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Marine and Petroleum Geology 49 (2014) 15e34

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Marine and Petroleum Geology

journal homepage: www.elsevier .com/locate/marpetgeo

A basin modeling and organic geochemistry study in the Vallecitossyncline, San Joaquin Basin, California

Meng He a,*, Stephan Grahama, Allegra Hosford Scheirer a, Kenneth E. Peters a,b

a Stanford University, Department of Geological & Environmental Sciences, Stanford, CA 94305, USAb Schlumberger, 18 Manzanita Place, Mill Valley, CA 94941, USA

a r t i c l e i n f o

Article history:Received 18 June 2012Received in revised form30 August 2013Accepted 2 September 2013Available online 3 October 2013

Keywords:Basin and petroleum system modelingBurial historyCalibrationOil familyOil-window maturityOileoil correlationBiomarkerKreyenhagenMorenoVallecitos syncline

* Corresponding author. 200 Westlake Park BoulevTel.: þ1 713 323 2788.

E-mail addresses: [email protected], meng.h

0264-8172/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.marpetgeo.2013.09.001

a b s t r a c t

The Vallecitos syncline is a westerly structural extension of the San Joaquin Basin. The Vallecitos oil field,comprised of eight separate areas that produce from Cretaceous and Paleogene reservoirs, accounted for5.4 MMB of oil and 5.6 BCF associated of gas through 2010. However, exploration for oil and gas in theVallecitos area is challenging due to structural complexity and limited data. The purpose of this study isto evaluate whether source rocks are actively generating petroleum in the Vallecitos syncline and toimprove our understanding of burial history and timing of hydrocarbon generation. We conductedbiomarker analysis on twenty-two oil samples from the Vallecitos syncline. Source-related biomarkersshow two genetic groups of oil, which originated from two different source rocks. These results differfrom earlier published interpretations in which the Kreyenhagen Formation is the only source rock in theVallecitos syncline, and suggest that the Cretaceous Moreno Formation in the syncline also is an activesource rock.

Stratigraphic evidence and modeling suggest that late Cenozoic episodes of erosion due to folding anduplift removed significant overburden on the flanks of the syncline. To better understand the petroleumsystems and clarify the total active source rocks in the area, 2D burial histories were generated throughthe Vallecitos syncline. A published cross-section through the deepest part of the syncline was selectedto conduct thermal history, basin evolution, and migration analyses. The 2D model results indicate thatthe lower Kreyenhagen Formation has various maturities within the formation at different locations inthe present-day syncline. The basal part of the Kreyenhagen Formation is in the dry gas window andmaturity decreases away from the central part to the flanks. It remains immature along shallow portionsof the present-day flanks. In contrast, the basal part of the Moreno Formation achieved extremely highmaturity (past the gas generation zone) but is in the oil generation zone on the flanks of the syncline atshallow depth. All of our geochemical and 2D model results suggest that there are two active sourcerocks in the Vallecitos syncline. Accordingly, we propose that there are two active petroleum systems inthe Vallecitos syncline.

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1. Introduction

The Vallecitos syncline is located on the western margin of theSan Joaquin Basin (Fig. 1a). Petroleum exploration in the Vallecitosarea commenced in the late 1880’s. Drilling activity increasedsignificantly thereafter (Wilkinson, 1960). By the end of 1959, theVallecitos field comprised eight separate producing areas (GriswoldCanyon, Franco, Cedar Flat, Ashurst, Silver Creek, Pimental Canyon,Central, and Los Pinos Canyon) (Fig. 1a), and had produced

ard, Houston, TX 77079, USA.

[email protected] (M. He).

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approximately 2.2 MMB of oil and 1.5 BCF of gas from Cretaceousand Paleogene reservoirs (Wilkinson, 1960). Cumulative oil and gasproduction through 2010 in the Vallecitos area reached 5.4 MMB ofoil and 5.6 BCF of gas, respectively (CDOGGR, 2010).

The Antelope shale member of the Monterey Formation is aprolific oil-source rock in the southern San Joaquin Basin (Grahamand Williams, 1985), but is absent in the Vallecitos area, suggestingthat the Eocene Kreyenhagen Formation is the only active sourcerock in the syncline according to the USGS in 2008 (Lillis andMagoon, 2008; Peters et al., 2008). However, our initial 1D modeland geochemical results indicate that there are two possible sourcerocks in the Vallecitos syncline: the Cretaceous Moreno Formationand the Eocene Kreyenhagen Formation (He et al., 2010). Theobjective of this study is to evaluate the organic matter content,

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Figure 1. a. Structural map of the top of Kreyenhagen Formation and study area with the distribution of the oil samples in the Vallecitos syncline. Star symbol indicates the 1D model location. GH is the 2D model cross-section line.NETEX1 well is the calibration well. K shows Kreyenhagen oils and M shows Moreno oils. Open symbol shows the reservoir where the oil is produced from. Blue rectangle indicates Yokut reservoir. Red circle indicates San Carlosreservoir. Green polygon indicates unknown reservoir. b. Generalized outcrop geology of the west side San Joaquin Basin. c. Generalized stratigraphic column of the Vallecitos syncline. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

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type and thermal history of the active source rocks, and petroleumsystems in the Vallecitos syncline by using basin modeling andorganic geochemistry, to incorporate the results into basinmodeling in order to determine the timing of hydrocarbon gener-ation. Maturity information from well cores was used to calibratethe model. Burial history, thermal maturity, and timing of hydro-carbon generation and expulsion were simulated in our 2D modelbased on a published cross-section through the deepest part ofsyncline.

This study improves understanding of petroleum systems in theVallecitos syncline through detailed study of oileoil correlationsand identification of effective source rocks in the region, where adetailed biomarker study has never been conducted. This workprovides input data that could be used for a future 3D model of thebasin. In particular, the biomarker analysis helps to understand theorigin of different oil types in the Vallecitos syncline and regionaldistribution of oil and gas. Maps of oil and gas distributions areneeded to accuratelymodel the petroleum systems in the Vallecitossyncline and the possible flowpaths for oil and gas migration. Thiswork improves the previous USGS model (Peters et al., 2008) andthus reduces risk for future oil and gas exploration in the Vallecitossyncline.

2. Geological and tectonic setting

The Vallecitos syncline is approximately 6.5 kmwide and 24 kmlong and is located in the southeastern part of San Benito County(Fig. 1a). It is one of the prominent en echelon fold sets that char-acterize the west side of the San Joaquin Basin (Harding, 1976;Dibblee, 1973) (Fig. 1a and b). The Vallecitos syncline is one of thefolds formed as a result of right lateral transpressional deformationalong the San Andreas fault (e.g., Graham, 1987) (Fig. 1b). Duringthe Late Cretaceous, the Vallecitos area was part of a large forearcbasin, the Great Valley basin, typified by deposition of submarinefans derived from the Sierra Nevada magmatic arc (Ingersoll, 1979).About 9 km thickness of Mesozoic throughmiddle Cenozoic marinestrata exposed in the syncline area were deposited in this setting(Clark, 1930; Donald, 1959; Miller, 1998; Anderson, 1998). TheVallecitos syncline is structurally underlain by Mesozoic accre-tionary complex assigned to the Franciscan complex (Bailey, 1960;Page, 1981; Wahrhaftig, 1984; Graham, 1987) (Fig. 1b). Submarinefan deposits of the Panoche Formation apparently blanketed theVallecitos area during CampanianeMaastrichtian time, althoughthey appear to have been stripped from the northern part of thearea before deposition of the MaastrichtianeDanian Moreno For-mation (Enos, 1961) (Fig. 1b and c), presaging initial formation ofthe Vallecitos syncline (see also Mitchell et al., 2010). The turbiditicCantua sandstone, a large lentil within the upper PaleoceneeEocene Lodo Formation, was deposited synchronously with earlyfolding of the Vallecitos syncline, as indicated by its restricteddistribution in the syncline, which likely was a trench-slope basin(Anderson,1998). Above these strata is a gradually shoaling upwardsuccession of Cenozoic deposits (Dibblee, 1981; Bartow, 1991). TheDomengine Formation comprises deltaic deposits reflecting up-ward shoaling and early uplift and unroofing of the subductioncomplex (Schulein, 1993), likely in response to flat-slab subduction(Moxon and Graham, 1987; Johnson et al., 2007). Nevertheless, themiddle and late Eocene saw a return to bathyal water depths withdeposition of the organic-rich, biosiliceous Kreyenhagen Formation(Milam, 1985). Stratigraphic relationships among Eocene forma-tions in the Vallecitos trough vary greatly over short distances,reflecting its character as a structurally confined seaway, and thiscomplicates correlation between the Vallecitos syncline and thelarger San Joaquin Valley (e.g., Graham and Berry, 1979; Schulein,1993).

Oligocene sediments are missing and probably were neverdeposited in the Vallecitos syncline, although Oligocene rocks existin the southernmost San Joaquin Basin (Bartow, 1991). Mioceneshallow-marine sedimentation in the subsiding Vallecitos troughwas strongly controlled by growth of the adjacent anticlinal uplifts(Rentschler, 1989). The Vallecitos syncline was an isolated, struc-turally controlled nonmarine depocenter in the late Neogene, asindicated by folded post-upper Luisian lacustrine sediments(Rentschler, 1985). As much as 2.1 km (7000 feet) of nonmarinesediment accumulated in the synclinal trough during Pliocene time(Bowersox, 2004; Dibblee, 1981).

