Organic geochemical correlation of Oklahoma crude oils using R- and Q-mode factor analysis

Org. Geochem. Vol. 12, No. 2, pp. 157-170, 1988 Printed in Great Britain. All fights reserved

0146-6380/88 $3.00+0.00 Copyright © 1988 PergamonPress pic

Organic geochemical correlation of Oklahoma crude oils using R- and Q-mode factor analysis MICHAEL H. ENGEL1., SCOTT W. IMBU$l and JOHN E. ZUMBERGE2J" ~School of Geology and Geophysics, Energy Center, 100 E. Boyd St., The University of Oklahoma, Norman, OK 73019, U.S.A. 2Cities Service Research Box 3908, Tulsa, OK 74102, U.S.A. (Received 27 July 1987; accepted 13 November 1987)

Al~tract--For the past several decades, there has been a significant amount of crude oil exploration and production throughout the state of Oklahoma. Publications with respect to biological marker compound distributions and stable isotopic compositions of Oklahoma crude oils, their potential genetic relationships and possible sources have, however, been very limited. In this study, a detailed organic geochemical investigation of 46 crude oils from throughout the state of Oklahoma is presented. In addition to assessing similarities and differences of the oils with respect to reservoir ages and geologic provinces, an attempt was made to establish possible genetic relationships on the basis of combined R- and Q-mode factor analysis of source-related geochemical parameters. While the oils from throughout the state were found to be remarkably similar in chemical and stable isotopic composition, four genetic families of oils have been delineated based on this statistical approach. The possible effects of thermal alteration, migration, and multiple sources, i.e. mixing of the oil groups, are discussed. Key words: Oklahoma oils, factor analysis, oil correlation, stable isotopes, carbon, n-alkanes, steranes, terpanes, biomarkers

INTRODUCTION Oklahoma comprises most of the southern part of the structurally complex Mid-Continent Province of the United States. Throughout Oklahoma and adjacent areas, significant oil and gas production is achieved from several Paleozoic stratigraphic horizons. Oil production in Oklahoma primarily occurs in the Anadarko Basin and its associated northern shelf areas, and in the Hugoton Embayment (Oklahoma and Texas panhandles and western Kansas). Production is also important in the Marietta, Ardmore and Arkoma Basins. The general locations of these basins are shown in Fig. 1. Hill and Clark (1980) have divided reservoirs in the Anadarko Basin and adjacent areas into five major time sequences: (1) MidCambrian Arbuckle to Silurian/Devonian PostHunton Orogeny (Arbuckle, Simpson, Viola and Hunton), (2) Mississippian, (3) Pennsylvanian, Morrow-Springer Series, (4) Post Morrowan or late Pennsylvanian, and (5) Permian. Despite significant production in the "mature" southern Mid-Continent Province, organic geochemical characterization of oils and potential source rocks has been limited and, to some extent, inconclusive. Cardweli (1977) reported that oils from Arbuckle and Pennsylvanian reservoirs had similar n-alkane distributions. Because Arbuckle-Ellenburger rocks *Author to whom correspondence should be addressed.

tPresent address: Ruska Laboratories, Box 742688, Houston, TX 77274, U.S.A.

were found to be poor source rocks and their extract compositions differ from that of the oils, Pennsylvanian shales were suggested as the source rocks for Arbuckle and Pennsylvanian oils. Curiale (1983) reported on the basis of n-alkane, sterane, hopane, 613C and V/Ni distributions that four oils and seven bitumens from the frontal and central Ouachita Mountains of southeastern Oklahoma had a common source. Based on source rock analyses, the Ordovician/Silurian Missouri Mountain-Polk CreekWomble rocks were suggested as the most likely source rocks for oils and bitumens in this area (Curiale, 1983). Hatch et al. (1986), in their study of 24 oils from the central Anadarko Basin, described three general types of oils. Type 1 oils, typified by a pronounced odd carbon preference for the Cll-C,9 n-alkanes and very low isoprenoid hydrocarbon abundances, are found in Ordovician reservoirs. Type 2 oils, found in Devonian and Mississipian reservoirs, have n-alkanes as the most abundant components in the gasoline range (C4--C7). Type 3 oils, produced from Pennsylvanian reservoirs, are similar to type 2 oils, except that methylcyclohexane is always greater in abundance than n-heptane. Oils of intermediate character were considered to be the result of mixing. Type 2 oils were also found in Cambro-Ordovician through Upper Pennsylvanian reservoirs in western Kansas and southwestern Nebraska (Hatch et al., 1986). The possibility of oil migration of up to 350-400 miles (563-644 km) was considered, as these oils were similar in chemical composition to extracts of the Woodford shale from below 6000 ft (1.8 km) in

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Fig. I. Index map of Oklahoma showing well locations of oil samples and boundaries of geologic provinces (modified from Johnson et al., 1972).

the central Anadarko Basin. Finally, while several, additional attempts have been made to establish genetic relationships between Oklahoma crude oils and potential source rocks (e.g. Baker, 1962; Silver et aL, 1980), these studies are restricted to geographic areas of limited extent and are not applicable for the region as a whole. The complex structural and thermal histories of Oklahoma basins further complicates the task of classifying oils and establishing their respective sources. Localized and regional folding and faulting may have resulted in mixing of oils from different sources. Varying thermal and migration histories within and between Oklahoma basins may have also, to some extent, altered the organic compositions of oils that initially had a common source. Also, a substantial portion of the oil that was initially produced and reservoired in central Oklahoma, may have been lost via subsequent erosional events 0Vebb, 1976, 1977; Stone, 1977). A variety of organic geochemical parameters (e.g. n-alkane and biological marker compound distributions, stable isotopes, trace element abundances) have been employed to determine the origins and genetic relationships of oils from individual basins (e.g. Tissot et al., 1971; Koons et al., 1974; Williams, 1974; Deroo et aL, 1977; Vogler et aL, 1981; Zumberge, 1983; Hitchon and Filby, 1984; Philp, 1985). The application of organic geochemical techniques for identifying distinct families of oils from larger geographic areas that encompass more than one basin is, however, at best, a difficult undertaking. Factors that have been reported to influence the composition of oils in specific basins, including source, maturity, migration, mixing, biodegradation, water-washing, are likely, to some extent, to further complicate

attempts at comparing oils between basins in regional studies. In this study, an overview of the chemical composition of crude oils of varying reservoir ages from throughout the state of Oklahoma is presented. Multivariate statistical techniques are employed to assess the similarities and differences among the oils. Variations in oil chemistry with respect to reservoir age and geologic province are discussed.

EXPERIMENTAL

AND STATISTICAL

METHODS

Forty-six crude oils (provided by Cities Service Corp., Tulsa, OK and the former Bartlesviile Energy Technology Center, Bartlesville, OK) from 21 counties in Oklahoma were analyzed for their hydrocarbon and stable carbon isotopic compositions. The well locations, reservoir formations and production depths of the wells are listed in Table 1. The oils were produced from reservoirs that range in age from Cambro-Ordovician (Arbuckle) to late Pennsylvanian (Virgilian). Production depths ranged from 1500 to > l l , 0 0 0 f t (457->3353m). As shown in Fig. 1, the oil samples came from wells that are situated throughout the state, and include samples from the Oklahoma Panhandle, the Anadarko Basin, the east and west portions of the Northern Self (as defined by the Nemaha Ridge), and the Marietta and Ardmore Basins. Initially, API gravity values (60°F) were determined for each oil using a Westphal Balance. Sulfur abundances were determined by X-ray fluorescence. Nitrogen abundances for some of the oils were provided by the former Bartlesville Energy Technology Center, Bartlesville, OK.

