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The depositional characteristics and oil potential of paleo Pearl river delta systems in the Pearl river mouth basin, South China Sea
Tectonophysics 235 (1994) 1- 11
The depositional characteristics and oil potential of paleo Pearl River delta systems in the Pearl River Mouth basin, South China Sea Chen Jishu, x1-1Shice, Sang Jinyu Research Institute
ofN~hai East Oil C~ny~
G~~~~
5lfo240, People’s ~~~1~
of China
(Accepted 18 November, 1993)
Abstract Delta systems in the Zhuhai, Zhujiang and Hanjiang formations are interpreted as having formed during periods of sea-level rise from the late Oligocene to mid-Miocene (30-10.5 My B.P.). Deltas in the Zhuhai Formation were developed in a shallow water environment on a sandy, wave- or fluvial-dominated coasts. Thick and blanket-like sandstones are common but mudstones are infrequent. Oil-bearing zones have been found only in the upper part of the systems, overlain by mudstone of the Zhujiang Formation, Deltas in the Zhujiang Formation are interpreted as having formed in a deeper water en~ronment on sandy, wave- or fluvial-dominated coasts. Carbonate platforms started growing on the Dongsha massif as the sea-level slowly rose. The moderate sandstone/mudstone ratio in the delta systems and a zone of secondary porosity in the carbonate rocks provide an excellent reservoir rock and seal rock for the basin. Deltas in the Lower Hanjiang Formation are similar to those of the Zhujiang Formation, but the upper part of the Formation is interpreted as having formed in a shelf en~ro~ent; the thick shelf mudstone, inter-bedded with offshore bar sandstones, would be one of the better regional seal rocks in the study area. No carbonate rock developed on the Dongsha massif due to the rapid rise in sea-level. All delta systems from the Zhuhai to the Zhujiang Formation are stacked in an onlap pattern indicating a more and more expanding marine transgression, which was caused by eustatic sea-level rise. This marine transgression is almost unique and contrasts to the progradational (offlap) pattern of Cenozoic delta systems in the Gulf of Mexico. It resulted in thick sandstone deposits, immature or poorly mature source rock and no ductile mudstones, with no growth faults or rollover structures. Hydrocarbons generated from Eocene-Oligocene lacustrine source rocks are trapped by drape structures related to local basement highs or by carbonate rock with secondary porosity. Most of the remaining undiscovered reserves in the basin will be found in non-structural, subtle traps, especially stratigraphic traps.
1.Introduction The evolution of the Pearl River Mouth basin can be divided into two stages: (1) a half-graben or faulted basins were filled before break-up led the cessation of deposition, causing an uncon0040-1951/94/$07.00 0 1994 Elsevier Science SW2 0040-1951(93)E0260-2
fort& (30 My B.P.), (2) after this break-up unconformity a continental marginal basin was formed (Fig. 1). During the first stage the separated half-graben or faulted basins are generally filled by fluviai, lacustrine and other no~arine elastic rocks, in&ding source rocks of the Wen-
B.V. All rights reserved
Chen Jishu et al. / Tectonophysics 235 (1994) 1-l 1
I
EUSTATIC
AGE
CURVER
WF
all previous continental basins. Thick carbonate rocks were also deposited on the Dongsha massif during the second stage. The study area of the Pearl River Mouth basin comprises a giant delta complex, with a total thickness of over 2,000 m and an area of 35,000 km* (Fig. 2). Both excellent potential reservoir rock and seal rock exist in the basin. However, the source rock is immature or of low maturity, I I I
m 1 a
Sandstone
m
Fig. 1. Generalized Basin.
-
Source
Mudstone
stratigraphy
G
I t-
SONIC LOG
Rocks
aCarbonate
rock
I 1
in the Pearl River Mouth
‘s-a% 2? _D_
I
chang and Enping formations. After the unconformity, the paleo Pearl River delta systems developed during a period of the spreading of the South China Sea and marine tr~sgression over
Fig. 2. Map of study area.
Fig. 3. Depositional facies of the Zhuhai Formation.
3
Chen Jishu et al. / Tea ~5n5~~ys~cs235 (1994) I-l 1
third-order sequence boundaries because data is limited. We divide the Formation into three members, based on biologic and lithologic data, as follows.
and there are no local structures generated by growth faults or deltaic sedimentation and several fault-controlled local structures contain little hydrocarbon. The reasons for this situation are analyzed and a suggestion as to further exploration directions are given in this paper.
2.1. Lower *.huhai Formation destructive delta systems
2. Destructive delta systems and related coastalnearshore systems in the Zhuhai Formation
This Member, which immediately overlies the unconformity, consists mainly of massive and blanket-like sandstones, including fhrvial, longshore bar and barrier bar sandstone (Fig. 3). The elastic facies resulting from fluvial, wave, tide and storm processes may have both fining-upward and coarsening-upward sequence with a high sandstone/mudstone ratio (3 : 1). Sandstones are often multi-storey and correlatable over much of the study area. They have a bell or cylinder shape log motif in gamma well logs (Fig. 3). Prodelta mudstone is very thin and very limited in distribution, the reason for this is probably that, at the very beginning of the sea-level rise,
Marine transgression began 30 My B.P. after a rapid and large eustatic sea-level fall, which is shown in the global cycle chart of Haq et al. (1988) (Fig. 1). The duration of the Zhuhai Formation is in response to Haq’s second-order super cycle, 30-21 My B.P. in age. The eustatic curve of the supercycle displays a sea-level rising step by step, which roughly coincides with the depositional background of the Zhuhai Formation. The supercycle includes five third-order sequences, but it is actuahy difficult to define the
l LU FENQ
/
SOUNCEANU p .
