Article history:Received 3 January 2013Received in revised form 18 July 2013Accepted 23 July 2013Available online 31 July 2013
Keywords:Algal bloomShahejie FormationEocene–OligoceneLacustrine petroleum source rockJiyang depressionEastern China
Algal blooms are responsible for the laminae in the lacustrine Shahejie Formation in the Jiyang depression, one ofthe sub-basins of the Bohai Gulf Rift Basin in eastern China. About 65 oil fields have been discovered in this de-pression during the past 40 years and the oil is mainly derived from three types of finely laminated lacustrinerocks of Shahejie Formation (Eocene–Oligocene). These lacustrine petroleum source rocks are oil shales, calcar-eous shales and calcareous laminatedmudstones. Optical, SEMand BSEI analyses of these source rocks reveal thatthere are algal (coccoliths and dinoflagellates) laminae in the rocks. The individual algal lamina is generally com-posed almostwholly of one species, such as Reticulofenestra bohaiensis in coccolith laminae, and Fromea aswell asBohaidina in dinoflagellate laminae. This distribution suggests that the algal laminae formed from algal blooms inthe ancient lake. Other components of source rocks includedorganicmatter laminae, carbonate laminae and clay-rich laminae. We also discuss the sedimentation of organic matter laminae and carbonate laminae which wererelated to algal blooms.We conclude that the laminated lacustrine petroleum source sediment of Jiyang depres-sion formed during episodes of algal blooms alternating with chemical and detrital sedimentation, and this rep-resents the primary mechanism responsible for the formation of lacustrine petroleum source rocks.
Asmore andmore oilfields have been discovered in lacustrine basinsaround the world, the importance of lacustrine petroleum source rockshas been widely recognized (Cabrera and Saez, 1999; Fleet et al., 1988;Katz, 1990a,b; Talbot and Kelts, 1989). Where these rocks were formedandwhat factors control their formation are the subjects ofmanypapers(e.g., Katz, 1990a,b, 1995; Kelts, 1988; Powell, 1986; Smith, 1990;Talbot, 1988). A variety of depositional models for lacustrine sourcerocks have been proposed. The three most important models are:(1) the playa lake, (2) deep anoxic lake, and (3) ephemeral lake(reviewed by Kelts, 1988; Talbot, 1988). In all of these models, twomechanisms considered to be essential for the formation of lacustrinepetroleum source rocks are: organic production (especially phytoplank-ton production) and preservation (Katz, 1990a,b, 1995). Although thereis a “production versus preservation controversy” (Demaison, 1991;Demaison and Moore, 1980; Parrish, 1995; Pedersen and Calvert,1990, 1991), Fleet et al. (1988) suggest that biological productivity isthe “first step” in the formation of lacustrine petroleum source rocksin all examples (Fleet et al., 1988: introduction).
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ghts reserved.
Factors influencing the level of primary productivity and the phyto-plankton productionmodes inmodern lakes have been studied in detail(Crossley and Owen, 1988; Hecky and Kling, 1981; Katz, 1990a,b, 1995;Kelts, 1988). The importance of algal blooms for the formation ofancient lacustrine and marine petroleum source rocks have also beenrecognized (Gallois, 1976; Loftus and Greensmith, 1988; O'Brien andSlatt, 1990; Parnell, 1988). For example, Gallois (1976) suggested thatKimmeridge Clay oil shales in England were formed from coccolithand dinoflagellate blooms. Parnell (1988) reported that the richestsource rocks in Middle Old Red Sandstone lacustrine sequences in theOrcadian Basin are the deposits of algal blooms in a stratified lake. Weexamine the role of algal blooms on source rock development basedon evidence from a well developed lacustrine formation in the Jiyangdepression of Bohai Gulf Rift Basin in eastern China.
2. The Jiyang depression
2.1. Geological setting
As a result of subduction of the Pacific Plate, a series of Meso-Cenozoic rift basins have formed in East China since the Late Jurassic(Ye et al., 1993). One of these rift basins is the Bohai Gulf Rift Basin,which is located in the vicinity of the Bohai Sea and surrounded by theYan Mountains in the north, the Taihang Mountains in the west, theLuxinan Uplift in the south and the Ludong Uplift as well as Liaodong
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Uplift in the east (Fig. 1). It consists of six sub-basins including theXialiaohe depression, the Bozhong depression, the Huanghua depres-sion, the Jizhong depression, the Linqing depression and the studiedarea of this paper — the Jiyang depression.
