites grading into

fractures7. Fracture plane filled with

matrix material8. Drilling mud and lost cir

culation material

Induced

1. Sharpness/jagged"fresh" appearance

2. Preferred orientation3. Often very long4. Often vertical5. Concoidal in very har

well cemented rock

FIGURE 10. Goniometer measurements of strike and dip of

fractures in Austin Chalk core

whereas the fracture density would be plotted as 3. In anextreme case, fifty, 6- to 8-inch echelon fractures mayoccur in a few feet of the core, with the remainder of corehaving two to three fractures per foot with other azimuths.The diagram ofpreferred direction would be dominated bythese fractures which could be misleading, especially if aregional fracture trend was being sought. These diagramsthen reflect two important characterizations of the fracturing: preferred direction of fracturing and preferred intensity of fracturing.



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CORE FRACTURE LOG

CORE LABORATORIES, INC.

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_Ll_iJJ iiilLLl i i i i i l iFIGURE 12. Example ofcore fracture log from fractured Austin Chalk Formation



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384 TRANSACTIONSGULFCOASTASSOCIATIONOFGEOLOGICALSOCIETIES VolumeXXVII, 1977

UTILIZATION OF FRACTURE DATA

The major utilization of the fracture log, in addition toidentifying dry holes, is to assist in the design of a completion program for potentially productive wells. Other applications of the fracture data include reserve calculations, calibration and interpretation of various down-

hole logs, assistance in locating development wells,and development of fracture block dimensions for reservoir simulation models.

In the Austin Chalk and Buda trend non-productivewells are rare. Commercially productive wells are determined by evidence of sufficient recoverable oil. Analysisof fracture logs may establish the presence of sufficientfracture systems to provide satisfactory flow rate and toestimate reserves.

Satisfactory production rates have occurred from anindividual high quality fracture. Larger fractured intervalsare required when the fractures diminish in quality. It canreadily be seen that the quality and quantity of individualfractures are equally important to well productivity. Partially mineralized fractures, with visible permeability,porosi ty, and oil stain are illust rated on figures 3 , 5, 7, and

NON-OMKNTED

'io"

FREQUENCY OF OCCURRENCE

MEASURABLE FRACTURES

AUSTIN CHALK FORMATION

SOUTH TEXAS

can aiauTMiis, mccmmmmti cmsurmtctn

FIGURE 13. Rose diagram of frequency of occurrence of fractures, Austin Chalk Formation, south Texas

8. Partially mineralized fractures are considered to be ofthe highest quality as mineralization serves as a proppingagent. This is confirmed by whole core permeabilitymeasurements on cores with this type of fracturing. Wellswith such fractures often flow oil after only light acidtreatment.

Completion intervals can be rated in decreasing order asfollows:

1. The best intervals are those with high quality fractures

2. Fractured intervals with high matrix oil saturation3. Fracture intervals without matrix oil saturation4. Finally, non-fractured intervals with matrix oil sat

uration

Normally, all of these conditions plus intervals without oilsaturation or fracturing occur in all wells studied.

Experience gained through the interpretation of coreanalysis, combined with fracture analysis, has resulted inthe development of a set of petrophysical conditions thatcan be used semi-quantitatively to determine the anticipated productivity of the Austin Chalk and Buda reservoirs.

NON-ORE NTED

FRACTURE DENSITY

MEASURABLE FRACTURES

AUSTIN CHALK FORMATION

SOUTH TEXAS

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FIGURE 14. Rose diagram of fracture density, Austin ChalkFormation, south Texas



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SNYDER, CRAFT 385

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Documents

1.Evaluation of Austin and Buda Formations From Core and Fractured Anaylisis