--
#
MASSIf \
BASIN YAROIN
(803,::~~&,,
Fig. 4. Destructive delta systems in the Lower Zhuhai Formation.
Chen Jishu et al. / Tectonophysics235 (1994) I-11
_.c.lnAN~UM 6ANOSTOWC
SYS-Y
PCnCLwrAaS OP SANDSTONE CONTSNT
Fig. 5. Destructive delta systems and transgressive sandstone sequences in Upper Zhuhai Formation. 2 2Pl-5
2 35-1-l
CARBONATE REEF
Fig. 6. Transgressive sandstone profiles across the Dong Sha Massif in the Upper Zhuhai Formation.
Chen Jishu et al. / Tectonophysics 235 (1994) l-l 1
the deltas prograded into a shallow water environment, where the sea floor is located above the wave base. The cuspate shape of the destructive delta systems (Fig. 4) indicates that the sediments carried through distributary channels were obviously reworked by wave and shelf currents: most of the suspension load was driven far away, while coarser sandy sediments were deposited along the shoreline. Another reason for the lack of mudstone is that the exposed erosional land consisted mainly of Mesozoic granite, which may have provided more sandy and less muddy elastic sediments for the basin. The Dongsha massif remains a elastic source region to this day (Fig. 4).
slope of the Dongsha massif in response to sealevel rise (Fig. 6). All delta systems in the Zhuhai Formation were developed during the initial relative sea-level rise phase after break-up unconformity in the northern South China Sea. They are generally destructive, have a cuspate geometry, a high sandstone/ mudstone ratio and contain well sorted and permeable sandstones which can be
IAMMA.RAY La
2.2. Middle Zhuhai Formation delta or beach-tooffshore sequences Four stacked coarsening-upward sequences without or with limited distributary facies in the upper part of each sequence are interpreted as being a set of incomplete delta sequences or beach-to-offshore sequences. Constructive delta systems may have been formed when the sea-level was at a still-stand or changing slowly, but they were strongly reworked by a subsequent marine invasion to the north when the sea-level rose rapidly. The marine influence is more striking in comparison with the Lower Member mentioned above: the sandstone/mudstone ratio is about 2: 1 and the cuspate shape geometry indicates that a destructive delta system was involved.
i
2.3. Upper Zhuhai Formation wave- and tidedominated destructive delta systems and transgressive sandstone sequences In this Member well sorted barrier bar sandstones, interbedded with thin mudstones and very thin limestones, are common; glauconite, anhydrite, barite, as well as some fragments of Foraminifera, Bryozoan and Echinodermata were also found, and several smaller cuspate-shape delta systems can be seen in the northern part of the area (Fig. 5). Transgressive sandstones, including beach, barrier bar and longshore bar sandstones were developing gradually along the
5
\
Fig. 7. Depositional facies of the Zhujiang Formation.
Chen Jishu et al. / Tectonophysics 235 (1994) l-l 1
e
louRcc DELTA
ARCA
8VSTCM
CARSONATE
PLATPORM
REEF/BANK IU~DUP PERCENTAQE OF SANDSTONE CONTENT
Fig. 8. Delta systems and carbonate platforms-reef/bank
systems in the Lower Zhujiang Formation.
SOURCRALREA FACIE8 BOUNDARY PERCENTAOL OF SANDSTONECONTCN
rig. Y. uestrucuve aelta and shelt systems in the Middle Zhujiang Formation.
7
Chen Jishu et al. / Tectonophysics 235 (1994) l-11
correlated over a large area. In general, the Zhuhai Formation is much richer in reservoir sandstones but poor in mudstone or seal rocks. 3. Delta systems and carbonate platforms in the Zhujiang Formation The period over which the Zhujiang Formation was laid down was about 7-8 My (21-13.8 My B.P.), and included four third-order eustatic cycles, shown in global cycle chart of Haq et al. (1988). However, three delta development stages can be distinguished, using biologic and lithologic data, in the study area (Fig. 7). 3.1. Lower Zhujiang Formation constructive delta systems and carbonate platforms
Three or four coarsening-upward delta sequences with a stronger fluvial influence were developed during the period of slow eustatic rise
(21-17.5 My B.P.). A large lobate-shape delta system and two other smaller birdfoot shaped delta systems were prograded into the shelf region from the northwest and southwest sides, respectively (Fig. 8). The Dongsha massif was drowned and, far away from the sediment source region, carbonate platforms, reef and bank complexes with a maximum thickness of 500 m, were deposited on the massif. Two or three zones of secondary porosity have been found within the carbonate interval. It is believed that the origin of the secondary porosity zones is closely related to eustatic sea-level falls at 17.5, 16.5 and 15.5 My B.P. (Haq et al., 1988). 3.2. Middle Zhujiang Formation destructive delta systems and shelf sandstone systems
The destructive delta systems have a slight cuspate shape and a wide shelf area with some
f
’ PYLI
k
PERCENTAOEOF SANOST~NECONTENT
r\ Fig. 10. Constructive
delta systems in the Upper
SOUSCS AREA
Zhujiang
FACIE8 SOUNOARV
Formation.
