Contentious Petroleum Geochemical Issues in China’s Sedimentary Basins

Petroleum geochemistry contributes to exploration successes by providing key constraints for geological models and critical input to exploration scenarios. One of the most important tasks in a typical exploration program is to identify the most effective source intervals or kitchens in a basin, through oil-source correlation. The results of correlation are valid only if the geochemical parameters used address adequately the genetic characteristics of the source rocks as well as the mass transport and mixing processes of hydrocarbon fluids occurring in the carrier beds and reservoirs. This manuscript discusses four of the major contentious petroleum geochemical issues in China’s sedimentary basins. It is suggested that marine incursions played a significant role in the formation of prolific petroleum source rocks in the gigantic, dominantly freshwater, Songliao Basin. Several models are proposed to account for the occurrence of immature oils in the Cathaysian rift system including the Bohai Bay Basin, thus immature source rocks are considered a mere minor contributor to the known economic immature oil resources. Both geological and geochemical evidence are reviewed to refute a dominantly coaly source for the petroleum discovered in the Turpan Basin. Results of case studies are presented to demonstrate the importance of recognizing petroleum fluid mixing to solve the oil-source correlation issues in the structurally complex Tarim Basin. In addressing the fundamental assumptions and potential flaws of the molecular geochemical parameters commonly used for oil-source correlation, the need of a mass fraction approach is proposed to deal with such contentious issues as marine versus lacustrine, coal versus lacustrine, and mature versus immature oils.

Generation of andesite through partial melting of basaltic metasomatites in the mantle wedge:Insight from quantitative study of Andean andesites

Continental crust in average exhibits a similar composition in both major and trace elements to andesites along active continental margins.For this reason,andesitic magmatism above oceanic subduction zones is considered to have played a key role in the generation of continental crust along convergent plate boundaries.With respect to the origin of andesites themselves,however,there is still a hot debate on how they have acquired their geochemcial compositions.The debate is mainly centralized on the relative contributions of crustal contamination,magma differentiation and source mixing,which reaches an impasse in the past decades.The essential reason for this kind of debates is that these three mechanisms only can account for some of the geochemical observations for andesites,leading to insufficient discrimination among them.Nevertheless,the geochemical features of andesites are primarily controled from early to late by the composition of their source rocks in addition to partial melting and magma differentiation processes.If source mixing and partial melting processes in the early stage of andesite magmatism can account for the first-order geochemical features of andesites,there is no need to invoke the late processes of magma differentiation and crustal contamination for andesite petrogenesis.This is illustrated by quantitative forward modeling of the geochemical data for Quaternary andesites from the Andean arc in South America based on an integrated interpretation of these data.The modeling has run with four steps from early to late:(1)dehydration of the subducting oceanic crust at forearc depths;(2)partial melting of the subducting terrigenous sediment and altered oceanic basalt at subarc depths to produce hydrous felsic melts;(3)the generation of basaltic metasomatites(e.g.,Si-excess pyroxenite)in the mantle wedge through reaction of the mantle wedge peridotite with large amounts of the hydrous felsic melts;(4)the production of andesitic melts by partial melting of the basaltic metasomatites.The results not only testify the hypothesis that the trace element and radiogenic isotope compositions of andesites can be directly produced by the source mixing and mantle melting but also demonstrate that partial melting of the basaltic metasomatites can reproduce the lithochemical composition of andesites.The compositional variations of Andean andesites within a single volcanic zone and among different volcanic zones can be explained by incorporating different amounts of heterogeneous hydrous felsic melts into their mantle sources,followed by different degree of partial melting under different pressures and temperatures.Therefore,the source mixing and partial melting processes at subarc depths can account for the first-order geochemical features of Andean andesites.In this regard,it may be not necessary for andesite petrogenesis to invoke the significant contributions from the processes of magma differentiation and crustal contamination.

Geology and mineralization of the Pulang supergiant porphyry copper deposit(5.11 Mt)in Shangri-la,Yunnan Province,China:A review

The porphyry copper belt in the Geza island arc in southwestern China is the only Indosinian porphyry copper metallogenic belt that has been discovered and evaluated so far.The Pulang porphyry copper deposit(also referred to as the Pulang deposit)in this area has proven copper reserves of 5.11×106 t.This deposit has been exploited on a large scale using advanced mining methods,exhibiting substantial economic benefit.Based on many research results of previous researchers and the authors’team,this study proposed the following key insights.(1)The Geza island arc was once regarded as an immature island arc with only andesites and quartz diorite porphyrites occurring.This understanding was overturned in this study.Acidic endmember components such as quartz monzonite porphyries and quartz monzonite porphyries have been identified in the Geza island arc,and the mineralization is mainly related to the magmatism of quartz monzonite porphyries.(2)Complete porphyry orebodies and large vein orebodies have developed in the Pulang deposit.Main orebody KT1 occurs in the transition area between the potassium silicate alteration zone of quartz monzonite porphyries and the sericite-quartz alteration zone.Most of them have developed in the potassium silicate alteration zone.The main orebody occurs as large lenses at the top of the hanging wall of rock bodies,with an engineering-controlled length of 1920 m and thickness of 32.5‒630.29 m(average:187.07 m).It has a copper grade of 0.21%-1.56%(average:0.42%)and proven copper resources of 5.11×10^(6) t,which are associated with 113 t of gold,1459 t of silver,and 170×10^(3) t of molybdenum.(3)Many studies on diagenetic and metallogenic chronology,isotopes,and fluid inclusions have been carried out for the Pulang deposit,including K-Ar/Ar-Ar dating of monominerals(e.g.,potassium feldspars,biotites,and amphiboles),zircon U-Pb dating,and molybdenite Re-Os dating.The results show that the porphyries in the Pulang deposit are composite plutons and can be classified into pre-mineralization quartz diorite porphyrites,quartz monzonite porphyries formed during the mineralization,and post-mineralization granite porphyries,which were formed at 223±3.7 Ma,218±4 Ma,and 207±3.9 Ma,respectively.The metallogenic age of the Pulang deposit is 213‒216 Ma.(4)The petrogeochemical characteristics show that the Pulang deposit has the characteristics of volcanic arc granites.The calculation results of trace element contents in zircons show that quartz monzonite porphyries and granite porphyries have higher oxygen fugacity.The isotopic tracing results show that the diagenetic and metallogenic materials were derived from mixed crust-and mantle-derived magmas.

