Genetic relationship between formation,accumulation and migration and dispersion of peat materials in Paleogene——Take the Qiongdongnan Basin as an example

Coal-type source rocks include both coal and terrigenous marine source rocks.By studying the distribution of secondary depressions,uplifts,as well as the characteristics of peat formation and accumulation in the northern marginal sea basin of the South China Sea,and combining them with coal formation characteristics observed in other basins,five genetic theories on the relationship between peat accumulation and dispersed organic matter accumulation are proposed.The northern marginal sea basin of the South China Sea is characterized by“disadvantageous coals formation and favorable terrigenous marine source rocks formation.”This paper provides a distribution map of coal seams and terrigenous marine source rocks in the Qiongdongnan Basin and determines their distribution patterns.Research shows that the migration of sedimentary facies in the basins and inner depressions led to the formation and migration of the peat accumulation centers.In addition,the vertical migration of the peat accumulation centers led to planar migration,which is actually a type of coupling relationship.Previous research results have revealed that the formation of coal-type source rock is multi-phased.The marginal sea basin is composed of several fault-depression basins,with each basin developing a second order of depression and uplift.There is no unified basin center or depositional center to be found.As a result,the concentration centers of coal-forming materials also vary greatly.Based on the distribution characteristics of coal-type source rocks in different basins within the marginal sea basins of the South China Sea,the research results have practical significance and provide guidance for exploring coal-type oil and gas reservoirs in this area.

Formation environment and hydrocarbon potential of the Paleogene Enping Formation coal measures in the ZhuⅠDepression of northern South China Sea

The coal-measure source rock in the Chinese sea area plays a significant role as a hydrocarbon source rock,with its genetic environment,development and distribution,and hydrocarbon generation potential serving as essential factors for the exploration of coal-type oil and gas fields.This study focuses on the coal-measure source rock of the Paleogene Enping Formation in the ZhuⅠDepression,located in the northern South China Sea.The main geological insights obtained are as follows.The coal measures of the Enping Formation are developed in a warm and wet tropical-subtropical climate.The development environment of the coal-measure source rock in the Enping Formation includes the braided river delta upper plain peat swamp,characterized by dry forest swamp coal facies with relatively thick coal seams and a small number of layers.The braided river delta lower plain swamp-interdistributary bay of braided river delta front represents a forest edge-wetland herbaceous swamp coal facies with numerous layers of thin coal seams and poor stability.The shore swamp corresponds to an open water swamp coal facies with multiple layers of thin coal seams and poor stability.The organic matter abundance in the braided river delta upper plain is the highest,followed by the braided river delta lower plain-braided river delta front,and the shore-shallow lake.The organic matter type is predominantly typeⅡ1.Thermal evolution analysis suggests that the organic matter has progressed into a substantial oil generation stage.The hydrocarbon generation potential of the coal-measure source rock in the Enping Formation is the highest in the braided river delta upper plain,followed by the braided river delta lower plain-braided river delta front and the shore-shallow lake.Overall,this study proposes three organic facies in the coal-measure source rock of the Enping Formation:upper-plain swamp-dry forest swamp facies,lower plain-interdistributary bay-forest-herbaceous swamp facies,and lake swamp-herbaceous swamp facies.

Astronomical influence of the development of Paleogene thin coal seam groups in offshore Lacustrine basins:A case study of the ZhuⅠDepression's Enping Formation located in the northern South China Sea

The development of the Paleogene coal seams in China's offshore basin areas generally had the characteristics of coal measures with large thicknesses,large numbers of coal seams,thin single coal seams,poor stability,scattered vertical distribution,and a wide distribution range.This study selected the Enping Formation of the ZhuⅠDepression in the northern section of the South China Sea as an example to determine the macro-control factors of the development of the Paleogene coal seam groups.An analysis was carried out on the influencing effects and patterns of the astronomical cycles related to the development of the thin coal seam groups in the region.A floating astronomical time scale of the Enping Formation was established,and the sedimentary time limit of the Enping Formation was determined to be approximately 6.15 Ma±.In addition,the cyclostratigraphy analysis results of the natural gamma-ray data of Well XJ in the Enping Formation of the Xijiang Sag revealed that the development of the thin coal seams had probably been affected by short eccentricity and precession factors.The formation process of coal seams was determined to have been affected by high seasonal contrast,precipitation,and insolation.During the periods with high values of short eccentricity,the seasonal contrasts tended to be high.During those periods,fluctuations in the precession controls resulted in periodic volume changes in precipitation and insolation of the region,resulting in the development of thin coal seams.It was also found that the periods with low precession were the most conducive to coal seam development.On that basis,combined with such factors as sedimentary environmental conditions conducive to the development of thin coal seam groups,this study established a theoretical model of the comprehensive influences of short eccentricity and precession on the development and distribution of Paleogene thin coal seam groups in offshore lacustrine basins.The patterns of the Paleogene astronomical periods and paleoclimate evolution,along with the control factors which impacted the development of thin coal seam groups in offshore lacustrine basins,were revealed.

