sparta b sandstones (eocene), fordoche field, pointe

KRUTAK, KIMBRELL 383

SPARTA B SANDSTONES (EOCENE), FORDOCHE FIELD, POINTE COUPEE PARISH, LOUISIANA—A COMPARTMENTALIZED, BARRIER-ISLAND OIL

AND GAS RESERVOIR

Paul R. Krutak1 and W. Clay Kimbrell2

ABSTRACT

Fine-scale correlation of sixty-two well penetrations of Sparta B sandstones (Middle Eocene) in Fordoche Field, Pointe Coupee Parish, Louisiana, shows the unit is compartmentalized into an upper and lower sandstone by a thin "shale" body that is resolvable on standard (1"= 100 ft) E-logs. Sedimentary structures and trace fossils preserved in conventional core of this thin "shale" indicate that it consists of interbedded fine to medium-grained sandstones and carbonaceous mudstones deposited as storm washover lobes, and tidal-inlet channels associated with a barrier-island complex. The bulk of Sparta B production (estimated at ~5.3 MM STB OIP and 2.6 BCF GIP) is from upper shoreface and storm-washover sandstones whose pore spaces contain diagenetic chlorite, kaolinite, and calcite. Enhanced oil recovery (EOR) techniques involving SHF (sequential hydrofluoric acid stimulation) have eliminated declining production rates caused by these diagenetic minerals. Remaining in-place oil and gas is 4,195,960 STB and 1,812 MMCF respectively. Sparta mudstone samples bear a pyrolysis signature confirming a barrier-island depositional regime. Tmax values indicate Sparta rocks are immature to marginally mature in south-central Louisiana. Combination of reservoir engineering and petrophysical techniques permits resolution of extremely thin, unconventional oil and gas reservoirs that contain large reserves of "by-passed" hydrocarbons.

INTRODUCTION STRUCTURE

Fordoche Field, Pointe Coupee Parish, Louisiana (Fig. 1), discovered 12/31/48 by Texaco, contains at least ten hydrocarbon-bearing zones: Sparta A and B; Sparta "strays"; Wilcox 4, 5,8,10-12,15 (Fig. 2). Developed further by Sun Oil Co. (now Oryx) in 1966, the main field has 44 wells. Wildcat and extension wells drilled subsequently by Clayton W. Williams, Jr. and others bring this to a total of sixty two penetrations (Table I, Fig. 3).

Wilcox 8 and 12 sandstones contain the majority of Fordoche reserves (-25 and 15 MM STB respectively). However, major reserves of "by-passed" hydrocarbons (oil and gas left in place after conventional development drilling) probably exist in Sparta B sandstones which are estimated to contain -5.6 MM STB OIP and 2.9 BCF GIP. Drilling depths to these sandstones average 11,565 ft with average reservoir thicknesses of 15 ft. Sparta B reservoirs average 24.7% porosity and -200 Md permeability. Other average values include: 7980 PSI BHP (abnormal range), 228 degrees F BHT, 2075 PSIG dew-or bubble point pressure, gas-oil ratio 500 SCF/BBL. Oil gravities are 40-44 API (Table 2).

Basin Research Institute, Louisiana State University, Baton Rouge, Louisiana 70803-4101 Department of Petroleum Engineering/Louisiana Geological Survey, 3516 CEBA Building, Louisiana State University, Baton Rouge, Louisiana 70803-6417

Fordoche Field is a deep-seated rollover anticline bordered on its north side by a high-angle, down-to-the-south normal fault (Fig. 4—see also Pierson, 1970). The northern (updip) limit of the field is defined by this fault, which strikes east-west to southeast. The major fault bifurcates eastward. This fault, and its smaller southeastern splay partitions Sparta B sandstones of the field into three reservoirs, a major one north of the fault, a smaller closed one south of the fault, and a possible minor one between the bifurcated section on the east.

East-west and northeast-southwest structure sections across the field at the Sparta B level (Figs. 5 and 6) illustrate the nature of the three Sparta B reservoirs. The largest northern reservoir (wells 2-12, Fig. 5) is delimited by an oil-water contact at -11,401 ft. It is apparently sealed against the fault southward. The oil-water contact (Combariza R., 1990, p. 73) of the larger southern reservoir closes northward against the fault at -11,553 ft. It is probable the single dry hole (well 7, Figs. 5 and 6) in the isolated eastern fault-splay block encountered Sparta B sandstones at approximately the same elevation as the oil-water contact (-11,401 ft), resulting in a water-wet reservoir.

STRATIGRAPHY

Geologists with Oryx Energy (personal communication, Nancy DeWitt /l 1/2/89/) noted that the Sparta is abnormally pressured (7500-8000 psi) and seems to have several "layers" or compartments defined by permeability barriers. All Sparta reservoirs (Sparta A and B, Sparta "strays") appear to be water drive and have relatively homogeneous liquids and gases. The

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386 TRANS ACTIONS—GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES VOLUME XLI, 1991

Figure 1. Location map, Fordoche Field, Pointe Coupee Parish, Louisiana.

apparent layering suggested certain Sparta units might constitute compartmentalized reservoirs.

