Short-term biomarker variability in the Monterey formation, Santa Maria Basin

Org. Geochem. Vol. 14, No. 1, pp. 1-13, 1989 Printed in Great Britain. All rights reserved.

0146-6380/89 $3.00+0.00 Copyright © 1989PergamonPress plc

Short-term biomarker variability in the Monterey Formation, Santa Maria Basin JOSEPH A. CURIALE and JOHN R. ODERMATT Unocal Science & Technology Division, P.O. Box 76, Brea, CA 92621, U.S.A. (Received 23 May 1988; accepted 14 August 1988) Abstract--Forty-eight samples of a 370-ft Miocene Monterey Formation core from the Union Leroy 5I-18 well, Santa Maria Basin, California, are examined for source rock potential and molecular distributions. Total organic carbon content, Rock-Eval pyrolytic yield, vitrinite reflectance, and distribution of regular steranes, tricyclic and pentacyclic terpanes, and aromatic steroids are measured. The present-day temperature range through the sampled interval is estimated to be less than 5°C; maturity differences among samples are negligible.Source rock potential and molecular distribution differences are considered to arise entirely from the effects of source input, diagenesis (including lithologic effects) and migration. The sampled section is comprised of siliceous and phosphatic/carbonate lithofacies, and covariances between geochemical parameters (source potential and biomarker distribution) and lithofacies are recognized. Several molecular parameters change systematically with depth/lithology, including flfl/(ctct + tiff) ethylcholestane ratios and C-20 and C-22 sterane and hopane epimer ratios. The siliceous lithofacies possesses higher moretane/hopane, tricyclic/pentacyclic terpane, and sterane/terpane ratios, and lower mono/mono + tri aromatic steroid ratios. Bisnorhopane/hopane ratios are up to 40 times higher in the phosphatic/carbonate lithofacies than in the siliceous lithofacies. Increased relative concentrations of aromatic steroids are attributed to sulfur-sterol reactions during very early diagenesis, whereas increased relative bisnorhopane amounts are considered to result from massive input of specific anaerobic biota. Although kerogen elemental ratios change abruptly at the lithofacies boundary, molecular distributions within the extractable organic matter change gradually, beginning about 30 ft (9 m) above this interface. This gradual change is tentatively attributed to upward migration of hydrocarbons originating in the phosphatic/carbonate lithofacies, probably through fractures within the lower part of the siliceous lithofacies interval. Key words--Monterey Formation, biomarkers, Santa Maria Basin, siliceous lithofacies, phosphate/ carbonate lithofacies, bisnorhopane, migration, lithologic variability.

characterizing biomarker variability (both lateral and temporal) in the available source rocks (e.g. Ji-Yang et al., 1982; Hughes et al., 1984; Mackenzie et al., 1983; Welte et al., 1982; Curiale et al., 1985; Wehner and Teschner, 1982; and many others). Recent work involving petroleum source rocks has indicated that short-term (temporal) geochemical variability, both for bulk (Macko and Quick, 1986) and molecular parameters (Moldowan et al., 1986), can be considerable. Wenger and Baker (1986) examined Pennsylvanian cyclothems from Kansas and Oklahoma, and demonstrated significant variations in total organic carbon content, vitrinite reflectance values, pristane/phytane ratios and vanadium concentration, over a 7.6 ft (2.3 m) interval. Considerable variability in molecular parameters was also noted by Moldowan et al. (1986) over a 5 m interval of the Toarcian shale in southwest Germany. Of particular interest in their study is the (relatively minor) variation noted in several maturation parameters, suggesting that diagenetic alterations may provide a secondary overprint to maturation-induced molecular variability. Each of these studies provides a strong indication that interpretations which seek to charac-

INTRODUCTION Biological markers in fine-grained sedimentary rocks have been the subject of considerable study since the term was first proposed by Speers and Whitehead (1969). Seifert (1977) initiated an increased awareness of the application of biomarkers in the petroleum industry, and their utility in exploration efforts is now widely recognized. Most of the classic studies involving biomarker applications (e.g. Seifert and Moldowan, 1978, 1979, 1981; Leythaeuser et al., 1977), as well as the many regional efforts produced in the 80s (reviewed by Mackenzie, 1984; Philp, 1985), have focussed upon large-scale variations in molecular composition, with adjacent samples separated laterally by many kilometers and/or vertically by hundreds of meters. Whereas this focus is often required because of high analytical expenses and the difficulties of handling very large amounts of data, the large-scale emphasis may leave the mistaken impression that short-term biomarker variability is minor. This often leads investigators to proceed with biomarker-centered source rock-oil correlation efforts without first fully 1 O(3, 14'1

A

2

JOSEPH A. CURIALE and JOHN R. ODERMATT

terize the organic composition of a source rock unit based upon a limited sample set can be subject to errors of unknown type or extent. The studies by Wenger and Baker (1986) and Moldowan et al. (1986) are of special interest because their results must be almost entirely maturationinvariant. Indeed, given the lack of non-indigenous (i.e. migrated) organic matter in the study section, observed geochemical variability must result exclusively from source input and diagenetic (including lithology-induced) influences. In the present paper, we study the effects of source input and diagenetic change on the organic matter within 370 ft (113 m) of continuously-cored Monterey Formation from southern California. Our purpose is to examine lithology-induced and source input-related molecular variability in extractable organic matter of a nonclastic petroleum source rock sequence in the absence of maturation effects. Both the economic potential (as source rock and reservoir) and the geographic extent of the Monterey Formation has kept interest in this mid-Miocene unit very strong. Studies involving source rock character and molecular distributions of the Monterey Formation were reviewed up through the mid-80s by Curiale et al. (1985). Theories about the origin of high sulfur concentrations in Monterey oils and associated organic matter were subsequently reviewed by Orr (1986). That same year, Bromley and Senftle (1986) examined and compared asphaltene pyrolyzates as indicators of organic matter character within Monterey kerogens, while Kruge (1986) used biomarker data to infer extensive vertical migration through mid-Miocene Monterey Formation equivalents in the San Joaquin Basin. Tannenbaum et al. (1986) examined the genetic relationship between extracts and pyrolyzates of a Monterey Formation diatomite in Belridge Field, and concluded that some portion of the bitumen present in this diatomite is non-indigenous. The migrated nature of soluble organic matter is a general concern throughout the Monterey, due to the often coincident sourcereservoir nature of the producing section (Isaacs, 1984; Curiale et al., 1985). Geologic f r a m e w o r k

The coincident source-reservoir character within the Monterey Formation is most pronounced in the mid-Miocene of the Santa Maria Basin, the location of our study core from the Union Leroy 51-18 well (Santa Maria Valley Field, western area, Santa Barbara County, Calif.--Fig. 1). The regional depositional environment of the basin during the Miocene was a sediment-starved dysaerobic to anoxic marine basin, based upon geologic (Pisciotto, 1978; Pisciotto and Garrison, 1981; Pisciotto, 1981) and geochemical evidence (Summerhayes, 1981; Magoon and Isaaes, 1983; Curiale et al., 1985; Lewan, 1980; Odermatt, 1986). The geometry of the stratigraphic units within the Santa Maria Valley Field suggests that the Mon-