The Jiyang depression is situated in the southeastern portion ofthe Bohai Gulf Rift Basin and is bounded by Chengning uplift in the north-west, the Luxinan uplift in the southeast and the Kendongqingtozhi upliftin the east (Fig. 2). The maximum width of the depression is about120 km and the length is more than 200 km, the total area is about25,000 km2,which covered most areas of the Yellow River Delta. About65 oil fields have been discovered in this depression over the past40 years and the oil yield is 2.7 × 107 t in 1998. The development of theJiyang depression can be divided into three phases (Wang and Qian,1992): (1) the initial rifting phase (the Late Jurassic to the Cretaceous);(2) the faulting and rapidly subsiding phase (Paleocene–Oligocene);and (3) the infilling phase (Miocene–Pliocene).
2.2. Stratigraphic framework
The Mesozoic deposits consist mainly of volcanic and red coarseclastic sediments. The Paleogene–Neogene sediments are wide-spread,
Fig. 1. Geographic and geologic location of the Bohai Gulf Rift Ba
distributed across the depression. The maximum thickness of thePaleogene–Neogene sedimentary rocks is about 7300 m, and hasbeen subdivided into five formations, from upper to lower, includingthe Minghuazhen Formation (Pliocene), the Guantao Formation(Miocene), the Dongying Formation (Oligocene), the Shahejie Formation(Eocene–Oligocene) and the Kongdian Formation (Eocene–Paleocene).These formations have been subdivided into members, for example, theShahejie Formation is divided into four members including first member(SHA1), secondmember (SHA2), thirdmember (SHA3) and fourthmem-ber (SHA4) (Fig. 3).
The Neogene (the Minghuazhen Formation and the GuantaoFormation) sequence is principally composed of alluvial and fluvialsediments. The Paleogene section is mainly composed of lacustrinesediments, especially in the Shahejie Formation. The thickness of thePaleogene is about 5500 m, of which 60% is the Shahejie Formation(3300 m).
2.3. Source rock sequence
Numerous studies have been carried out on the oil potential of thePaleogene sediments in the Jiyang depression (Hong, 1983; Wang and
sin. The boxed area corresponds to the area shown in Fig. 2.
Fig. 2. Geographic and geologic location of the Jiyang depression, and the location of the studied well.
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Qian, 1992). The results show that there are mainly three oil-producingsequences including the lower andmiddle parts of SHA1, the lower andmiddle parts of SHA3 and the upper part of SHA4 (Fig. 3). In this paper,we choose three typical boreholes for study — GN27 well, Niu38 welland Chun11well (Fig. 2). The upper SHA4 of Chun11 is composed of cal-careous shales, dark gray massive mudstone, limestone, dolomite andsiltstone (Fig. 3). The middle and lower parts of SHA3 of Niu38 consistof dark gray massive mudstone, red and gray laminated mudstone, cal-careous laminated mudstone, oil shale and a slump unit (turbidite)(Fig. 3). The lower and middle parts of SHA1 of GN27 are composed ofcalcareous shales, dark gray massive mudstone, dolomite and shellylimestone (Fig. 3). Organic geochemical analyses have shown that thethree most important source rocks in the Jiyang depression are the oilshales, calcareous shales and calcareous laminated mudstones. Theirorganic geochemical characteristics are summarized as follows: (1) oilshales: TOC: 5.69%, Bit.A: 0.968%, HC.: 6003 ppm, Type I kerogen;(2) calcareous shales: TOC: 1.39%, Bit.A: 0.135%, HC.: 875 ppm, type Ikerogen; (3) calcareous laminated mudstone: TOC: 3–6.3%, Bit.A: 0.2–1%, HC.:1350 ppm, type I kerogen (Hong, 1983).
3. Source rock lithologies
3.1. Oil shales
The oil shales are dark brown, of characteristically of lowdensity andcontain conspicuous fibrous calcite layers (Fig. 4). Combined optical mi-croscopy and backscattered electron imagery (BSEI) reveals that theyare predominantly composed of couplets of alternating organic-richand clay-rich laminae (Fig. 5). The clay-rich laminae range in thicknessfrom 15 to 100 μm and are predominantly composed of illite (80%). Inaddition, they have subsidiary components which comprise ankerite(5–10%), pyrite (1–3%), quartz (1–2%) and feldspar (1–2%), along withtrace amounts of apatite (0.03%). In contrast, the organic-rich laminaeare thinner than the clay-rich laminae, ranging from 6 to 50 μm andare predominantly composed of amorphous organic matter (20–70%).