E-log correlation of sixty two well penetrations (Table 1) of Sparta B sandstones resulted in construction of a detailed fence diagram (Fig. 7). This indicates Sparta B sandstones of Fordoche Field are compartmentalized. Perforated zones in three producing Sparta B wells (Fig. 7: 3,6, 12) straddle a thin (4-7 ft) shale body. Sandstones above and below this shale originally produced. Perforated zones in two other wells (Fig. 7: 4, 5) are in a sandstone below the same shale unit. The sandstone above this shale produced Sparta B oil and gas in another well (Fig. 7: 20).

This thin shale body compartmentalizes the Sparta B into an upper and lower sandstone. Three wells produced from the upper sandstone (IP -205-390 BOPD, 92-172 MCFGPD, 0-80 BWPD). Two wells produced from the lower sandstone (IP -283^126 BOPD, 119-171 MCFGPD, 415 BWPD). The thin shale unit in the upper part of the Sparta B sandstone appears to provide a seal for the top of the lower Sparta B sandstone, particularly in updip wells in the northern fault block of Fordoche Field. Upper Sparta B sandstones are sealed by overlying shales enclosing Sparta A sandstones. Similar thin shales compartmentalize the Sparta B in the southern fault block of Fordoche Field; however, Sparta B sandstones are apparently water-wet below ~11,536 ft (Fig. 4).

Table 2: FORDOCHE FIELD* Basic Reservoir and Fluid Data

Reservoir/Fluid Properties Reservoir Sparta A Sparta B

Reservoir Type Oil Oil Average Depth, TOS, Ft 11 ,300 11,565 Initial BHP, PSIG 8,370 7,980 Principal Drive Mechanism Water- Water-

drive drive Initial GOR, SCF/BBL 500-2200 500 Porosity, % 24.7 24.7 Initial Water Saturation, % 45 48 Average Permeability, Md -200 -200 Average net pay, Ft 21 15 BHT, degrees F 226 228 Bubble Point Pressure, PSIG 2450 2075 Initial FVF BO, RB/STB 1.38 1.28 Initial FVF, BG, RB/MCF .521 .521 Oil Gravity, °API 40-53.5 40-44 Sep. Gas Gravity .62 .62 Tertiary Recovery Project No No Original OIP, MMSTB 3.5 5.3 Original GIP, BCF 1.8 2.6 Est. Recovery, %OOIP 60 50 Type Completions Natural Natural Total # of Completions 6 5

*Modified from Oryx Energy Company, Richardson, Tex-as, 11/2/89.

Table 2. Basic reservoir and fluid Data, Sparta A and B sandstones, Fordoche Field, Pointe Coupee Parish,

Louisiana (modified from Charles Mayne, Oryx Energy, 11/2/89).

A gross sandstone isopach map of the Sparta B (Fig. 8) indicates a sand body aligned northwest-southeast. It thins southeastward. A local thick occurs in the center of strati-graphic section C-C (Fig. 9) and extends northward where maximum thicknesses (more than 100 ft) are attained. Sparta B sandstone percentages (Fig. 10) behave similarly. Curiously, lower percentages (70-80) are on the "top" of the sand trend (southeastern portion of map, Fig. 10) with higher values (100) on its flanks (Fig. 11). Sand/Shale ratios (Fig. 12) reflect this also, with infinity values (100% sand) displaced outward on the flanks of the field (Fig. 12).

PALEOENVIRONMENTAL INTERPRETATION

Lemoine et al. (1988, p. 21: 1989a, p. 379) showed that Sparta production is associated with Eocene sandstone barrier-island complexes. They demonstrated that porosity and permeability trends could be predicted if detailed E-log, core


TYPICAL WELL LOG FORDOCHE FIELD, POINTE COUPEE PARISH, LA.

SUN OIL CO. J. W. Andrews W-8 RASUA #1

Sec. 32., T6S. R8E T.D. 14,051'

11,300

11,550

12,770

12,980

13,180 13,260 13,400

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13,860

RESERVOIR AVERAGE NET SAND

SPARTA A

"w~1?>"HIGH PRESSURE WATER SANDS - ^ 34*

W-12 OIL

LAYER 1 LMIRJ LAYER

Figure 2. Typical well log, Fordoche Field, Pointe Coupee Parish, Louisiana. Note position and average thickness of Sparta B reservoir sandstones (modified from Eckles et al., 1981, Fig. 2, p. 972). Well log corresponds to #17, Fig. 3, Table 1.


Figure 3. Well base map, Fordoche Field, Pointe Coupee Parish, Louisiana. See Table 1 for operators, total depths, Sparta B stratigraphic details and production data for coded wells (1-62). Figs. 5 and 6 illustrate Sparta B structure sections along lines A-A', B-B'. Figs. 9 and 11 portray Sparta B stratigraphic sections along lines C-C, D-D'.

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IP 426 BOPD, 171 MCFGPD, 4 BWPD

C.W.W.. JR. J.GREEN«.aL*l

IP 205 BOPD. 92 MCFGPD. 80 BWPD

C.W.W. JR. C H A l M N HEIRS # 1

IP IBS BOPD, 119 MCFGPD, 5 BWPD

O SPARTA '«• OIL WELL AND CODE NUMBER

\ " 0 S P A B T A "*" O I L * GAS WELL AND CODE NUMBER

INITIAL PRODUCTION BARRELS OIL PER DAY THOUSAND CUBIC FEET GAS PER DAY BARRELS WATER PER DAY. BARRELS CONDENSATE PER DAY.

