1. 3 Shenshen Branch of CNOOC Ltd., Guangzhou, 510240, China.
Paleoenvironmental Reconstruction and Sequence Stratigraphic Analysis Using Element Geochemical Analysis in Pear River Mouth Basin, South China Sea
Zhang Changmin1, Zhu Rui1, Yu Ye2, Li Shaohua1, Shi Hesheng3, Du Jiayuan3
1 School of Geosciences, Yangtze University, Wuhan, Hubei, , China; 2 Hunan Provincial Key Laboratory
of Shale Gas Resource Utilization, Hunan University of Science and Technology, Xiangtan, , China;
3 Shenshen Branch of CNOOC Ltd., Guangzhou, , China.
Introduction
Sedimentary structures and their assembladges, well logging curves and seismic profiles are dominated in sedimentary paleoenvironmental interpretation and sequence stratigraphic correlation. But there has lost very valuable information from the laboratory chemical analysis materials. Here we introduce the application of element geochemical analysis using for paleoenvironmental and sequence stratigraphic study in Pear River Mouth Basin, South China Sea.
43 mudstone samples and 37 carbonate samples from 10 wells of Tertiary Zhuhai formation (Paleogene) to Hanjiang formation (Neogene) of Pear River Mouth Basin have been tested for C, O isotopic and other geochemical analysis. The sampling well position is shown in the figure below.
Results and Discussion
1 Paleoevironmental reconstruction
1.1 Paleoclimate
1.5 Oxidation reduction
The Al2O3/Na2O, CIA (chemical index of alteration), Rb/Sr and Sr/Cu are very important indexes to reveal the intensity of chemical weathering. The chemical weathering is mainly influenced by paleoclimate. Therefore, those indexes can also indicate the paleoclimate. The Al2O3/Na2O, CIA, Rb/Sr gradually diminish from Zhuhai formation to Hanjiang formation and the Sr/Cu is just contrary in figure. The result show the paleoclimate changing to be dryer from Zhuhai stage to Hanjiang stage of this area..
The &U(&U=U/(0.5×(Th/3+U))) and MnO/TiO2 are very important indexes to reveal oxidation reduction. The paleoenvironmental oxidation-reductional condition changed from oxidizd at Zhuhai period to reductive at the period of Zhujiang and Hanjiang.
2 Sequence stratigraphic analysis
Elemental Fe and Al are rich on the sequence boundary of ZHSQ6 and NSQ1 and mainly oxidized. The ratio of Th/U is large, and the ratio of Sr/Ca is little on the sequence boundary of ZHSQ6 and NSQ1. The Al, Fe, Mn, Mn/Fe, Sr/Ca, (La/Ce)N and ΣREE gradually incresae during the LST period until the maximum, and they gradually decrease during the HST period until the minimum. The Th/U is contrary.
There are 25 samples in W1 well, which were taken from Zhuhai formation(7 samples), the lower section of Zhujiang formation(4 samples) , the upper section of Zhujiang formation(7 samples), and Hanjiang formation(7 samples). The W2 well has 37 samples in Zhujiang formation, NSQ2-1 is 11, NSQ2-2 is 11, NSQ3 is 9, NSQ4 is 4, respectively. In the lower of Zhujiang formation, there are 3 samples in well X1, 10 samples in well S1-5, and 5 samples in well D1-2.
1.2 Paleosalinity
The geochemistry indicators of Fe family element, Th/U, Cr/VCo/Ni, Sr/Ba and δ(Ce) gradually increase during the LST period until the maximum, and they gradually decrease during the TST period until they get the minimum at the moment of the maximum flooding period, then they gradually increase during the HST period. The geochemistry indicators of Mn family element, Mn/Fe, Mn/Ti, Sr/Ca, ∑REE and (La/Ce)N counter to those geochemistry indicators.
Aim
Reconstruct paleoenvironment of Pear River Mouth Basin, including paleoclimate, paleosalinity, sea level change, ancient water depth, paleotemperature of seawater and oxidation reduction.
Identify and demarcate sequence stratigraphy by using element geochemistry in Pear River Mouth Basin.
Enrich the research content of element geochemistry and propose a new method of sequence stratigraphic identification.
The 100×(MgO/Al2O3), Sr/Ba increase gradually from Zhuhai to Hanjiang formation, which indicate that paleosalinity increase gradually from Zhuhai to Hanjiang formation. The value of Z shows that the limestone of Zhujiang formation (from NSQ2-1 to NSQ4) is marine limestone, and the paleosalinity increases during early and then decrease at late stage of Zhujiang period. The results show that the paleosalinity of the D well area was higher while the paleosalinity of the X and S well area was lower in early Zhujiang period.
Conclusion
The paleoclimate changing to be dryer from zhuhai to Hanjiang stage, the paleosalinity increased from Zhuhai to Hanjiang formation. Paleoenvironment in Zhuhai stage is mainly oxidation, but it is reductive during Zhujiang and Hanjiang stages.
The paleotemperature of seawater varied in 18～24℃, and has the same variation as the paleosalinity and sea level which are the highest at the time of NSQ3 in the Zhujiang period.
The water of S well area was relatively deep and in low salinity, X well area was medium, D well area was relatively shallow and in high salinity.
The geochemical element is abnormal in the vicinity of sequence boundary and marine flooding surface, which is a new method to demarcate sequence stratigraphy.
Contact: Ph:
1.3 Sea level change and ancient water depth
Eustasy control the transformation of carbon oxidation and reduction (δ13C) and the value ofδ18O is influenced by the sea level fluctuation in seawater. The δ13C and δ18O show that the sea level rises during early and then fall at late stage of Zhujiang period and it reached to the highest at the time of NSQ3. The 1000×(Sr/Ca) and V/Cr pointed out that the water of S well area was relatively deep, X well area was medium and D well area was relatively shallow.
Methods
The major and trace elements were analyzed using ICP-AES and ICP-MC at the State Key Laboratory of Marine Geoglogy, Tongji University.
The C and O isotopic compositions were measured with MC-ICP-MC at the State Key Laboratory of Isotope Geochemistry in Guangzhou Institute of Geochemistry, Chinese Academy of Sciences.
Geochemical element is very sensitive to the depositional environment, and the content of geochemical element exists anomaly response in the vicinity of sequence boundary and marine flooding surface.
1.4 Paleotemperature of seawater
Acknowledgements
The authors appreciate the help of Professor Shao Lei and Key Laboratory of Marine Geology (Tongji University) with the geochemical analysis, and Shenzhen Branch of CNOOC L td. with project support.
The paleotemperature of seawater rised and then falled from the early to the late of the Zhujiang period and the paleotemperature reached to their highest value at time of NSQ3, and temperature ranges from 18 to24.