terey Formation has an on-lap relationship to a middle Miocene high to the north and thickens to the south (Woodring and Bramlette, 1950). The subsurface distribution of the Miocene section is further complicated by the presence of east-west and northwest-southeast trending normal faults, and the possibility of regional thrust faults as interpreted from seismic data (Crouch et al., 1984). The Monterey Formation of the Santa Maria Basin is generally comprised of a variety of lithologies, including phosphatic shale, thinly bedded chert and cherty shale, diatomaceous shale, siliceous shale, carbonate beds (dolomite) and foraminiferal shales (Woodring and Bramlette, 1950; Pisciotto, 1978). The lithologies present in the Monterey of the Union Leroy 51-18 core include dolomites, siliceous shales, rhythmites (Pisciotto, 1978), volcanic ash beds, black shales, phosphatic shales, phosphatic mudstones and bedded silica (Odermatt, 1986; Odermatt et al., 1989). A generalized stratigraphic column from the Union Leroy 51-18 core, based upon the lithologies of samples selected for geochemical study, is shown in Fig. 1. The core from the Union Leroy 51-18 well contains two major lithofacies, defined after the nomenclature of Pisciotto (1978) and Pisciotto and Garrison (1981). The lithofacies are comprised of (a) siliceous shales, bedded silica and volcanic ash beds (siliceous lithofacies), and (b) phosphatic shales, black shales, dolomite beds and phosphatic mudstones (phosphatic shale-carbonate lithofacies). For the purposes of the present study, the two lithofacies are defined using inorganic geochemical techniques, and can be distinguished by three major criteria: the abundance of biogenic silica; abundance of carbonate; and the presence of phosphatic shale (Odermatt, 1986; Odermatt et al., 1988). In the following, we briefly describe the major lithologies and mineralogies of the two lithofacies; further details are available in Odermatt (1986). In the phosphatic shale/carbonate lithofacies, phosphate occurs in fine laminae and irregular to elongated globules (commonly referred to as blebs), and it is particularly abundant in this well from the contact with the overlying siliceous lithofacies at 4645ft (1416m) at least to a depth of 4774ft (1455 m). Bentonites are also present, and white ash beds are noted at 4610.8ft (1405.4m), 4702.6ft (1433.4m) and 4779.9ft (1456.9 m). Most of the carbonate observed in the section occurs in an interval from 4725 to 4875 ft (1440-1486 m). Dolomite occurs as beds (up to 1 ft thick) in this interval, and it also makes up a significant proportion of the finer grain lithologies (e.g. shales, mudstone). Samples from this interval are generally either relatively soft shales and mudstones, or hard dolomite beds which are commonly associated with macroscopic fractures. In the siliceous lithofacies, silica-rich lithologies (having higher biogenic silica contents) are confined to the shallow portion of the core (<4645ft;

Short-term biomarker variability in the Monterey Formation, Santa Maria Basin

3

Arroyo SILICEOUS SHALE LITHOFACIES GUADALUPE

t~aria, SAN MA., .

.

.

.

__ __ Z--.--:-T . . .

.

.

.

.

vA,LEY

Z ~

Siliceous/Phosphate m

--~ . . .

~

.

-_~-----------~ = =2-Z ----2----2

Shale Boundary LEGEND

SILICEOUS SHALE L ~

r

Los Alamos

DOLOMITE

~):

LOMPO(

PHOSPHATIC-SHALE MUDSTONE

CORE UNAVAILABLE

',

PHOSPHATE/ CARBONATE LITHOFACIES

~Buellton

,rUCBS

31NT CONCEPTION

/ /

0 t

8 I

16 t

Miles

Fig. 1. Location map for the Union Leroy 51-18 well, Santa Maria Valley Field, Santa Maria Basin, Calif. Sample locations are designated by arrows on the lithofacies column; top and bottom sample depth are noted. Lithofacies column and map adapted from Odermatt (1986).

<1416m), and occur in beds which are very well indurated and finely laminated. The laminated character of these rocks results from alternating layers of biogenic silica, and thin intercalated clay laminae. In many cases, samples from this lithofacies are hard, brittle and exhibit a number of fractures. The siliceous rocks of the Monterey Formation in this well, as for those of the Monterey throughout the Santa Maria Basin, are present at different diagenetic levels. Pisciotto (1978) and Isaacs (1983) discuss crystallographic phase changes from biogenic opal-A, through diagenetic opal-CT and finally to cryptocrystalline quartz. In the Leroy 51-18 well, only the opal-CT and quartz phases are present, with the diagenetic boundary approximately coinciding with the lithofacies boundary (Odermatt, 1986). Such a coincidence is fortuitous in this case, as this phase change is commonly reported as discordant to lithologic alterations elsewhere in the basin (Isaacs, 1983; Odermatt, 1986, and references therein). Each of the two predominant lithofacies is organically distinctive. The remainder of this paper deals with these distinctions, and discusses details of the source rock potential and molecular distribution within the Monterey Formation of the Union Leroy

51-18 well. Following a section concerning sampling extent and methodology, we present molecular data and supporting source rock screening information for the core samples. The subsequent discussion will (a) present information concerning lithology-biomarker relationships, (b) question the utility of certain molecular maturity parameters, and (c) provide specific evidence for fluid migration within the cored interval. METHODS

Forty-eight samples were collected from the Union Leroy 51-18 core; sampling size and frequency varies with lithology. Three sample classes were collected. (1) Samples spaced evenly at 10 ft (3 m) intervals, beginning at a randomly chosen location at the top of the core (4484-4864 ft; 1367-1483 m). The actual placement of these samples was determined by core availability (e.g. the interval from 4725.5 to 4774 ft was not available). (2) Spot samples located within specific lithologies or particularly interesting stratigraphic features within the cored interval. Some of these samples were located to examine whether migrated bitumen is associated with the extractable organic matter of dolomites and fractured siliceous

4

JOSEPHA. CURIALEand JOHN R. ODERMATT

shales. Others were located at spacings of less than 1 ft, in order to study the variation of bitumen composition over a very closely spaced interval. (3) Samples chosen as precision checks (i.e. replicate samples; e.g. 4524.5 and 4804 ft). Total organic carbon and Rock-Eval data were obtained by conventional methods (DGSI, Houston) on all of these samples; vitrinite reflectance determinations were made on selected whole rock samples by Dr J. M. Jones (Newcastle, U.K.), and are reported where primary vitrinite maceral counts exceeded 20 readings per sample (Table 1). Each of the 48 core samples was crushed and powdered prior to extraction; replicate samples were handled independently from this step onwards. Rocks were extracted and subsequently processed as described in Curiale et al. (1985). Chromatographically-obtained branched and cyclic aliphatic hydrocarbon fractions were analyzed by gas chromatography and gas chromatography-mass spectrometry (GCMS) according to conditions described in Curiale et al. (1985), except that a 25 m DB-5 (0.25 mmi.d.) column was used for GCMS analyses. Certain biomarker data are displayed as described in Fig. 11 of Curiale (1986). All ratios discussed are derived from area integration of mass chromatographic data (uncorrected), and have an estimated relative precision of 7%. RESULTS Published results for the total organic carbon (TOC) content, extractable organic matter (EOM) yield and EOM/TOC ratios for the Monterey Formation in the Union Leroy 51-18 well suggest that this unit is an excellent petroleum source rock in the Santa Maria Valley Field (Curiale et al., 1985). Our detailed TOC and Rock-Eval analyses at an average interval of 7ft (2.4m) over 370ft (113 m) of continuous Monterey section in this well confirm the source conclusions published previously. Table 1 lists TOC contents and primary ($1, $2, $3, Tmax) and secondary ($I/S~ + $2, HI, OI) Rock-Eval data, for both whole rock and extracted rock samples throughout the Monterey section of the well. Whole rock TOC and HI values range up to 17% and 750mg hydrocarbon/g organic carbon, respectively. Relatively low S~/S I + $2 values (generally 0.05-0.15%, Table 1) suggest that the EOM in these samples is largely indigenous, despite the occurrence of EOM contents in excess of 4% of the rock (by weight; Table 2 of Curiale et al., 1985). Modified van Krevelen diagrams for the data in Table 1 are presented in Fig. 2 for both whole rock samples (left) and extracted rock samples (right). The relatively wide range of oxygen indices in the whole rock data set is diminished considerably in the extracted rock data, with extracted rock oxygen indices often being about half of whole rock oxygen indices (Table 1). In terms of chemical kerogen type (Tissot