Fordochc Field Point Coupee P i r U h

Loot l iana

Figure 7. Fence diagram, Sparta B sandstones, Fordoche Field, Pointe Coupee Parish, Louisiana. See Fig. 3 for coded wells and section, township, and range locations. Table 1 lists details of Sparta B stratigraphy upon which this diagram is based.

description, and petrographic data were available. These trends depend on the geometry, thickness variability, petro-physical properties and lateral development of the various fluid flow units.

The gross sandstone isopach map (Fig. 8) suggests a northwest-southeast aligned barrier-island sand body (Fig. 13) with a "pass" (flood-tide delta?) developed across its northwestern end. The lower sandstone percentages on the "top" of this barrier and the higher flank values (Figs. 10, 11) suggest strips reflecting the muddier lagoonal side of a barrier island complex.

ORGANIC GEOCHEMISTRY

Programmed pyrolysis of Sparta mudstone samples (Chinn et al., 1988a,b) demonstrated that depositional environments associated with a barrier-island complex are recognizable within the Sparta Formation.

More recently, Chinn et al. (1991) presented pyrolysis results of 150 representative Sparta source rock analyses. These analyses average 2.19% total organic carbon (TOC) and range up to 8.14% TOC in south-central Louisiana (Sassen, 1990). Rock-Eval analyses indicate that Sparta hydrogen index values average 233 mg HC/g OC, with oxygen index values being moderate (average 31 mg CO^g OC. These values indicate Sparta shales contain oil and gas generative Type II kerogen, a fact substantiated by visual kerogen assessment (Sassen, 1990).

Sparta kerogens are amorphous and are derived from terrestrial sources (Chinn et al., 1991). Atypically, Sparta kerogens fluoresce under ultraviolet light. Gregory et al. (1991, in press) showed that fluorescence intensity correlates positively with hydrogen index. The intensity and proportion of fluorescing organic particles in a sample increases as hydrogen index rises.

Sparta Tmax values average 433 degrees C. This indicates that the Sparta Formation is immature to marginally mature in


2000 4000 " F T 1000 2000

M

ISOPACH, SPARTA "B" FORDOCHE FIELD, POINTE COUPEE

PARISH, LOUISIANA ISOPACH INTERVAL 10' ti&& >100'

Ao oA' STRUCTURE SECTION A-A" Bo oB' STRUCTURE SECTION B-B' C o o C . STRATIGRAPHIC SECTION C-C* D ° °D' STRATIGRAPHIC SECTION D-D'

61 D

f \) FORDOCHE FIELD

INDEX MAP SOUTHERN U.S.

- -..".to to

—So -60

Figure 8. Sparta B isopach map, Fordoche Field, Pointe Coupee Parish, Louisiana. See Table 1 for well codes and raw data. Figs. 5 and 6 are structure sections A-A', B-B'. Figs. 9 and 11 depict stratigraphic sections C-C, D-D'

south-central Louisiana where it is normally buried to 11,132-11,660 ft (3,393-3,554 m) (Chinn et al., 1991).

PETROPHYSICS

The lithology and sedimentary structures of conventional core cut from 11,330-11,380 ft in the lower part of the Sparta B producing interval in the Clayton W. Williams, Jr., Seymour Dreyfus #1 (Well 12, Figs. 3 and 7, respectively), indicate this section consists of upper shoreface sandstones capped by storm washover sandstones and lagoonal mudstones. Down-core, the remaining lithofacies are lower shoreface and inner shelf sandstones (Lemoine et al., 1988; Lemoine, 1989b, Fig. 12, p. 36).

The perforated interval in this well (Fig. 14, 11,307-11,346 ft) produced from upper shoreface and storm washover sandstones and lagoonal mudstones. Petrographic, SEM (scanning electron microscopy), and EDX (energy dispersive X-ray) study of samples from these intervals (Fig. 14, samples 2-11) reveals the following down-core trends.

High porosity (20-25%), high permeability (250-500+ Md), fine to very fine-grained quartz arenites whose pore throats average about 10 microns make up the storm washover-la-goonal mudstone petrofacies. Detrital glauconite is abundant; detrital chert is less common; potassium feldspars and sodic plagioclases are rare. Organic lamellae and kerogen clots occur. Cements (quartz overgrowths, calcite) are poorly developed. Samples 2-8 (11,331.54-11,338.25 ft) are in the lower

KRUTAK, KIMBRELL

study of samples from these intervals (Fig. 14, samples 2-11) reveals the following down-core trends.

High porosity (20-25%), high permeability (250-500+ Md), fine to very fine-grained quartz arenites whose pore throats average about 10 microns make up the storm washover-la-goonal mudstone petrofacies. Detrital glauconite is abundant; detrital chert is less common; potassium feldspars and sodic plagioclases are rare. Organic lamellae and kerogen clots occur. Cements (quartz overgrowths, calcite) are poorly developed. Samples 2-8 (11,331.54-11,338.25 ft) are in the lower part of the perforated interval. Fig. 15 illustrates a sample with the highest porosity and permeability within this petrofacies.