159

Organic geochemistry o f O k l a h o m a oils Table 1. Oklahoma exude oils analyzed in this study Sample

No.

County (geologic province; see Fig. I)

1 2 3

Texas (PH) d Texas (PH) Beaver (PH)

4

Beaver (PH)

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Harper (NSW) Woods (NSW) Woods (NSW) Woods (NSW) Woods (NSW) Woods (NSW) Woods (NSW) Grant (NSW) Grant (NSW) Osage (NSE) Washington (NSE) Noble (NSE) Payne (NSE) Creek (NSE) Oklahoma (NSW) Caddo (ANB) Grady (ANB) Grady (ANB) Grady (ANB) MeClain (ANB) MeClain (ANB) McClain (ANB) MeClain (ANB) MeClain (ANB) Seminole (NSE) Garvin (ANB) Garvin (ANB) Garvin (ANB) Garvin (ANB) Garvin (ANB) Garvin (ANB) Pontotoe (NSE) Cotton 0,VM B) Kingfisher (NSW) Love (MAB) Love (MAB) Love (MAB) Carter (MAB) Carter (MAB) Love (MAB) Love (MAB) Love (MAB)

Production

Field Carthage Postle Dower, N. Grand Valley, E. Gate Lake Oakdal¢ N.W. Oakdale N.W. Oakdale N.W. Oakdale N.W. Oakdale N.W. Oakdale N.W. Webb, W. Sooner Trend Burbank Bartlesville-Dewey

Formation Morrow Morrow Morrow U. Morrow Red Fork Red Fork Red Fork

Cherokee Chester Hunton Red Fork Red Fork Oswego Burbank Bartlesville

Lucien Paradise

Simpson Misissippi

Glennpool Jones, W.

Bartlesville Cleveland Marchand

Binger, E. Golden Trend Golden Trend Golden Trend Payne, N. Golden Trend Golden Trend Golden Trend Golden Trend Wewoka Golden Trend Golden Trend Golden Trend Golden Trend Golden Trend Golden Trend Fitts, N. Cache Creek Sooner Trend b b b b b b b b

Bromide Arbuckle

Bromide Hunton Viola

Simpson Hunton Dcese Hunton Hunton Oil Creek Viola Viola Viola Bromide

McAlester Cisco Hunton Viola Viola Viola Viola Viola Viola Viola Viola

Age

depth in feet

Penn.-Morrow Penn.-Morrow Penn.-Morrow Penn.-Morrow Penn.-Des Moines Penn.-Des Moines Penn.-Des Moines Penn.-Des Moines

Miss. Sil.-Dev. Penn.-Des Penn.-Des Penn.-Des Penn.-Des Penn.-Des Ord.

Moines Moines Moines Moines Moines

Miss. Penn.-Des Moines Penn.-Missouri Penn.-Missouri Ord. Cam.-Ord. Ord. Sil.-Dev. Ord. Ord. Sil.-Dev. Penn.-Des Moines Sil.-Dev. Sil.-Dev. Ord. Ord. Ord. Ord. Ord. Penn.-Des Moines Penn.-Virgil Sil.-Dev. Ord. Ord. Ord. Ord. Ord. Ord. Ord. Ord.

4338 6152 7428 6948-6987 6192 6200~240 ~ 6200-6240" 6200-6241P > 6201P > 6200~ 6200-6240" 4464-4482 5725--6200 2700-2760 5184-5214 b 4776-4787 1500 4750--4760 9982-10055 11000-11075" 11075a 11000-11075 a 8728-8748 11000-11075 ° 11000-11075 ° 8600-8745 a 660ff4~30" 3859-3995 8600-8745 a 11000-11075 a 11000-11075 a 11000-11075 a 11000-11075 a 110(}0-11075a 1651-1822 1300 6300" ~ 8700 c ~ 8000 c ~ 8700 ~ ~ 7800 ~ ~ 6300c ~ 800ff ~ 8700c ~ 8000c

(m) (1322) (I 875) (2264) (2118-2130) (1887) (1890-1902) (1890-1902) (1890-1902) ( > 1890) ( > 1890) (1890-1902) (1361-1366) (1745-1890) (823) (1580-1589) (1456-1459) (457) (1448-1451) (3043-3065) (3353-3376) (3376) (3353-3376) (2660-2666) (3353-3376) (3353-3376) (2621-2665) (2012-2021) (1176-1218) (2621-2665) (3353-3376) (3353-3376) (3353-3376) (3353-3376) (3353-3376) (503-555) (396) (1920) (2652) (2438) (2652) (2377) (1920) (2438) (2652) (2438)

qnferred from Nehring (1981). q~lot available. qnferred from Petroleum Information Corp. (1979-1983). 'tPH: Panhandle; NSW: Northern Shelf-West; NSE: Northern Shelf-East; ANB: Anadarko Basin; WMB: Western Marietta Basin; MAB: Marietta/Ardmore Basin.

Approximately 0.5 g samples of each oil were placed in individual, preweighed glass vials and concentrated under a stream of N2 in a sandbath (40°C) until additional weight loss was less than 1% ( ~ 1 h). Twenty-five milliliters of distilled pentane was added to each sample, and the samples were maintained at room temperature for 24 h to precipitate asphaltenes. The samples were subsequently passed through pre-cleaned (distilled pentane) glass fiber filters. An additional 25-50 ml of pentane was passed through each filtration unit until the eluents were clear. The pentane soluble fractions were concentrated under N 2 to ~ 5mi volumes. The asphaltene fractions were eluted from the filters with a mixture of chloroform and methanol (9:1, v/v) and evaporated to dryness under N2 in preweighed vials. The pentane soluble fractions were separated into

their respective saturate, aromatic and NSO fractions by column chromatography using the procedure described by Rohrback (1983). Next, the saturate, aromatic and NSO fractions were concentrated in preweighed vials. Approximately 1 mg portions of the saturate and aromatic hydrocarbon fractions of the oils were prepared for analysis of their stable carbon isotopic compositions using the static combustion method described by Sofer (1980). A VG 602C Micromass mass spectrometer equipped with a 90 ° sector magnetic deflection instrument was used for these analyses. The 61ac (%0) values for the samples are reported relative to the PDB standard and are corrected for 170 contribution. Portions of the saturate and aromatic crude oil fractions were redissolved in distilled heptane and

MICHAEL H. ENGEL et al.

160

Table 2. Normalized abundances of selected n-alkanes used for combined R-mode and Q-mode factor analysis

Sample No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

n-Crt 10.6 9.7 11.3 11.0 9.0 47.5 9.8 44.0 23,4 11.0 16.2 15.2 10.8 12.3 13.1 13.4 12.8 13.9 12.4 14.1 12.1 22,8 11.3 13.9 11.6 8.3 12.1 10.4 16.0 11.8 14.3 12.5 I 1.0 11.0 12.8 10.3 14.2 11.6 14.0 15.8 15.1 16,5 15.2 15.0 14.5 16.5

n'Ci8 9.7 9.0 10.3 10.0 8.5 19,4 9.1 21.3 14.1 10.1 12.9 12.6 10.1 11.0 11,7 11.7 11.3 12.1 11.4 12.0 10.5 132 9.7 11.7 9.7 7.6 10.3 10.1 13.4 10.4 10.0 10.5 9.6 9.2 10.8 9.8 12.4 10.5 11.2 9.6 11,7 9.8 10.4 9,5 10.3 9.3

n'CI9 9.7 8.4 9.4 9.2 8.3 8.7 9.2 10.8 10.5 10.0 10.8 11.1 10.3 10.0 10.8 10.8 10.1 10.5 10.3 11.2 9.9 16,3 9.5 10.3 9.8 7.2 9.8 10.2 I 1.1 9.6 12.3 10.5 9.9 9.3 10.7 I0.0 10.9 10.4 10,5 10.0 10.9 10.2 10.8 10.4 10.7 10.3

n-C~.