The Vallecitos syncline has a complex tectonic history due to itslocation at the western margin of North America where a sub-duction zone existed throughout the late Mesozoic and earlyCenozoic. The Paleogene in California represents a transitionalperiod between a Mesozoic Andean-style convergent marginsetting and the Neogene transform setting (Graham, 1987; Bartow,1991). As noted above, the syncline likely initiated as a discretefeature in the Paleocene, as suggested by the distribution of theEocene Cantua Sandstone (e.g., Graham and Berry, 1979). TheMendocino Triple Junction migrated northwestward past the Val-lecitos area in the mid-Miocene, inducing renewed tectonism innearby regions (Dickinson and Snyder, 1979; Ingersoll, 1979). Initialfolding was pre-late Saucesian, and continued uplift is reflected inthe deposition of the mid-Miocene Temblor Formation (Rentschler,1985). The folding that shaped the modern Vallecitos synclineoccurred between the early Miocene and middle Pliocene (Bartow,1991). In summary, a complex history of erosion, uplift and sedi-mentation in the Vallecitos syncline resulted in petroleum-producing structural traps in the Vallecitos oil field. Cenozoicstrata in the Vallecitos syncline contain petroleum, although theyare buried less than 1.5 km (Rentschler, 1985).

Heating, manifested as local volcanism, affected the Vallecitosarea and other areas adjacent to the evolving San Andreas fault inthe Miocene in response to the encroachment and eventual colli-sion of the East Pacific Rise with the California margin and subse-quent migration of the Mendocino triple junction (e.g., Stanley,1987). Southwest of Panoche Valley, a few kilometers northwestof the Vallecitos syncline, flat-lying basalt and basaltic andesite,about 0.6 km (w2000 feet) thick, unconformably overlie foldedrocks of the Franciscan Complex and the Great Valley Group(Dibblee, 1981) (Fig. 1b). Those volcanic rocks have been radio-metrically dated about 10 Ma or late Miocene (Prowell, 1974). Inaddition, a small intrusion of syenite (ca. 12.8 Ma) occurs along themargin of the New Idria intrusive serpentinite body (Obradovichet al., 2000), which forms the core of the Coalinga anticline(trending from the New Idria serpentinite SE through the JoaquinRidge and Christie wells in Fig. 1b) about 5 km south of the Valle-citos syncline (Dibblee, 1981). The New Idria serpentinite body hasbeen inferred to be a diaper that breached the surface at the core ofNew Idria anticline during the late middle Miocene (Casey andDickinson, 1976).

3. Petroleum geochemistry

3.1. Samples and methods

A total of twenty-two black oil samples, two oil seeps, and onegas condensate covering most of the syncline were selected for thisstudy (Fig. 1a). Some well studied samples and end member oilsamples of Kreyenhagen and Moreno shales were also used forcomparison and oileoil correlation. One Kreyenhagen source-rockextract (SJ121) was included in this study to confirm the origin ofoil from the Kreyenhagen source rock. End member oil samplesfrom Kreyenhagen source rock include sample SJ122 and sample


SJ119 from Big Tar Canyon near the Coalinga oil field (Fig. 1b).Sample SJ118 is believed to be the end member oil from Morenosource rock in Oil City, which has also been well studied in theliterature (Peters et al., 1994; Lillis and Magoon, 2008) (Table 1;Fig. 1b). In addition, source-rock extracts from the KreyenhagenFormation, and end member oil samples from the Kreyenhagen,and Moreno formations from outside of the Vallecitos synclinewere included in this study to be compared with oil samples fromthe Vallecitos syncline.

The chemical composition of oil is directly determined by thetype of organic matter and the depositional environment of sourcerocks (Tissot and Welte, 1984). Oil chemistry is widely used topredict the character of the source rocks (Peters et al., 2005).Compared with most bulk geochemical parameters, source-relatedand age-related biomarker ratios for oil samples are particularlyuseful to describe facies and depositional environment of thesource rock, while also successfully establishing oileoil and oilesource rock correlations. All of the crude oil samples were quanti-tatively analyzed by gas chromatographyeflame ionization detec-tion (GCeFID). The biomarkers (e.g., terpanes, steranes, aromaticsteroids) in the branched, cyclic, and aromatic fractions wereanalyzed by selected ion recording-gas chromatographyemassspectrometry (SIR-GCeMS) (Fig. 2a, b and c).

Table 1Oil samples by field, well ID, reservoir age, longitude, latitude, oil type and township.

Oil sampleID

Field name Well API Well name Reservoirage

SJ102 Vallecitos 4069200250 Bunker 34-4 EoceneSJ104 Vallecitos 4069000290 Ashurst 1A-5 Eocene

SJ105 Vallecitos 4069001710 Ashurst 43-5 Eocene

SJ106 Vallecitos 4069001550 F & I 37-31 EoceneSJ107 Vallecitos 4069000030 Bryant-U S L 16A-28 EoceneSJ108 Vallecitos 4069000030 Bryant-U S L 16A-28 EoceneSJ109 Vallecitos 4069200310 Ashurst 38-28 EoceneSJ112 Vallecitos 4069001030 Cal-O-Tex

Exploration Co. 1Eocene

SJ117 Vallecitos 4069200560 Tannehill Oil CoF&I 12X well.

Unknown

SJ119 Big Tar Canoy 4031006810 Seep SurfaceSJ121 Kreyenhagen

extract(5)N/A N/A N/A

SJ122 Stanford3(kreyenhagen)

N/A N/A Unknown

SJ124 Vallecitos 4069001350 Ashurst 52A-33(Pfau, Pfau &fau,LLC)

Unknown

SJ125 Vallecitos 4069001670 F&I 86-29 UnknownSJ126 Vallecitos 4069001500 F&I 16-31(Patriot

Resources LLC)Unknown

SJ128 Vallecitos 4069001510 F&I 26-31(PatriotResources LLC)

Unknown

SJ129 Vallecitos 4069000040 16B-28(PanocheVallecitos oil company)

Unknown

SJ130 Vallecitos N/A Seep 1 (F&I) SurfaceSJ131 Vallecitos N/A Seep 2(F&I) SurfaceSJ103 Vallecitos 4069001870 Ashurst 2 PaleoceneSJ110 Vallecitos 4069200470 Olson-McDonald 1 Late

CretaceouSJ111 Vallecitos 4069001120 Panoche 1 PaleoceneSJ113 Vallecitos N/A Ash 1,2,3,5,6,9 PaleoceneSJ114 Vallecitos 4069000900 Ash 6 PaleoceneSJ115 Vallecitos 4069000950 Nicholas 5 PaleoceneSJ118 Coalinga 4019004400 Coast Range Oil

Co. 1 well (Oil City)Unknown

SJ123 Vallecitos 4069001870 Ashurst 2-27 UnknownSJ127 Vallecitos 4069000940 Nicholas 3 Unknown

*N/A: not applicable. EK ¼ Eocene Kreyenhagen, CM ¼ Cretaceous Moreno.

Chemometric analysis was used to classify and assign confi-dence limits for identifying genetic oil families in the Vallecitossyncline. Chemometrics is a collection of multivariate statisticalmethods for recognizing patterns in large data sets. It allows us toidentify and remove noise from the data, show affinities amongsamples or parameters, and make accurate predictions about un-known samples. Principal components analysis (PCA) is one com-mon technique in chemometric analysis. For the chemometricanalysis in this study, we selected twenty-five source- and depo-sitional environment-related biomarkers (Table 2; Fig. 3) based onour geochemical expertise to differentiate oil families in terms ofage, organic input and depositional environment of the sourcerocks. PCAwas completed using Pirouette (Infometrix, Inc., Bothell,WA). PCA reduces the dimensionality of the data set, whileretaining most of the information. It transfers a number of possiblyrelated variables into a smaller number of uncorrelated variables,the principal components (Fig. 3). This transformation retains thestructural relationships underlying the data. The first three prin-cipal components generally describe most of the variance withinthe dataset. Three-dimensional plots that differentiate families arebuilt by generally using the first three principal components as axesto generate the best three-dimensional separation of the samplesinto genetic families (Fig. 3).