STRATIGRAPHIC SECTIONS C - C

Datums - Top Sparta "B" (Eocene)

FT 50-

-100--150-

% - 50-

c NE

M *• #

4* =v: sw

Thicknesses in Feet

NE .E B ^

FT - 50--100-•150-

% • 50-•100-

Sand Percentages

SS/SH RATIOS Q

: I: NE iz:

SS/SH ry RATIOS

sw : \:

'4* SS/SH Ratios

Geometric Progression Interval = 2 FT 200

100

0 2000

1000

4000 FT

2000 M

VE = 10x*

*FT - M scale & VE applies to top section only

Figure 9. Stratigraphic sections C-C, Sparta B, Pointe Coupee Parish, Louisiana. See Figs. 3 and 8 for line of section.

393

Vermicular chlorite occludes porosity in sample 8 (11,338.25 ft). Fig. 16 shows the occlusion effect this has on porosity and permeability. Initial production (12/5/86) in this interval was 395 BOPD, 79 BWPD. Later production (4/1/89) was 140 BOPD, 130 BWPD.

An interval of very fine-grained quartz arenites with varying porosity (3.3-21.8%) and permeability (0.01^*2.00 Md) characterizes the upper shoreface sandstone petrofacies. Pore throats range from 10-20 microns (Fig. 17). Porosity and permeability drop substantially in this interval (sample 11 has only 0.01 Md permeability, 3.3% porosity). Samples 9 and 10 (11,342.21 ft, 11,344.79 ft) are in the lowest segment of the perforated interval. Kaolinite occludes porosity in these samples. Chlorite reduces porosity in sample 11 (11,346.00 ft). Below the base of the perforated interval, calcite completely cements the porosity (sample 12, 11,347.50 ft—Fig. 18). Porosity and permeability increase again in sample 13(11,350.46 ft). Reddish bitumen? occurs in patches; EDX (energy dispersive X-ray) indicates that quartz grains are coated with titanium and vanadium-rich clays.

The "shale" interval present in the Clayton W. Williams, Jr. Seymour Dreyfus #1 well (Fig. 7, Well 12) includes the storm washover sandstone and lagoonal mudstone petrofacies of Fig. 16. This petrofacies is correlative across the northern tier of wells in Fordoche Field (Fig. 7, Wells 2-6; 12; 48-51). Presumably, petrographic details in these wells are similar to those observed above in the Seymour-Dreyfus #1 well.

The existence of diagenetic clays (chlorite, kaolinite) and carbonates (calcite) within Sparta B sandstones of Fordoche Field presents completion and stimulation problems. Chlorite contains significant amounts of iron and magnesium and is extremely sensitive to acid and to oxygenated waters. It dissolves easily in dilute HCL and the iron liberated during dissolution can reprecipitate as ferric hydroxide crystals larger than existing pore throats (Davies and Associates, personal communication). Kaolinite is loosely attached to host grains and can easily dislodge in areas of high fluid turbulence. Carbonate minerals plug porosity and can almost obliterate permeability.


7 PERCENT SANDSTONE, SPARTA "B" $ FORDOCHE FIELD, POINTE COUPEE PARISH, LOUISIANA

2000 4000

FT

1000 2000

Ao-Bo-C°-Dc-

CONTOUR INTERVAL 10%

-oA' STRUCTURE SECTION A-A' -oB ' STRUCTURE SECTION B-B' - ° C STRATIGRAPHIC SECTION C-C - ° D ' STRATIGRAPHIC SECTION D-D'

-90

H/JLf FORDOCHE FIELD

INDEX MAP SOUTHERN U.S.

Figure 10. Percent sandstone map, Sparta B, Fordoche Field, Pointe Coupee Parish, Louisiana. See Table 1 for well codes and raw data. Figs. 5 and 6 are structure sections A-A', B-B'. Figs. 9 and 11 depict stratigraphic sections C-C, D-D'.


<?

, STRATIGRAPHIC / SECTIONS D - D'

^ Datums - Top Sparta "B" (Eocene) , < < / *fr

#>

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D NW

^ T ^ —-« & '

FT

f ^ S E ,S 150-

Thicknessas in Feet

.£• Nwr 100 H—

_"U- I iD' %

[ _ J S E M

SS/SH RATIOS D

:f: NW

Rand Percentages

lA/ SS/SH Ratios

Geometric Progression Interval = 2

FT 200-, M .50

V y r\

100-

SS/SH n , RATIOS U . 2 .

S E - 4 -

V 32

100

0 25

2000 4000 FT

2000 M 1000

VE = 70 x* 'FT - M scale & VE applies to top section only

Figure 11. Stratigraphic sections D-D', Sparta B, Fordoche Field, Pointe Coupee Parish, Louisiana. See Figs. 3 and 10 for line of section.

Table 3: FORDOCRE FIELD* Sparta B Production Before and After SHF+ Stimulation

Before After Increase Well Outcome Reservoir (BOPD) (BOPD) (Factor)

J. Blanchard 4 Success Sparta B 20 122 6.1 (Code #11)

N. Smith, Jr. 4 Success Sparta B 31 214 6.5 (Code #20) 12.6

Average = 6.3

Before After Increase (BWPD) (BWPD) (Factor)

J. Blanchard 4 Success Sparta B 50 212 4.2 (Code #11)

N. Smith, Jr. 4 Success Sparta B 2 46 23.0 (Code #20) 27.2

Average = 13.6

*Coded wells are Sun (Oryx Energy) wells. Data from Table 4 in Holden et al., 1981, p. 1488. See Table 1 for exact location and initial production data on coded wells.