Chen Jishu et al. / Tectonophysics 235 (1994) l-11
8
shelf sandstone ridges (Fig. 9). Thick shelf mudstone and moderate shelf sandstone are common, the sandstone/mudstone ratio is about 1: 2. The previous carbonate platform on the Dongsha massif was overlain by sediments deposited in an outer-shelf to hemipelagic environment. Some condensed sections have been found in the cores above the top of carbonate rock. It is clear that the marine transgression further expanded after
;AMMA.RAY
u)
-I-
zLl-WtOWG
soNlcLoo
!
YUD BTDNC
\
\
the deposition of the carbonate rock, although there has been no obvious global sea-level rise since 15.5 My B.P. (Haq et al., 1988). The strong subsidence in the study area is possibly the main reason for the rapid marine transgression after 15.5 My B.P 3.3. Upper Zhujiang Formation constructive delta systems Three coarsening-upward delta sequences were deposited in a slow marine transgression environment in response to the global sea-level fall from 15 to 13.8 My B.P. (Haq et al., 1988). The deltaic geometry (Fig. 10) is similar to that of constructive delta systems (Fig. 8) in the Lower Zhujiang Formation, but no carbonate rock was developed as the water depth was too deep for coral reefs to grow on the Dongsha massif. Delta systems in the Zhujiang Formation are developed as the relative sea-level rose further, the rate of relative sea-level rise exceeded the rate of deposition, causing shoreline and delta systems to move northward. Some small constructive delta systems were developed and more sands were deposited as barrier bars, longshore bars and shelf sandstone ridges. Thick and widespread shelf mudstones might have been formed as a regional seal rock. Thick carbonate rock with a secondary porosity was deposited on the Dongsha massif. Both reservoir rock (sandstone and carbonate rock) and seal rock are well developed in the Zhujiang Formation and most of the hydrocarbons discovered are concentrated in the Lower Zhujiang Formation. 4. Delta systems in the Haqjiang Formation The duration of this Formation is about 3.3 My (13.8-10.5 My B.P.), based on biological data, it can be divided into two members (Fig. 111, the Lower and the Upper.
/’
4.1. Lower Hanjiang Formation constructive delta systems
.’ /
i
Fig. 11. Depositional facies of the Hanjiang Formation.
The sea-level fall at 13.8 and 12.5 My B.P. (Haq et al., 1988) may possibly have resulted in
9
Chen Jishu et al. / Tectonophysics 235 (1994) l-l 1
distributary channel sandstones deposited directly on the previous shelf region. Coarseningupward delta sequences with some inter-distributary mudstones and thin coal beds ( < l-2 m> are common, the sequences have thicknesses of 55-75 m and a sandstone/mudstone ratio of 2 : 1. Two lobate delta systems developed on the southwest and northwest sides of the area (Fig. 12).
In general, the elastic lithology of the Hanjiang Formation is similar to that of the Zhujiang Formation, it has both reservoir rocks and cap rocks, but only a few oil-bearing zones have been found up to now.
4.2. Upper Hanjiang Formation and sandstone systems
Although many periods of eustatic sea-level fall since 30 My B.P. are shown in the global cycle chart of Haq et al. (19881, the paleo Pearl River Delta Systems are interpreted as having developed during a period of relative sea-level rise, possibly caused by more rapid subsidence after the break-up unconformity. The stacked delta systems from the Zhuhai to Hanjiang Formations (about 30-10.5 My B.P.) have a ‘backstopping
shelf mudstones
The interval consists of thicker shelf mudstones interbedded with thin-moderate shelf sandstones (Fig. 11). Some thick and massive beach-to-offshore sandstones, containing fossils of Foraminifera and Echinodermata, were found in the northern region.
5. Discussion on the hydrocarbon accumulation in the paleo Pearl River delta systems
e
SOURCE
#c
BASIN
r\
FACIES
6
PERCENTAGE SANDSTONE
/
Fig. 12. Constructive
delta system in the Lower Hanjiang
Formation,
AREA MARGIN BOUNDARY OF CONTEN
Chen Jishu et al. / Tectonophysics 235 (1994) I-11
10
SE
NW
V
-@-
QEOLOOIC AQEha)
--g-MAXIMUM
Tc/
MAR I NEMUDSTOME
nCOASTAL
m
REEF
/7
PIAN
PLAMbELtA
$L~PE FAN/BASIN PL~ORFAN
Fig. 13. Diagrammatic
COASTAL
FLOODIMQ SURFACE
cross-section
ZHU II DEPRESSION
showing onlap across the Pearl River Mouth basin.