Formation name Longitude Latitude Oiltype

Sec-Twn-Rng

Yokut Sandstone �120.75057 36.50051 EK 34-16S-11EKreyenhagenFormation

�120.78556 36.48111 EK 5-17S-11E

KreyenhagenFormation

�120.78944 36.483268 EK 5-17S-11E

Yokut Sandstone �120.80945 36.49057 EK 31-16S-11EYokut Sandstone �120.77813 36.50562 EK 28-16S-11EYokut Sandstone �120.77813 36.50562 EK 28-16S-11EYokut Sandstone �120.77214 36.50236 EK 28-16S-11EKreyenhagenFormation

�120.78793 36.461361 EK 8-17S-11E

Unknown �120.797183 36.484441 EK 5-17S-11E

Unknown �120.170633 35.933485 EK 18-23S-17EN/A N/A N/A EK N/A

Unknown N/A N/A EK N/A

Unknown �120.768901 36.499554 EK 33-16S-11E

Yokut Sandstone �120.77945 36.50484 EK 29-16S-11EYokut Sandstone �120.815045 36.492078 EK 31-16S-11E

Yokut Sandstone �120.812963 36.492338 EK 31-16S-11E

Unknown �120.779128 36.505529 EK 28-16S-11E

Unknown �120.80866 36.49139 EK N/AUnknown �120.80822 36.47607 EK N/ALodo Formation �120.75949 36.51058 CM 27-16S-11E

sMoreno Formation �120.81325 36.52833 CM 19-16S-11E

Lodo Formation �120.82552 36.52622 CM 24-16S-10ELodo Formation �120.672 36.502 CM 28-16S-12ELodo Formation �120.67026 36.50278 CM 28-16S-12ELodo Formation �120.670265 36.502308 CM 28-16S-12EMoreno Formation �120.3668 36.269198 CM 17-19S-15E

San Carlos �120.75989 36.51021 CM 27-16S-11ESan Carlos �120.672139 36.503302 CM 28-16S-12E


3.2. Results and discussion

Most of the oil samples are non-biodegraded and a few showslight biodegradation. One condensate and three oil seeps that areseverely biodegraded are in the oil collection for this study. The twoidentified oil families have broadly similar geochemical character-istics, but could originate from different source rocks or different

a

b

Figure 2. a. Whole-oil gas chromatogram and 191 m/z ion mass fragmentogram for Kreyenchromatogram and 191 m/z ion mass fragmentogram for Moreno end-member oil and selfragmentogram for Kreyenhagen source rock extract, Moreno end-member oil, and selecte

organic facies (Jones, 1987) of the same source rock. In addition togenetic information, age, lithology and depositional environment ofthe source rock can be inferred from the detailed biomarkercomposition, as discussed below. Two oil families were defined andare shown in the PCA plot (Fig. 3; Table 2). The two outliers in thePCA plot are biodegraded oil seeps from Big Tar Canyon (SJ119) andVallecitos syncline (SJ131). The Eocene Kreyenhagen source-rock

hagen source rock extract and selected Kreyenhagen oil in Vallecitos. b. Whole-oil gasected Moreno oil in Vallecitos. c. Whole-oil gas chromatogram and 217 m/z ion massd Kreyenhagen and Moreno oils in Vallecitos.

c

Figure 2. (continued).


extract (SJ121) clustered with oil samples inferred to originate fromthe Kreyenhagen Formation. Sample SJ118 considered to be the endmember oil from Moreno source rock, clustered with the inferredMoreno oil in the Vallecitos syncline (Fig. 3).

3.2.1. Oileoil correlation and geochemical characteristics of oilfamilies

We define a genetic oil family in this paper as a group of oilsamples that were generated and expelled from a single source rockorganofacies. Some oil samples may show evidence of mixing, butare dominated by input from only onemajor source rock. Variationsin oil characteristics due to maturity or post-expulsion alterationprocesses, such as biodegradation and phase separation, should notalter genetic relationships. Representative gas chromatograms andterpane mass chromatograms for each oil family are shown inFig. 2a, b, and c. Key biomarker and non-biomarker parametersused in this study to differentiate oil families include pristane/phytane (Pr/Ph), C29/C30 hopane, and C35/C34 homohopane, thedistribution of C27, C28, and C29 steranes, and age-related biomarkerratios, such as the extended tricyclic terpane ratio (ETR) and triar-omatic dinosteranes/(3-methystigmastane 20R) (Table 2).Maturity-related biomarker ratios, such as C29 20S/(20S þ 20R), C29bb/(bbþ aa), and Ts/(Ts þ Tm), were also examined in this study. Inaddition, Ts/(Ts þ Tm) ratio could be affected by salinity and beused as an indicator of a saline environment. The Pr/Ph ratio is usedas an indicator of oxicity of the water column during a given sourcerock deposition (Tissot and Welte, 1984). Although higher Pr/Phratio may reflect terrigenous plant input, maturity can also increasethis ratio (Mackenzie, 1984). Philp (1994) noted that only Pr/Phgreater than four may be taken as an indicator of oxic depositionalenvironment. Pr/Ph ratios less than one are still widely used in theliterature as an indicator of anoxic depositional environments,whereas, Pr/Ph greater than three indicates a suboxic to oxic

depositional environment (Mello et al., 1988). Pr/Ph between 1.25and 2.13 could indicate anoxic to suboxic water column during thesource rock deposition.

Key biomarker parameters used to distinguish different oilfamilies are summarized in Table 2 and details of the biomarkerparameters that characterize each oil family are described below.

3.2.2. Kreyenhagen oil familyFourteen oil and two oil seep samples in our dataset originated

from the Kreyenhagen Formation. The samples in this family arecharacterized by 2.2< Pr/Ph< 3.2 (Fig. 2a), which typically denotesa suboxic to oxic source rock depositional environment. The sam-ples have relatively low ratios of pristane to n-C17 and phytane to n-C18. The distribution, relative abundance, and stereochemistry ofpentacyclic terpanes obtained fromm/z 191 ion chromatograms areshown in Fig. 2a and b, and Table 3, and the derived parameters arein Table 2. Tricyclic terpanes usually occur in small amounts(maximum 5e10% of C30 hopane). Samples in this family have lowtricyclic terpane concentrations, indicating little terrigenousorganic matter input. In addition, low tricyclic terpanes in theKreyenhagen oil and Kreyenhagen source-rock extract samplessupport an oil-source rock correlation for those samples. Significantco-eluting 18a (H)-oleanane and 18b (H)-oleanane in this family (asoleanane content usually 5e10% of C30 hopane) indicates someterrigenous organic matter input from angiosperms, and suggestsan Upper Cretaceous or Tertiary source rock (Moldowan et al.,1994). C29 hopane is less than C30 hopane (C29/C30 < 0.6), whichcould indicate a clay-rich source rock (shale). According to Melloet al. (1988), Ts/Tm below unity indicates a saline environment,whereas Ts/Tm above unity indicates a brackish to fresh waterenvironment. The Ts/Tm ratios of all the oil samples in the Valle-citos syncline are below unity (Fig. 2a and b; Table 2) indicating asaline environment. Homohopane distributions were widely used

Table 2Geochemical parameters of oil samples in the Vallecitos syncline and the reference samples.

SampleID

Family aC19/C19 þ C23

aC22/C21 aC24/C23aC26/C25 aTet/C23

a%C27 a%C28 a%C29 aETR aC31R/H aC29/C30 Hop

aDiaH/(H þ diaH)

aC35/(C34 þ C35)Hop

aGA/H aDiast aOI

SJ102 EK 0.17 0.21 0.58 1.11 0.61 0.32 0.34 0.34 1.75 0.30 0.49 0.04 0.39 0.02 0.25 0.20SJ104 EK 0.12 0.18 0.50 1.02 0.68 0.33 0.35 0.32 1.64 0.25 0.51 0.03 0.42 0.02 0.27 0.16SJ105 EK 0.14 0.13 0.61 1.20 0.55 0.33 0.36 0.31 1.80 0.24 0.46 0.05 0.42 0.03 0.26 0.16SJ106 EK 0.15 0.16 0.69 1.14 0.70 0.32 0.35 0.33 1.49 0.29 0.57 0.04 0.49 0.03 0.28 0.16SJ107 EK 0.16 0.19 0.46 0.96 0.79 0.33 0.34 0.33 2.07 0.40 0.61 0.03 0.41 0.01 0.26 0.23SJ109 EK 0.15 0.22 0.47 0.98 0.79 0.32 0.34 0.34 1.71 0.35 0.56 0.02 0.37 0.01 0.25 0.21SJ112 EK 0.13 0.15 0.68 1.18 0.56 0.32 0.35 0.33 1.52 0.22 0.43 0.04 0.40 0.01 0.30 0.11SJ117 EK 0.15 0.16 0.69 1.22 0.47 0.32 0.35 0.32 1.93 0.23 0.46 0.05 0.39 0.02 0.31 0.15SJ119 EK 0.10 0.23 0.64 0.90 0.39 0.46 0.38 0.16 20.51 0.40 0.80 0.62 0.19 0.78 0.68 0.82SJ121 EK 0.12 0.21 0.50 0.88 0.68 0.31 0.36 0.33 1.28 0.26 0.49 0.03 0.41 0.01 0.39 0.15SJ122 EK 0.12 0.11 0.77 1.11 0.28 0.33 0.36 0.31 2.16 0.24 0.43 0.07 0.44 0.02 0.35 0.17SJ124 EK 0.16 0.16 0.50 1.06 0.74 0.33 0.34 0.34 1.58 0.48 0.54 0.02 0.41 0.01 0.25 0.24SJ125 EK 0.14 0.14 0.48 0.88 0.82 0.34 0.35 0.31 1.39 0.39 0.53 0.03 0.44 0.02 0.28 0.23SJ126 EK 0.13 0.12 0.66 1.08 0.45 0.32 0.37 0.31 1.76 0.26 0.46 0.05 0.40 0.01 0.31 0.19SJ128 EK 0.13 0.16 0.60 1.14 0.48 0.32 0.37 0.32 1.99 0.26 0.45 0.04 0.42 0.01 0.30 0.20SJ129 EK 0.16 0.15 0.45 1.06 0.77 0.32 0.35 0.33 1.61 0.42 0.53 0.02 0.41 0.02 0.23 0.23SJ130 EK 0.14 0.09 0.71 1.14 0.45 0.32 0.37 0.30 1.89 0.27 0.45 0.05 0.42 0.01 0.30 0.19SJ131 EK 0.12 0.24 0.56 1.43 0.71 0.18 0.37 0.44 0.99 0.48 5.79 0.37 0.38 0.21 0.64 0.53SJ103 CM 0.09 0.22 0.47 0.96 0.35 0.32 0.45 0.24 2.91 0.24 0.45 0.03 0.47 0.02 0.30 0.08SJ110 CM 0.10 0.18 0.70 1.00 0.29 0.34 0.42 0.25 3.69 0.21 0.42 0.06 0.43 0.01 0.35 0.08SJ111 CM 0.08 0.15 0.74 1.02 0.27 0.33 0.42 0.25 3.52 0.21 0.42 0.07 0.42 0.01 0.37 0.08SJ113 CM 0.10 0.24 0.53 1.08 0.33 0.33 0.41 0.25 2.60 0.22 0.42 0.06 0.42 0.02 0.36 0.08SJ114 CM 0.08 0.25 0.55 1.08 0.31 0.34 0.42 0.24 2.31 0.23 0.43 0.06 0.43 0.01 0.37 0.08SJ115 CM 0.07 0.23 0.52 1.02 0.32 0.33 0.42 0.25 2.39 0.22 0.43 0.06 0.42 0.01 0.35 0.06SJ118 CM 0.42 0.25 0.42 0.88 0.58 0.34 0.41 0.24 2.96 0.22 0.58 0.04 0.36 0.01 0.36 0.06SJ123 CM 0.07 0.11 0.44 0.86 0.36 0.32 0.44 0.24 2.74 0.27 0.43 0.03 0.49 0.02 0.31 0.08SJ127 CM 0.09 0.30 0.56 1.06 0.33 0.34 0.41 0.24 2.30 0.24 0.42 0.06 0.42 0.01 0.35 0.08