n-C25

7.7 7.2 7,5 7,2 7.2 2.5 7.8 2.3 5.8 7.4 6.9 7.2 7.5 7.6 7.6 7.5 7.5 7.5 7.8 7.4 7.2 5.7 7.5 7.3 7.1 5.1 7.1 7.6 7.4 7,1 7.0 6.8 7.4 7.5 7.0 7.8 7.7 7.8 6.9 6.0 6.9 6.0 6.3 6.0 6.1 5.9

6.8 6.3 6.2 6.0 6.1 1.6 5.9 1.4 3.9 5.5 4.6 4.5 5.2 5.3 4.7 4.9 5.1 4.7 5.2 4.7 5.4 3.6 5.5 4.9 5.5 3.9 5,0 5,4 4.2 5.1 5.0 5.0 5.3 5.4 5.0 5.7 4.7 5.2 5.1 4.9 4.7 4.8 4.9 4.9 4.9 4.8

n-C-~8 n-C33 4.0 4.6 4.2 4.2 4.8 1.1 4.1 1.0 2.4 3.7 2.9 3.0 3.4 3.4 2.9 3.3 3.4 3.0 3.3 3.0 3.7 2.2 3.9 3.3 4,0 3.7 3.5 3.6 2.6 3.7 3.4 3.9 3.8 4. | 3.5 3.8 2.8 3.6 3.2 3.8 3.1 3.9 3.7 3.9 3.7 3.7

0.5 1.3 0.6 1.0 1,6 0.5 1.0 0.4 0.8 1.2 0.9 0.9 1.3 1.0 1.0 1.0 1.1 0.9 0.9 0.9 1.2 0,5 1.2 1.1 1.3 5.6 1.5 1.3 0.8 1.4 1.0 1.2 1.4 1.4 1.2 1.2 0.9 1.1 l.l 1.7 1.1 1.6 1.4 1.7 1.6 1.6

Kratos MS-25 mass spectrometer (MS) equipped with a Nova 4X computer and a DS-55 data system. The MS was run in the electron impact (El) mode using multiple peak monitoring (MPM). The magnetic field was kept constant while varying the accelerating voltage (2 kV range) in order to monitor masses 191.1799 (terpanes) and 217.1956 (steranes). In addition to qualitative comparisons of crude oil compositions, combined R-mode and Q-mode factor analysis was used in an attempt to reduce the dimensionality of the data to a few important components that best describe variations in the data set. Details of this statistical method and its applicability for crude oil correlations have been previously discussed (Zhou et al., 1983 and Zumberge, 1987, respectively). Combined R-mode and Q-mode factor analysis using only the relative n-alkane abundances (C~7--C36) of the Oklahoma oils (Table 2) indicated that seven n-alkanes (ClT, Czs, CI9, C22, C2s, C28, C33) accounted for much of the variation. A second factor analysis in which the distribution of these seven n-alkanes and several other parameters (Table 3) that are primarily source related (rl3Csat, 613CArom, Pr/Ph, Pr/C~7, %S, CPI, Para/Naph) was attempted. The results of this statistical approach are discussed below.

RESULTS AND DISCUSSION

The gross compositions of the crude oils are listed in Table 4. The AP! gravity values, stable carbon isotope values, elemental (S, N) abundances, carbon preference indices (CPI) and hydrocarbon ratios, e.g. pristane/phytane (Pr/Ph), for the individual oils are listed in Table 3. Selected biological marker compound ratios (steranes, terpanes) for 18 representative oils analyzed by GC-MS are listed in Table 5. (A ) Assessment of maturity and alteration processes

analyzed by gas chromatography (GC) using a Hewlett-Packard 5880A GC equipped with an FID detector and a 15 m × 0.25 mm i.d. fused silica capillary column coated with dimethyl polysiloxane (SP2100). The column temperature programs for the saturate and aromatic hydrocarbon analyses were 130--270°C at 8°C/min and 100-270°C at 4°C/rain, respectively. The carrier gas (He) flow rate for all analyses was 2.5 ml/min. Saturate hydrocarbon fractions of 18 of the crude oils were analyzed for their terpane and sterane distributions by combined gas chromatography-mass spectrometry (GC-MS). Each saturate hydrocarbon fraction was injected into a Carlo Erba GC (split, 5:1) equipped with a 30 m x 0.25 m m i.d. fused silica capillary column coated with SE-54. The column temperature program for the analysis was 100°C (isothermal for 2 min) to 280°C at 2.5°C/min. The carrier gas (He) flow rate was 0.5 ml/min. The GC column was directly inserted into the source of a

Prior to attempting to establish genetic relationships between Oklahoma oils, it is important to assess the extent to which the individual oils have been thermally altered, biodegraded and/or deasphalted. The 18 Oklahoma oils that were analyzed for biological marker distributions all appeared to be mature (Table 5): Sterane ratios, e.g. 5~t(H) ethylcholestane (20S)/5~(H) ethylcholestane (20R), have approached equilibrium (Mackenzie, 1984). Whereas pentacyclic triterpane ratios are also employed as indicators of thermal maturity, the Tm/T, ratios in the present study were somewhat variable (Table 5). This may be a function of source in addition to maturity (Seifert and Moldowan, 1978). In addition to biological marker distributions, a plot of API gravity values vs % < C15 hydrocarbon composition was constructed (Fig. 2) to attempt to distinguish relative thermal maturities for the Oklahoma oils. Oils 6, 8, 9 and I 1 are condensates. The remaining oils were of moderate to relatively high maturity.

Organic geochemistry of Oklahoma oils

161

Table 3. Analytical data for Oklahoma oils API

Sample

gravity

No.

(60°F)

1 2 3 4 5 6 7 8 9 lO Il 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

37.4 38.0 37.2 39.6 34.8 53.9 39.9 64.8 64.2 43.4 64.4 41.1 43.2 38.6 35.2 38.8 41.1 41.5 37.4 47.2 50.8 55.3 46.7 43.0 46.8 51.7 43.6 41.0 32.8 44.1 47.8 29.0 44.7 46.9 42.1 34.0 34.8 46.9 41.8 36.6 39.2 36.2 36.4 37.8 35.0 36.4

aNot

%S 0.06 0.00 0.10 0.09 0.05 < 0.01 0.08 < 0.01 0.06 0.09 0.03 0.24 O.ll 0.20 0.22 0.12 0.24 0.19 0.36 0.05 O.Ol 0.02 0.06 0.03 O. 11 0.09 0.06 0.22 0.18 O.lO 0.22 0.23 0.14 0.17 0.27 0.34 0.45 0.14 0.23 0.46 0.22 0.49 0.94 0.44 0.72 0.47

%N --° --0.158 -------0.054 -0.049 0.088 0.054 0.072 0.091 0.106 0.018 ---0.043 ----0.061 ------0.175 0.140 ----------

Pr/Ph 2.28 2.20 2.07 2.08 1.35 3.33 1.32 2.74 2.35 1.69 1.87 1.40 1.61 1.45 1.64 1.56 1.60 1.64 1.54 1.63 1.49 1.45 1.47 1.46 1.44 1.23 1.38 1.26 1.32 1.20 1.36 1.21 1.25 1.37 1.28 1.50 1.60 1.46 1.30 1.23 1.40 1.16 1.26 1.16 I. 11 1.20