-----+
CONTlNENTAL
SUB-TERTIARY
SHELF +
SLOPE
SECTION
Fig. 14. Schematic cross-section of the Texas portion of the northern Gulf of Mexico basin illustrating the progradational pattern. Arrows indicate principal direction of fluid movement through pressure shale section (Bruce, 1984).
strata1
Chen Jishu et al. / Tectotwphysics 235 (1994) I-11
pattern’ (onlap) (Fig. 13), indicating the basin has been expanding since 30 My B.P. One delta system progressively deepens upward as younger delta sequences retreat farther landward, this statal pattern is quite different from the forestepping (or progradational) pattern of the Cenozoic delta systems deposited in the northern Gulf of Mexico (Fig. 14). Other Cenozoic delta systems, for example the Niger delta, West Africa, Baram delta or Mahakam delta, Borneo, also have a progradational pattern. The relative sea-level rising background, coupled with a sand-rich erosional land surface, have led to the following results in the study area: (1) Most of the delta systems are destructive, they provide more sandstones and less prodelta mudstones. The thick and extensive shelfal mudstones in each formation are generally silty, hard and fragile. Ductile mudstones are poor in all three formations, which is probably the main reason for the lack of growth faults and rollover structures, the many normal faults only add to the problem rather than trapping hydrocarbons: some drilling wells on local fault-controlled structures are dry holes; therefore, ‘faults mean high risk’ is a saying of some exploration operators. (2) Most prodelta mudstones in the three formations are immature or poorly mature source rocks. The hydrocarbon discovered in the past has mainly been generated from lacustrine source rock in the Wenchang and Enping formations (Eocene-Oligocene). Two striking features of hydrocarbon accumulation can be summarized based on the data from oilfields: (1) the oil production zones are more concentrated in the Upper Zhuhai to Lower Zhujiang Formations, including both elastic and carbonate rocks; (2) hydrocarbons were generally trapped in two simple structures: reef-bank carbonate buildups and drape structures related to local basement highs. (3) It is expected that most of the remaining undiscovered reserve in the basin will be found in non-structural subtle traps, particularly in stratigraphic traps close to lacustrine source rocks.
11
References Abbott, W.O., 1985. The recognition and mapping of basal transgressive sand from outcrop, subsurface, and seismic data. AAPG Mem. 39: 157-168. Bruce, C.H., 1984. Smectite dehydration-its relation to structural development and hydrocarbon accumulation in northern gulf of Mexico Basin. AAPG Bull., 68 (6): 673683. Chart, M.A. and Dott, R.H., Jr., 1986. Depositional facies and progradational sequences in Eocene wave-dominated deltaic complexes, southwestern Oregon. AAPG Bull., 70 (4): 415-429. Chen Shizhong and Hu Pingzhong, 1987. Tertiary reef and its significance on exploration in Pearl River Mouth Basin. China Offshore Pet., 1 (1): 3-10. Coleman, J.M. and Prior, D.B., 1982. Deltaic environments of deposition. AAPG Mem., 31: 139-178. Dolan, J.F., 1989. Eustatic and tectonic controls on deposition of hybrid siliciclastic/carbonate basinal cycles: discussion with examples. AAPG Bull., 73 (10): 1233-1246. Haq et al., 1988. Mesozoic and Cenozoic chronostratigraphy and cycles of sea-level change. In: C.K. Wilgus, B.S. Hastings, C.G.St.C. Kendall, H.W. Posamentier, CA. Ross and J.C. Van Wagoner (Editors), Sea-level changes: An Integrated Approach. SEPM Spec. Publ., 42: 71-108. Libby-French, J., 1984. Stratigraphic framework and petroleum potential of northeastern Baltimore Canyon trough, Mid-Atlantic outer continental shelf. AAPG Bull., 68 (1): 50-73. Ioutit, T.S., Hardenbol, J., Vail, P.R. and Baum, G.R., 1988. The key to age dating and correlation of continental margin sequences. In: C.K. Wilgus, B.S. Hastings, C.G.St.C. Kendall, H.W. Posamentier, C.A. Ross and J.C. Van Wagoner (Editors), Sea-level Changes: An Integrated Approach. SEPM Spec. Publ., 42: 183-213. Shelton, J.W., 1984, Listric normal faults: an illustrated summary. AAPG Bull., 68 (7): 801-815. Vail, P.R., Audernard, F., Bowman, S.A., Eisner, P.N. and Perez-Cruz, G., 1990. The Stratigraphic signatures of tectonics, eustasy and sedimentation (unpubl.). Van Wagoner, J.C., Mitchum, R.M., Campion, K.M. and Rahmanian, V.D., 1990. Siliciclastic sequence stratigraphy in well logs, cores, and outcrop. (AAPG Methods in Exploration Series, No. 7.) Am. Assoc. Pet. Geol., Tulsa, Okla., USA. Wang Xie-pei, Fei Qi and Zhang Jia-Hua, 1985. Cenozoic diapiric traps in Eastern China. AAPG Bull., 69 (12): 2098-2109.