SampleID

Family bBisnorH

b25-Nor bTADMD3/C28S

bTADino b24TET/H

bPr/Ph

b27MA b28MA b29MA C26T/Ts

3-/(3- þ 4-MeS 20R)

Ts/(Ts þ Tm)

C29 bb/(bb þ aa)

C29 20S(20S þ 20R)

TA C20/(C20 þ C28,20R)

TA(I)/TA(I þ II)

SJ102 EK 0.16 0.06 0.09 0.74 0.03 2.60 0.22 0.51 0.27 0.43 0.51 0.39 0.33 0.34 0.16 0.09SJ104 EK 0.15 0.01 0.10 0.80 0.02 2.40 0.23 0.52 0.25 0.41 0.42 0.39 0.27 0.34 0.12 0.06SJ105 EK 0.27 0.02 0.10 0.81 0.03 2.00 0.24 0.51 0.25 0.52 0.40 0.44 0.32 0.33 0.15 0.08SJ106 EK 0.12 0.06 0.09 0.75 0.04 2.30 0.22 0.50 0.27 0.36 0.49 0.48 0.37 0.42 0.25 0.15SJ107 EK 0.21 0.07 0.09 0.77 0.03 2.64 0.21 0.50 0.29 0.48 0.46 0.29 0.28 0.32 0.08 0.04SJ109 EK 0.23 0.09 0.09 0.73 0.03 2.80 0.22 0.46 0.32 0.44 0.51 0.33 0.29 0.35 0.11 0.06SJ112 EK 0.15 0.02 0.10 0.74 0.03 2.10 0.24 0.51 0.24 0.44 0.52 0.54 0.44 0.46 0.10 0.10SJ117 EK 0.11 0.06 0.09 0.72 0.03 2.46 0.25 0.49 0.26 0.56 0.53 0.46 0.39 0.42 0.26 0.16SJ119 EK 0.57 3.99 0.09 0.74 0.88 1.50 0.25 0.49 0.26 5.81 0.51 0.56 0.55 0.77 0.06 0.03SJ121 EK 0.03 0.03 0.11 0.81 0.02 1.00 0.23 0.48 0.29 0.33 0.42 0.50 0.37 0.50 0.48 0.07SJ122 EK 0.04 0.04 0.10 0.76 0.04 1.75 0.30 0.44 0.26 0.73 0.48 0.63 0.30 0.50 0.34 0.24SJ124 EK 0.23 0.04 0.10 0.75 0.03 2.61 0.24 0.49 0.27 0.46 0.50 0.43 0.11 0.33 0.09 0.05SJ125 EK 0.23 0.01 0.09 0.79 0.03 2.25 0.22 0.51 0.26 0.45 0.44 0.45 0.16 0.33 0.08 0.04SJ126 EK 0.15 0.01 0.09 0.77 0.03 2.34 0.25 0.52 0.24 0.61 0.46 0.47 0.26 0.37 0.22 0.13SJ128 EK 0.14 0.00 0.10 0.76 0.03 2.36 0.23 0.51 0.27 0.56 0.49 0.43 0.30 0.39 0.22 0.13SJ129 EK 0.20 0.06 0.09 0.78 0.03 2.36 0.22 0.49 0.29 0.46 0.46 0.32 0.26 0.31 0.08 0.04SJ130 EK 0.15 0.02 0.10 0.77 0.03 1.56 0.22 0.52 0.26 0.58 0.47 0.51 0.30 0.40 0.22 0.13SJ131 EK 0.48 0.16 0.09 0.67 0.45 1.00 0.22 0.52 0.25 0.33 0.60 0.47 0.58 0.77 0.04 0.03SJ103 CM 0.09 0.07 0.09 0.88 0.02 2.13 0.19 0.64 0.18 0.89 0.28 0.32 0.30 0.24 0.06 0.03SJ110 CM 0.04 0.03 0.11 0.84 0.03 2.09 0.20 0.61 0.19 1.00 0.36 0.36 0.39 0.37 0.31 0.16SJ111 CM 0.06 0.02 0.11 0.84 0.03 2.00 0.20 0.61 0.19 1.00 0.36 0.46 0.37 0.40 0.31 0.16SJ113 CM 0.10 0.05 0.10 0.85 0.03 1.90 0.22 0.59 0.20 0.76 0.34 0.49 0.39 0.38 0.27 0.14SJ114 CM 0.10 0.05 0.10 0.85 0.02 2.00 0.21 0.59 0.20 0.70 0.34 0.50 0.37 0.37 0.28 0.14SJ115 CM 0.08 0.05 0.10 0.85 0.03 1.70 0.22 0.59 0.20 0.71 0.34 0.49 0.34 0.38 0.27 0.13SJ118 CM 0.08 0.06 0.09 0.85 0.05 3.20 0.20 0.61 0.19 1.00 0.34 0.31 0.32 0.39 0.21 0.10SJ123 CM 0.06 0.08 0.09 0.89 0.02 1.72 0.18 0.63 0.19 0.84 0.26 0.34 0.30 0.23 0.06 0.03SJ127 CM 0.09 0.05 0.11 0.85 0.03 1.96 0.22 0.58 0.19 0.75 0.34 0.48 0.36 0.39 0.25 0.13

a Parameter was used in the Chemometric analysis. C19/C19 þ C23, C22/C21, C24/C23, and C26/C25 tricyclic terpanes; Tet/C23 ¼ C24 tetracyclic terpane/C23 tricyclic terpane; %C27 ¼ %C27 steranes/(%C27e%C29 steranes); %C28 ¼ %C28 steranes/(%C27e%C29 steranes); %C29 ¼ %C29 steranes/(%C27e%C29 steranes); ETR ¼ (C28 þ C29 tricyclics)/trisnorneo-hopane; C31R/H ¼ C31 homohopane/hopane; C29/C30Hop ¼ C29 30-norhopane/hopane; DiaH/(H þ diaH) ¼ C30 diahopane/(hopane þ diahopane); C35/(C34 þ C35)Hop ¼ C35homohopanes/(C34 þ C35 homohopanes); GA/H ¼ gammacerane/hopane; Diast ¼ diasterane/(diasteranes þ sterane); OI ¼ oleanane/(oleanane þ hopane). Many of theseparameters are discussed in Peters et al. (2005).

b Parameter was used in the Chemometric analysis. BisnorH ¼ bisnorhopane/hopane; 25-Nor ¼ 25-nor-hopane/hopane; TA-DMD3/C28S ¼ C28 triaromatic demethyldi-nosterane/C28 stigmastane; TA-Dino ¼ C29 triaromatic dinosteranes/(C29 dinosteranes þ 3-methylstigmastane 20R); 24TET/H ¼ C24 tetracyclics/hopane;27MA ¼ monoaromaticC27/monoaromatic(C27 þ C28 þ C29); 28MA ¼ monoaromaticC28/monoaromatic(C27 þ C28 þ C29); 29MA ¼ monoaromaticC29/monoaromatic(C27þ C28þ C29); C26T/Ts¼ C26 tricyclic terpane/trisnorneohopane; 3-/(3-þ 4-MeS 20R)¼ 3-/(3-þ 4-methylstigmastane 20R); C24Tet/C26¼ C24 tetracyclics/C26

trycyclics; TA C20/(C20 þ C28, 20R) ¼ desmethyl triaromatic steroids; TA(I)/TA(I þ II)¼(C21 þ C22)/(C21 þ C22 þ C26 þ C27 þ C28) desmethyl triaromatic steroids. Many of theseparameters are discussed in Peters et al. (2005). See EK & CM in Table 1.