Pr/n-Ci7

Ph/n-Cis n-Cls/n-Cl9 Para/Naph CPI

0.25 0.21 0.22 0.23 0.30 0.64 0.52 0.48 0.58 0.71 0.58 0.43 0.64 0.47 0.69 0.40 0.58 0.50 0.57 0.43 0.30 0.09 0.37 0.36 0.34 0.35 0.35 0.67 0.31 0.36 0.26 0.34 0.39 0.37 0.36 0.99 0.45 0.51 0.36 0.15 0.35 0.15 0.35 0.15 0.25 0.14

0.12 0.10 0.12 0.12 0.24 0.47 0.43 0.36 0.41 0.46 0.39 0.37 0.42 0.36 0.47 0.29 0.41 0.35 0.41 0.31 0.23 O. I l 0.30 0.29 0.28 0.32 0.30 0.55 0.28 0.34 0.27 0.33 0.36 0.32 0.33 0.69 0.33 0.39 0.35 0.20 0.32 0.21 0.41 0.20 0.32 0.21

0.99 1.07 1.10 1.09 1.03 2.23 0.99 1.97 1.34 1.02 1.20 1.13 0.97 I.lO 1.08 1.09 l.ll 1.16 l.lO 1.07 1.06 0.8 I 1.03 1.14 0.99 1.05 1.06 1.00 1.22 1.07 0.81 1.00 0.97 0.99 1.02 0.98 1.13 1.01 1.06 0.96 1.07 0.95 0.97 0.91 0.96 0.90

0.78 0.84 0.55 0.59 0.71 0.92 0.47 1.03 0.62 0.65 0.57 0.52 0.50 0.43 0.35 0.46 0.37 0.45 0.42 0.42 0.87 1.86 0.73 0.34 0.69 0.79 0.51 0.36 0.29 0.49 0.69 0.55 0.79 0.54 0.46 0.51 0.36 0.88 0.40 0.66 0.37 0.76 0.57 0.71 0.50 0.71

1.06 1.02 1.02 1.02 1.02 0.99 l.OI 1.09 1.03 1.03 1.02 1.02 1.03 0.98 1.00 1.00 0.98 l.O0 1.00 1.03 1.01 1.24 1.00 0.99 1.02 0.99 1.03 1.03 0.99 1.00 1.10 1.03 1.02 1.O0 1.04 1.03 1.00 1.01 1.06 1.04 1.02 1.04 1.04 1.05 0.93 1.07

6 t3CpD s Sat. (960)

Aromatic

6 ~3Cme

- 29.28 - 29.77 - 28.87 - 28.89 - 28.42 - 30.72 - 30.01 - 30.11 - 30.42 - 30.58 - 30.36 - 29.81 - 29.68 - 30.63 - 29.86 - 30.70 - 30.99 - 30.92 - 30.99 - 30.49 - 30.39 - 31.52 - 30.66 - 30.60 - 30.79 - 30.56 - 30.54 - 31.12 - 30.44 - 30.52 - 31.46 - 30.88 - 30.87 - 31.02 - 30.74 - 31.38 - 30.66 -31.17 - 30.03 - 30.85 -- 30.09 - 30.91 - 30.75 - 30.91 - 30.61 - 30.94

- 28.24 - 28.56 - 27.86 - 27.89 - 27.64 - 29.32 - 28.95 - 28.33 - 29.49 - 29.59 - 29.53 -- 29.26 - 29.04 - 29.86 - 29.17 - 29.76 - 30.24 - 29.92 - 30.37 - 29.35 - 29.17 - 29.33 - 29.28 - 29.97 -- 29.72 -- 29.76 - 29.87 - 30.58 - 29.86 - 29.98 - 30.47 - 30.49 - 30.10 - 30.00 - 30.25 - 30.21 - 30.27 -30.36 - 26.69 - 30.06 - 29.71 - 30.21 - 30.46 - 30.06 - 30.59 - 30.21

(%o)

determined.

Based on the percent saturate hydrocarbons, all of the oils, with the exception of No. 21, are paraffinic or paraffinic-naphthenic, according to the classification scheme of Tissot and Welte (1984). A crossplot of &t3C saturate hydrocarbon vs 313C aromatic hydrocarbon values (Fig. 3) shows that all of the non-condensate oils, with the exception of Nos 21, 22, 23 and 36, are nonwaxy. In addition to thermal alteration, processes such as biodegradation, deasphalting and migration can alter crude oil compositions. Several lines of evidence indicate that none of the oils have been biodegraded. Gas chromatograms for all samples show that the n-alkane distributions are apparently unaltered. The Pr/n-Cl7 and Ph/n-Cls values are low ( < 1.00) and X(n-Cls-n-C24)/n-C25 + values are high, with the exception of oil 26, which clearly shows a bimodal distribution. Also, all samples plot in the "normal crude oil" area on a paraffin/naphthene/aromatic

ternary diagram (Tissot and Welte, 1984). Oil No. 32, as evidenced by its abnormal API vs % < C,s relationship (Fig. 2), may have been altered by deasphalting or, as suggested by Sofer (personal communication), by contamination with corrosion prevention materials during production. The effects of migration on correlation parameters has received some attention (e.g. Leythaeuser et al., 1984; Bonilla and Engel, 1986). In a structurally complex area such as Oklahoma, however, it is at present, very difficult to distinguish migration effects from the other alteration phenomena discussed above. (B) Assessment of genetic relationships for the Oklahoma crude oils (1) General observations. Given the similarity of the geochemical parameters (Table 3) for the Oklahoma oils, it initially appeared unlikely that genetic families of oils could be unambiguously distinguished solely

.

.

.

,

% Paraffins % Naphthenes % Aromatics

.

_

% Paraffins % Naphthenes % Aromatics .

Ct~+ hydrocarbon composition

% NSO compounds % Asphaltenes % Nonhydrocarbons

% Saturate hydrocarbons % Aromatic hydrocarbons % Hydrocarbons

% Less than C~5+ % C,s + C js + composition

_

C~s+ hydrocarbon composition

% Saturate hydrocarbons % Aromatic hydrocarbons % hydrocarbons % NSO compounds % Asphaltenes % Nonhydrocarbons

Gross oil composition

T

.

Gross oil composition % Less than C~s + % Cas+ C,5 + composition

.

.

.

.

.

.

.

.

.

17

.

.

20.3 54.5 25,2

13.0 0.4 13.4

64,8 21.8 86.6

~

33.6 43.2 23.2

-

-

.

22.7 50.3 27,0

15.8 1.2 17.0

60.6 22.4 83,0

42.7 57.3

18

.

34.4 41.0 24.6

13.1 1.5 14.6

12.5 0.4 12.9

.

.

24.2 75,8

2

64.4 21.0 85.4

.

66.9 20.2 87.1

25.1 74.9

1

52.7 47.3

.

21.4 50.8 27,8

12.8 2.0 14.8

61.5 23.7 85.2

36.6 63.4

19

26,6 48,7 24.7

12.5 0.6 13,1

65.4 21.5 86,9

26.4 73.6

3

.

.

20. l 79,9

5

.

24.9 59.9 15.2

8.4 1.0 9.4

76.8 13.8 90.6

47.4 52.6

20

27.9 47.3 24.8

8.9 0.0 8.9

44.1 50.6 5,3

7.3 0.9 8.2

86.9 4.9 91.8

53,8 46.2

21

32.9 46.3 20.8

15.8 0.1 15,9

68.5 66.6 22.6 17.5 91.r-~ 84.1

33.8 66,2

4

.

m

.