and Welte, 1984), both Types I and II organic matter are present in the samples, and appear to be lithologically correlative. Samples from the phosphate/ carbonate lithofacies (below 4645 ft; 1416 m) contain predominantly Type I kerogen, whereas those in the siliceous lithofacies (above 4645 ft; 1416 m) contain predominantly Type I/II kerogen; this distinction is particularly evident in the extracted rock data (Table 1; Fig. 2, right). Our data indicate that thermal maturity is relatively low for these samples. Tmax values range between 396 and 413°C in the whole rock, and vitrinite reflectance (VR) values (determined on whole rock preparations) are 0.27-0.48% Ro. No particular depth trend is evident in the T~aXand VR data (Table 1), or in the kerogen H/C atomic ratio data (Odermatt, 1986), suggesting that these thermal maturity parameters are not sensitive enough to indicate monotonic thermally-induced chemical distinctions over this limited depth range (370ft; 113 m). This result is not particularly surprising: using a mean annual surface temperature of 13.8°C and the 42°C/km geothermal gradient proposed for the Santa Maria Valley Field area (French, 1940; Pisciotto, 1981), the in-place present-day temperature range of the study core is only 71-76°C. Such a small temperature range (particularly for rocks deposited only 12-15mybp) is evidently insufficient to yield any perceptible Tm~x or VR trend. In such situations, thermal maturity trends are only observable with highly sensitive molecular maturity parameters (e.g. Curiale, 1988). The biomarker distribution in the extractable organic matter of the Monterey Formation in the coastal basins of southern California is distinctive (Seifert et al., 1978; King and Claypool, 1983), and is characterized by the almost complete absence of diasteranes, the dominance of cholestanes (relative to C28, C29 and C30-desmethyl steranes), and the prominence of monoaromatic steroid hydrocarbons and certain nuclear demethylated pentacyclic triterpanes (including 25,28,30-trisnorhopane and 28,30bisnorhopane; Curiale et al., 1985). Almost all of these features (except for the occurrence of 25,28,30-trisnorhopane) are evident in the EOM of the Union Leroy 51-18 Monterey samples. Detailed molecular distributions for nine representative Monterey rock extracts from this well were presented by Curiale et al. (1985); these distributions are very similar to those observed for the 48 core samples of the present study, selected examples of which are discussed below.

DISCUSSION Short-term variability o f molecular " m a t u r i t y " parameters

The 71-76°C temperature range over this core indicates that conventional thermal maturity param-

Short-term biomarker variability in the Monterey Formation, Santa Maria Basin Table 1. Source rock screening data--whole rock/extracted rocka Depth (ft)

Sample

TOC/TOC

SI/S I

$2/S 2

$3/S 3

HI/HI

OI/OI

PI/PI

Tma~/Tmax TCC

VR

4484.00 4494.00 4504.00 4514.00 4524.00 4524.50 4534.00 4544.30 4554.00 4564.00 4574.00 4584.00 4587.00 4589.30 4594.00 4594.75 4604.00 4614.00 4624.00 4634.00 4645.00 4654.00 4664.00 4674.00 4684.00 4692.40 4693.00 4700.30 4704.00 4713.50 4714.00 4725.30 4774.00 4784.00 4793.20 4793.30 4793.50 4794.00 4804.00 4804.00 4804.00 4814.00 4825.80 4828.80 4834.00 4844.00 4845.40 4847.50 4854.00

01 02 03 04 05 05 06 07 08 09 10 I1 12 13 14 15 16 17 18 19 20 21 22 23 25 26 27 28 29 30 31 32 33 34 35 36 37 38 50 50 39 40 41 49 42 43 44 45 46

3.68/3.21 5.85/4.87 12.55/11.60 6.70/5.66 4.30/4.33 5.85/4.87 2.09/1.80 1.71/1.48 12.01/11.28 4.61/3.74 12.45/11.74 3.50/3.06 2.39/1.83 0.25/0.20 5.12/4.07 1.76/1.56 11.02/10.19 16.91/14.67 3.88/4.62 16.15/15.13 9.11/8.16 17.61/16.02 6.72/5.39 11.43/10.17 8.67/-3.59/3.17 14.17/12.23 15.39/13.66 6.74/6.13 5.54/4.05 7.80/7.01 1.14/I.08 12.83/11.17 5.20/3.95 9.08/8.27 13.23/11.77 14.48/12.59 5.01/3.85 11.93/10.79 11.93/10.79 11.99/10.65 2.07/I.58 5.68/4.68 10.32/8.94 2.04/1.58 6.36/5.64 4.56/3.93 1.52/1.35 6.04/5.06

2.9/0.8 3.7/2.1 5.3/2.8 4.5/1.2 3.2/1.0 3.7/2.1 1.6/0.5 1.6/0.4 5.9/3.4 3.3/1.1 6.3/2.7 2.6/0.8 1.7/0.4 0.1/0.0 3.0/1.0 1.0/0.2 6.8/2.8 9.6/2.9 2.1/1.0 8.7/4.2 3.9/2.5 8.9/7.4 2.2/0.4 3.4/2.9 2.8/-2.7/0.5 10.6/7.1 5.8/2.2 2.5/0.8 5.4/1.0 2.7/1.4 0.4/0.1 10.0/3.1 5.0/0.1 5.9/2.3 8.0/4.1 7.9/3.6 3.6/1.1 7.4/3.7 7.4/3.7 8.5/2.5 1.8/0.1 4.1/1.0 6.4/2.8 1.5/0.1 3.2/0.9 1.9/0.4 0.5/0.1 4.3/1.1

22.7/19.6 30.1/26.8 75.6/68.2 36.5/30.2 27.9/26.4 30.1/26.8 11.2/9.6 8.5/7.5 81.8/75.9 28.0/21.8 86.9/78.4 19.8/18.9 12.9/9.2 0.6/0.3 28.5/23.0 10.3/9.6 81.4/71.0 114.5/102.7 23.6/28.3 114.9/102.3 66.7/56.5 125.1/108.9 43.0/39.0 83.7/71.1 63.2/-25.3/20.2 99.4/86.2 99.6/82.4 50.2/46.6 40.9/27.5 56.0/50.2 5.5/5.5 98.4/85.8 38.9/27.4 66.1/60.9 98.8/88.2 108.5/89.4 36.9/28.5 88.5/81.3 88.5/81.3 90.6/77.5 14.2/9.0 42.1/36.5 79.7/67.3 13.4/9.8 48.1/42.4 22.8/18.9 5.6/4.7 44.9/37.0

0.9/0.8 1.0/1.0 2.0/1.9 1.1/1.3 1.3/1.1 1.0/1.0 0.7/0.6 0.7/0.4 1.6/1.6 2.0/0.8 2.0/I.7 1.1/0.8 1.0/0.6 0.8/0.4 1.0/0.8 1.0/0.7 1.6/I.4 2.6/I.9 2.1/I.2 2.3/I.9 2.5/I.7 3.9/2.5 3.1/I.6 3.3/1.9 1.8/-1.9/1.1 2.1/1.8 2.8/2.3 2.3/1.5 1.8/1.2 2.2/1.4 1.7/0.9 1.9/I.5 1.4/I.0 1.8/I.3 2.2/I.6 2.5/I.9 2.0/1.1 1.9/1.5 1.9/1.5 2.1/1.6 1.6/0.9 1.4/I.0 1.5/I.2 1.4/0.8 1.5/I.1 1.3/0.9 1.4/0.7 1.3/I.0

616/611 514/551 603/588 544/533 648/610 514/551 535/534 499/508 682/673 608/584 698/668 565/619 538/502 228/150 557/565 585/616 739/697 677/700 609/614 711/676 732/693 711/680 639/724 733/699 729/-705/636 701/705 647/603 745/760 739/680 718/716 482/512 767/768 749/694 728/737 746/749 749/710 737/740 742/754 742/754 755/728 686/571 742/780 773/753 657/620 756/752 501/481 369/347 743/732

24/24 17/21 16/16 16/23 30/26 17/21 35/34 39/30 13/14 43/21 16/14 31/25 44/33 304/195 20/19 60/42 14/14 15/13 55/25 14/12 28/21 22/16 47/31 29/19 21/-52/36 15/14 18/17 34/25 32/29 28/20 147/85 15/14 28/27 19/15 17/14 17/15 40/29 16/14 16/14 18/15 79/56 24/21 15/14 67/49 24/19 28/23 93/52 21/20

