Thermal and exhumation history of the Songnan Low Uplift,Qiongdongnan Basin:constraints from the apatite fission-track and zircon(U-Th)/He thermochronology

Significant advancements have been made in the study of Mesozoic granite buried hills in the Songnan Low Uplift(SNLU)of the Qiongdongnan Basin.These findings indicate that the bedrock buried hills in this basin hold great potential for exploration.Borehole samples taken from the granite buried hills in the SNLU were analyzed using apatite fission track(AFT)and zircon(U-Th)/He data to unravel the thermal history of the basement rock.This information is crucial for understanding the processes of exhumation and alteration that occurred after its formation.Thermal modeling of a sample from the western bulge of the SNLU revealed a prolonged cooling event from the late Mesozoic to the Oligocene period(~80-23.8 Ma),followed by a heating stage from the Miocene epoch until the present(~23.8 Ma to present).In contrast,the sample from the eastern bulge experienced a more complex thermal history.It underwent two cooling stages during the late Mesozoic to late Eocene period(~80-36.4 Ma)and the late Oligocene period(~30-23.8 Ma),interspersed with two heating phases during the late Eocene to early Oligocene period(~36.4-30 Ma)and the Miocene epoch to recent times(~23.8-0 Ma),respectively.The differences in exhumation histories between the western and eastern bulges during the late Eocene to Oligocene period in the SNLU can likely be attributed to differences in fault activity.Unlike typical passive continental margin basins,the SNLU has experienced accelerated subsidence after the rifting phase,which began around 5.2 Ma ago.The possible mechanism for this abnormal post-rifting subsidence may be the decay or movement of the deep thermal source and the rapid cooling of the asthenosphere.Long-term and multi-episodic cooling and exhumation processes play a key role in the alteration of bedrock and contribute to the formation of reservoirs.On the other hand,rapid post-rifting subsidence(sedimentation)promotes the formation of cap rocks.

Application of C_(30)tetracyclic polyprenoids as effective biomarker in oil-to source rock correlation in the ZhuⅢdepression,Zhujiangkou Basin,northern South China

The northern South China Sea,including the Zhujiangkou Basin and the Beibuwan Basin,developed high-quality lacustrine source rocks during the Eocene rifting period.These source rocks are vital for hydrocarbon generation in the northern South China Sea.The ZhuⅠdepression in the Zhujiangkou Basin and the Beibuwan Basin typically exhibit high abundance of C_(30)4-methyl steranes.However,shales in the Eocene Wenchang Formation in the ZhuⅢdepression of the Zhujiangkou Basin contains lower quantities of high-quality lacustrine source rocks with 4-methyl steranes,which often co-elute with some pentacyclic triterpanes in gas chromatography-mass spectrometry(GC-MS).Therefore,the single 4-methylsterane parameter based on GC-MS cannot accurately distinguish organic source in the deep to semi-deep water lacustrine source rocks of the Wenchang Formation from other source rocks,thus impeding the recognition of their contributions to petroleum reservoirs.In this study,GC-MS of aliphatic hydrocarbons,palynofacies and algal identification,as well as stable carbon isotope data of organic matter were used to identify the algal species and construct the paleoclimate during deposition of the Wenchang Formation source rocks in the ZhuⅢdepression of the Zhujiangkou Basin.It is suggested that during the Wenchang Formation period,freshwater green algae prevailed in the lake,which is likely account for the relatively low content of 4-methyl steranes in the high-quality lacustrine source rocks.Controlled by the algal species,it is proposed that the content of C_(30)tetracyclic polyprenoids(TPP)can better indicate the quality of the Wenchang source rocks than C_(30)4-methyl steranes.Consequently,a relationship between the TPP index and the quality of the lacustrine source rocks in the Wenchang Formation of the ZhuⅢdepression was established.A higher TPP index indicates higher organic matter abundance and hydrogen index of the lacustrine source rocks.When applied to the origin analysis of oils in the ZhuⅢdepression,it is believed that the organic-rich deep lacustrine source rocks in the Wenchang Formation made great contribution to the transitional zone crude oils in the Wenchang A and Wenchang B depressions.