22

46.0 50.2 3.8

29.1 2.1 31.2

66.2 2.6 68.8

95. l 4.9

6

62,9 33.9 3,2

12,9 O. 1 13.0

84.2 2.8 87.0

64.1 35.9

.

_

.

7

39,4 53.7 6.9

9.3 0.4 9.7

84.1 6.2 90,3

49.9 50,1

23

27.1 57.5 15.4

10.9 0,8 11.7

74.7 13.6 88.3

33.4 66.6

.

_ .

34.7 55.9 9.4

33.6 0.4 34.0

59.8 6.2 66.0

99.8 0.2

9

20.2 59.5 20.3

I |.i 0.3 11.4

,

70.6 18.0 88.6

43.0 57.0

24

36.2 52. I i 1.7

6,0 0.7 6.7

82.4 10.9 93.3

51.6 48.4

25

Sample number

46.2 45.0 8.8

30,7 0,0 30,7

63.2 6.1 69.3

99.6 0.4

8

Sample number

Table 4. Gross oil compositions

40.2 50.8 9.0

7.6 1.4 9.0

82.8 8.2 91.0

69. I 30.9

.

26

31.7 48.7 19.6

8.3 0.8 9.1

73.1 17.8 90.9

56.7 43.3

10

.

_

.

28.9 56.9 14.2

6.8 1.6 8.4

78,6 13.0 91.6

52,8 47,2

.

27

.

32.4 56.6 11.0

15,2 1,2 16.4

74.4 9.2 83.6

98.9 1.1

11 ,

.

20.3 56.8 22,9

9.5 1.1 10.6

68.9 20.5 89.4

40. I 59,9

28

.

27,5 52.6 19.9

I 1.9 0.6 12,5

70,1 17.4 87.5

43. l 56.9

.

12 _

29

.

t6.3 57.0 26.7 .

12.4 0.8 13.2

63,6 23,2 86.8

26.7 73.3

.

26.7 53.0 20.3

10.2 0.7 10.9

71.0 18.1 89.1

49,0 51.0

13

.

.

.

26.9 55.2 17.9

8.9 0.6 9.-5

74.3 16,2 90,5

50.7 49.3

30

23. I 54.3 22.6

18.5 0.5 19,0

62.7 18.3 81.0

35, l 64.9

14

33.0 47.9 19.1

1 !.7 0,7 12.4

70.9 16,7 87.6

51.2 48,8

31

19.2 55.4 25.4

13,6 0.8 14,4

63.9 21.7 85.6

33.4 66.6

15

29.7 54.3 16.0

9.1 8.0 17.1

69.6 13,3 82.9

63.3 36.7

32

24.5 53,2 77.3

! 1,3 0.3 11,6

68.7 19.7 88,4

43.7 56.3

16

31.8 58.5 9.7

7.5 1.0 8.5

82.6 8.9 91.5

49.8 50.2

34

25.0 54.0 21.0

10.7 1.5 12.2

69.4 18.4 87.8

45.8 54.2

35

22.9 44.6 32.5

18.9 2.9 21.8

52.8 25.4 78.2

28.9 71.1

36

18.8 52.1 29.1

18.0 4.3 22.3

55.1 22.6 77.7

33.9 66.1

37

40.7 46.0 13.3

8.5 1.3 9.8

78.2 12.0 90.2

64.6 35.4

38

22.2 54.9 22.9

12.9 0.5 13.4

66.8 19.8 86.6

43.0 57.0

39

27.4 41.5 31.1

15.2 2.6 17.8

56.6 25.6 82.2

34.4 65.6

40

19.4 52.7 27.9

11.9 1.1 13.0

62.7 24.3 87.0

49.3 50.7

41

26.8 35.2 38.0

13.6 2.5 16.1

52.0 31.9 83.9

41.7 58.3

42

20.5 36.0 43.5

22.8 2.3 25.1

42.3 32.6 74.9

35.4 64.6

43

28.3 39.6 32.1

32.0 2.7 34.7

43.4 21.9 65.3

33.6 66.4

44

I~

0.47 0.43 0.39 0.20 0.23 0.27 0.21 0.19 0.68 0.25 0.21 0.20 0.19 0.21 0.26 0.25 0.25 0.37

1 3 5 7 l0 13 14 19 22 25 30 32 37 38 39 41 43 46

16.7 16.8 16.4 15.6 16.7 15.1 18.0 20.1 12.0 17.6 15.3 16.5 18.8 16.8 16.3 17.9 18.1 23.3

5z

cl + c2 + c 3

7.0 7.8 6.9 7.5 8.6 8.1 6.6 9.6 18.6 6.6 7.0 6.5 7.4 7.0 7.2 6.6 6.0 5.9

5z

dl + d 2

20.5 19.2 21.0 23.8 26.6 23.3 21.9 33.3 8.2 20.9 22.3 24.8 23.4 24.7 24.8 22.9 25.3 17.0

y~

e

f

12.4 12.3 15.1 17.5 13.8 17.1 16.5 24.9 5.2 14.2 16.6 13.7 14.6 15.3 15.5 15.2 14.8 12.1

~

q

6.9 6.9 9.2 8.9 10.0 8.4 7.8 10.8 4.5 6.7 8.0 7.8 7.1 9.4 9.1 9.4 9.3 7.8

~ 11.6 13.5 16.0 16.0 13.2 16.5 10.2 18.4 6.7 10.3 12.9 11.5 10.4 16.7 16.7 16.0 15.9 17.4

5z

hl + h z

0.50 0.80 0.32 0.26 0.52 0.25 0.27 1.25 1.40 0.50 0.37 1. I I 1.41 0.65 0.89 0.83 1.62 1.20

~-

T~

1.62 1.44 1.83 1.25 1.33 1.29 2.00 1.67 1.60 1.00 1.17 1.45 1.50 1.71 1.44 1.75 1.42 1.50

L-

K4

Pentacyclic triterpanes b

27.9 39.3 32.8

16.5 2.6 19.1

54.4 26.5 80.9

32.8 67.2

46

1.90 1.95 1.56 1.58 1.45 1.71 2.33 1.64 1.50 1.78 1.69 1.77 1.70 1.61 1.29 1.80 1.91 1.80

I/2 S

1.41 1.27 1.00 1.00 1.00 l.I 1 0.85 0.82 -1.11 0.93 1.09 1.00 1.06 0.88 1.23 1.03 1.12

19/226

Steranes ~

17.9 36.0 46.1

16.0 4.2 20.2

43.0 36.8 79.8

35.4 64.6

45

¢Pcak designations a through h refer to the Cj9 through C~s homologs, respectively. ILongman and Palmer (1987); source indicator. ~Zumberge (1983), Reed (1977), Simoneit (1977), Richardson and Miller (1981); source indicator. ~Peak designations as follows: T , / T ~ - - I 7 a , 21p-22, 29, 30-trisnorhopane/18% 21fl-22, 29, 30-trisnorhopane; K / L - - 1 7 a , 21fl-30, 31-bishomohopane (22S)/17at, 21fl-30,31-bishomohopane (22R). ~seifert and Moldowan (1978, 1981), McKirdy et al. (1983); maturity indicator if have same source, carbonate source indicator. 4seifert and Moldowan (1980), Seifert et al. (1980), Mackenzie (1980); maturity indicator. ~Peak designations as follows: I/2--13fl, 17a-diacholestane: 20S/20R; 19/22--5a-ethylcholestane: 20S/20R. 5Mackenzie et al. (1980); maturity indicator. 6Seifert and Moldowan (1981); maturity indicator.