These laminae also contain detrital clay (predominantly illite), silt-sizedquartz, feldspar (1–2%), ankerite and pyrite (1–3%). Displacive calcitelayers were also present. Calcite is low-Mg, non-ferroan calcite.
Results of dinoflagellate distribution in the laminae are summarizedin Table 1 and indicate that dinoflagellate cysts are absent from both theorganic-rich laminae and the clay-rich laminae.
3.2. Calcareous shales
The calcareous shales are light gray and consist of alternating lightand dark laminae. The light colored laminae are either fine or coarsegrained calcite and the dark laminae are organic-rich (Figs. 6, 8a). Scan-ning electron microscope (SEM) analysis indicates that the fine grainedcalcite laminae are coccolith laminae composed almost wholly of onespecies, Reticulofenestra bohaiensis (Zhong et al., 1988) (Fig. 7). Thecoarse grained calcite laminae are composed of calcite (10–20 μm)and also contain coccoliths (Fig. 8b). Within the coccolith laminae, thesize of coccoliths ranges from 2 to 5 μm, and have a good preservation.Within the coarse grained calcite laminae, the calcite is sparry calcitewith overgrowths. Coccoliths display poor preservation suggestingdissolution. Thus it indicates that the coccolith-bearing coarse calcitelaminaemay be recrystallized from the dissolution of coccolith laminae.SEM examination also demonstrated that organic-rich laminae aremainly composed of amorphous organic matter (N80%) and containtrace amounts of detrital clay.
The abundance of dinoflagellates in organic-rich laminae is400–500/100 mg and is –250/100 mg in light laminae (Table 1). Consid-ering that the light laminae are almost composed of coccoliths or coarsecalcite, the abundance of dinoflagellates in these laminae is expected tobe lower than the observed values. This may be the result of the mixingof two lamina samples because individual laminae are too thin to sampleaccurately. Another important phenomenon is that there are dinoflagel-late laminae in the organic-rich laminae. These dinoflagellates laminaeare also composed of one species, Fromea (Fig. 9a).
Fig. 3. Stratigraphical columnof the Paleogene–Neogene (left) and the source rocks sequences (right: a, b, c)within the Jiyang depression. Panel (a) represents themiddle and lower SHA1of GN27 well; panel (b) represents the middle and lower of SHA3 of NIU38 well and panel (c) represents the upper of SHA4 of CHUN11 well.
Fig. 4. Photograph of oil shale polished core. Niu38 well, 3360.03 m, lower of SHA3.
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3.3. Calcareous laminated mudstones
The calcareous laminatedmudstones are much darker gray than theoil shales and calcareous shales and lamination is not obvious onpolished core samples. Combined optical microscope and BSEI revealsthat they are predominantly composed of couplets that consist of thinlayers of alternating fine grained carbonated-rich and clay-rich laminae(Fig. 10). The clay-rich laminae range in thickness from 50 to 250 μmand are predominantly composed of fine grained calcite (1–4 μm, 5%)and silt-sized detrital quartz (3%), illite (70%) and organic matter(20%). In contrast, the carbonate-rich laminae are predominantly com-posed of micron sized fine grained calcite (1–4 μm) which form layers30–100 μm thick. In addition to the fined grained calcite, some of thecarbonate-rich laminae also contain coarser (10–20 μm), anhedralcalcite crystals.
Fig. 5. (a) Thin-section photomicrograph of oil shale, consisting of clay-rich laminae (light) and organic-rich laminae (dark). Fibrous calcite layer is on the bottom; (b) backscatteredelectron image of the oil shale. The light layers are clay-rich laminae and the dark layers are organic laminae. Niu38 well, 3366 m, lower of SHA3.
Table 1The abundance of dinoflagellates in different laminae of lacustrine petroleum source rocks.
Source rock type Lamina type Abundance(number/100 mg)
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The abundance of dinoflagellates in organic-bearing clay-rich lami-nae is 250/100 mg and is 67/100 mg in the carbonate-rich laminae(Table 1). The dinoflagellate laminae also occur in the organic-bearingclay-rich laminae (Fig. 9b).
Fig. 6. Thin-section photomicrograph of calcareous shale. The light is fine grained calcitelaminae, and dark is organic laminae.
4. Discussion
In summary, the three most important lacustrine petroleum sourcerocks in the Jiyang depression are predominantly composed of fourtypes of laminae: (1) algal laminae (coccolith laminae and dinoflagellateslaminae), (2) organic-rich laminae, (3) carbonate-rich laminae and (4)clay-rich laminae. These laminae are also common in both modern lakeand ocean sediments. By analogy with the depositional processes oflaminae in modern lakes and oceans, the mechanisms of formation ofthe laminae in ancient lacustrine sediments can be useful in determiningmechanisms for the formation of lacustrine petroleum source rocks.
