Differences and identification on multi-time hydrocarbon generation of carboniferous-permian coaly source rocks in the Huanghua Depression,Bohai Bay Basin

Coal is a solid combustible mineral,and coal-bearing strata have important hydrocarbon generation potential and contribute to more than 12%of the global hydrocarbon resources.However,the deposition and hydrocarbon evolution process of ancient coal-bearing strata is characterized by multiple geological times,leading to obvious distinctions in their hydrocarbon generation potential,geological processes,and production,which affect the evaluation and exploration of hydrocarbon resources derived from coaly source rocks worldwide.This study aimed to identify the differences on oil-generated parent macerals and the production of oil generated from different coaly source rocks and through different oil generation processes.Integrating with the analysis of previous tectonic burial history and hydrocarbon generation history,high-temperature and high-pressure thermal simulation experiments,organic geochemistry,and organic petrology were performed on the Carboniferous-Permian(C-P)coaly source rocks in the Huanghua Depression,Bohai Bay Basin.The oil-generated parent macerals of coal's secondary oil generation process(SOGP)were mainly hydrogen-rich collotelinite,collodetrinite,sporinite,and cutinite,while the oil-generated parent macerals of tertiary oil generation process(TOGP)were the remaining small amount of hydrogen-rich collotelinite,sporinite,and cutinite,as well as dispersed soluble organic matter and unexhausted residual hydrocarbons.Compared with coal,the oil-generated parent macerals of coaly shale SOGP were mostly sporinite and cutinite.And part of hydrogen-poor vitrinite,lacking hydrocarbon-rich macerals,and macerals of the TOGP,in addition to some remaining cutinite and a small amount of crude oil and bitumen from SOGP contributed to the oil yield.The results indicated that the changes in oil yield had a good junction between SOGP and TOGP,both coal and coaly shale had higher SOGP aborted oil yield than TOGP starting yield,and coaly shale TOGP peak oil yield was lower than SOGP peak oil yield.There were significant differences in saturated hydrocarbon and aromatic parameters in coal and coaly shale.Coal SOGP was characterized by a lower Ts/Tm and C31-homohopane22S/(22S+22R)and a higher Pr/n C17compared to coal TOGP,while the aromatic parameter methyl dibenzothiophene ratio(MDR)exhibited coaly shale TOGP was higher than coaly shale SOGP than coaly TOGP than coaly SOGP,and coal trimethylnaphthalene ratio(TNR)was lower than coaly shale TNR.Thus,we established oil generation processes and discriminative plates.In this way,we distinguished the differences between oil generation parent maceral,oil generation time,and oil production of coaly source rocks,and therefore,we provided important support for the evaluation,prediction,and exploration of oil resources from global ancient coaly source rocks.

Decipher hydrocarbon generation and accumulation based on fluid inclusion and chronology:A case study from the Upper Paleozoic buried-hills in Huanghua Depression,Bohai Bay Basin

Deciphering hydrocarbon generation and accumulation stage is of significance to understand oil and gas evolution and seek exploration targets.Taking the Upper Paleozoic buried-hills in the Huanghua Depression,Bohai Bay Basin,as a case study,hydrocarbon generation environment and detailed accumulation process are revealed by fluid inclusions observations,Laser Raman spectroscopy,Fourier Infrared spectroscopy,and K-Ar isotope measurements.The results show that both oil and gas inclusion were captured in the quartz overgrowth,dissolved feldspar and calcite microfractures,showing blue to dark brown fluoresce.The grains containing oil inclusions index(GOI)of oil,oil&gas and gas being 25%,65%,and 10%and the inclusions with abundant methyl groups and short chains,both indicate high thermal maturity.One series of fluids inclusion is generally observed,evidenced by the concentrated homogenization temperature of 135-145℃ and salinity of 3%-15 w.t.%NaCl equiv,indicating one primary charging stage.The gas and gas&liquid inclusions mainly contain CH_(4),with also peaks indicating CO_(2) and N_(2.)The Carboniferous and Permian biomarkers show reducing environment with brackish water,with organic matter sources both from marine and continental.The relative content ofααα20RC_(27),ααα20RC_(28),andααα20RC_(29) exhibit source contributions both from algae and higher plants,and mainly of II2 to III kerogen.Both coal derived gas and oil associated hydrocarbons are identified from most of the buried-hills.Combining the fluid homogenization temperature and salinity,as well as the thermal evolution history,the hydrocarbon generated from the Upper Paleozoic was concentrated at the end of the Eocene(40 Ma±),while the beginning of charging is 60 Ma±.The Wumaying Buried-hill is of only coal derived gas and has potential for inner coal measure natural gas exploration.The results provide a detailed understanding of hydrocarbon accumulations in the study area,which can also be reference for improving petroleum exploration efficiency in similar basins.