Crystallization Conditions and Mineral Chemistry in the East of Tafresh, Central Iran, with Insights into Magmatic Processes

The Tafresh granitoids are located at the central part of the Urumieh-Dokhtar Magmatic Arc(UDMA)in Iran.These rocks,mainly consisting of diorite and granodiorite,were emplaced during the Early Miocene.They are composed of varying proportions of plagioclase+K-feldspar+hornblende±quartz±biotite.Discrimination diagrams and chemical indices of amphibole phases reveal a calc-alkaline affinity and fall clearly in the crust-mantle mixed source field.The estimated pressure,derived from Al in amphibole barometry,is approximately 3 Kb.The granitoids are I-type,metaluminous and belong to the calc-alkaline series.They are all enriched in light rare earth elements and large ion lithophile elements,depleted in high field strength elements and display geochemical features typical of subduction-related calc-alkaline arc magmas.Most crystal size distribution(CSD)line patterns from the granitoids show a non-straight trend which points to the effect of physical processes during petrogenesis.The presence of numerous mafic enclaves,sieve texture and oscillatory zoning along with the CSD results show that magma mixing in the magma chamber had an important role in the petrogenesis of Tafresh granitoids.Moreover,the CSD analysis suggests that the plagioclase crystals were crystallized in a time span of less than 1000 years,which is indicative of shallow depth magma crystallization.

Production of Poly(3-Hydroxybutyrate-co-3-Hydroxyhexanoate) Using Aeromonas hydrophila 4AK4 Grown in Mixed Carbon Source

Aeromonas hydrophila 4AK4 was grown on mixed substrates of soybean oil and lauric acid for the production of polyhydroxyalkanoate copolymer consisting of 3 hydroxybutyrate (3HB) and 3 hydroxyhexanoate (3HHx). A maximal poly(3 hydroxybutyrate co 3 hydroxyhexanoate) (PHBHHx) content of 49.13% in dry cells was obtained in a shake flask culture. PHBHHx of 6.26 g/L was produced in a fermentation experiment over 48 h on a sole carbon source containing 100 g/L soybean oil, while 12.40 g/L PHBHHx was produced on a mixed carbon source containing 80 g/L soybean and 20 g/L lauric acid over the same period of time, resulting in a polyhydroxyalkanoate (PHA) productivity of 0.25 g/(L·h). The results show that mixed carbon sources are suitable for industrialized production of PHBHHx from A. hydrophila 4AK4, as the mixed carbon sources also overcome the foaming problem that occurs when lauric acid is employed as a sole carbon source in PHBHHx production.

Provenance of the Early Permian bauxitic claystone in Huayingshan region,Sichuan Basin,South China:Constraints from U-Pb ages and trace elements of detrital zircons

The Early Permian bauxite in the Sichuan Basin is an important part of karst-type bauxite deposits in South China Block and its provenance is poorly understood.Here we present results of mineralogy,wholerock geochemistry,detrital zircon U-Pb ages and trace element compositions of the bauxitic claystone from the Lower Permian Dazhuyuan Formation and those of the siltstone from the underlying Lower Silurian Hanjiadian Formation in Huayingshan region,Sichuan Basin,South China Block to constrain the provenance of the bauxitic claystone.Detrital zircons from bauxitic claystone and siltstone have similar age spectra,and the ages of bauxitic claystone are classified into five groups of 2600-2200 Ma,1800-1600 Ma,1200-900 Ma,900-650 Ma,and 650-400 Ma respectively.Those of the siltstone are classified into four groups of 2600-2200 Ma,1200-900 Ma,900-650 Ma,and 650-400 Ma respectively.We propose that the bauxitic claystone in the study region has mixed sources:the siltstone of the Lower Silurian Hanjiadian Formation probably constitutes the dominant source,and the Late Paleoproterozoic magmatic rocks along the western margin of the Yangtze Block are probably the secondary source.Moreover,the source materials of the Hanjiadian Formation siltstone are mainly sourced from Grenville-aged magmatic rocks in the Cathaysia Block and Neoproterozoic magmatic rocks in the Jiangnan orogen,with minor contributions from the underlying clastic sediments in the Cathaysia Block and Pan-African magmatic rocks in the orogen in eastern Gondwana.