Figure 3. PCA for all oil samples and seeps in our dataset based on selected biomarkerand isotope ratios (Table 2). CM: oil from Cretaceous Moreno Formation; EK: oil fromEocene Kreyenhagen Formation; EKE: Eocene Kreyenhagen source rock extract. Opendiamond and solid square symbols indicate the oil samples in the Vallecitos syncline.Biomarkers have been altered in biodegraded Seep 2 and SJ119.


to assess source rock redox conditions and also can be used forcorrelation. When the concentration of extended homohopanesC31eC35 decreases with poor preservation of C35 homohopane, thisindicates a suboxic to oxic open marine environment. TheKreyenhagen oils in the Vallecitos syncline show poor preserva-tions of C35 homohopane, indicating a suboxic to oxic open marineenvironment (Fig. 2a and b; Table 2). C29 25-norhopane/Ts 18a (H)-norneohopane in this family is greater than one. In addition, C29normoretane/18a (H)-oleanane is less than one (Fig. 2a). The dis-tribution and relative abundances of steranes as well as their ste-reochemistry was obtained from m/z 217 ion chromatogramsshown in Figure 2c and Table 3, and derived parameters are listed inTable 2. Source and maturity related parameters in this studyinclude aaa 20R isomers of C27eC28eC29 steranes and diasteranes.High diasteranes in the samples indicates a clay-rich source rock(shale).

Sterane ternary diagrams are used extensively to show re-lationships between oils and/or source rock bitumen (Peters et al.,2005). The principal use of sterane ternary diagrams is to distin-guish groups of crude oils from different source rocks or differentorganic facies of the same source rock. In addition, monoaromaticsteroid ternary diagrams are useful to distinguish petroleum basedon the depositional setting of the source rock (Moldowan et al.,

Table 3Identification of labeled biomarker peaks shown in Figures 2a, b and c.

Peak ID Sterane m/z ¼ 217 Peak ID Terpane m/z ¼ 191

1 C27 ba 20S diacholestane 1 Ts 18a(H)-trisnorhopane2 C27 ba 20R diacholestane 2 Tm 17a(H)-trisnorhopane3 C28 ba 20S diasterane a 3 C28 17a,18a,21b(H)-

bisnorhopane4 C28 ba 20S diasterane b 4 C29 25-nor-hopane5 C28 ba 20R diasterane a 5 C29 Ts 18a(H)-norneohopane6 C28 ba 20R diasterane b 6 C30 17a(H)-diahopane7 C27 aa 20S cholestane 7 C29 normoretane8 C27 bb 20R cholestane 8 a-oleanane9 C27 bb 20S cholestane 9 b-oleanane10 C27 aa 20R cholestane 10 C30 17a(H) hopane11 C29 ba 20R

diastigmastane11 C31 22S 17a(H) homohopane

12 C28 aa 20S ergostane 12 C31 22R 17a(H) homohopane13 C28 aa 20S ergostane 13 C32 22S 17a(H) bishomohopane14 C28 bb 20R ergostane 14 C32 22R 17a(H) bishomohopane15 C28 bb 20S ergostane 15 C33 22S 17a(H) trishomohopane16 C28 aa 20R ergostane 16 C33 22R 17a(H) trishomohopane17 C29 aa 20S stigmastane 17 C34 22S 17a(H) extended hopane18 C29 bb 20R stigmastane 18 C34 22R 17a(H) extended hopane19 C29 bb 20S stigmastane 19 C35 22S 17a(H) extended hopane20 C29 aa 20R stigmastane 20 C35 22R 17a(H) extended hopane

1985). On both of the ternary diagrams, Kreyenhagen oil samplesin the Vallecitos syncline cluster near the area defined by marineshale on the ternary plots fromMoldowan et al. (1985) (Fig. 4a andb). The inferred Kreyenhagen oil correlates with Kreyenhagensource-rock extract on the sterane ternary diagrams. Several plotsof selected saturate and aromatic biomarker parameters, such asextended tricyclic terpanes (ETR), oleanane index, TA-Dino/(TA-Dino þ 3-methylstigmastane 20R) (i.e., TA-Dino ¼ C29 triaromaticdinosteranes), and TA 3-/(3- þ 4-methylstigmastane 20R), indicatethat Kreyenhagen sourced oil is well separated fromMoreno sourceoil (Fig. 4c and d).

3.2.3. Moreno oil familyEight oil samples originated from the Moreno Formation in the

Vallecitos syncline. This family is characterized by 1.6 < Pr/Ph < 2.2, which indicates an anoxic to suboxic water columnduring the source rock deposition. The samples have relativelyhigh ratios of pristane to n-C17 and phytane to n-C18 (>2) (Fig. 2b).Samples in this family have low tricyclic terpane concentrations,indicating terrigenous organic matter input. In addition, low tri-cyclic terpanes in the Moreno samples from the Vallecitos syn-cline and Oil City oil (Fig. 1b) support an oileoil correlation forthose samples. Very low 18a (H)-oleanane and 18b (H)-oleananein this family could also indicate little terrigenous organic matterinput. C29 hopane is less than C30 hopane (C29/C30 < 0.4), whichcould indicate a clay-rich source rock (shale). In addition, highconcentrations of diasteranes also suggest a clay-rich source rock.The concentration of extended homohopanes (C31eC35) decreaseswith poor preservation of C35 homohopane, indicating a suboxicto oxic open marine source rock environment. C29 25-norhopane/Ts 18a (H)-norneohopane in this family is much less than one. Inaddition, C29 normoretane/18a (H)-oleanane is also greater thanone (Fig. 2b). On both of the sterane and monoaromatic steroidternary diagrams, Moreno samples cluster near the area definedby marine shale (Moldowan et al., 1985) (Fig. 4a and b). Theinferred Moreno samples correlate with the Oil City oil on thesterane ternary diagrams, which is believed to be from Morenosource rock (Peters et al., 1994; Lillis and Magoon, 2008). Thedistribution of C27 aa 20R cholestane, C28 aa 20R ergostane andC29 aa 20R stigmastane of the Kreyenhagen and Moreno samplesare clearly distinguished on Figure 2c (peak ID 10, 16 and 20 inTable 3) and Figure 4b.

3.2.4. Thermal maturityMaturity of the samples was assessed using the following pa-

rameters: C29 sterane isomerization ratios C29 aaa 20S/(S þ R), andC29 abb 20(S þ R)/(abb þ aaa) 20(S þ R); Ts/(Ts þ Tm), TA [C20/(C20 þ C28 20R)] and TA (I)/TA (I þ II). The isomerization ratio at theC-20 position (20S/20S þ 20R) in the C29 aaa steranes, which istheoretically zero at the depositional surface, increases with ther-mal maturity, reaching the endpoint value of about 0.55 at peak oilgeneration. The abb-iso steranes relative to aaa-normal steranesratio increases with maturity, reaching an endpoint at w0.75(Mackenzie, 1984; Grantham, 1986). The Kreyenhagen family hasC29 20S/(20S þ 20R) sterane ratios of 0.3e0.4 and C29 bb/(bb þ aa)20R sterane ratios of 0.11e0.44, indicating early oil-windowmaturity (Table 2). Ts/(Ts þ Tm), TA (I)/TA (I þ II), and TA [C20/(C20 þ C28, 20R)] for the Kreyenhagen family (w43%, w7%, andw16%, respectively) also support this conclusion. The Morenofamily mostly has typical oil-window maturity and some sampleshave early oil windowmaturity. C29 20S/(20S þ 20R) for this familyisw0.32, and C29 bb/(bbþ aa) 20R isw0.35. Ts/(Ts þ Tm), TA (I)/TA(I þ II), and TA [C20/(C20 þ C28, 20R)] for Moreno family (w42%,w8%, and w26%, respectively) are also consistent with thisconclusion.

a b

c d

Figure 4. a. Ternary diagram of C27eC29 C-ring monoaromatic steroid distributions in crude oils and rock extract samples shows the comparison of oils from the Vallecitos synclinewith well studied Kreyenhagen and Moreno oil samples. Open symbols indicate the oil samples in the Vallecitos syncline (see Fig. 3 for CM, EK, and EKE). b. Regular sterane 5a, 14a,17a(H) 20R (m/z ¼ 217) ternary diagram shows the comparison of oils from the Vallecitos syncline with well studied Kreyenhagen and Moreno oils. Open symbols indicate the oilsamples in the Vallecitos syncline (see Fig. 3 for CM, EK, and EKE). c. Comparison of oils from the Vallecitos syncline with well studied Kreyenhagen and Moreno oil samples. Opensymbols indicate the oil samples in the Vallecitos syncline (legend see Figure 4a). d. Comparison of oils from the Vallecitos syncline with well studied Kreyenhagen and Moreno oilsamples. Open symbols indicate the oil samples in the Vallecitos syncline (legend see Figure 4a).


4. Basin and petroleum system modeling

In the past decade, three-dimensional (3D) basin and petroleumsystemmodeling (BPSM) of the subsurface through geological timehas evolved as a major research focus of both the petroleum in-dustry and academia. BPSM continues to grow in importance as atool to understand subsurface geology and basin evolution byintegrating key aspects of geochemistry, geology, geophysics, andstratigraphy. Among the above, geochemistry is themost importanttool to understand the processes affecting petroleum systems. Apetroleum system is composed of a pod of active source rock and allrelated oil and gas, including all geological elements and processes:source rock, reservoir rock, seal, overburden rock, and favorabletiming of petroleum generation, migration, accumulation and trapformation (Magoon and Dow, 1994). The first step in identifyingpetroleum systems is to characterize and map the geographic dis-tribution of oil and gas types. Geochemical tools, such as bio-markers, diamondoids, and carbon isotope analysis are used toconduct oileoil and oilesource correlation, which is the key tounderstand and determine the geographic extent of petroleumsystems. It is recommended to start with a 1D model to solve theprincipal timing and understand the burial history in a particular

setting. Then 2D or 3D models can be chosen based on the purposeof the study. In this study, our objective was to understand thethermalmaturity of the two hypothesized source rocks. A 2Dmodelwas then created to examine scenarios of erosion thickness in moredetail and to understand the sensitivity of the thermal maturity ofsource rock to assumed erosion thickness.