Characteristics of the Cretaceous Magmatism in Huanghua Depression and Their Relationships with Hydrocarbon Enrichment

The Huanghua depression located in the hinterland of the Bohai Bay Basin in eastern China is a typical area for the research of multistage magmatic activities with hydrocarbon enrichment,many high-yield wells related to igneous rocks were discovered within the Cretaceous strata in recent years.However,the spatial and temporal distribution of Cretaceous igneous rocks remains unclear,and the relationships among magmatic sequence,lithology,and hydrocarbon enrichment have been poorly studied.In order to solve these problems,core observation,logging analysis,major element analysis,zircon U-Pb chronology,oil-bearing grade statistics and reservoir spaces analysis were used to subdivide the magmatism cycles and to investigate the characteristics of igneous reservoirs.Our results show that the Mesozoic magmatism in Huanghua depression started in 140.1±1.4 Ma and could be divided into two stages including the Early Cretaceous stage and Late Cretaceous stage.The Early Cretaceous magmatism principally developed basic-intermediate rocks in the north zone,and could be subdivided into 3 cycles with their forming ages of 140,125–119,and 118–111 Ma,respectively.By contrast,the late stage mainly developed intermediate-acid rocks centralized in the south zone and formed at 75–70 Ma.The GR and SP curves are good indicators for the contrast of different lithologies,lithofacies and for magmatic sequences division.Intensive magmatism may have an advantage to form reservoirs,since basalt in cycle III in the Early Cretaceous and dacite porphyrite in the Late Cretaceous have great exploration potential.Lithology and tectonic fractures have an important influence on the formation of reservoir spaces and hydrocarbon enrichment.The characteristics of Cretaceous magmatism and igneous reservoirs in Huanghua depression and adjacent areas are summarized,providing important information for igneous reservoirs research and oil-gas exploration in the Cretaceous in related areas.

Discriminating hydrocarbon generation potential of coaly source rocks and their contribution:a case study from the Upper Paleozoic of Bohai Bay Basin,China

Although various coaly source rocks widely developed in the Carboniferous-Permian(C-P)of the Bohai Bay Basin,their geochemical characteristics and hydrocarbon generation potential are poorly understood.This study aims to discriminate the contribution of hydrocarbon generation from different C-P coaly source rocks and clarify the differences within generated oils using organic geochemistry,organic petrology,and thermal simulation experiments.The coaly source rocks containt coal clarain and durain,carbonaceous shale,and shale deposited in deltaic and lagoonal environment.The results indicated that clarain,durain,and carbonaceous shale exhibited higher hydrogen index and liquid-gas hydrocarbon yields than lagoonal and deltaic shales,which was mainly associated with the concentrations of sporinite,cutinite,and hydrogen-rich collodetrinite.Aliphatic hydro-carbons originated firom coal and carbonaceous shale presented lower Ts/(Ts+Tm),Ga/17α(H)21β(H)-C_(30)hopane,18α(H)-oleanane/17α(H)21β(H)-C_(30)hopane ratios,and higher 17β3(H)21α(H)-C_(30)Morane/17α(H)21β(H)-C_(30)hopane than deltaic lagoonal shales.Parameters of aromatic hydrocarbons generated from five lithologies of coaly source rocks trended as clear group distribution,e.g.,clarain and durain showing lower MNR,DBT/Fluorene(F)ratios and higher DBF/F ratio than coaly shales.The distinct descending trend of hydrocarbon potential is obtained from clarain,durain,carbonaceous shale to lagoonal and deltaic shales,implying dominated the petroleum and natural gas supplement from coal and carbonaceous shale.The difference between aliphatic and aromatic hydrocarbons provides a significant contribution to analyze the generic relationship between coaly source rock and lacustrine shale.Our results illustrate the importance of coaly source rocks for the in-depth oil-gas exploration of the Bohai Bay Basin and understanding hydrocarbon generation potential of source rocks in coal bearing strata.