11/4 11/7 7/4 11/4 10/4 11/7 13/5 16/5 7/4 11/5 7/3 12/4 12/4 14/12 10/4 9/2 8/4 8/3 8/3 7/4 5/4 7/6 5/1 4/4 4/-10/3 10/8 6/3 5/2 12/3 5/3 8/I 9/3 11/1 8/4 7/4 7/4 9/4 8/4 8/4 9/3 11/2 9/3 7/4 10/1 6/2 8/2 9/3 9/3

401/402 390/392 398/397 396/394 399/403 390/392 402/404 401/402 401/401 402/403 404/403 403/403 403/404 413/417 401/400 407/408 405/405 404/403 404/403 405/402 403/404 405/404 404/404 407/406 404/-409/410 405/406 403/401 407/409 402/409 405/409 407/410 410/411 412/411 408/409 411/410 411/410 407/408 411/410 411/410 411/410 412/410 409/411 411/411 410/411 408/411 399/401 404/403 407/411

-0.41 0.38 ----------

11.5 6.9 12.4 8.2 52.3 6.9 3.7 4.0 7.9 7.8 15.8 4.2 3.8 93.9 7.0 83.4 40.7 7.2 2.4 6.5 44.7 27.6 41.4 38.5 56.4 81.5 34.0 17.1 47.5 11.7 44.8 95.1 43.5 79.1 58.0 36.8 28.4 65,4 52.4 52,4 52.8 90.1 55.9 41.7 89.6 39.9 11.8 30.1 20,8

----0.29 --0.27 -0.43 ----------

0.42 0.48 0.48 --0.36 ------

~'I'OC = total organic carbon (%); S~, $2 and $3 in mg hydrocarbons per gram rock; HI and OI in mg hydrocarbon per gram organic carbon; PI: S~/S I + $2 (%); Tm~xin "C; TCC = total carbonate content (%); VR = vitrinite reflectance (% Ro).

eters should show little to no difference in thermal maturity. This is indeed the case for Tma~ and VR values discussed earlier, and it is also the case for molecular maturity parameters. Furthermore, the expected lack of variation in thermal maturity indicates that this sample set would be ideal for use in assessing the reliability of various indicators as maturation parameters (Moldowan et al., 1986; Curiale et al., 1988). Several molecular indicators of thermal immaturity have been noted in both oils and rocks of the Monterey Formation, including the occurrence of sterenes and hopenes (Giger and Schaffner, 1980, 1981), 5fl-steranes (King and Claypool, 1983) and unusually low 20S/20R-5~ (H),14~t (H),I 7~ (H)ethylcholestane ratios (Seifert and Moldowan, 1981). Each of these has also been observed in our work on mid-Miocene rocks and oils from several southern

California basins (Curiale et al., 1985). In addition, we now report the occurrence of spirosterenes in Monterey extracts from the Leroy 51-18 well (identified via mass chromatography and comparison with literature spectra; Peakman et al., 1984; Brassell et al., 1984). The C27, C28 and C29 spirosterenes are present with the C28 compounds dominant; approximately equal amounts of the 20S and 20R epimers were observed for each homologue. In addition to the occurrence of these unsaturated aliphatic components, conventional sterane and hopane maturity parameters were monitored throughout the study section. Fig. 3 shows the depth variation of four such parameters. The moretane/hopane ratio (Panel I) shows significant and non-monotonic variation with depth; values in the upper siliceous interval of the core are generally higher than those in the lower phosphate/carbonate interval.

6

JOSEPH A. CURIALE and JOHN R. ODERMATT 1000

!

900 W H O L E ROCK DATA

E X T R A C T E D ROCK DATA

800 700

"r."., ." ;,,

X hi

600

~

In'elm ° Ill •

H Z I.U 5 0 0 O >."1-

400

11I

500

/

200 100

[ : pSIHLIoCEpOHUSTELITHO;ACNIA. ETS .......

i

0

25

50

75

100

125

150

OXYGEN

25

50

75

100

CIE$ ]

125

150

INDEX

Fig. 2. Van Krevelen diagrams for whole rock (left) and extracted rock (right) samples from Monterey Formation, Union Leroy 51-18. Hydrogen index and oxygen index values are reported as mg hydrocarbons per gram organic carbon and mg carbon dioxide per gram organic carbon, respectively. Samples from the phosphate/carbonate lithofacies contain chemical Type I kerogen, whereas siliceous lithofacies samples contain chemical Type I/II kerogen; see text. Data from Table 1.

The most c o m m o n l y used epimer-based molecular maturation parameters, 22S/22S + 2 2 R - h o m o h o p a n e and 20S/20S + 20R-ethylcholestane, are presented in the center two panels o f Fig. 3. These values are generally near (for the h o m o h o p a n e ratio) or (for the

44~0 J_

sterane ratio) slightly below recognized equilibrium values (Mackenzie, 1984), although anomalous data points are evident (particularly at the top and middle o f the core for the 22S/22S + 22R and 20S/20S + 20R ratios, respectively). A n unusual finding, especially

I.JL

.

I1.,~1

•

•

,v

4500.

miD

Nstnl~

m Ill

III.IClOUI

•

LITHOFACIII

n mR

Do

PliOSPHATE/CARIlONATE LITI4OFACIE$ aid

4700

O• •

eO

OO

• oeo~ e

O O

IO

•

Oo

•d o oq

oo

4000

&rio

&05

1110 MORETANE HOPANE

0.15

n00 ~

~

0.20

n40 225 22S+22R-C~

~80

n00

~20

&40

20~ ~ 4 k - - - - - 2 0 S + 2 0 R -Ca

870 ~

~

080

~g0

MONOMONO_+TRIAROMATI c STEROIOS

Fig. 3. Variation of conventional molecular maturation parameters with depth in the Leroy 51-18 well. Lithofacies boundary between siliceous and phosphate/carbonate lithofacies (Odermatt, 1986) is shown as a horizontal line. Panel I: Moretane/hopane ratio, measured by area counts on m/z 191 mass chromatogram, Panel II: 22S/(22S + 22R)-homohopane ratio, measured by area counts on m/z 191 mass chromatogram, Panel III: 20S/(20S + 20R)-5c( (H),I4~ (H), 17~ (H)-ethylcholestane ratio, measured by area counts on m/z 217 mass chromatogram. Panel IV: Ratio of monoaromatic to (monoaromatic and triaromatic) steroid hydrocarbons. "Monoaromatics" are considered as the area sum (m/z 253) of the ten major (C27-C29) monoaromatic steroid peaks, while "triaromatics" are the area sum (m/z 231) of the five major (C26-C28) triaromatic steroid peaks. The peaks/compounds used in this ratio are those defined in Fig. 11 of Curiale (1986).