The depositional characteristics and oil potential of paleo Pearl River delta systems in the Pearl River Mouth basin, South China Sea Chen Jishu, x1-1Shice, Sang Jinyu Research Institute
ofN~hai East Oil C~ny~
G~~~~
5lfo240, People’s ~~~1~
of China
(Accepted 18 November, 1993)
Abstract Delta systems in the Zhuhai, Zhujiang and Hanjiang formations are interpreted as having formed during periods of sea-level rise from the late Oligocene to mid-Miocene (30-10.5 My B.P.). Deltas in the Zhuhai Formation were developed in a shallow water environment on a sandy, wave- or fluvial-dominated coasts. Thick and blanket-like sandstones are common but mudstones are infrequent. Oil-bearing zones have been found only in the upper part of the systems, overlain by mudstone of the Zhujiang Formation, Deltas in the Zhujiang Formation are interpreted as having formed in a deeper water en~ronment on sandy, wave- or fluvial-dominated coasts. Carbonate platforms started growing on the Dongsha massif as the sea-level slowly rose. The moderate sandstone/mudstone ratio in the delta systems and a zone of secondary porosity in the carbonate rocks provide an excellent reservoir rock and seal rock for the basin. Deltas in the Lower Hanjiang Formation are similar to those of the Zhujiang Formation, but the upper part of the Formation is interpreted as having formed in a shelf en~ro~ent; the thick shelf mudstone, inter-bedded with offshore bar sandstones, would be one of the better regional seal rocks in the study area. No carbonate rock developed on the Dongsha massif due to the rapid rise in sea-level. All delta systems from the Zhuhai to the Zhujiang Formation are stacked in an onlap pattern indicating a more and more expanding marine transgression, which was caused by eustatic sea-level rise. This marine transgression is almost unique and contrasts to the progradational (offlap) pattern of Cenozoic delta systems in the Gulf of Mexico. It resulted in thick sandstone deposits, immature or poorly mature source rock and no ductile mudstones, with no growth faults or rollover structures. Hydrocarbons generated from Eocene-Oligocene lacustrine source rocks are trapped by drape structures related to local basement highs or by carbonate rock with secondary porosity. Most of the remaining undiscovered reserves in the basin will be found in non-structural, subtle traps, especially stratigraphic traps.
1.Introduction The evolution of the Pearl River Mouth basin can be divided into two stages: (1) a half-graben or faulted basins were filled before break-up led the cessation of deposition, causing an uncon0040-1951/94/$07.00 0 1994 Elsevier Science SW2 0040-1951(93)E0260-2
fort& (30 My B.P.), (2) after this break-up unconformity a continental marginal basin was formed (Fig. 1). During the first stage the separated half-graben or faulted basins are generally filled by fluviai, lacustrine and other no~arine elastic rocks, in&ding source rocks of the Wen-
B.V. All rights reserved
Chen Jishu et al. / Tectonophysics 235 (1994) 1-l 1
I
EUSTATIC
AGE
CURVER
WF
all previous continental basins. Thick carbonate rocks were also deposited on the Dongsha massif during the second stage. The study area of the Pearl River Mouth basin comprises a giant delta complex, with a total thickness of over 2,000 m and an area of 35,000 km* (Fig. 2). Both excellent potential reservoir rock and seal rock exist in the basin. However, the source rock is immature or of low maturity, I I I
m 1 a
Sandstone
m
Fig. 1. Generalized Basin.
-
Source
Mudstone
stratigraphy
G
I t-
SONIC LOG
Rocks
aCarbonate
rock
I 1
in the Pearl River Mouth
‘s-a% 2? _D_
I
chang and Enping formations. After the unconformity, the paleo Pearl River delta systems developed during a period of the spreading of the South China Sea and marine tr~sgression over
Fig. 2. Map of study area.
Fig. 3. Depositional facies of the Zhuhai Formation.
3
Chen Jishu et al. / Tea ~5n5~~ys~cs235 (1994) I-l 1
third-order sequence boundaries because data is limited. We divide the Formation into three members, based on biologic and lithologic data, as follows.
and there are no local structures generated by growth faults or deltaic sedimentation and several fault-controlled local structures contain little hydrocarbon. The reasons for this situation are analyzed and a suggestion as to further exploration directions are given in this paper.
2.1. Lower *.huhai Formation destructive delta systems
2. Destructive delta systems and related coastalnearshore systems in the Zhuhai Formation
This Member, which immediately overlies the unconformity, consists mainly of massive and blanket-like sandstones, including fhrvial, longshore bar and barrier bar sandstone (Fig. 3). The elastic facies resulting from fluvial, wave, tide and storm processes may have both fining-upward and coarsening-upward sequence with a high sandstone/mudstone ratio (3 : 1). Sandstones are often multi-storey and correlatable over much of the study area. They have a bell or cylinder shape log motif in gamma well logs (Fig. 3). Prodelta mudstone is very thin and very limited in distribution, the reason for this is probably that, at the very beginning of the sea-level rise,
Marine transgression began 30 My B.P. after a rapid and large eustatic sea-level fall, which is shown in the global cycle chart of Haq et al. (1988) (Fig. 1). The duration of the Zhuhai Formation is in response to Haq’s second-order super cycle, 30-21 My B.P. in age. The eustatic curve of the supercycle displays a sea-level rising step by step, which roughly coincides with the depositional background of the Zhuhai Formation. The supercycle includes five third-order sequences, but it is actuahy difficult to define the
l LU FENQ
/
SOUNCEANU p .