4.1. Algal blooms and coccolith/dinoflagellate laminae
Coccolith laminae and dinoflagellate laminae present in the lacus-trine petroleum source rocks of the Jiyang depression are characterizedby their monospecific character, indicating that they formed fromcoccolithophore and dinoflagellate blooms, respectively.
In the modern oceans, coccolithophores are wide-spread and abun-dant, and they often bloom during a given period. There is only onegenus (Hymenomonas) of calcareous nannoplankton which has beenfound inmodern lakes (Dean, 1981). Analyses of “whitewater” samplesfrom the Gulf of Maine have shown that there was only one species(Emiliania huxleyi) and the maximum densities of cells and detachedcoccoliths were about 3.5 × 103 ml−1 and 1.2 × 105 ml−1 respectively
Fig. 7. (a) SEMmicrograph shows alternating organic matter laminae (O) and coccolith laminae (C) in calcareous shale; (b) close-up SEMmicrographs of coccolith laminae in calcareousshale. Notice there is only one species (Reticulofenestra bohaiensis) and it has a good preservation.
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(Ackleson et al., 1988). Coccolith laminae are very common in modernand late Quaternary marine sediments (Kemp, 1996), and they arealso dominated by one species, such as light laminae in Holocenesediments of the Black Sea, which are overwhelmingly comprised ofEmiliania huxleyi (Hay, 1988; Jones, 1994).
By analogy with modern and late Quaternarymarine sediments, thecoccolith laminae in the Jiyang depression appear to represent coccolithblooms. 87Sr/86Sr measurements of these coccolith laminae have beencarried out by Liu and Cheng (1996). The results show that 87Sr/86Srratio of coccolith laminae in SHA1 (Oligocene) is 0.7114 and is 0.7111in SHA4 (Eocene). These values are higher than the 87Sr/86Sr of the seawaters in the same geologic time (Oligocene: 0.7076–0.7084; Eocene:0.7076–0.7099; Koepnick et al., 1988). This indicates that the coccolithsin Jiyang depression are non-marine coccoliths (i.e., lake deposits).
Dinoflagellates are abundant in modern oligo-mesosaline lakes, andtheir blooms have been recorded from a number of localities (Kelts,1988). Studies on dinoflagellates in the lacustrine sediments of Jiyangdepression show that they are non-marine dinoflagellates (Xu et al.,1997). The dinoflagellate laminae in the source rocks, therefore alsoformed from dinoflagellate blooms in the ancient lakes.
Fig. 8. (a) Thin-section photomicrograph of calcareous shale consisting of coarse calcite laminacoarse calcite laminae.
4.2. Algal blooms and organic-rich laminae
Some algal blooms may also be deposited in the form of diatoms,such as Chrysophyceae and certain highly resistant taxa (Livingstone,1984). Other common algae remains such as the Chlorophyceae andthe Cyanophyceae are rare or absent in these settings, although bloomsof these algae are very common inmodern lakes (Kelts, 1988). This phe-nomenon ismore prominent in the ancient lacustrine sediments, whichalso include algal laminae of dinoflagellates and coccoliths, Pediastrumlaminae and Botryococcus laminae (Kelts, 1988).
Organic-rich laminae are dominantly by amorphous organic matter,and are suggested to have been derived from algal blooms, includingdinoflagellate blooms, cyanophytes and chlorophyte blooms. Thismecha-nism is similar to that described by Bradley (1970) for the lacustrineGreen River Formation.
4.3. Algal blooms and carbonate-rich laminae
As early as 1901, Forel recognized the biological role for lacustrinecarbonate sedimentation. Later, Minder (1922, 1926, as referenced in
e (light) and organic matter laminae (dark); (b) close-up SEM micrograph of coccolith in
Fig. 9. (a) Fromea lamina in the organic-rich laminae of calcareous shale; (b) Bohaidinalamina in the organic-bearing clay laminae of calcareous laminated mudstone. Twoimages are photomicrographs of palynological preparation.