1.00

Short-term biomarker variability in the Monterey Formation, Santa Maria Basin for the homohopane ratio in Fig. 3, is the slight but definite trend away from equilibrium with increasing depth. Whereas epimerization "reversals" were observed by Lewan et al. (1986) in the course of hydrous pyrolysis experiments, trends away from equilibrium with depth are not common in natural data. Because the epimer ratio difference from top to bottom of this core is so small, and because the present-day temperature range through the sampled section is only 5°C, it is likely that the epimer ratio variations with depth recorded in Fig. 3 are not maturity-induced (Moldowan et al., 1986). Panel IV in Fig. 3 shows the variation of the relative content of mono- and triaromatic steroid hydrocarbons in the core extracts. These hydrocarbon types are represented predominantly by the monoaromatic steroids in the lower portion of the core, whereas relatively significant amounts of triaromatic steroids are present toward the top. This observation is opposite that expected for maturityinduced aromatic steroid variability (Mackenzie, 1984): in this case, the more mature section of the core contains the lowest relative triaromatic steroid content. It is possible that alternate (direct) sources of triaromatic steroids (other than from aromatization of monoaromatic analogs) must be considered (Curiale et al., 1985). Relationship between biomarkers and lithofaeies

The Monterey Formation in the Union Leroy 51-18 core shows significant lithologic variability, resulting in the division of the core interval into two major lithofacies. As noted earlier, the siliceous lithofacies extends from the top of the core to approx. 4645 ft (1416m), whereas the phosphate/carbonate lithofacies is found below. The present section will discuss observed relationships between lithofacies and biomarker distribution. The major lithofacies boundary is denoted in Fig. 3 as a horizontal line at 4645ft (1416m). Whereas the epimer ratios in this figure (Panels II and III) show no apparent discontinuity at this depth, definite differences are noted above and below this boundary for the moretane/hopane ratio (Panel I) and the aromatic steroid ratio (Panel IV). Specifically, both moretane and triaromatic steroid hydrocarbons appear to be relatively enriched in the siliceous interval. The differences are dramatic for the mono/mono + triaromatic steroid ratio, although it is interesting that the ratio contrast at the lithofacies boundary is not sharp, but gradational. This point is significant, and may suggest past fluid migration within the core, as discussed later. The relationship between relative monoaromatic steroid content and lithofacies in this core is of particular interest in light of the unusually high concentration of monoaromatic steroids in both Monterey rock extracts and reservoired oils (Seifert et al., 1983; Curiale et al., 1985, and references therein). The high concentration of these compounds within

7

the Union Leroy 51-18 core is usually accompanied by relatively high 28,30-bisnorhopane content [note that all references to this compound in this paper refer to the total of the 17~(H),18~(H),21fl(H) and 17fl(H),18~(H),21fl(H) isomers; Moldowan et al. (1984)]. Curiale et al. (1985) noted unusually high bisnorhopane/hopane ratios in selected Monterey extracts of the Leroy well. Fig. 4 illustrates these observations in one display, showing (a) the shortterm variation of the bisnorhopane/hopane ratio with depth, and (b) typical hydrocarbon gas chromatograms for the two lithofacies present in the core. Of initial interest in Fig. 4 are (a) the large range (two orders of magnitude) of the bisnorhopane/ hopane ratio, and (b) the large increase in this ratio in the upper portion of the phosphate/carbonate lithofacies. Variations in this ratio in oils and rock extracts have been attributed to either early diagenesis (Grantham et al., 1981) or a combination of original source input and thermal maturity (Curiale et al., 1985; Moldowan et al., 1984; Cornford et al., 1983; Katz and Elrod, 1983). Within our sample set, samples from the siliceous lithofacies have the lowest bisnorhopane/hopane ratios measured in the core [note that the siliceous lithofacies samples in the gradational zone between 4615ft (1407m) and 4645 ft (1416m) will be discussed separately later], whereas values above 50 are only found in the phosphate/carbonate lithofacies. The absence of a significant maturity gradient in this core, and the confinement of bisnorhopane to a twenty foot interval within a single lithofacies, suggest that neither thermal maturity nor diagenetic alteration could completely account for the variability observed. We therefore interpret the variability in the bisnorhopane/hopane ratio in this core as resulting predominantly from distinctive original organic matter input (Katz and Elrod, 1983). Several workers, using results from pyrolytic studies, have proposed that 28,30-bisnorhopane (along with certain other distinctive compounds) is introduced to the depositional surface as a hydrocarbon, and thus may never become polymerized into kerogen (Moldowan et al., 1984; Noble et al., 1985; Philp and Gilbert, 1987, and references therein). Katz and Elrod (1983), prior to the publication of most of these pyrolytic deductions, suggested an anaerobic bacterial source for this terpane. Both the wide variation in bisnorhopane/hopane ratio and the limited stratigraphic extent of the high values for this ratio in the 51-18 Monterey core (Fig. 4) are consistent with an unusually large molecular input from a single life form. The molecular distinction between the siliceous and phosphate/carbonate lithofacies is not limited to relative bisnorhopane content. Also shown in Fig. 4 are typical aliphatic hydrocarbon gas chromatograms for a siliceous sample (top) and a phosphate/ carbonate sample (bottom). Molecular differences are obvious in the chromatograms. The phosphate/

8

JOSEPH A. CURIALE and JOHN R. ODERMATT

4450. 45004550. 4600. 0

4

8

IZ

IB

3~

~ 14 T+ml < M I ~ )

3e

4o

.....................

4650.

+4

~!.LJ.9. fP..U..S..

=:===~m~AHOSPHATE/ RBONATE

8 4700. o 4750. 4800,

28,30BISNORHOPANE

M

48504900-

~'o

zb

3"o

40 5'0 BISNORHOPANE/HOPANE

5'0

70

80

0'0

Fig. 4. Depth trend of the 28,30-bisnorhopane/hopane ratio, measured as the ratio of area counts from the m / z 191 mass chromatogram. Lithofacies boundary shown as a horizontal line (siliceous above, phosphate/carbonate below). Typical aliphatic hydrocarbon gas chromatograms (of the 500-900°F distillate cut of the rock extract) are shown for each of the lithofacies. Pristane/phytane ratios are 0.81 (siliceous sample) and 0.39 (phosphate/carbonate sample). See Curiale et al. (1985) for GC conditions.

carbonate interval, in comparison to the siliceous interval, is relatively low in n-alkane and isoprenoid content, has a lower pristane/phytane ratio (0.39, in contrast to 0.81 for the siliceous sample) and much higher relative amounts of monoaromatic steroid hydrocarbons (see also Fig. 3, right panel) and 28,30-bisnorhopane. Both the variation in n-alkane content and the relationship between pristane/ phytane ratio and relative bisnorhopane content have been observed previously (Curiale et al., 1985, and Katz and Elrod, 1983, respectively). When the steranes and hopanes are examined, the distinction between the samples highlighted in Fig. 4 becomes even more dramatic. Figure 5 shows histograms of the relative distributions of 58 biomarkers (when present) in typical samples from the siliceous (upper) and phosphate/carbonate (lower) lithofacies. The distribution in the lower lithofacies indicates that, compared to the monoaromatic steroids, bisnorhopane and hopane, all other measured biomarkers are present in relatively small amounts (see caption to Fig. 5). In contrast, the typical biomarker distribution found in the siliceous lithofacies shows significantly higher relative amounts of steranes, triaromatic steroids and tricyclic terpanes. The contrast between the two lithofacies is highlighted in the ternary diagram of Fig. 6, showing the normalized (relative) distribution of regular steranes, terpanes (tri- and pentacyclic) and monoaromatic steroids.

The predominance of monoaromatic steroids in the phosphate/carbonate lithofacies is reminiscent of a similar distribution noted in heavy Monterey oils (Curiale et al., 1985). A noteworthy feature of Fig. 5 is that, despite compound family differences and bisnorhopane/ hopane ratio differences between the two samples, the internal distributions of monoaromatic steroids within the two samples are similar. This is probably indicative of a similar sterol input to the two depositional environments, whereas the difference in the ratio of steranes to monoaromatic steroids in the two samples may be indicative of differing diagenetic processes. Changing extents of anoxia (accompanied by changes in the availability of ferric iron) could have made different amounts of elemental sulfur available to the depositional systems corresponding to the two different lithofacies. Ensuing sulfur-sterol reactions could then directly produce aromatic steroids (Douglas and Mair, 1965) in different amounts in each environment. This scenario would yield similar distributions of aromatic steroids in the two lithofacies (due to similar organic matter input), but widely different total concentrations of such steroids. The anoxia which would give rise to such relatively high aromatic steroid concentrations in the phosphate/carbonate lithofacies could also have supported a distinctive anaerobic biota containing 28,30-bisnorhopane or its precursors (Katz and El-

Short-term biomarker variability in the Monterey Formation, Santa Maria Basin 1.00' MONOAROMATIC STEROIDS

9

T|RPAN|I

AN 8611 SILICEOUS LITHOFACIES TRIAR OMA TIC STEROIDS TRICYCLIC TERPANES

Z 0

~.