--
#
MASSIf \
BASIN YAROIN
(803,::~~&,,
Fig. 4. Destructive delta systems in the Lower Zhuhai Formation.
Chen Jishu et al. / Tectonophysics235 (1994) I-11
_.c.lnAN~UM 6ANOSTOWC
SYS-Y
PCnCLwrAaS OP SANDSTONE CONTSNT
Fig. 5. Destructive delta systems and transgressive sandstone sequences in Upper Zhuhai Formation. 2 2Pl-5
2 35-1-l
CARBONATE REEF
Fig. 6. Transgressive sandstone profiles across the Dong Sha Massif in the Upper Zhuhai Formation.
Chen Jishu et al. / Tectonophysics 235 (1994) l-l 1
the deltas prograded into a shallow water environment, where the sea floor is located above the wave base. The cuspate shape of the destructive delta systems (Fig. 4) indicates that the sediments carried through distributary channels were obviously reworked by wave and shelf currents: most of the suspension load was driven far away, while coarser sandy sediments were deposited along the shoreline. Another reason for the lack of mudstone is that the exposed erosional land consisted mainly of Mesozoic granite, which may have provided more sandy and less muddy elastic sediments for the basin. The Dongsha massif remains a elastic source region to this day (Fig. 4).
slope of the Dongsha massif in response to sealevel rise (Fig. 6). All delta systems in the Zhuhai Formation were developed during the initial relative sea-level rise phase after break-up unconformity in the northern South China Sea. They are generally destructive, have a cuspate geometry, a high sandstone/ mudstone ratio and contain well sorted and permeable sandstones which can be
IAMMA.RAY La
2.2. Middle Zhuhai Formation delta or beach-tooffshore sequences Four stacked coarsening-upward sequences without or with limited distributary facies in the upper part of each sequence are interpreted as being a set of incomplete delta sequences or beach-to-offshore sequences. Constructive delta systems may have been formed when the sea-level was at a still-stand or changing slowly, but they were strongly reworked by a subsequent marine invasion to the north when the sea-level rose rapidly. The marine influence is more striking in comparison with the Lower Member mentioned above: the sandstone/mudstone ratio is about 2: 1 and the cuspate shape geometry indicates that a destructive delta system was involved.
i
2.3. Upper Zhuhai Formation wave- and tidedominated destructive delta systems and transgressive sandstone sequences In this Member well sorted barrier bar sandstones, interbedded with thin mudstones and very thin limestones, are common; glauconite, anhydrite, barite, as well as some fragments of Foraminifera, Bryozoan and Echinodermata were also found, and several smaller cuspate-shape delta systems can be seen in the northern part of the area (Fig. 5). Transgressive sandstones, including beach, barrier bar and longshore bar sandstones were developing gradually along the
5
\
Fig. 7. Depositional facies of the Zhujiang Formation.
Chen Jishu et al. / Tectonophysics 235 (1994) l-l 1
e
louRcc DELTA
ARCA
8VSTCM
CARSONATE
PLATPORM
REEF/BANK IU~DUP PERCENTAQE OF SANDSTONE CONTENT
Fig. 8. Delta systems and carbonate platforms-reef/bank
systems in the Lower Zhujiang Formation.
SOURCRALREA FACIE8 BOUNDARY PERCENTAOL OF SANDSTONECONTCN
rig. Y. uestrucuve aelta and shelt systems in the Middle Zhujiang Formation.
7
Chen Jishu et al. / Tectonophysics 235 (1994) l-11
correlated over a large area. In general, the Zhuhai Formation is much richer in reservoir sandstones but poor in mudstone or seal rocks. 3. Delta systems and carbonate platforms in the Zhujiang Formation The period over which the Zhujiang Formation was laid down was about 7-8 My (21-13.8 My B.P.), and included four third-order eustatic cycles, shown in global cycle chart of Haq et al. (1988). However, three delta development stages can be distinguished, using biologic and lithologic data, in the study area (Fig. 7). 3.1. Lower Zhujiang Formation constructive delta systems and carbonate platforms
Three or four coarsening-upward delta sequences with a stronger fluvial influence were developed during the period of slow eustatic rise
(21-17.5 My B.P.). A large lobate-shape delta system and two other smaller birdfoot shaped delta systems were prograded into the shelf region from the northwest and southwest sides, respectively (Fig. 8). The Dongsha massif was drowned and, far away from the sediment source region, carbonate platforms, reef and bank complexes with a maximum thickness of 500 m, were deposited on the massif. Two or three zones of secondary porosity have been found within the carbonate interval. It is believed that the origin of the secondary porosity zones is closely related to eustatic sea-level falls at 17.5, 16.5 and 15.5 My B.P. (Haq et al., 1988). 3.2. Middle Zhujiang Formation destructive delta systems and shelf sandstone systems
The destructive delta systems have a slight cuspate shape and a wide shelf area with some
f
’ PYLI
k
PERCENTAOEOF SANOST~NECONTENT
r\ Fig. 10. Constructive
delta systems in the Upper
SOUSCS AREA
Zhujiang
FACIE8 SOUNOARV
Formation.