19.6 17.4 10.9 9.3 9.3 9.3 17.4 16.9 40.3 22.2 14.3 13.7 15.6 12.3 10.3 12.1 10.5 16.3

a + bz

al

a + b

Tricyclic diterpanes ( x 100)~

Table 5. Selected tricyclic diterpane, pentacyclic triterpane and sterane ratios for 18 Oklahoma oils

36.9 46.6 16.5

8.7 0.8 9.5

75.6 14.9 90.5

53.3 46.7

33

Sample No.

% Paraffins % Naphthenes % Aromatics

Cis + hydrocarbon composition

% NSO compounds % Asphaltenes % Nonhydrocarbons

% Saturate hydrocarbons % Aromatic hydrocarbons % Hydrocarbons

Cls + composition

Gross oil Composition % Less than Ci5 + %Ci5+

Sample number

o.

o

O

o

O

164

MIC~L H. ENGELet al.

-- 6 0

|

,°

20

10

20

30

40

50

60

70

80

90

100

% LESS THAN Cls HYDROCARBONS

Fig. 2. Cross plot of API Gravity (60°F) vs % less than C~5 hydrocarbon content of oils.

on the basis of source-related criteria. Noticeable exceptions, however, were (1) the PennsylvanianMorrowan oils from the Oklahoma Panhandle (oil Nos 1, 2, 3, 4), which appear to be geochemically unique with respect to Pr/Ph values and stable carbon isotope values (Table 3) and (2) Ordovicianreservoired oils from the Marietta/Ardmore Basin Province (oil Nos 39-46), the majority of which were found to have n-C~a/n-C,9 values less than 1.0, moderate to high sulphur content and low Pr/Ph and Pr/n-C]7 values. An attempt was also made to establish genetic relationships for the Oklahoma oils on the basis of similarities in n-alkane distributions. The n-alkane distributions for representative oils from the Oklahoma Panhandle and the Marietta Basin are shown in Fig. 4. A chromatogram is also shown that is representative of the remaining oils that were investigated. Oils with similar n-alkane distributions, however, often had other source-related parameters that were dissimilar. Clearly, an integrated statistical approach was required to assess possible genetic relationships based on the available organic geochemical data. Prior to discussing the results of the combined R- and Q-mode factor analysis, similarities and differences in oil compositions are discussed with respect to reservoir ages and geologic provinces. (2) Reservoir age. Oils are frequently referred to in the literature by the age of the reservoir rock from which they were produced. This does not, however, imply that they were sourced from rocks of a similar age. Nevertheless, secular trends in crude oil composition as a function of reservoir age have been reported. For example, Stahl (1976, 1977) observed a secular trend for 6J~C values of oils that were produced from reservoirs that ranged in age from Cambrian to Tertiary. This is consistent with previous work which shows that there is a general depletion in ~3C abundance of oils with increasing geologic age (e.g. Degens, 1969; Welte et al., 1975). Zumberge (1983) delineated six distinct genetic groups of oils from the WiUiston Basin. In general, it was observed that oils produced from the same formation were similar regardless of their location in the basin.

Longman and Palmer (1987) reported that fifteen oils produced from Ordovician reservoirs associated with six U.S. basins have several distinctive geochemical characteristics in common. In structurally complex regions such as the southern Mid-Continent, however, there is reason to suspect that extensive faulting and fracturing may have complicated migration paths or allowed mixing of reservoired oils. In addition, important source rocks such as the Woodford Formation in the Anadarko Basin and adjacent shelf areas vary in quantity and character in different parts of the basin (e.g. Sullivan, 1985; Comer and Hinch, 1987). To determine what geochemical parameters may be considered characteristic of oils produced from reservoirs of different geologic ages and if any secular trends (Cambro-Ordovician to Pennsylvanian-Virgilian) can be established for Oklahoma oils, regardless of geologic province, source-related geochemical parameters (Tables 3 and 5) were compiled. Comparisons of the means for each parameter with respect to geologic age group with the collective mean and standard deviation for all age groups were made. Possible maturation differences, as discussed previously, may limit the applicability of some source-related parameters. The Cambro-Ordovician oil (No. 22) is somewhat unique, having high Para/Naph and high tricyclic diterpane (Cl9/(Cz9 + C20) and (Ci9 + C20)/y-) ratios and low Pr/n-C]7 and n-Cls/n-Cj9 ratios. Also, the CPI for this oil is high and the saturate hydrocarbon fraction is depleted in ~3C. It is suspected, however, on the basis of API gravity and biological marker ratios (Table 5), that this oil may be thermally altered. The Ordovician-reservoired oils (Nos 16, 21, 23, 25, 26, 31-35, 39-46) are distinctive only by their mean high sulphur content and low Pr/Ph ratio. The Silurian-Devonian reservoired oils (Nos I0, 24, 29, 30, 38) are low in sulphur content. Mississippian oil -27.0

27.5

~"

~ 2S.O

/

°,

-28,5 i -29~ i

-29,5

~ -30.0

-3(15

-31.0 32.0

///. ~;~%. .

.

.

.

-31.8

.

. . . . . ' ......... ... i .... , . . -31~ -3G8 -30.0 -29.5 -29~) -28~ 8-C SAIIJAATE HYDROCARBONS [Y.]

. .

280

Fig. 3. Cross plot of ~13C values for the saturate and aromatic fractions of the Oklahoma oils. The diagonal line represents a statistical separation between waxy and nonwaxy oils (Sorer, 1984).

Organic geochemistry of Oklahoma oils

165

PANHANDLE

I

LU Z hi

ILl I-" W 121

J

lb

L_ 2b

3b

TIME (MINUTES)

MARIETTA

[I o

n. t~

BASIN

I

I.iJ Z

j T I M E ( MINUTES )

ul ne

b.l I--W 0

OTHER O I L S

J

,o

20

TIME ( MINUTES )

Fig. 4. Gas chromatograms showing the n-alkane distributions of representative oils from (l) the Oklahoma Panhandle, (2) the Marietta Basin and (3) the remaining oils of this study.

No. 17 has a high Pr/n-Ct7 value (Table 3). The remaining oils are from Pennsylvanian reservoirs and, while having somewhat varied compositions, have saturate and aromatic hydrocarbon fractions that are enriched in 13C, and have relatively high Pr/Ph values. Among the subgroups of the Pennsylvanian oils, the Morrowan oils (Tables 3 and 5) are enriched in nitrogen content, have high Pr/Ph, Para/Naph and tricyclic terpane (C,9/(C~9+C20)) values and low sulphur content and Pr/n-Cl7 values. These oils are also unique with respect to their enrichment in ]3C. Pennsylvanian-Desmoinesian oils are only distinctive with respect to having low tricyclic terpane ((C~9+C~0)/Y.) values. Finally, the Pennsylvanian-Virgilian oil (No. 37) has a high sulphur content, a high n-C~s/n-Cl9 value and a low CPI value. Among the reservoir groups, no trends that could

be described as secular were observed. This is no doubt in part attributable to complications such as maturity, migration and mixing. The n-Cls/n-Cl9 values are low for the Cambro-Ordovician and many Ordovician oils. High quantities of n-C,9 relative to n-Cls may be attributed to the presence of an ancient form of the extant alga Gloeocapsamorpha prisca, prior to the evolution of vascular land plants (Foster et al., 1986). Despite the diversity of geologic settings and the large geographical area from which these oil samples were obtained, the mean stable carbon isotope values for the saturate hydrocarbon fractions of all reservoir age groups fall within the narrow range of - 2 9 . 2 0 to -31.52%o. If it is assumed that the saturate hydrocarbon fractions are no more than l ~ depleted relative to the whole oil isotopic values, this range may be considered typical for Paleozoic oils (Stahl,

166

MICHAELH. ENGELet al.