PENTACYCLIC

o.1o

I,<

STIRANE$

iiiii

MONOAmOMATIC STEROIDS

Fig. 6. Ternary diagram depicting relative distribution of steranes, terpanes and monoaromatic steroids for extracts of the Monterey Formation in Union Leroy 51-18. Note the trend toward monoaromatic steroids, and the lithofacies distinction. Compare with Fig. 17 of Curiale et al. (1985).

:.:.:.:.:.:.: ::::::::::::::::::5

1.00

AN 8 6 2 8 14OSPHATE/CARBONATE LITHOFACIES Z O

0.10 a

I-

t

Fig. 5. Biomarker histograms for the same two lithofacies-representative samples shown in Fig. 4. A suite of 58 aliphatic and aromatic hydrocarbon biomarkers are displayed in relative distribution, derived from mass chromatograms--see caption to Fig. 11 of Curiale (1986) for detailed compound identification. Note that vertical scales are logarithmic.

rod, 1983). Thus the complete model requires (a) the creation of aromatic steroids via diagenetic sulfur reactions below the seawater-sediment interface, and (b) a bisnorhopane-producing biota dependent upon an anoxic environment. Relative compound class differences within Monterey extractable organic matter in the Union Leroy 51-18 well (Fig. 4) can then be interpreted according to such a model: The decreased relative concentration of n-alkanes and isoprenoids in the phosphate/carbonate lithofacies (Fig. 4) results from a dilution effect (i.e. massive input of anaerobic biota), whereas an increase in the monoaromatic/regular sterane ratio in this lithofacies (Fig. 5) is due to diagenetic sulfur-sterol reactions. It should be noted, before leaving the subject of diagenetic sulfur reactions, that our model specifically relates these reactions with products cur-

rently present in the extractable organic matter of the Monterey. Thus the bulk of the organic matter, namely the kerogen, has not been considered. This is critical, in that there appears to be little difference in the organic sulfur contents of the kerogens from the two lithofacies (12.0_+2.1% in the siliceous lithofacies; 11.0_+ 1.7% in the phosphate/carbonate lithofacies). The question of the ultimate effects of kerogen sulfur on the monoaromatic/regular sterane ratio cannot be directly addressed until the molecular sulfur associations in kerogen are understood. A pyrolytic approach to understanding the nature and distribution of organosulfur compounds in Monterey kerogens is currently underway in this laboratory. The difference in relative tricyclic terpane contents of the two lithofacies shown in Fig. 5 is not readily understandable. Although Fig. 5 suggests that tricyclic terpanes are absent in the carbonate/phosphate lithofacies, detailed examination reveals that they are indeed present, albeit in relative amounts less than 1% of the monoaromatic steroids. When the c a r b o n number distribution of these terpanes is examined, results similar to those observed for oils and Monterey samples from several basins in southern California are evident (Curiale et al., 1985). More importantly however, internal differences are evident, which further distinguish the lithofacies (Fig. 7). Specifically, samples from the siliceous lithofacies have marginally (but consistently) lower relative amounts of the C23 component. As direct biochemical precursors of these compounds are unknown, this particular lithofacies distinction cannot yet be incorporated into a model based upon extreme anoxia and input of specific biota. Before leaving the question of covariance between lithofacies and organic geochemical parameters, it should be noted that this relationship is not strictly limited to molecular constituents. As the van Krevelen diagrams of Fig. 2 indicate, the phosphate/

I0

JOSEPH A. CURIALE and JOHN R. ODERMATT 5O% C:4

80%

80% C=s

C.

Fig. 7. Ternary diagram depicting relative distribution of the C23, C24 and C25 (sum of both epimers) tricyclic terpanes in Leroy 51-18 Monterey extracts. Distinction between samples from differing lithofacies is noted.

carbonate lithofacies possesses slightly but distinctly better petroleum source rock potential than the siliceous lithofacies, a determined by Rock-Eval and total organic carbon data. Direct molecular evidence for vertical migration

The covariance between lithofacies and molecular composition does not correspond exactly to the siliceous-phosphate/carbonate lithofacies boundary (4645 ft; 1416 m) of Odermatt (1986). This is evident in the gradational trend of the aromatic steroid ratio in Fig. 2 (Panel IV) and, more clearly, in the gradual increase in the bisnorhopane/hopane ratio beginning at about 4615ft (1407m), 30ft (9m) above the lithofacies boundary (Fig. 3). Indeed, virtually every measured EOM parameter which reveals a lithofacies dependence (including n-alkane content, pristane/

phytane ratio, tricyclic/pentacyclic terpane ratio and relative C23 tricyclic terpane content) shows a gradational change just above the lithofacies boundary, beginning at approx. 4615 ft (1407 m). The onset of a gradational change in EOM parameters at 4615 ft (1407 m) stands in contrast to (a) the actual occurrence of the lithofacies break at 4645 ft (1416 m), and (b) the distinct discontinuity in kerogen parameters (Odermatt, 1986) known to occur coincidently with the lithofacies boundary in this well. Odermatt (1986) and Odermatt et al. (1988) have shown that several kerogen parameter trends, including V/(V + Ni), Mo/(Mo + Cr), V/C and Ni/C ratios, are discontinuous precisely at the lithofacies boundary. Whereas the kerogen/lithofacies discontinuity can be explained in terms of original depositional processes, the gradational EOM trend starting approx. 30ft (9m) above the lithofacies boundary suggests migration of soluble organic matter by this amount. This proposed extent of migration is most clearly seen in Fig. 4, where the bisnorhopane/hopane ratio gradually increases from a siliceous lithofacies average of 3 at about 4615 ft (1407m), until a value in excess of 40 is reached just above the lithofacies boundary at 4645ft (1416m). Values below the boundary eventually reach as high as 80, before dropping to 10-30 through the remainder of the core (Fig. 4). We propose that EOM has vertically migrated (across bedding) approx. 30 ft (9 m) from the uppermost portion of the phosphate/carbonate lithofacies. (Note that lateral migration is no doubt also a factor here, although our data can neither confirm nor deny its extent.) Migration of soluble organic matter from shales to adjacent sandstones has been thoroughly documented by Leythaeuser et al. (1987, and references to past work therein; see also Leythaeuser et al., 1988) in a variety of settings. Their studies are molecularly

Table 2. Lithofacies comparison chart for Monterey Formation, Union Leroy 51-18~ General Litbofacies Depth Hydrogen Index EOM Sterane/terpane C27/C28/C29 Bisnorhopane/hopane 22S/(22S + 22R) 20S/(20S + 20R) Moretane/hopane Kerogen V/(V + Ni) Mo/(Mo + Cr)

Siliceous 4484-4614 ft (21) 577 mg/g (19)

Phosphate/carbonate 4654-4864 ft (28) 692 mg/g (28)

0.27 (21) 50/27/23 (21) 3.42 (2 l ) 0.55 (20) 0.43 (20) 0.11 (19)

0.15 (28) 52/27/21 (27) 21.18 (27) 0.54 (20) 0.36 (24) 0.07 (16)

0.67 (18) 0.80 (17)

0.39 (28) 0.47 (27)

~Hydrogen index, EOM and kerogen value are presented as averages, with number of samples in parentheses. Sterane/terpane is the area ratio of major cholestanes, methylcholestanes and ethylcholestanes to major tricyclic and pentacyclic terpanes, derived from m/z 217 and m/z 191 mass chromatograms, respectively. C27/C28/C29 is the distribution of the 5~t(H),I4ct (H),17~t (H),20R steranes by carbon number (area count, m/z 217). Bisnorhopane/hopane, 22S/(22S+22R)-homohopanes and moretane/hopane are ratios derived from m/z 191 area counts; 20S/(20S + 20R) is derived from m/z 217 area counts for the 5~t(H),14~t(H),17ct(H)-ethylcholestane. Both kerogen ratios are from Odermatt (1986).