Zircon U-Pb geochronology,Hf isotopes,and geochemistry constraints on the age and tectonic affinity of the basement granitoids from the Qiongdongnan Basin,northern South China Sea

Studies in the northern South China Sea(SCS)basement remain important for understanding the evolution of the Southeast Asian continental margin.Due to a thick cover of sediments and scarce borehole penetration,little is known about the age and tectonic affinity of this basement.In this study,an integrated study of zircon U-Pb geochronology,Hf isotopes,and whole-rock major and trace elements on seven basement granitoids from seven boreholes of Qiongdongnan Basin has been carried out.New zircon U-Pb results for these granitoids present middle-late Permian((270.0±1.2)Ma;(253±3.4)Ma),middle to late Triassic((246.2±3.4)Ma;(239.3±0.96)Ma;(237.9±0.99)Ma;(228.9±1.0)Ma)and Late Cretaceous ages((120.6±0.6)Ma).New data from this study,in combination with the previous dataset,indicates that granitoid ages in northern SCS basement vary from 270 Ma to 70.5 Ma,with three age groups of 270–196 Ma,162–142 Ma,and 137–71 Ma,respectively.Except for the late Paleozoic-Mesozoic rocks in the basement of the northern SCS,a few old zircon grains with the age of(2708.1±17)Ma to(2166.6±19)Ma provide clues to the existence of the pre-Proterozoic components.The geochemical signatures indicate that the middle Permian-early Cretaceous granitoids from the Qiongdongnan Basin are I-type granites formed in a volcanic arc environment,which were probably related to the subduction of the Paleo-Pacific Plate.

The influence of organic sources and environments on source rock deposition during the periods of Cretaceous–Eocene and Oligocene–Miocene,northern Kalimantan

The sedimentary system of Kalimantan has undergone significant development since the Oligocene.Previous research have largely ignored the capacity of the Cretaceous–Eocene sediments to produce hydrocarbons,focusing instead primarily on the Oligocene–Miocene coal as the principal source rocks.Shales and coals from the outcrops in the northern margin of Kalimantan were analyzed with palynological and geochemical methods to characterize the palaeoenvironmental and palaeoecological differences between the Cretaceous–Eocene and the Oligocene–Miocene samples.The high proportion of Cheirolepidoaceae,Schizaeoisporites and Ephedripites in the pollen assemblage from the Cretaceous–Eocene outcrops reflects an arid tropical/subtropical climate.The relatively low abundances of gymnosperm-derived biomarkers including isopimarane,β-phyllocladane,β-kaurane,suggest the gymnosperm features in flora.High C_(27)/C_(29)ααα20R sterane ratios,(C_(19)–C_(29))tricyclic terpanes/C_(30)αβhopane and extremely low oleanane/C_(30)αβhopane,bicadinane T/C_(30)αβhopane,and diterpenoid abundance indicate that there was a dominance of algae relative to higher plants in the organic matter.The gymnosperm-derived biomarkers,including isopimarane,β-phyllocladane,β-kaurane,suggest that palaeovegetation during this period was dominated by gymnosperms.The saline and reducing conditions in the bathyal and abysmal sea,evidenced by rather low Pr/Ph and high Gammarerane index,are beneficial for the preservation of hydrogen-rich organic matter.It is presumed that the Cretaceous–Eocene shales had great hydrocarbon generation potential in the southern South China Sea.During the period of Oligocene to Miocene in the Zengmu Basin and the Baram-Sabah Basin,the climate changed to a dominant humid and warm condition,which is corroborated by abundant pollen of Florschuetzia and Magnastriatites hawardi.Low C_(27)/C_(29)ααα20R sterane ratios,(C_(19)–C_(29))tricyclic terpanes/C_(30)αβhopane,and high oleanane/C_(30)αβhopane,bicadinane T/C_(30)αβhopane suggest that the palaeovegetation was dominated by angiosperms including the mangrove plants.The extremely abundant higher plants provide ample terrigenous organic matter for the formation of coal-measures in delta facies.The low gammacerane index and high Pr/Ph indicate the fresh and sub-oxic water in delta-neriticabysmal faces,which is not beneficial for the accumulation of hydrogen-rich organic matter.Thus,the Oligocene–Miocene marine argillaceous rocks can be potential sources of natural gas.