+SHF = sequential hydrofluoric acid stimulation treatment.

Table 3. Production before and after well stimulation, Sparta B reservoir, Fordoche Field, Pointe Coupee Parish, Louisiana (data from Holden et al., 1981, p. 1488). See Table 1 for exact location and initial production data on coded wells.


Figure 12. Sand/Shaie ratios, Sparta B, Fordoche Field, Pointe Coupee Parish, Louisiana. See Table 1 for well codes and raw data. Figs. 5 and 6 are structure sections A-A', B-B'. Figs. 9 and 11 depict stratigraphic sections C-C, D-D'.

RESERVOIR STIMULATION

Oryx engineers have apparently solved some of these Sparta B production and stimulation problems by utilizing innovative EOR techniques, these techniques have been extensively employed on both Wilcox and Sparta reservoirs in Fordoche Field. Wilcox sandstones are very water sensitive. Their primary drive mechanism is fluid expansion and solution gas (Holden et al, 1981, Table 1, p. 1486). Sparta reservoir rocks are much less water sensitive and have strong water drives. Sparta fluid production declines slowly because of bottom hole pressure loss, increasing water cuts, formation damage, and diagenetic mineral reactions. Plugging in both Wilcox and Sparta reservoirs limits production and necessitates frequent stimulation to restore wells to commercial production (Holden etal., 1981, p. 1485).

Sun Gas Co. and Halliburton petroleum engineers developed a SHF stimulation technique to maximize recovery from Fordoche Field wells. Although most of their efforts involved Wilcox reservoirs, the Sparta B reservoir in two wells was also involved (Holden et al., 1981, Table 4, p. 1488).

The treatment (detailed in Holden et al. , 1981, p. 1487) consists of adding HC1, NH4F, diesel oil or ammonium chloride water as well as a water-wetting nonemulsifying agent. An organic polymer is also added to the SHF treatment fluids to obtain clay and silica stabilization. The addition of the polymer allows deep clay dissolution with the SHF, and clay and silica stabilization in that part of the formation where inadequate dissolution occurs. Polymer usage results in higher production rates for longer periods after well stimulation.


Block Diagram of Sparta Depositional Environments Fordoche Field, Pointe Coupee Parish, Louisiana'

Figure 13. Barrier island depositional model for Sparta B sandstones, Clayton W. Williams, Jr., Seymour: Dreyfus #1, Fordoche Field, Pointe Coupee Parish, Louisiana. Note position of Seymour Dreyfus #1 well analogue (star) and two other potential sand-rich sites (dotted circles). See Figs. 7 (code #12) and 14 for stratigraphic details.

Figure 14. Interpreted petrofacies and position of Sparta B samples and thin sections (#s 2-20) from Clayton W. Williams, Jr., Seymour Dreyfus #1 well conventional core, Sec. 31, T6S, R8E, Pointe Coupee Parish, Louisiana (well code #12, Fig. 3, Table 1).

Table 3 is a summary of Sparta B well production rates before and after SHF stimulation in Sun's J. Blanchard #4 and N. Smith, Jr. #4 (code numbers 11, 20, Table 1). Oil rates (per day) increased an average of 6.3-fold, water rates 13.6-fold.

RESERVOIR DESCRIPTION AND PRODUCTION ANALYSIS

Using data obtained from volumetric calculations and decline curve observations, a reservoir description and production analysis was performed on Sparta B sandstones (reservoir B) in the larger southern reservoir (Fig. 19).

Four Oryx (Sun) wells perforated the Sparta B reservoir: the J. Blanchard #4, the N. Smith #7D, the N. Smith #9, and the N. Smith #10 (code numbers 11, 13, 22 and 34, Table 1). At present, only the J. Blanchard #4 well is producing from this reservoir. The N. Smith #10 produced the reservoir for only two months and was recompleted in a Sparta stray sand. The N. Smith #9 well was recompleted into the Sparta A sand June, 1990. The N. Smith #4 was shut-in January, 1986. Oryx originally put the N. Smith #4 well into a discrete, separate Sparta B reservoir interpreted to be bounded by a permeability barrier. However, for the purpose of this paper, it is treated as part of a four well reservoir.

Fig. 20 indicates that by May, 1990, cumulative production bad reached 1,089,830 STB of oil and 830.899 MCF of gas. Additionally, 2,785,314 BBLS of water had been produced by May, 1989. Historically, the oil production has displayed a 20% decline, which includes the SHF stimulation previously mentioned. Water production has leveled to approximately 20,000 bbls per month over the last 9 years (Fig. 21). Gas production has approximated 2.5 MMCF per month since late 1979. Recently a large increase occurred in gas production.

The cumulative gas-oil ratio has risen from an average 500 SCF/STB during its early life to 762 SCF/STB at present (Fig. 22). The average monthly gas-oil ratio has risen from approximately 500 SCF/STB during the first 7 years of life to the 103,875 SCF/STB noted during May, 1990 (Fig. 23).