Chen Jishu et al. / Tectonophysics 235 (1994) l-11
8
shelf sandstone ridges (Fig. 9). Thick shelf mudstone and moderate shelf sandstone are common, the sandstone/mudstone ratio is about 1: 2. The previous carbonate platform on the Dongsha massif was overlain by sediments deposited in an outer-shelf to hemipelagic environment. Some condensed sections have been found in the cores above the top of carbonate rock. It is clear that the marine transgression further expanded after
;AMMA.RAY
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soNlcLoo
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YUD BTDNC
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the deposition of the carbonate rock, although there has been no obvious global sea-level rise since 15.5 My B.P. (Haq et al., 1988). The strong subsidence in the study area is possibly the main reason for the rapid marine transgression after 15.5 My B.P 3.3. Upper Zhujiang Formation constructive delta systems Three coarsening-upward delta sequences were deposited in a slow marine transgression environment in response to the global sea-level fall from 15 to 13.8 My B.P. (Haq et al., 1988). The deltaic geometry (Fig. 10) is similar to that of constructive delta systems (Fig. 8) in the Lower Zhujiang Formation, but no carbonate rock was developed as the water depth was too deep for coral reefs to grow on the Dongsha massif. Delta systems in the Zhujiang Formation are developed as the relative sea-level rose further, the rate of relative sea-level rise exceeded the rate of deposition, causing shoreline and delta systems to move northward. Some small constructive delta systems were developed and more sands were deposited as barrier bars, longshore bars and shelf sandstone ridges. Thick and widespread shelf mudstones might have been formed as a regional seal rock. Thick carbonate rock with a secondary porosity was deposited on the Dongsha massif. Both reservoir rock (sandstone and carbonate rock) and seal rock are well developed in the Zhujiang Formation and most of the hydrocarbons discovered are concentrated in the Lower Zhujiang Formation. 4. Delta systems in the Haqjiang Formation The duration of this Formation is about 3.3 My (13.8-10.5 My B.P.), based on biological data, it can be divided into two members (Fig. 111, the Lower and the Upper.
/’
4.1. Lower Hanjiang Formation constructive delta systems
.’ /
i
Fig. 11. Depositional facies of the Hanjiang Formation.
The sea-level fall at 13.8 and 12.5 My B.P. (Haq et al., 1988) may possibly have resulted in
9
Chen Jishu et al. / Tectonophysics 235 (1994) l-l 1
distributary channel sandstones deposited directly on the previous shelf region. Coarseningupward delta sequences with some inter-distributary mudstones and thin coal beds ( < l-2 m> are common, the sequences have thicknesses of 55-75 m and a sandstone/mudstone ratio of 2 : 1. Two lobate delta systems developed on the southwest and northwest sides of the area (Fig. 12).
In general, the elastic lithology of the Hanjiang Formation is similar to that of the Zhujiang Formation, it has both reservoir rocks and cap rocks, but only a few oil-bearing zones have been found up to now.
4.2. Upper Hanjiang Formation and sandstone systems
Although many periods of eustatic sea-level fall since 30 My B.P. are shown in the global cycle chart of Haq et al. (19881, the paleo Pearl River Delta Systems are interpreted as having developed during a period of relative sea-level rise, possibly caused by more rapid subsidence after the break-up unconformity. The stacked delta systems from the Zhuhai to Hanjiang Formations (about 30-10.5 My B.P.) have a ‘backstopping
shelf mudstones
The interval consists of thicker shelf mudstones interbedded with thin-moderate shelf sandstones (Fig. 11). Some thick and massive beach-to-offshore sandstones, containing fossils of Foraminifera and Echinodermata, were found in the northern region.
5. Discussion on the hydrocarbon accumulation in the paleo Pearl River delta systems
e
SOURCE
#c
BASIN
r\
FACIES
6
PERCENTAGE SANDSTONE
/
Fig. 12. Constructive
delta system in the Lower Hanjiang
Formation,
AREA MARGIN BOUNDARY OF CONTEN
Chen Jishu et al. / Tectonophysics 235 (1994) I-11
10
SE
NW
V
-@-
QEOLOOIC AQEha)
--g-MAXIMUM
Tc/
MAR I NEMUDSTOME
nCOASTAL
m
REEF
/7
PIAN
PLAMbELtA
$L~PE FAN/BASIN PL~ORFAN
Fig. 13. Diagrammatic
COASTAL
FLOODIMQ SURFACE
cross-section
ZHU II DEPRESSION
showing onlap across the Pearl River Mouth basin.