5. Numerical modeling and input

Our geochemical results indicate that there are two different oilfamilies in the study area generated from the Eocene Kreyenhagenand Cretaceous Moreno formations. These geochemical resultswere anticipated based on our initial 1D model results. The base ofthe Moreno Formation in the deepest part of the syncline reachedthermal maturity as early as 42 Ma and the synclinal KreyenhagenFormation became thermally mature as early as 19 Ma (Fig. 5a) (Heet al., 2010). The results from the 1D model show that the MorenoFormation already passed the dry gas window, and part of thelower Kreyenhagen Formation is in the oil window at present-dayin the deepest part of syncline (He et al., 2010). The transformationratio (TR) shows that the Moreno Formation (TR ¼ 100%) isdepleted, while most of the Kreyenhagen Formation (TR ¼ 70%) in


the deepest part of the syncline is at the peak of oil generation atpresent-day (Fig. 5b) (He et al., 2010). In addition, most of structuraltraps in this area were formed due to relatively recent folding anduplift of the basin (<10 Ma) (Graham and Williams, 1985;Rentschler, 1989; Peters et al., 2008). The Moreno Formation inthe deepest part of syncline had equivalent vitrinite reflectance (Ro)of 4% at the time the young traps were formed (Fig. 5a) (He et al.,

Figure 5. a. Plot of time versus vitrinite reflectance of the two source rocks for the calibratsource rocks for the calibrated model at 1D location. c. Geological profile along the line GH

2010). The question is how those young traps could captureearlier generated Moreno oil. Is some part of theMoreno Formationwithin the oil-window maturity charging the traps in the synclineat present-day? The maximum present-day burial depth of theKreyenhagen Formation is about 2.9 km (9500 ft) in the syncline(Fig. 5c). How did this shallowly buried source rock become amajorsource rock in the Vallecitos study area if the deeper Moreno

ed model at 1D location. b. Burial history curves with transformation ratio (TR) of two(Figure 1a) in the Vallecitos syncline.

Figu

re5.

(con

tinu

ed).


Table 4General input data for 1D and 2D models.

Age of interval PetroModstratigraphicunit

Deposition (Ma) Presentthickness(M)

Erosion/Hiatus (Ma) Erosionthickness(M)

Lithology Petroleum systemelement

From To From To

Cenozoic Tertiary Pliocene Unnamed 0.6 0.2 0 0.2 0 25 Sandstone (typical) Overburden RockUnnamed 2.5 2 0 2 0.6 381 Shale (typical)San Joaquin Fm. 4.5 3 0 3 2.5 381 Shale (typical)

Miocene toPliocene

Etchegoin Fm. 5.5 4.5 1570.3 Sandstone (typical) Reservoir Rock

Miocene Monterey Fm. 13 5.5 0 Shale (typical) Overburden RockTemblor Fm. 22 14 701 14 13 152 Sandstone (clay poor) Reservoir Rock

Oligocene Unnamed 37 30 0 30 22 610 Shale (typical) Overburden RockEocene U. Kreyenhagen Fm. 43 37 457 Sandstone (clay rich)

L. Kreyenhagen Fm. 48.5 43 366 Shale (organic rich) Source RockDomengine Fm. 49 48.6 46 48.6 48.5 23 Sandstone (typical) Reservoir Rock

Paleocene toEocene

Yokut Sandstone 49.5 49 213 Sandstone (clay poor)Arroyo Hondo shale(Lodo Fm.)

51.5 49.5 183 Shale (organic lean) Seal Rock

Cantua Sandstone(Lodo Fm.)

53 51.5 427 Sandstone (clay poor) Reservoir Rock

Cerros Shale(Lodo Fm.)

55.5 53 107 Shale (organic lean) Seal Rock

San Carlos Sandstone(Lodo Fm.)

58.5 55.5 274 Sandstone (clay poor) Reservoir Rock

Mosozoic Cretaceous to Paleocene Moreno Fm. 73.5 61 594 61 58.5 100 Shale (organic rich) Source RockPanoche Fm. 83.5 73.5 1000 Shale (organic lean) Underburden Rock


Formation had no contribution to the oil pools in the syncline atpresent-day? How can the shallowly buried Kreyenhagen Forma-tion mature? In order to better answer these questions and un-derstand the burial history of the source rocks in the syncline, a 2Dgeohistory reconstruction of the Vallecitos syncline was completedusing PetroMod 2D involving erosion scenarios on the flanks of thesyncline, which could not be simulated in our initial 1D model.

5.1. 2D model input and burial history of source rocks

The 2D model was developed based on a published cross-section through the deepest part of the Vallecitos syncline(Fig. 5c). We recognize that the structure at depth in the synclinelikely is more complicated than is depicted in Figure 5c, but itnevertheless serves as a basis for evaluating maturation scenarios.Discretization was conducted by defining vertical grid and hori-zontal event lines. Inputs for the 2D model are geological layerthicknesses, depositional duration, and type (depositional, non-depositional and erosional) of the different modeling events. In

Table 5Boundary condition and model calibration data from well NETEX1 for 1D and 2D model

Age (Ma) Paleowaterdepth (m)

Age (Ma) SWIT (sedimentwater interfacetemperature) (�C)

Model

Age (Ma)

0 �150 0 17.3 04.5 �100 4.5 16.3 95.5 �80 5.5 16.33 1416 �50 16 20.4 1618 600 18 5.69 1837 1000 37 24.47 2543 600 43 12.7 3048.5 200 48.5 19.48 3549 1000 49 24.6 4049.5 1000 49.5 13.32 4551.5 1000 51.5 13.63 5053 1000 53 13.88 5555.5 1000 55.5 14.17 6058.5 1000 58.5 24.13 6573.5 1000 73.5 14.82 73.5

addition, lithologies, paleowater depths, paleotemperatures, heatflow estimates, as well as hydrocarbon generation kinetics arerequired. These numerical data, including thickness, age at upperand lower boundary, lithological properties and heat flow wereassigned to each grid cell. Source rock properties, such as totalorganic carbon (TOC) content, hydrogen index (HI), and kineticparameters for petroleum generation, were also required modelinput. In general, the basic input data for the 2D model are verysimilar to that for the initial 1D model, which was located in thedeepest part of the Vallecitos syncline and was built from a com-posite pseudo-well based on the nearby the NETEX1 well (API:06900250) (Fig. 1a; Table 4). The stratigraphy, lithostratigraphy,erosion and hiatus events were based on the USGS database(Hosford Scheirer and Magoon, 2008) and on updated studies fromprevious graduate studies at Stanford University (e.g., Anderson,1998; Bac, 1990; Enos, 1961; McGuire, 1988; Milam, 1985;Rentschler, 1989; Schulein, 1993) (Fig. 1c). All calibration datawere obtained from the NETEX1 well (Table 5). The depositionaldurations of the respective units are based on published data,

s.

Depth (m) Ro% Tmax (�C) Ro% (convertedfrom Tmax)HF (heat flow,

mW/m2)

68 1100 428 0.54145 1119 0.54 426 0.51155 1228 427 0.53155 1301 0.59160 1347 432 0.6275 1393 431 0.6070 1430 430 0.5865 1503 0.6160 1512 431 0.6054.5 1539 432 0.6253 1594 434 0.6551.5 1646 432 0.6250 1673 433 0.6348.546


completion logs, and sequence stratigraphic work at StanfordUniversity (e.g., Anderson, 1998; Enos, 1961; Milam, 1985;Rentschler, 1989; Schulein, 1993). The lithologies were character-ized using standard physical and chemical parameters available inthe PetroMod software. The thickness for each layer is determinedfrom the cross-section profile GH (Fig. 5c).

All of the boundary conditions are consistent with those used inour initial 1D model (Table 5). Paleobathymetry is critical forpaleosubsurface topography and structural configurations, whichaffect burial and maturation of the source rocks, as well as migra-tion pathways. Paleowater depth (PWD), one of the boundaryconditions necessary for basin and petroleum system modeling,was estimated from previous graduate work in biostratigraphy atStanford University (e.g., Anderson,1998; Bac,1990; McGuire,1988;Milam, 1985; Rentschler, 1989; Schulein, 1993) and the literature(Graham and Berry, 1979). This allowed calculation of paleo-meansurface temperatures from the sediment-water-interface depthand paleo-latitude (Wygrala, 1989). Paleotemperatures are anotherimportant input parameter because they affect the geothermalgradient and the temperature history of the basin. Sedimentewa-ter-interface temperature (SWIT), another boundary condition forBPSM, can be derived from paleomean surface temperature andpaleowater depth. Heat flow history is an important input for the2D model, but it is difficult to determine. Heat flow is not directlymeasured, but related to the geothermal gradient. The present-daytemperature is the only known parameter in a heat flow history.The remaining thermal history has to be determined by adjustingsimulated model results to the temperature profiles (calibrationdata), such as vitrinite reflectance, bottom-hole temperature, andRock-Eval Tmax temperature. An alternative way to determineheatflow history is based on subsidence and thermo-mechanicalmodels (e.g., McKenzie, 1978). The heat flow profile starts withbasal heatflow at a certain geological time and is forward modeledthrough time to achieve a fit with present-day heatflow.