The volumetrics of the reservoir were based on planimeter values from the net pay isopach map (Fig. 19). Using these values in Simpson's Rule equation (Salvadori and Baron, 1963, p. 93):

V = h/3( AO + 4A1 + 2A2 + 4A3 +... + 4An-l + An) + havgAn

where v = bulk volume, acre-feet

h = contour interval, feet

A = successive areas enclosed by isopach lines, acres

havg = average thickness above maximum thickness isopach line, feet

the initial bulk volume of the reservoir was 6790 acre-feet. Substituting this value in the following equation (Craft and Hawkins, 1*959, p. 106):

THIN SECTIONS (2 ETC.) & PETROFACIES

11.307

n11.330'

CLAYTON WILLIAMS SEYMOUR DREYFUS #1

SEC.31,T6S,R8E PERF:11,307,-11,346' CORE: 11,330'-11,380'

IP 395 BOPD, 79 BWPD (12-5-86) 140 BOPD, 130 BWPD (4-1-89)

SPARTA B (EOCENE) FORDOCHE FIELD

POINTE COUPEE PARISH, LA

INTERSEDDED FINE TO MEDIUM-GRAINED SANDSTONE AND CARBONACEOUS MUDSTONE

MASSIVE-APPEARING AND BURROWED FINE-GRAINED SANDSTONE

BKDTURBATED VERY FINE TO FINE-GRAINED SANDSTONE

INTERBEDDED BURROWED VERY FINE-GRAINED SANDSTONE AND LAMINATED MUDSTONE

11,380'

I m

I

398 TRANSACTIONS—GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES VOLUME XLI, 1991

Figure 15. Sparta B petrographic and SEM (scanning electron micrograph) details of sample 5 (11,334.71 ft—storm washover sandstone-lagoonal mudstone petrofacies) from Clayton W. Williams, Jr. , Seymour Dreyfus #1 well conventional core. Sample represents highest measured porosity (25. 2%)-permeability (696 Md) values in this petrofacies. See Fig. 14 for location of sample. Petrographic photomicrographs above, SEM micrographs below. PL = plane light; PP = plane-polarized light; Q = quarts; DF = dissolution feldspar (probably albite); P = porosity; PT = pore throats.

OOIP = (7758 * P * (1 - Sw) * V) / Boi

where OOIP = original oil in place, stock tank barrels (STB)

P = porosity, fraction

Sw = water saturation, fraction

V = bulk volume, acre-feet

Boi = formation volume factor, reservoir barrel/stock tank barrel

the original oil in place was 5,285,790 STB. Using the initial COR of 500 SCF/STB, as given by Oryx, the original solution gas in place is 2,642,895 MCF.

Based upon correlations, an empirical equation (American Petroleum Institute, 1967, p. 19):

BAF = 4259 * (P*(l-Sw)/Boi)+1.0422 * (k*vwi/voi) + 0.0770

* (Sw) -0.1903 * (pi/pa)-0.2159

where BAF = STB of oil per acre-foot

4259 = e+8.3569

P = porosity, fraction

Sw = water saturation, fraction

Boi = formation volume factor of oil, reservoir bbls/ STB

k = permeability, millidarcies

vwi = viscosity of interstitial water, centipoises

voi = viscosity of initial oil, centipoises

pi = initial reservoir pressure, psig

pa = abandonment pressure, psig (assume 500 psig)

may be used to arrive at an expected recovery of 221 STB/acre-foot. The multiple coefficient for this equation is 0.958 and the standard error of estimate is 17.6% (the standard error of estimate applies only at the mean of distribution). Using the bulk volume value of 6790 acre-feet and a GOR of 762 SCF/


Figure 16. Sparta B petrographic and SEM (scanning electron micrograph) details of sample 8 (11,338.25 ft-near base of storm washover sandstone-iagoonal mudstone petrofacies) from Clayton W. Williams, Jr., Seymour Dreyfus #1 well conventional core. Sample displays lower measured porosity (24.3 %)—permeability (463 Md) values than sample 5 (Fig. 15). See Fig. 14 for location of sample. Petrographic photomicrographs above, SEM micrographs below. PL = plane light; PP=plane-polarized light; Q = quartz; CH=chlorite; P=porosity; PT = pore throats. Presence of Fe in SEM chlorite micrographs confirmed by EDX (energy dispersive x-ray) analysis.

STB, the total oil reserve is 1,500,000 STB of oil and 1,143 MMCF of gas.

face, storm washover, and tidal-inlet channel sandstone litho-somes.

Based upon its decline curve (Fig. 21) the reservoir is very near, if not at the end of its primary life. The J. Blanchard #4 well is averaging 8-10 BOPD, which should be close to the economic limit for that well. An accelerated blowdown of the remaining recoverable gas from this well is evident from the last few reported months of production. Therefore, the primary recovery of the reservoir will be 20.6% of the in-place oil, 31.4% of the in-place gas, and 72.6% of the originally estimated primary oil and gas reserve. These observations alone indicate a large amount of "by-passed" unrecovered hydrocarbons. Based upon the above values, remaining in-place oil and gas is 4,195,960 STB and 1,812 MMCF, respectively.

SUMMARY

Sparta B sandstones in south-central Louisiana were deposited as a prograding barrier-island complex. Four sedimentary facies have reservoir potential: lower shoreface, upper shore-

Detailed correlation of 62 Sparta B well penetrations in Fordoche Field, Pointe Coupee Parish, Louisiana indicates the Sparta B is compartmentalized into an upper and lower sandstone by a thin "shale" body that is resolvable on conventional E-logs (scale 1"= 100'). Conventional core of this thin "shale" indicates that it consists of interbedded fine to medium-grained sandstones and carbonaceous mudstones deposited as storm washover lobes, and tidal-inlet channels.