Biomarkers reveal the terrigenous organic matter enrichment in the late Oligocene-early Miocene marine shales in the Ying-Qiong Basin,South China Sea

The increase of total organic carbon content of the late Oligocene-early Miocene terrigenously-dominated marine shales in the shallower depth intervals was reported in the Ying-Qiong Basin,South China Sea.The organic enriched lower Sanya Formation shales(early Miocene)have biomarker characteristics of tropical/subtropical plants,with abundant high molecular weight n-alkanes,angiosperm-derived oleanane,rearranged oleananesⅠ,Ⅱ,Ⅱ,tricyclic/tetracyclic terpanes including des-A-oleanane,X,*,Y,Z,Z1 and bicadinanes W,T,T1,R.The biomarker characteristics are suggestive of larger influx of the dominant tropical/subtropical angiosperms in flora under a warming and more humid climate during depositions of the lower Sanya Formation(early Miocene)than the older Lingshui Formation(late Oligocene).The tropical/subtropical angiosperm input was thought as the prime control of terrigenous organic matter enrichment relative to the redox condition,and the coeval sea level changes and seafloor spreading in the South China Sea.Enrichment of the terrigenous organic matter in the early Miocene shales is likely in association with the coeval peak East Asian summer monsoon intensity in the South China Sea.

Tectonic subsidence of the Zhu I Sub-basin in the Pearl River Mouth Basin, northern South China Sea

The Pearl River Mouth Basin, which is situated on the northern margin of the South China Sea, has attracted great attention not only because of its tectonic setting but also because of its abundant hydrocarbon resources. We have analyzed the Cenozoic tectonic subsidence history of 4 drilled wells and 43 artificial wells from the Zhu 1 Sub-basin of the Pearl River Mouth Basin by back-stripping, using newly interpreted seismic profiles. We also calculated the average tectonic sub- sidence rates of the four sags in the Zhu 1 Sub-basin. The rifting and post-rifting stages are separated by abrupt changes in the tectonic subsidence curves and average subsidence rates. In the eastem sags of the Zhu 1 Sub- basin, tectonic subsidence started to slow at ca. 30 Ma, compared with ca. 23.8 Ma in the western sags. This probably corresponds to the timing of break-up and suggests that tiffing in the Pearl River Mouth Basin ended earlier in the eastern sags than in the western sags. Anomalously accelerated tectonic subsidence occurred at 17.5-16.4 Ma during the post-tiffing stage, with average subsidence rates as high as 301.9 m/Myr. This distin- guishes the Pearl River Mouth Basin from classical Atlantic passive continental marginal basins, which demonstrate exponentially decaying post-rift tectonic subsidence.

Thermal-history reconstruction of the Baiyun Sag in the deep-water area of the Pearl River Mouth Basin, northern South China Sea

The Baiyun Sag, located in the deep-water area of the northern South China Sea, is the largest and deepest subbasin in the Pearl River Mouth Basin and one of the most important hydrocarbon-accumulation depression areas in China. Thermal history is widely thought to be of great importance in oil and gas potential assessment of a basin as it controls the timing of hydrocarbon generation and expulsion from the source rock. In order to unravel the paleo-heat flow of the Baiyun Sag, we first analyzed tectonic subsidence of 55 pseudo-wells constructed based on newly interpreted seismic profiles, along with three drilled wells. We then carried out thermal modeling using the multi-stage finite stretching method and calibrated the results using collected present-day vitrinite reflectance data and temperature data. Results indicate that the first and second heating of the Baiyun Sag after 49 Ma ceased at 33.9 Ma and 23 Ma. Reconstructed average basal paleo- heat flow values at the end of the rifting periods are -57.7- 86.2 mW/m2 and -66.7-97.3 mW/m2, respectively. Following the last heating period at 23 Ma, the study area has undergone a persistent thermal attenuation phase, and basal heat flow has cooled down to ～64.0-79.2 mW/m2 at present.