Key for the 2D model is the erosion events proposed byRentschler (1985). Major folding of the Vallecitos syncline occurredabout 10 Ma and a series of erosion events took place on the flanksof the syncline caused by that folding (Bartow, 1991). Those erosionevents could not be simulated in our initial 1Dmodel. This certainlywill affect the burial history of the basin and thermal history of thesource rocks.

5.2. Source rocks and kinetics

We propose two active source rocks in the Vallecitos synclinebased on our initial 1Dmodel and geochemical results, which differfrom previously published studies. The active source rocks includeshale of the Cretaceous Moreno and Eocene Kreyenhagen forma-tions. The Moreno Formation is a marine sedimentary sequencedeposited in the central San Joaquin basin during the Late Creta-ceous to early Tertiary. The Moreno Formation was deposited in anupper-slope to outer-shelf marine environment, with a portion ofthe unit influenced by upwelling oceanographic conditions andsedimentation under anoxic or low-oxygen conditions (McGuire,1988). This unit covers much of the northwestern San Joaquin ba-sin, including outcrop exposures west of Coalinga and northward tothe Laguna Seca Hills; subsurface occurrences from the KettlemanHills northward to the Chowchilla gas field; westward into theVallecitos syncline and eastward beyond the axis of the San Joaquinbasin (McGuire, 1988; Reid,1988; Bac, 1990; Peters et al., 2007). Thesediments are composed primarily of shale, mudstone, diatoma-ceous shale, diatomite and silty turbidite deposits. The shale in thelower part of this sequence is organic-rich source rock and the siltyturbidite deposits are potential reservoir rocks. The Moreno For-mation has highly variable marine autochthonous input from algae

and bacteria (Bac, 1990). Total organic carbon (TOC) of MorenoFormation ranges from 1 to 3.95% and hydrogen index varies from38 to 376mg HC/g TOC (McGuire, 1988). Pyrolysis results plotted ona modified van Krevelen diagram indicate that Moreno source rocksamples contain predominantly Type II kerogen (McGuire, 1988).

The Kreyenhagen Formation is a widespread middle Eocene toOligocene bathyal marine sequence of shale, diatomaceous shale,porcelanite and diatomite, with minor turbiditic sandstone (Milam,1985). The laminated character of these biogenic units and highTOC values indicate deposition under low-oxygen conditionsassociated with an oxygen-minimum zone (Milam, 1985). Kreyen-hagen source rock is a diatomaceous and foraminiferal fine-grainedshale and clay shale bodywithin the lower Kreyenhagen Formation,and it covers most of the northern San Joaquin basin (Peters et al.,2007). Kreyenhagen shale contains type II/III kerogen, and consistsof algal and terrigenous organic matter. Total organic carbon (TOC)of the Kreyenhagen shale ranges from 1.55 to 3.88%. The hydrogenindex of the Kreyenhagen Formation ranges from 137 to329 mg HC/g TOC (Milam, 1985).

Kerogen kinetics control the timing of hydrocarbon generationand zones of diagenesis, catagenesis, and metagenesis. Kinetics foreach source rock (kerogen) can be determined by laboratory ex-periments. Laboratory pyrolysis at different heating rates yieldsactivation energies and a pre-exponential factor for the kerogen. It isassumed that the conversion of kerogen to oil and gas is irreversibleand is defined by a series of parallel pseudo-reactions. For our 2Dmodel in this study, the organicmatter was assumed to bemainly ofmarine origin and the source rocks has an average TOC of 2.5%. HIvalues measured on kerogen concentrates range between 137 and329 mg HC/g TOC for the Kreyenhagen shale, and between 38 and376 mg HC/g TOC for Moreno shale, suggesting type II kerogen(Milam, 1985; McGuire, 1988). In this study, we used standard re-action kinetics in PetroMod for Type II kerogen based on Behar et al.(1997), average 2% TOC for Kreyenhagen Formation, and 3% TOC forMoreno Formation. The average HI for both Kreyenhagen andMoreno Formation is 305 and 350 mg HC/g TOC, respectively(Milam, 1985; McGuire, 1988). The petrophysical data, includingdensity, initial porosity, compressibility, thermal conductivity, heatcapacity, permeability, and capillary pressure, were associated witheach different lithology type in PetroMod software. Those petro-physical data are recalculated in each geological time step, andreflect the dynamic model with the burial or erosion events.

5.3. Calibration data

Calibration of the model is required to understand the burialhistory and to determine the timing of petroleum generation andexpulsion in the Vallecitos syncline. Various parameters can beused to achieve calibration, such as heatflow, thermal and physicalproperties of different rock lithologies, surface temperature, andsediment deposited (burial) or eroded (uplift). However, many ofthese parameters, such as thermal and physical properties, areconstrained within narrow ranges of values. We chose heatflow tobe the primary model calibration parameter, as it is normallyassumed in the other well-studied basin models that heatflowhistory is less constrained than any other parameter. Forwardmodeling of petroleum systems requires basal rather than surfaceheatflow as input. Basal heatflow may include heat supplied fromthe deep mantle, radiogenic heat from the crust, and any transientheat provided by a thermal event.

Use of two independent calibration tools, such as vitrinitereflectance (Ro, percent) and Rock-Eval pyrolysis Tmax is recom-mended for temperature history reconstruction. The 2D model wascalibrated by comparing measured vitrinite reflectance and Tmax inthe selected wells with the corresponding values predicted by the

Figure 6. a. Plot of calculated (line) and measured (symbols) %Ro versus depth for the models with three different heatflow scenarios. Open circles indicate measured %Ro data, crosssymbols indicate converted Ro from measured Tmax data in the NETEX1 well. b. Plot of calculated (line) and measured (symbols) %Ro versus depth for the model. Open circlesindicate measured %Ro data, cross symbols indicate converted Ro from measured Tmax data in the NETEX1 well.


Figure 7. Modeled present-day thermal maturity of source rocks expressed as vitrinite reflectance (% Ro) in the 2D calibrated model.


model at those locations. The model calculates vitrinite reflectancevalues using the “Easy% Ro” method of Sweeney and Burnham(1990). The vitrinite reflectance and Tmax from the NETEX1 wellwere used in this study for heatflow calibration (Fig. 6a and b;Table 5). Rock-Eval analyses of the cuttings were completed byGeoMark Research, Ltd. The vitrinite reflectance results are fromWeatherford Laboratories. Quality control on measured Tmax datawas achieved by rejecting some data affected by contamination ormigrated oil, according to rules developed by Peters and Cassa(1994). We checked the quality of measured vitrinite reflectancedata used for calibration by comparing it to vitrinite reflectance(percent) calculated from Tmax (�C) using the following formula:

Vitrinite reflectanceðcalculatedÞ ¼ ð0:018ÞðTmaxÞ � 7:16

The measured Tmax data were converted to vitrinite reflectanceby using this formula as well. This formula is based on data for acollection of shales containing low-sulfur Type II or Type III kerogen(Jarvie and Lundell, 2001). The curve generated by the formulacorresponds reasonably well with empirical observations of Tmaxversus vitrinite reflectance for Type III kerogen (Teichmüller andDurand, 1983). Use of the formula is not recommended for low orhigh maturity samples (where Tmax is less than 420 �C or greaterthan 500 �C) or when S2 is less than 0.5 mg hydrocarbon/g rock.“Caving” of rock cuttings from shallow to deeper parts of thewellbore during drilling can invalidate these calculations becausethe caved material represents a contaminant that is less thermallymature than the deeper rock cuttings.

6. Modeling results and discussion

Heatflow is the most uncertain parameter among all of themodel input parameters. We can model the heatflow history of theVallecitos area by considering the effects of two tectonic settings.

Because the Vallecitos syncline is located above a former subduc-tion zone in the western US, the subducting slab may have cooledthe syncline in its early history (late Cretaceous) (Dickinson andSeely, 1979; Hamilton, 1988). Later, collision of the East PacificRise mid-oceanic spreading center with North America andnorthward migration of the Mendocino Triple Junction heated thearea of the Vallecitos syncline during the Miocene (Atwater andMolnar, 1973; Engebretson, 1982). Based on this history, we pro-pose three models with different heat flow scenarios: 1) a “hotmodel” with a linear regression heatflow profile followed by athermal pulse due to the northward progression of the MendocinoTriple Junction; 2) a “cold model”with a linear regression heatflowprofile and low starting heat flow due to cooling by the subductedslab below the syncline; and 3) a “combination model” with lowstarting heatflow followed by a thermal pulse due to both north-ward progression of the Mendocino Triple Junction and cooling ofsubducted slab below the syncline. The first heat flow trend rep-resenting the “hot model” starts with a low heatflow of 46 mW/m2

and has a rapid increase of heatflow from 20 to 10Ma, followed by adecline to an average present-day value of 68 mW/m2. The “coldmodel” has a low starting heatflow of 30 mW/m2 at 73.5 Ma anddoes not dramatically increase until 25 Ma. Then, the trend has alinear increase from 25 Ma to present day at 68 mW/m2 (Peterset al., 2008). The “combination model” has a low starting heat-flow of 30 mW/m2 at 73.5 Ma, but there is an additional heatingevent from 18 to 9 Ma. However, the model results based on abovethree scenarios all show a cooler profile than expected based on themeasured vitrinite reflectance data (Fig. 6a). That could indicatethat “hot model” is closer reality than the other twomodels and theMendocino Triple Junction has a stronger affect on thermal historyof the syncline than cooling by the subducted slab.