The bulk of Sparta production in Fordoche Field comes from Sparta B sandstones deposited in upper shoreface and storm-washover environments. Diagenetic clays (vermicular chlorite and kaolinite) occlude porosity in both storm-washover and upper shoreface sandstone reservoirs. Secondary calcite may completely occlude porosity in upper shoreface sandstones.

Sparta mudstone samples carry a pyrolysis signature indicative of barrier-island depositional environments. Total organic carbon and Rock-Eval analyses show that Sparta shales contain oil- and gas-generative Type II kerogen that atypically


M« Figure 17. Sparta B petrographic and SEM (scanning electron micrograph) details of sample 10 (11,344.79 ft—upper part of upper sho-reface sandstone petrofacies) from Clayton W. Williams, Jr., Seymour Dreyfus#1 well conventional core. Sample has relatively high porosity (21.3 %) but very low permeability (7.1 Md). See Fig. 14 for location of sample. Petrographic photomicrographs above, SEM micrographs below. PL=planelight;PP=plane-polarizedlight;Q=quartz;HV=heavy mineral; AL=albiteplagioclase;CH=chert;HA=kao-linite; P = porosity; PT pore throats. EDX (energy dispersive X-ray) confirms concentration of Si and Al and lack of Fe in SEM kaolinite micrographs.

fluoresces under ultraviolet light. Tmax values indicate Sparta rocks are immature to marginally mature in south-central Louisiana.

Enhanced oil recovery (EOR) techniques involving sequential hydrofluoric acid stimulation (SHF) have eliminated problems associated with diagenetic chlorite, kaolinite, and calcite in Sparta B sandstone reservoirs of Fordoche Field. These innovative EOR procedures increased Sparta B oil production by a factor of 6.3-fold.

Volumetric calculations and decline curve analyses indicate that the Sparta B reservoir in Fordoche Field contains 5,285,790 STB of original oil in place (OOIP) and 2,642,895 MCF of original solution gas in place (OGIP). The Sparta B reservoir contains -remaining reserves of 4,195,9609 STBO and 1,812 MMCFG.

Fine-scale correlation of conventional E-logs (1"-100'), combined with conventional core analysis, reservoir engineer

ing and petrophysical (petrographic-SEM-EDK) studies, permits resolution of extremely thin, unconventional oil and gas reservoirs that contain large reserves of "by-passed" hydrocarbons.

ACKNOWLEDGMENTS

Funding for the work leading to this paper came from several sources: Louisiana Education Quality Support funds (8g monies), Dept. of Energy funds, and matching funds from L.S.U. (Principal Investigator, Dr. Adam T. Bourgoyne, Jr., Dept. of Petroleum Engineering, L.S.U.). Oryx Energy Company (Richardson, Texas) donated the core material and introduced the writers to the field. Dr. Roger Sassen and Elizabeth W. Chinn supplied preprints, and Rowdy C. Lemoine core information. Mr. John Echols and Dr. Adam T. Bourgoyne critiqued an early draft of the manuscript.


Figure 18. SpartaBpetrographicandSEM(scanningelectronmicrograph)detailsofsamplel2(ll,347.50ft—1.5 ft below bottom of producing interval in upper shoreface sandstone petrofacies) from Clayton W. Williams, Jr., Seymour Dreyfus #1 well conventional core. Sample has very low porosity (3.1%) and permeability (0.01 Md). See Fig. 14 for location of sample. Petrographic photomicrographs above, SEM micrographs below. PL=plane light; PP=plane-polarized light; Q=quartz, C A=calcite; MI=microcline. Note almost complete occlusion of porosity by calcite. EDK (energy dispersive x-ray) indicates relative abundance of Ca in SEM calcite micrographs.

i &'/ .*& ! ,-i ? i2?

T 6 S

.^s^SlM; ..V-VV^?<&=B--YVT-V.l !?—-

Fordoche Field Point* Coup** Pariah, Loubiana

Not Pay Isopach Sparta "B" Suinbtorf

Isopocn Intotvoi 1™

0 4000 feet

N

Hi£ An£e NamlFuH

Figure 19. Sparta B net pay isopach map, Fordoche Field, Pointe Coupee Parish, Louisiana. See Fig. 4 for structure contour map of same unit.


loooooa DC UU

I CO CO

•=> o 08 _J

o co

O

100000:

FORDOCHE FIELD SPARTA B RESERVOIR B

1000000

100000 c/2

1000

10- IIII runnimiiirnJiinimuiniuIlanmnimniniiiiui l oo MAR-73 JAN-76 JAN-78 JAN-80 JAN-82 JAN-84 JAN-86 JAN-88 MAY-90

Li.

o LI. O

O

TIME IN MONTHS

CUM STB OIL CUMMCFGAS CUM BBLS WATER

Figure 20. Cumulative oil, gas and water production, Sparta B reservoir (reservoir B), Fordoche Field, Pointe Coupee Parish, Louisiana.