1977). If the Morrowan age group is excluded, the range of means for the remaining age groups is narrowed to - 30.0 to - 31.0%o. The distribution of tricyclic diterpanes in oils has been suggested by several workers to be source-related (e.g. Zumberge, 1983). The C~9 and C~0 homologs have, according to Reed (1977), Simoneit (1977) and Richardson and Miiller (1981), a possible vascular plant origin. It is interesting to note that the Cambro-Ordovician group appears to have the highest relative abundance of these biological markers. Although this was also observed for Ordovician oils in the Williston Basin (Zumberge, 1983), the possibility of removal of higher homologs by cracking cannot be discounted. While true secular trends with respect to age of source may exist in Oklahoma, the geologic complexity of the region and the present uncertainty of the identities (and ages) of the source rocks for the respective oils obscure them. It is clear from the data, however, that in attempting to establish secular trends for oils in this or any other region, thick and geologically complex sequences such as the Pennsylvanian should be divided into their respective stages. (3) Geologic province. Well locations for the 46 oils were assigned to six geologically defined areas, or provinces. These geologic provinces (Fig. 1) have been modified from those defined by Johnson et al. (1972). The northern shelf areas were divided into the "East" and "West" Northern Shelf Provinces, with the Nemaha Ridge as a boundary and the Panhandle Province as defined by the Oklahoma Panhandle. Included in the Anadarko Basin Province are oils from Garvin, Grady, McClain and Caddo Counties. Finally, the oils from Carter and Love Counties and the oil from Cotton County were assigned as the "Marietta/Ardmore Basin Province" and the "western Marietta Basin Province," respectively. From the compilation of general geochemical and biological marker parameters, it is apparent that numerous similarities and differences exist among the oils from the various geologic provinces. Most of the source-related parameters appear to be equivalent for the East and West Northern Shelf Provinces. This would indicate, perhaps, a similar source, and that the development of the Nemaha Ridge during the Morrow-Atokan Wichita orogeny did little to cause independent chemical evolution of oils in the Northern Shelf areas. A small discrepancy among tricyclic diterpane values, i.e. ( C t 9 + C 2 0 ) / ~ , w a s observed between oils of the Northern Shelf East (Nos 14, 19) and Northern Shelf West (Nos 5, 7, 10, 13, 38) Provinces. This may have resulted from maturity differences or relatively minor contributions of other oils (especially in the Western Northern Shelf, as indicated by the low n-Cls/n-Cl9 ratios). Oils from the Anadarko Basin Province show a few dissimilarities to the West and East Northern Shelf Province oils. This may result from, in addition to maturity dissimilarities and possibly the effects of migration, a basically common source (e.g. Wood-

ford shale) with local contributions of oils from Pennsylvanian shales found in the Northern Shelf

Provinces. Disparities exist among the tricyclic diterpane parameters between the primarily Ordovician oils of the Anadarko Basin Province and the Ordovician oils of the Marietta/Ardmore Basin Province, although most oils in both provinces display n-alkane distributions typical of what has previously been described as characteristic for Ordoviciansourced oils [i.e. strong odd-over-even predominance among C~5-C19 n-alkanes as previously indicated by Martin et al. (1963). Curiale (1983), Nunn et al. (1984), Fowler and Douglas (1984), and Longman and Palmer (1987)]. Mixing may have altered the character of oils from the highly faulted southeastern portion of the Anadarko Basin Province (e.g. Grady and Garvin Counties). Most intriguing of the contrasts that can be drawn among the provinces are those between the Panhandle Province oils and oils in all other provinces. The Panhandle Province oils are clearly distinct even from the oils of the neighboring Western Northern Shelf Province. In particular, the Panhandle oils exhibit a high Pr/Ph ratio and enriched 6~3C values of the saturate hydrocarbon fractions. Three possible scenarios that might account for these differences are (1) Panhandle Province oils have the same fundamental source as do most of the other oils, but they have significant contributions from the Pennsylvanian shales (e.g. Morrow) in the Northern Shelf areas (Hill and Clark, 1980), (2) Panhandle Province oils may reflect a facies change of the Woodford Shale, i.e. marine to terrigenous (e.g. Sullivan, 1985), and (3) Panhandle Province oils were sourced independently of the other Oklahoma oils. A possible source may be located in the Palo Duro and Dahlhart basins of the Texas Panhandle. Although the compilation of geochemical parameters by geologic province cannot be considered conclusive by reason of incomplete sampling of all major fields in each province, it is clear that changes in oil chemistry do occur from province to province. More importantly, one source bed, even one as extensive and organic-rich as the Woodford Shale (Comer and Hinch, 1987), is not likely to have sourced all Oklahoma oils (Burruss and Hatch, 1987). Grouping of oils on the basis of combined R- and Q-mode factor analysis using source-related analytical parameters and selected n-alkane abundances, as will be discussed below, further supports this conclusion. (4) Combined R- and Q-mode factor analysis. Combined R- and Q-mode factor analysis was performed on the Oklahoma oils using the seven source-related analytical parameters and normalized n-alkane abundances discussed above. The correlation matrix (Table 6) for the 14 variables provides the linear correlation coefficients for each combination of variables. Not surprisingly, there are several very positive and negative correlation coefficients among the n-alkane parameters. This is especially true for the

Organic geochemistry of Oklahoma oils

+1

..: . . .

r)

A I I I I

o

d

~

, IIII

lilt t~

,¢

~D~l.

eq¢,,i¢,,i

-+++'1111 e

,m

I

I I

I a~

I I

I

I I

E e,

8-~

....

IIII

I

II

I

~ - ' ~ I

PI

I

I I

III

~

167

negative correlations between low versus high molecular weight n-alkanes. Since the n-alkanes are norrealized to one another, this negative correlation may, in part, be attributed to closure effects. Correlations between n-alkanes and other parameters are more difficult to explain. Given the moderate maturities of most of the Oklahoma oils (e.g. Fig. 2), low n-C33 values are expected to be associated with low Pr/Ph ratios. If low Pr/Ph ratios are indicative of marine origin, one would expect the values of longerchain n-alkanes, which typically reflect a terrigenous origin, to be low as well. Another important correlation elucidates the relationship between 613Csat VS 613CArom. The highly positive c~ 13Csat/~'13CArom (+0.88) correlation is consistent with the conclusions of previous workers (Fuex, 1977; Stahl, 1977, 1979). Sofer (1984) used the relationship between stable carbon isotope values of the saturate and aromatic fractions of oils to distinguish oils of marine versus terrigenous origins. The 6t3CArom parameter also shows a significant correlation with two other parameters: Pr/Ph (+0.61) and %S (-0.57). These correlations may be artifacts of the data from the Panhandle and Marietta/Ardmore Basin Provinces. Panhandle oils have high Pr/Ph values and are enriched in ~3C. The Marietta/Ardmore Basin oils are rich in %S yet depleted in t3C. The simultaneous R- and Q-mode factor analysis of the analytical and n-alkane data are presented in Fig. 5. Together the first two factors account for about 60% of the total variation in the data set (factor 1, 37%; factor 2, 23%). Factors 3, 4 and 5 describe 18, 9 and 4% of the variation, respectively. The relative contribution of each of the 14 geochemical parameters to factors 1 through 5 are listed in Table 7. Normal alkane variables n-C~7, rt-Cl9 and n-C25 account for most of the loadings (82%) of factor 1, while Pr/Ph, %S and t / ' C 3 3 comprise most (63%) of factor 2. Considering only factors 1 and 2, the n-alkanes are four times as important as the remaining parameters are for describing variations in the data (i.e. n-alkanes=80%, other parameters = 20% of factor 1 and 2 loadings). Factor