Geological evidence suggests a thermal anomaly close to theVallecitos syncline. Intrusive basalt located about 9 km to thenorthwest of the Vallecitos syncline is dated to middle Miocene age

Figure 8. a. Events chart for the Kreyenhagen-Yokut(.) petroleum system shows the time of deposition of the essential elements and processes of this petroleum system. b. Eventschart for the MorenoeSan Carlos(.) petroleum system shows the time of deposition of the essential elements and processes of this petroleum system.


(Dibblee, 1979) (Fig. 1b). Similarly, the intrusive serpentinite diapirnear New Idria, located about 5 km south of the Vallecitos syncline,created a thermal pulse that may have provided an additional heatsource in the Vallecitos syncline (Vermeesch et al., 2006). By usingvitrinite reflectance in the Joaquin Ridge #1 well (Fig. 1b), whichwas not affected by the thermal anomaly, we calculated maximumerosion thickness ofw1 km in the New Idria area (Vermeesch et al.,2006; Horstman, 1984). The Christie #1 well (Fig. 1b), which wasaffected by the New Idria thermal anomaly, has vitrinite reflectancevalues of w1.9%e2% or maximum paleotemperature of w180 �C atabout 1.25 km depth (Vermeesch et al., 2006). Using the above dataas input, the calculated heatflow value was about 180 mW/m2,assuming the thermal conductivity for magmatic intrusions is1.95 W/m/K, when Mendocino Triple Junction passed this area(Delaney, 1988). In addition, modern examples of high heatflowvalues associated with hot spots and spreading centers offer pointsof comparison. The present-day heatflow in Iceland is around

220 mW/m2 (Stein and Stein, 2003). Another example morereminiscent of the Vallecitos syncline is the Salton Sea, whichrepresents the transition between oceanic spreading in the Gulf ofCalifornia to the south and the San Andreas continental transformfault system to the north. Heatflow values in the U.S. portion of thetrough are >100 mW/m2 (Lachenbruch et al., 1985). In addition,some isolated areas have especially high heatflow, >200 mW/m2

(Newmark et al., 1988). Plate reconstructions indicate that theMendocino Triple Junction passed the Vallecitos area around 19 Ma(Atwater and Stock, 1998). Considering all of the above factors, weused 160 mW/m2 as the heatflow value starting at 18 Ma anddecreased it slowly over the next 9 Ma following McKenzie (1978)(Table 5) in order to better match the model results with measuredRo data (Fig. 6b).

Temperature (magnitude and duration) is one of the mostimportant factors to determine the reaction rate of organic matter.The conversion of kerogen into hydrocarbons (oil and gas)


(transformation ratio, TR) can be determined when the tempera-ture history and kinetics are known. Maturity values are calculatedby using the “Easy% Ro” method of Burnham and Sweeney (1989).Computed vitrinite reflectance (%Ro) values are similar to the actualmeasurements from the well based on the above consideration in acalibrated model (Fig. 6b). Our initial 1D modeling shows that thepresent-day lower Kreyenhagen Formation still has potential for oilgeneration, whereas the present-day lower Moreno Formation is adepleted source rock (He et al., 2010). A sudden increase of thermalmaturity in Pliocene and Pleistocene time is related to rapid sedi-mentation in the syncline as its flanks were uplifted, as well as thethermal pulse associated with the Mendocino Triple Junction. Thus,the tectonic evolution of the region significantly influenced burialand thermal history of the study area. However, the simple 1Dmodel fails to simulate complex tectonic events, such as subsi-dence, uplift and erosion.

Therefore, a 2D model was created to simulate the thermalhistory of the source rock in the Vallecitos syncline involving pro-posed erosion scenarios from relatively recent folding of the syn-cline and the three different heatflow scenarios discussed earlier.Interestingly, the 2Dmodeling results show that the deepest part ofthe lower Kreyenhagen Formation is in the dry gas window atpresent-day, but that the rest of the lower Kreyenhagen Formationremains immature on the very shallow flanks of the syncline(Fig. 7), at the location of the chosen cross-section. The lowerKreyenhagen Formation has significant remaining hydrocarbonpotential in the study area. The deepest part of Moreno Formationis overmature and the shallower sections of Moreno Formation onthe flanks are in the late oil window at present-day. The modelpredicts that the Moreno Formation has no remaining hydrocarbonpotential over much of the Vallecitos syncline.

We identified two petroleum systems: 1) the Eocene Kreyen-hageneYokut(.) petroleum system; and 2) the Cretaceous MorenoeSan Carlos(.) petroleum system, based on our geochemical andmodeling results. The stratigraphic position of reservoirs which theoils were produced from is noted in Figure 1a. The KreyenhageneYokut is a significant petroleum system in the Vallecitos syncline.The Kreyenhagen oil type is widely distributed along the flanks ofthe syncline. The present-day pod of active Kreyenhagen sourcerock is close to the center of the syncline at a shallow depth and thepossible migration paths are most likely from the center of thesyncline to the flanks, where all structural traps occur at a shallowdepth. The basal part of the Kreyenhagen Formation at w2.9 kmdepth is in the dry gas window in the deepest part of the synclinebased on our 2D model results and it should be less mature alongthe basin axis to the west at shallow depth. The young structuraltraps (<10 Ma) may have only captured a small portion of earlygenerated Kreyenhagen oil, since the onset of petroleum genera-tion for lower Kreyenhagen Formation in the deepest part of thesyncline occurred at w19 Ma, as shown on the burial history plot(He et al., 2010) (Fig. 8a). The basal lower Kreyenhagen Formationreached peak generation or expulsion very quickly at w9 Ma andthe present-day TR is 80%, indicating significant remaining gener-ation potential in the source rock (He et al., 2010). Most of the oil inthe present-day traps was recently generated oil from Kreyenhagensource rock.

In contrast, the MorenoeSan Carlos petroleum system is prob-ably a gas-prone system in the syncline. Moreno oil is restricted tothe flanks at a shallow depth. The burial history of the MorenoFormation shows that the onset of petroleum generation started at39 Ma in the deepest part of the syncline and reached peak gen-eration about 34 Ma (He et al., 2010). The results show that MorenoFormation is mostly depleted and only the very top part of MorenoFormation on the flank is still in the late oil window at present-day.Most parts of Moreno Formation are in the wet gas zone in the

deepest part of the syncline at present-day (Fig. 7). The MorenoFormation could be a major source for gas accumulations in thesyncline. Earlier generated Moreno oil could have been lost due tostructural damage of the old traps. Younger traps could only pre-serve recently generated Moreno oil from the top part of formationon the flank in the syncline (Fig. 8b). Moreno oil was also likelycracked to gas at high maturity. The MorenoeSan Carlos petroleumsystem should have little present-day oil generation potential, butcould be a major gas system in this area.

7. Conclusions

A numerical 2D model of the Vallecitos syncline was developedbased on thorough analysis of stratigraphy and basin evolution. Themodel results have significant value to evaluate the timing andmagnitude of hydrocarbon generation and the remaining hydro-carbon potential in the Vallecitos syncline. In general, organic-richpotential source rocks can not become thermally mature withoutsignificant burial. However, the Vallecitos syncline appears to be anexception in that shallowly buried source rock matured as a resultof a thermal anomaly without deep burial. Geological relations inthis area and our detailed model results suggest that passage of theMendocinoTriple Junction near the Vallecitos syncline had a strongthermal impact on the maturity of both source rocks. The currentdistributions of the oil families established by our geochemicalstudy support the modeling results suggesting that a thermalanomaly passed near the syncline in the middle Miocene. Theyoung structural traps seem to preserve the recently generatedKreyenhagen and Moreno oil. The 2D modeling results predict thatthe deepest part of lower Kreyenhagen Formation is in the dry gaswindow at present-day, but the lower Kreyenhagen Formation,especially on the flanks, is less mature. Kreyenhagen source rockremains immature in the very shallow parts of the syncline, at leastat the location of the cross-section.

The BPSM model and detailed geochemical results show thatboth the Eocene Kreyenhagen and Cretaceous Moreno formationswere active source rocks in the Vallecitos syncline. The geographicextents of the both active source rocks likely cover most of thedeeper syncline. Biomarker analyses of oil samples support thecontention that the two different oil families originated from theKreyenhagen and Moreno formations. In addition, the EoceneKreyenhagen Formation still has great potential for petroleumgeneration, especially in those areas buried at shallow depth. TheCretaceous Moreno Formation is a depleted source rock for oilgeneration, but it remains a source rock for hydrocarbon gas in theVallecitos syncline.

Acknowledgments

This work was completed with financial support from the Basinand Petroleum SystemModeling (BPSM) and theMolecular OrganicGeochemistry Industrial Affiliates (MOGIA) programs at StanfordUniversity. We greatly appreciate the assistance from ProfessorMike Moldowan and staff in sample separation, GC, and GCeMS inMolecular Organic Geochemistry laboratory at Stanford University.We are also grateful to Leslie Magoon (Stanford), Paul Lillis (USGS)and David Suek (Stephens Energy Co LLC) who provided the oilsamples. We also thank the California Core Repository in Bakers-field for providing the well core samples for the Rock-Eval pyrolysisand vitrinite reflectance analyses. Schlumberger is acknowledgedfor providing the PetroMod petroleum system modeling software.The authors would also like to thank anonymous reviewers whoprovided useful comments that improved the revised manuscript.


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