-----+
CONTlNENTAL
SUB-TERTIARY
SHELF +
SLOPE
SECTION
Fig. 14. Schematic cross-section of the Texas portion of the northern Gulf of Mexico basin illustrating the progradational pattern. Arrows indicate principal direction of fluid movement through pressure shale section (Bruce, 1984).
strata1
Chen Jishu et al. / Tectotwphysics 235 (1994) I-11
pattern’ (onlap) (Fig. 13), indicating the basin has been expanding since 30 My B.P. One delta system progressively deepens upward as younger delta sequences retreat farther landward, this statal pattern is quite different from the forestepping (or progradational) pattern of the Cenozoic delta systems deposited in the northern Gulf of Mexico (Fig. 14). Other Cenozoic delta systems, for example the Niger delta, West Africa, Baram delta or Mahakam delta, Borneo, also have a progradational pattern. The relative sea-level rising background, coupled with a sand-rich erosional land surface, have led to the following results in the study area: (1) Most of the delta systems are destructive, they provide more sandstones and less prodelta mudstones. The thick and extensive shelfal mudstones in each formation are generally silty, hard and fragile. Ductile mudstones are poor in all three formations, which is probably the main reason for the lack of growth faults and rollover structures, the many normal faults only add to the problem rather than trapping hydrocarbons: some drilling wells on local fault-controlled structures are dry holes; therefore, ‘faults mean high risk’ is a saying of some exploration operators. (2) Most prodelta mudstones in the three formations are immature or poorly mature source rocks. The hydrocarbon discovered in the past has mainly been generated from lacustrine source rock in the Wenchang and Enping formations (Eocene-Oligocene). Two striking features of hydrocarbon accumulation can be summarized based on the data from oilfields: (1) the oil production zones are more concentrated in the Upper Zhuhai to Lower Zhujiang Formations, including both elastic and carbonate rocks; (2) hydrocarbons were generally trapped in two simple structures: reef-bank carbonate buildups and drape structures related to local basement highs. (3) It is expected that most of the remaining undiscovered reserve in the basin will be found in non-structural subtle traps, particularly in stratigraphic traps close to lacustrine source rocks.
11
References Abbott, W.O., 1985. The recognition and mapping of basal transgressive sand from outcrop, subsurface, and seismic data. AAPG Mem. 39: 157-168. Bruce, C.H., 1984. Smectite dehydration-its relation to structural development and hydrocarbon accumulation in northern gulf of Mexico Basin. AAPG Bull., 68 (6): 673683. Chart, M.A. and Dott, R.H., Jr., 1986. Depositional facies and progradational sequences in Eocene wave-dominated deltaic complexes, southwestern Oregon. AAPG Bull., 70 (4): 415-429. Chen Shizhong and Hu Pingzhong, 1987. Tertiary reef and its significance on exploration in Pearl River Mouth Basin. China Offshore Pet., 1 (1): 3-10. Coleman, J.M. and Prior, D.B., 1982. Deltaic environments of deposition. AAPG Mem., 31: 139-178. Dolan, J.F., 1989. Eustatic and tectonic controls on deposition of hybrid siliciclastic/carbonate basinal cycles: discussion with examples. AAPG Bull., 73 (10): 1233-1246. Haq et al., 1988. Mesozoic and Cenozoic chronostratigraphy and cycles of sea-level change. In: C.K. Wilgus, B.S. Hastings, C.G.St.C. Kendall, H.W. Posamentier, CA. Ross and J.C. Van Wagoner (Editors), Sea-level changes: An Integrated Approach. SEPM Spec. Publ., 42: 71-108. Libby-French, J., 1984. Stratigraphic framework and petroleum potential of northeastern Baltimore Canyon trough, Mid-Atlantic outer continental shelf. AAPG Bull., 68 (1): 50-73. Ioutit, T.S., Hardenbol, J., Vail, P.R. and Baum, G.R., 1988. The key to age dating and correlation of continental margin sequences. In: C.K. Wilgus, B.S. Hastings, C.G.St.C. Kendall, H.W. Posamentier, C.A. Ross and J.C. Van Wagoner (Editors), Sea-level Changes: An Integrated Approach. SEPM Spec. Publ., 42: 183-213. Shelton, J.W., 1984, Listric normal faults: an illustrated summary. AAPG Bull., 68 (7): 801-815. Vail, P.R., Audernard, F., Bowman, S.A., Eisner, P.N. and Perez-Cruz, G., 1990. The Stratigraphic signatures of tectonics, eustasy and sedimentation (unpubl.). Van Wagoner, J.C., Mitchum, R.M., Campion, K.M. and Rahmanian, V.D., 1990. Siliciclastic sequence stratigraphy in well logs, cores, and outcrop. (AAPG Methods in Exploration Series, No. 7.) Am. Assoc. Pet. Geol., Tulsa, Okla., USA. Wang Xie-pei, Fei Qi and Zhang Jia-Hua, 1985. Cenozoic diapiric traps in Eastern China. AAPG Bull., 69 (12): 2098-2109.