Short-term biomarker variability in the Monterey Formation, Santa Maria Basin based, and involve primary migration from within the source rock to the source/sand contact; i.e. migration through a conventional connected pore network. In the case of the Monterey Formation in the Union Leroy 51-18 well, fractures within the section, rather than conventional pore frameworks, are probably responsible for the observed migration (Odermatt, 1986). Such a fracture network, obvious from visual examination of the core face, results in significantly increased permeability immediately above the lithofacies boundary. It is therefore probable that the molecular fractionation aspects of these previous studies are inapplicable to the vertical migration observed in our study, resulting in the conclusion that the observed gradations are caused by dilution effects. The extent of the vertical migration evidenced here (and the currently unknown extent of molecular fractionation involved during such migration), indicate that caution must be exercised in oil-source rock correlation efforts. This is particularly true within the Monterey Formation, where the fractured reservoir rock can also act as the petroleum source rock (Curiale et al., 1985). This characteristic of the Monterey generally precludes the use of common bulk migration indicators, such as EOM/TOC, which have such utility for clastic source rocks. In the Leroy 51-18 section, EOM/TOC ratios (Odermatt, 1986) range from 0.07 to 0.58, and the values do not correlate with obvious migration pathways (such as core fractures). Values above the lithofacies break (0.25 average) are not significantly different from those below the break (0.30 average). Clearly the migrational variability proposed here is not gross enough to be observed in bulk extraction and TOC values. This suggests that molecular distribution variability, while subtle, must nevertheless be accurately measured and interpreted in order to avoid correlations that are in error because of the unrecognized effects of migrational dispersal. CONCLUSIONS

Chemical characteristics of the two major lithofacies present in the Monterey Formation in the Leroy 51-18 well are summarized in Table 2. In a general sense, the phosphate/carbonate lithofacies is a better petroleum source rock than the siliceous lithofacies, based upon higher hydrogen indices at constant thermal maturity. The EOMand kerogen-based distinctions between the two lithofacies are also summarized in Table 2, including sterane/terpane, bisnorhopane/hopane, V/(V + Ni) and Mo/(Mo + Cr) ratios. Of particular interest is the nearly constant carbon number distribution of the regular steranes, regardless of lithofacies. The results of this study document short-term biomarker variations within a non-clastic petroleum source rock, and impose further limitations upon the indiscriminate use of biomarkers as maturation indi-

11

cators in iithologically variable sections. In addition, several observed molecular features, brought to our attention due to their covariance with lithofacies, provide insight concerning deposition during the mid-Miocene in the Santa Maria Basin. The relatively high concentrations of aromatic steroids and 28,30-bisnorhopane can be attributed to, respectively, diagenetic sulfur-sterol reactions and massive input of specific anaerobic biota. Such features could eventually allow us to evaluate both the extent of anoxia in the depositional environment and the extent of microbial influence on the chemical composition of preserved organic matter. Thus the unusual features of the Monterey Formation as a petroleum source rock may ultimately be deciphered using the techniques of molecular geochemistry. Acknowledgements--The manuscript was improved significantly following reviews by J. M. Moldowan, M. Engel, S. Palmer and L. Wenger. Carolyn Tucker and Michael Hartley assisted in laboratory fractionation work; Dan Cardin operated the TSQ-MS. This work grew out of a master's thesis at California State University, Los Angeles (by J. R. Odermatt). We thank G. H. Smith, J. B. Dunham, R. E. Sweeney and especially B. W. Bromley for many helpful discussions concerning the geochemistry and geology of the Monterey Formation. D. W. Ahlborn assisted with the drafting. We also thank Unocal management for encouraging this work, and for permission to publish the results.

REFERENCES

Brassell S. C., McEvoy J., Hoffmann C. F., Lamb N. A., Peakman T. M. and Maxwell J. R. (1984) Isomerisation, rearrangement and aromatisation of steroids in distinguishing early stages of diagenesis. Org. Geochem. 6, 11-23.

Bromley B. W. and Senftle J. T. (1986) Quality variations in middle Miocene oils of southern California--Source organic matter controls. American Chemical Society 192nd National Meeting, Anaheim, CA, GEOC97. Abstract. Cornford C., Morrow J. A., Turrington A., Miles J. A. and Brooks J. (1983) Some geological controls on oil composition in the U.K. North Sea. In Petroleum Geochemistry and Exploration of Europe (Edited by Brooks J.), pp. 175-194. Blackwell, Oxford. Crouch J. K., Bachman S. B. and Shay S. T. (1984) Post-Miocene compressional tectonics along the Central California margin. In Tectonics and Sedimentation along the California Margin (Edited by Crouch J. K. and Bachman S. B.), pp. 37-55. Pacific Section SEPM Publication. Curiale J. A., Cameron D. and Davis D. V. (1985) Biological marker distribution and significancein oils and rocks of the Monterey Formation, California. Geochim. Cosmochim. Acta 49, 271-288. Curiale J. A. (1986) Origin of solid bitumens, with emphasis on biological marker results. Org. Geochem. 10, 559-580. Curiale J. A. (1988) Molecular geneticmarkers and maturity indices in intermontane lacustrine facies: Kishenehn Formation, Montana. Org. Geochem. 13, 633~538. Curiale J. A., Larter S. R., Sweeney R. E. and Bromley B. W. (1988) Molecular thermal maturity indicators in oil and gas source rocks. In Thermal History of Sedimentary Basins (Edited by Naeser N. D. and McCulloh T. H.). Springer, New York. In press.

12

JOSEPH A. CURIALE and JOHN R. ODERMATT

Douglas A. G. and Mair B. J. (1965) Sulfur: role in genesis of petroleum. Science 147, 499-501. French Jr R. W. (1940) Geothermal gradients in California oil wells. Am. Pet. Inst. Drill Prod. Prac. 653-658. Giger W. and Schaffner C. (1980) Molecular fossils in Miocene Monterey shales, California (astr). In Proc. 26th Int. Geological Congr., Vol. 2, p. 772. Giger W. and Schaffner C. (1981) Unsaturated steroid hydrocarbons as indicators of diagenesis in immature Monterey shales. Naturwissenshaften 68, 37-39. Grantham P. J., Posthuma J. and De Groot K. (1980) Variation and significance of the C27 and C28 triterpane content of a North Sea core and various North Sea crude oils. In Advances in Organic Geochemistry 1979 (Edited by Douglas A. G. and Maxwell J. R.), pp. 29-38. Pergamon Press, Oxford. Hughes W. B., Holba A. G. and Miller D. E. (1984) North Slope Alaska oil-rock correlation study. In AAPG Studies in Geology No. 20, pp. 379-402. Isaacs C. M. (1984) Monterey--key to offshore California boom. Oil Gas J. 82, 75-81. Isaacs C. M. (1983) Compositional variation and sequence in the Miocene Monterey Formation, Santa Barbara coastal area, California. In Cenozoic Marine Sedimentation Pacific Margin U.S.A. (Edited by Larue D. K. and Steel R. J.), pp. 117-133. SEPM, Los Angeles. Ji-Yang S., Mackenzie A. S., Alexander R., Eglinton G., Gowar A. P., Wolff G. A. and Maxwell J. R. (1982) A biological marker investigation of petroleums and shales from the Shengli oilfield, The People's Republic of China. Chem. Geol. 35, 1 3 l. Katz B. J. and Elrod L. W. (1983) Organic geochemistry of DSDP Site 467, offshore California, Middle Miocene to Lower Pliocene strata. Geochim. Cosmochim. Acta 47, 389--386. King J. D. and Claypool G. E. (1983) Biological marker compounds and implications for generation and migration of petroleum in rocks of the Point Conception deep-stratigraphic test well, OCS-CAL 78-164 No. 1, offshore California. In Petrolum Generation and Occurrence in the Miocene Monterey Formation, California (Edited by Isaacs C. M. and Garrison R. E.), pp. 191-201. Pacific Section, SEPM. Kruge M. A. (1986) Biomarkers geochemistry of the Miocene Monterey Formation, West San Joaquin Basin, California: implications for petroleum generation. Org. Geochem. 10, 517-530. Lewan M. D (1980) Geochemistry of vanadium and nickel in organic matter of sedimentary rocks. Unpublished Ph.D. Dissertation, University of Cincinnati. Lewan M. D., Bjoroy M. and Dolcater D. L. (1986) Effects of thermal maturation on steroid hydrocarbons as determined by hydrous pyrolysis of Phosphoria Retort Shale. Geochim. Cosmochim. Acta 50, 1977-1987. Leythaeuser D., Hollerbach A. and Hagemann H. W. (1977) Source rock/crude oil correlations based on distribution of C27+ cyclic hydrocarbons. In Advances in Organic Geochemistry 1975 (Edited by Campos R. and Gofii J.), pp. 429-442. Empresa Nacional Adaro de Investigaciones Mineras, Madrid. Leythaeuser D., Schaefer R. G. and Radke M. (1987) On the primary migration of petroleum. In Proc. 12th Worm Petroleum Congr., Vol. 2, pp. 227-236. Wiley, New York. Leythaeuser D., Schaefer R. G. and Radke M. (1988) Geochemical effects of primary migration of petroleum in Kimmeridge source rocks from Brae field area, North Sea. I: Gross composition of C15+ soluble organic matter and molecular composition of C15+ saturated hydrocarbons. Geochim. Cosmochim. Acta 52, 701-713. Mackenzie A. S. (1984) Applications of biological markers in petroleum geochemistry. In Advances in Petroleum Geochemistry (Edited by Brooks J. and Welte D.), pp. 115-214. Academic Press, London.