1 0 V1JuilJiikTIIIHi'iin'ilUJiiiinI^IUiniktiutiniiIIIIIIIIIiIIniitnjniKijiinjaiiituiuuiiin»nniniinniiirivntniiimuniuiiuninJiDiirinLIIIHinuiinniinmiiiDinTlIITIlTrrnr" 1 0 0

MAR-73 JAN-76 JAN-78 JAN-80 JAN-82 JAN-84 JAN-86 JAN-88 MAY-90

TIME IN MONTHS

STB OIL MCFGAS - * - BBLS WATER

Figure 21. Monthly oil, gas and water production, Sparta B reservoir (reservoir B), Fordoche Field, Pointe Coupee Parish, Louisiana.


1000O


100 i ii ii 111 ini I! iiti I IIIIII iini ii IIIIIIIIII 11 IIII niiii IIIIIIIII MIIII ill ii IIIIMIIII IIIIIIIII IIII ) UIII LIIII i IIIIIIIII i IIIIIB ii IIII IIIII uii i F IIII i iiniiiu 1111 IIIII ill i MIIIIIII iiiimiu i inn inn linn1

MAR-73 JAN-76 JAN-78 JAN-80 JAN-82 JAN-84 JAN-86 JAN-88 MAY-90

TIME IN MONTHS

Figure 22. Cumulative produced gas-oil ratio (SCF/STB), Sparta B reservoir (reservoir B), Fordoche Field, Pointe Coupee Parish, Louisiana.


TIME IN MONTHS Figure 23. Average monthly gas-oil ratio (SCF/STB), Sparta B reservoir (reservoir B), Fordoche Field, Pointe Coupee Parish, Louisiana.

404 TRANSACTIONS—GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES VOLUME XLI, 1991

REFERENCES

American Petroleum Institute, 1967. A Statistical Study of Recovery Efficiency. Bulletin D14.

Chinn, Elizabeth W., Lemoine, R. C. and Sassen, R., 1988a. Organic Geochemical Contrasts in Marine and Lagoonal Mudstone, Middle Eocene Sparta Formation, South Central Louisiana. 1988 Geological Society of America Meeting, Denver, Colorado, Abstracts with Programs, p. A259.

Chinn, Elizabeth W. , Lemoine, R. C. and Sassen, R., 1988b. Recognition of depositional environments using programmed pyrolysis. Transactions, Gulf Coast Association of Geological Societies, vol. 38, p. 589.

Chinn, Elizabeth W., Sassen, R. and Cole, G.A., 1991. Triter-pane Biomarkers in Lower Tertiary Source Rocks: South Louisiana. Bulletin American Association of Petroleum Geologists, manuscript submitted.

Combariza R., Gabriel A., 1990. By-Passed Oil Due to Misinterpretation of Well Logs. Unpublished M.S. Thesis, Dept. of Petroleum Engineering, Louisiana State University, Baton Rouge, LA, 138 p.

Craft, B.C. and Hawkins, M.F., 1959. Applied Petroleum Reservoir Engineering: Prentice-Hall, 437 p.

Eckles, W. W., Jr., Prihoda, C , and Holden, W.W., 1981. Unique Enhanced Oil and Gas Recovery for Very High-Pressure Wilcox Sands Uses Cryogenic Nitrogen and Methane Mixture. Journal of Petroleum Technology, Vol. 33, June 1981, p. 971-983.

Gregory, W.A., Chinn, E. W., Sassen, R. and Hart, G. F., 1990. Fluorescence Microscopy of Particulate Organic Matter,

Sparta Formation and Wilcox Group, south-central Louisiana. Organic Geochemistry, in press.

Holden, W. W., Ill, Prihoda, C.H. and Hall, B.E. , 1981. Successful Stimulation of Fordoche Field With a Retarded HF Acid. Journal of Petroleum Technology, Vol. 33, August 1981, p. 1485- 1490.

Lemoine, R. C , 1989. Stratigraphic Framework and Sedimentary Facies of the Sparta Formation (Middle Eocene), South-Central Louisiana. Unpublished M.S. Thesis, Dept. of Geology and Geophysics, Louisiana State University, Baton Rouge, LA, 138 p.

Lemoine, R. C., Moslow, T. F. and Ferrell, R. E., 1988. Facies Control on Reservoir Properties within a Barrier Island Complex, Sparta Formation, Pointe Coupee Parish, Louisiana. Bulletin American Association Petroleum Geologist, vol. 72, no. 2, p. 211.

Lemoine, R. C , Moslow, T. F., Sassen, R. and Ferrell, R. E., 1989. Sparta Sandstones: Future Exploration Potential in South-Central Louisiana. Bulletin American Association Petroleum Geologists, vol. 73, no. 3, p. 379.

Pierson, J.R., Jr., 1970. Fordoche Field, Pointe Coupee Parish, Louisiana, in Typical Oil and Gas Fields of Southeast Louisiana. Lafayette Geological Society, vol. 2, p. 9-9g.

Salvadori, M.G. and Baron, M.L., 1963. Numerical Methods in Engineering: Prentice-Hall, 258 p.

Sassen, R. , 1990. Lower Tertiary and Upper Cretaceous Source Rocks in Louisiana and Mississippi: Implications to Gulf of Mexico crude oil. Bulletin American Association Petroleum Geologists, vol. 74, no. 6, p. 857-878.

sparta b sandstones (eocene), fordoche field, pointe

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