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Kelts and Hsü, 1978) developed the concept of inorganic, biogenicallyinduced calcite precipitation. Kelts and Hsü (1978) reviewed carbonatesedimentation in fresh-andbrackish-water lakes. Kelts andHsü suggestedthat assimilation of CO2 by photosynthesis of algae is the most importantmechanismof CaCO3 precipitation in lakes. Because the exchange equilib-rium of atmospheric CO2 with water is a comparatively slow reaction,
Fig. 10. (a) Thin-section photomicrograph of calcareous laminated mudstone consisting of finecalcareous laminated mudstone. The light layers are fine grained calcite laminae and the dark
carbon dioxide in surface water during active photosynthesis may be-come very depleted in eutrophic lakeswithin a short time, and thewatersbecome supersaturatedwith respect to calcite. Dean (1981) further dem-onstrated that the rate of precipitation of CaCO3 from water is directlyproportional to rates of assimilation of CO2 by photosynthesis of algae.Thus when algal blooms occur in lakes, the rate of assimilation of CO2
by photosynthesis is high and this results in the rapid sedimentation ofcarbonate minerals which deposited in the lake bottom and formed thecarbonate laminae. Thompson et al. (1997) found that an annual whitingevent occurs each year in lateMay to early June in Fayetteville Green Lake,and this event correlates with exponential growth of the Synechococcuspopulating (Synechococcus blooms). They concluded that Synechococcusblooms can play an important role in the precipitation of carbonate inthe lake. Dittrich and Obst (2004) summarized the origin of lacustrinecalcite from both field and laboratory studies: (1) field studies showthat calcite precipitation in oligotrophic lakes is strongly linked withpicocyanobacteria blooms, and (2) laboratory experiments led to the for-mulation of the mechanism of precipitation induced by microalgae.Brauer et al. (2008) suggested that micritic calcite (b5 μm) layers in lam-inated sediment sequence from Piànico palaeolake in the southern Alpsformeddue tomassive calcite precipitation events described as “whiting”.
In Jiyang depression, the carbonate-rich laminae are predominantlycomposed of micritic calcite, indicating carbonate chemical precipitation.Considering the couplet association of carbonate-rich laminae withorganic-rich laminae and/or organic-bearing clay laminae, it is suggestedthat algal blooms play an important role of the carbonate-rich laminationtype in the oil shale.
There are also fibrous calcite layers in the oil shales. This fibrouscalcite may have been displaced from fine-grained calcite laminae dur-ing post-depositional digenesis. If this is true, fine grained calcite mayhave existed in oil shales, andmay also indicate that the level of primaryproductivity was high in ancient lakes.
4.4. Summary
Thus, from the above discussion, the formation of the three mostimportant lacustrine petroleum source rocks in Jiyang depression areall closely related to algal blooms.
calcite laminae (light) and clay-rich laminae (dark); (b) backscattered electron image oflayers are organic-bearing clay laminae.
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Calcareous shales composed of coccolith laminae and dinoflagellatelaminae indicate that they may be formed from coccolith and dinofla-gellate blooms. Within the oil shales, the organic matter laminae mayindicate algal blooms, and the fibrous calcite may replace original fine-grained calcite laminae, thus also indicating algal blooms. Calcareouslaminated mudstones contain dinoflagellate laminae which formedfrom algal blooms and the precipitation of calcite laminae was also in-duced by algal blooms.
5. Conclusions
(1) Three most important lacustrine petroleum source rocks in theJiyang depression are: oil shales, calcareous shales, and calcareouslaminated mudstones. All these source rocks are finely laminatedsediments consisting of different types of laminae and different or-igin.
(2) Oil shales are predominantly composed of organic-rich laminaeand clay-rich laminae, also contain fibrous calcite layers; calcare-ous shales are predominantly composed of coccolith laminae anddinoflagellate laminae. Calcareous laminated mudstones are pre-dominantly composed of fine grained calcite laminae and clay-rich laminae.
(3) Coccolith laminae and dinoflagellate laminae are characterized bytheir monospecific nature. This suggests that they formed fromcoccolithophore blooms and dinoflagellate blooms in the ancientlakes. Organic-rich laminae are mainly composed of amorphousorganic matter. This type of organic matter is derived from algae.Their preservation in the form of laminae may indicate that thesewere the result of algal blooms in the ancient lakes. Assimilationof CO2 by photosynthesis of algae is the most important mecha-nism of CaCO3 precipitation in lakes, and the fine grained calcitelaminae therefore also indicate that the level of productivity inthe ancient lake was high.
(4) Algal blooms are the primary mechanisms of the formation of thelacustrine petroleum source rocks of the Jiyang depression.
Acknowledgments
We are grateful to Jinli Xu who identified dinoflagellates. We thankN.R. O'Brien and an anonymous reviewer for their comments, whichhelped improve the manuscript.
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