Mackenzie A. S., Maxwell J. R., Coleman M. U and Deegan C. E. (1983) Biological marker and isotope studies of North Sea crude oils and sediments. In Proc. llth World Petroleum Congr., Lond, Vol. PDI(4), pp. 1-12. Macko S. A. and Quick R. S. (1986) A geochemical study of oil migration at source rock reservoir contacts: Stable isotopes. Org. Geochem. 10, 199-205. Magoon L. B. and Isaacs C. M. (1983) Chemical characteristics of some crude oils from the Santa Maria Basin, California. In Petroleum Generation and Occurrence in the Miocene Monterey Formation, California (Edited by Isaacs C. M. and Garrison R. E.), pp. 201-213. Pacific Section SEPM. Moldowan J. M., Sundararaman P. and Schoell M. (1986) Sensitivity of biomarker properties to depositional environment and/or source input in the Lower Toarcian of SW-Germany. Org. Geochem. 10, 915-926. Moldowan J, M., Seifert W. K., Arnold E. and Clardy J. (1985) Structure proof and significance of stereoisomeric 28,30-bisnorhopanes in petroleum and petroleum source rocks. Geochim. Cosmochim. Acta 48, 1651 1661. Noble R., Alexander R. and Kagi R. I. (1985) The occurrence of bisnorhopane, trisnorhopane and 25-norhopanes as free hydrocarbons in some Australian shales. Org. Geochem. 8, 171-176. Odermatt J. R. (1986) Transition element geochemistry of organic matter from the Monterey Formation, Union Leroy 51-18 well, Santa Maria Valley Field, Santa Barbara County, California. Unpublished M.S. Thesis, California State University at Los Angeles. Odermatt J. R., Curiale J. A. and Hurst R. (1989) Metal ratios in organic matter of the Monterey Formation: Transition element-lithofacies correlation. AAPG Bull. In press. Orr W. L. (1986) Kerogen/asphaltene/sulfur relationships in sulfur-rich Monterey oils. Org. Geochem. 10, 499-516. Peakman T. M., Lamb N. A. and Maxwell J. R. (1984) Naturally occurring spiro steroid hydrocarbons. Tetrahedron Lett. 26, 349-352. Philp R. P. (1985) Biological markers in fossil fuel production. Mass Spectrom. Rev. 4, 1-54. Philp R. P. and Gilbert T. D. (1987) A review of biomarkers in kerogens as determined by pyrolysis-gas chromatography and pyrolysis-gas chromatography-mass spectrometry. J. Anal. Appl. Pyrol. 11, 93-108. Pisciotto K. A. (1978) Basinal sedimentary facies and diagenetic aspects of the Monterey Shale, California. Unpublished Ph.D. Dissertation, University of California at Santa Cruz. Pisciotto K. A. (1981) Diagenetic trends in the siliceous facies of the Monterey Shale in the Santa Maria region, California. Sedimentology 28, 547-571. Pisciotto K. A. and Garrison R. E. (1981) Lithofacies and depositional environments of the Monterey Formation. In The Monterey Formation and Related Siliceous Rocks of California (Edited by Garrison R. E., Douglas R. G., Pisciotto K. A., Isaacs C. A. and Ingle J. C.), pp. 97-122. Pacific Section SEPM. Seifert W. K. (1977) Source rock-oil correlations by C27-C30 biological marker hydrocarbons. In Advances in Organic Geochemistry 1975 (Edited by Campos R. and Gofii J.), pp. 21-44. Empresa Nacional Adaro de Investigaciones Mineras, Madrid. Seifert W. K., Moldowan J. M., Smith G. W. and Whitehead E. V. (1978) First proof of structure of a C28-pentacyclic triterpane in petroleum. Nature 271, 436-437. Seifert W. K. and Moldowan J. M. (1978) Applications of steranes, terpanes and monoaromatics to the maturation, migration and source of crude oils. Geochim. Cosmochim. Acta 42, 473-484. Seifert W. K. and Moldowan J. M. (1979) The effect of

Short-term biomarker variability in the Monterey Formation, Santa Maria Basin biodegradation on steranes and terpanes in crude oils. Geochim. Cosmochim. Acta 43, II 1-126. Seifert W. K. and Moldowan J. M. (1981) Paleoreconstruction by biological markers. Geochim. Cosmochim. Acta 45, 783-794. Seifert W. K., Carlson R. M. K. and Moldowan J. M. (1983) Geomimetic synthesis, structure assignment and geochemical correlation application of monoaromatized petroleum steroids. In Advances in Organic Geochemistry 1981 (Edited by Bjor~y M. et al.), pp. 710-724. Wiley, Chichester. Speers G. C. and Whitehead E. V. (1969) Crude petroleum. In Organic Geochemistry: Methods and Results (Edited by Eglinton G. and Murphy M. T. J.), pp. 638~567. Springer, New York. Summerhayes C. P. (1981) Oceanographic controls on organic matter in the Miocene Monterey Formation, offshore California. In The Monterey Formation and Related Siliceous Rocks o f California (Edited by Garrison R. E., Douglas R. G., Pisciotto K. A., Isaacs C. M. and Ingle J. C.), pp. 213 219. Pacific Section SEPM.

13

Tannenbaum E., Ruth E., Huizinga B. J. and Kaplan I. R. (1986) Biological marker distribution in coexisting kerogen, bitumen and asphaltenes in Monterey Formation diatomite, California. Org. Geochem. 10, 531-536. Tissot B. P. and Welte D. H. (1984) Petroleum Formation and Occurrence. Springer, Berlin. Wehner H. and Teschner M. (1982) Correlation of crude oils and source rocks in the German Molasse Basin-Applicability of chromatographic techniques. J. Chromatogr. 204, 481-490. Welte D. H., Kratochvil H., Rullkotter J., Ladwein H. and Schaefer R. G. (1982) Organic geochemistry of crude oils from the Vienna Basin and an assessment of their origin. Chem. Geol. 35, 33-68. Wenger L. M. and Baker D. R. (1986) Variations in organic geochemistry of anoxic-oxic black shale~arbonate sequences in the Pennsylvanian of the Midcontinent, U.S.A. Org. Geochem. 10, 8~92. Woodring W. P. and Bramlette M. N. (1950) Geology and paleontology of the Santa Maria District, California. USGS Professional Paper 222.