ITable 7. R-mode relative contributions to factor Ioadings

I I I I t I I

I I

o

Factor l

Factor 2

Factor 3

Factor 4

Factor 5

Other parameters 6 ~3Cs~t t5UCArom Pr/Ph Pr/n-Ci7 %S Para/Naph CPI

0.35 0.24 0.50 0.02 0.06 0.12 0.27

0.23 0.46 0.64 0.03 0.50 0.03 0.27

0.06 0.20 0.00 0.57 0.01 0.60 0.04

0.25 0.05 0.03 0.10 0.11 0.13 0.41

0.03 0.01 0.22 0.06 0.12 0.01 0.14

n -Alkanes n-Ci7 n-Ci8 n-el9 n-C22 n-C25 n-C2s n-C33

0.82 0.49 0.79 0.35 0.85 0.68 0.05

0.01 0.25 0.03 0.35 0.00 0.12 0.74

0.02 0.18 0.01 0. l 1 0.04 0.17 0.03

0.11 0.06 0.06 0.30 0.03 0.00 0.01

0.01 0.01 0.00 0.16 0.04 0.01 0.01

7q

168

MICHAELH. ENGELet al.
major loadings on factor 1 (Table 7) with a high negative correlation (Table 6); 6 ~3Cs=and n-C]s have approximately equal loadings between factors 1 and 2. The length of the vectors in Fig. 5 correspond to the importance of the variables for describing variation in the data. Figure 6 shows a plot of the oil samples in the same factor space as Fig. 5 (Zhou et al., 1983). From this figure, two domains of oils are readily distinguished. These domains, or families, include the Pennsylvanian-Morrowan oils from the Panhandle Province (Family I) and the Ordovician oils from the Marietta/Ardmore Basin Province (Family IV). Most of the remaining oils cluster in two areas designated as Families II and III. While it is apparent that some overlap may exist between Families II and III, their separation is justified on the basis of several geochemical parameters discussed below. Oils not included in one of the four families, if not condensates, appear to be either mixes of oils or to have unique characteristics that preclude grouping. Oils 39 and 41 have normalized n-alkane distributions almost identical to those of Family IV oils. Oils 32, 34 and 35 are identical to oil 33 with respect to normalized n-alkane distribution, but they fail to cluster readily into Family III. Oils 22 and 31 have the same very low n-C]s/n-Cl9 ratios but do not plot close together. These oils have n-alkane patterns similar to the Family IV oils but thermal processes may have altered their chemical parameters.

2>

PR/PH

/ / "..o,,

018

Cp~

"

-110

/. ii)

.

,o ~0=8

:./

\

-1.0

\C33

Fig. 5. Combined R- and Q-mode factor analysis of selected n-alkanes and source-related geochemical parameters. Two condensates (oils 6 and 8) and oil 26 (possibly contaminated) were not included in the factor analysis.

3 is made up primarily of Pr/n-C~7, Para/Naph and 613C~om, while factor 4 is comprised mostly of CPI, 613Cs= and n-C22. The primary components of factor 5 are Pr/Ph and n-C22. In Fig. 5, the 14 variables are related to the factor 1 and 2 space. For example, n-C]7 and n-C25 are



1.0-

.2L

@

.a

•" ~ 1

FAMILY~

--<,.~s'2~

AMILY

is.1 ,21 10

-11o

I

r

I

/" 31

.3

"~-~_~/B17 * " - ~ 1-3a

3s

•

FAMILY

123-

7 I

,=~ .=~_/FAMILY III ,3o

.3z ~ . 3 4

'l

I 1.0

"

IV~

-1.0 Fig. 6. Location of the oil samples in the factor space shown in Figure 4.

Organic geochemistry of Oklahoma oils Initial work using n-alkane distributions to group the oils showed that oils 14, 17, 19, 21, 27 and 30 of Family III were more closely related to the Family II oils than to the remaining Family III oils. These oils may represent a mix of the aforementioned oils. The existence of intermediate values of several Family III analytical parameters to those of Family II and IV oils suggests that Family III may represent a mixture of the two families. Family I oils include the Pennsylvanian-Morrowan oils from the Panhandle Province as well as Pennsylvanian-Desmoinesian oils from the adjacent West Northern Shelf Province. Relative to the collective means and standard deviations of all the families, these oils may be described as having high %N, high Pr/Ph and tricyclic Cl9/(Cl9 + C20) ratios and depleted 6 13C saturate and aromatic values. The oils of Family II were produced from reservoirs of varying ages, but most are from the Anadarko Basin Province and the adjacent Northern Shelf-East Province. These oils are typified by a high n-C~a/n-C19 ratio and a low Para/Naph ratio. Family III oils represent no predominant reservoir age but all are from either the Anadarko Basin Province or the East and West Northern Shelf Provinces. These oils are distinguished only by their high Pr/n-Cl7 ratios. Ordovician oils from the Marietta/Ardmore Basin Province comprise all of the oils of Family IV. These oils are very high in %S and have low Pr/Ph, Pr/n-Cl7 and n-Cls/r/-Cl9 ratios. SUMMARY

169

providing the crude oils for this study. We thank J. A. Curiale and Z. Sofer for their critical reviews of the manuscript. M. H. Engel wishes to acknowledge the National Science Foundation, Division of Earth Sciences (Grant No. EAR-8352055) and industrial contributors to his Presidential Young Investigator Award (Atlantic Richfield, Mobil, Phillips Petroleum, Texaco, Inc.) for support of this research. We thank R. Benthien for assistance with the drafting and M. Starr for typing the manuscript. REFERENCES

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Organic geochemical analyses of oils from Paleozoic reservoirs from throughout the State of Oklahoma indicate that the majority of these oils are of remarkably similar composition, given the structural complexities of the region as a whole and the likelihood of compositional differences arising from mixing, migration, slight differences in maturity and the possibility of multiple sources. Combined R- and Q-mode factor analysis resulted in the delineation of four relatively distinct geochemical families of oils. It is not, however, implied that these families necessarily reflect unique sources. Explanations for subtle differences in composition that resulted in the statisUSGS Research on Energy Resources--1986 Program and tical formation of these groupings are complex. In Abstracts (Edited by Carter L. M. H.). USGS Geological addition to analyzing other oils from throughout Survey Circular 974, pp. 21-23. the region as they become available, an organic Hill G. W. Jr and Clark R. H. (1980) The Anadarko geochemical investigation of potential source rocks in Basin--A regional petroleum aceumulation--A model Oklahoma is planned. It is anticipated that additional for future exploration and development. Shale Shaker 31, 238-251. work on this problem will not only assist in the clarification of the origin(s) and genetic relationships Hitchon B. and Filby R. H. (1984) Use of trace elements for classification of crude oils into families---example from of Oklahoma oils, but will serve as a guide when Alberta, Canada. Bull. Am. Assoc. Pet. Geol. 68, 838-849. attempting to correlate oils from different reservoirs Johnson K. S., Branson C. C., Curtis N. M. Jr., Ham W. E., Harrison W. E. Marcher M. V. and Roberts J. F. in regions of large geographic extent. (1972) Geology and earth resources of Oklahoma--An atlas of maps and cross sections. Oklahoma Geol. Surv. Acknowledgements--We wish to thank C. Sehiefelbein, A. Educ. Publ. 1, I-8. Jones, S. Sellers, M. Heard and J. Williamson for their assistance with the analyses. We thank Cities Service Corp. Koons C. B., Bond J. G. and Peirce F. L. (1974) Effects of depositional environment and postdepositional history on and the former Bartlesville Energy Technology Center for

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