Control effects of the synsedimentary faults on the basin-marginal fans in the central part of the deep-water area of early Oligocene Qiongdongnan Basin, South China Sea

The synsedimentary faults and basin-marginal fans located in the central part of the deep-water area of the early Oligocene Qiongdongnan Basin have been investigated using seismic profiles,boreholes,and well-log data.Through the formations of the characterized paleogeomorphology,such as transverse anticlines,fault ditches,and step-fault belts,the synsedimentary faults are known to have controlled the development position,distribution direction,and extension scales of the basin-marginal fans.For example,at the pitching ends of two adjacent faults,transverse anticlines developed,which controlled the development positions and distributions of the fans.During the early Oligocene,the faults controlled the subsidence center,and fault ditches were formed at the roots of the faults.In the surrounding salient or low salient areas,which were exposed as provenance areas during early Oligocene,the fault ditches acted as the source channels and determined the flow paths of the clastics,where incised valley fills were obviously developed.The fault ditches which developed in the sedimentary basins were able to capture the drainage systems and influenced the distributions of the fans.The large boundary faults and the secondary faults generated two fault terraces and formed step-fault belts.The first fault terrace caused the clastics to be unloaded.As a result,fans were formed at the entrance to the basin.Then,the second fault terrace caused the fans to move forward,with the fans developing in a larger extension scale.The results obtained in this study will potentially be beneficial in the future prospecting activities for reservoirs and coalmeasure source rocks in the basins located in the deep-water areas of the South China Sea.

Cenozoic giant coal-bearing basin belt discovered in China's sea and its adjacent areas

Sea area is an important area of oil and gas exploration in China.It has been found that China's sea area mainly consists of coal type oil and gas,and the exploration of coal-bearing series source rocks has become an important part of oil and gas exploration there.Through years of comprehensive geological research in China's sea area,it has been revealed that it has undergone multiple occurrences of tectonic opening and closing movements in varying degrees in the Paleogene,forming 26 Cenozoic sedimentary basins of various types,such as active continental margin,passive continental margin,transitional continental margin and drift rift basins.In the present study,it is observed for the first time that coal type source rocks are mainly developed in 14 continental margin basins in China's sea area,revealing that a very large C-shaped coal-bearing basin group developed there in the Cenozoic.Next,based on the coupling analysis of paleoclimate,paleobotany,paleogeography and paleostructure,it is observed that there are five coal-forming periods in China's sea area,namely the Paleocene,Eocene,early Oligocene,late Oligocene and Miocene-Pliocene,and the coal-forming age is gradually new from north to south.It is also found that the coal seams in the sea area are mainly developed in three coal-forming environments in Cenozoic,namely delta,fan delta and tidal flat-lagoon.The coal seams developed in different environments are characterized by thin thickness,many layers and poor stability.However,the coal-bearing series source rocks in China's sea area have a wide distribution range,very high thickness and large amount,thus forming a material basis for the formation of rich coal type oil and gas.

Peat formation and accumulation mechanism in northern marginal basin of South China Sea

In the present study,the coal-rock organic facies of Oligocene Yacheng Formation of the marginal basin in the South China Sea were classified and divided.In addition,through the correlations of the large-scale coal-bearing basins between the epicontinental sea and the South China Sea,it was concluded that the coal forming activities in the South China Sea presented particularity and complexity.Furthermore,the coal forming mechanisms also presented distinctiveness.The marginal basins in the South China Sea consist of several large and complex rift or depression basins,which are distributed at different tectonic positions in the South China Sea.Therefore,the marginal basins in the South China Sea are not simple traditional units with onshore continental slopes extending toward the deep sea.The marginal basins are known to consist of multi-level structures and distinctive types of basins which differ from the continental regions to the sea.During the Oligocene,the existing luxuriant plants and beneficial conditions assisted in the development of peat.Therefore,the Oligocene was the significant period for the formation and aggregation of the peat.However,the peat did not form in unified sedimentary dynamic fields,but instead displayed multi-level geographical units,multiple provenance areas,instability,and nonevent characteristics.As a result,the marginal basins in the South China Sea are characterized by non-uniform peat aggregation stages.In another words,the majority of the peat had entered the marine system in a dispersive manner and acted as part of the marine deposits,rather than during one or several suitable coal-forming stages.These peat deposits then became the main material source for hydrocarbon generation in all of the marginal basins of the South China Sea.The study will be of much significance for the hydrocarbon exploration in the marginal basins of the South China Sea.

Depositional Environments of the Upper Permian Quartzose Sandstone(Shandong Province, North China): Insight from Trace Element Geochemistry

The depositional environment of the Upper Permian quartzose sandstone （Kuishan sandstone in Shihezi Formation of Upper Permian） in the North China epicontinental basin is controversial. In order to test the previous hypotheses, we analyzed sedimentological characteristics of the Kuishan sandstones in outcrops and boreholes, and carried out trace element geochemical analysis by electron probe microanalyzer. Three lithofacies were recognized, including normal-graded conglomerate （Cng）, trough and planar cross-bedded coarse sandstone （CStpc）, and planar cross-bedded medium sandstone （MSpc）. Normal-graded conglomerate （Cng） formed in the meandering river or deltaic distributary channels. Trough and planar cross-bedded coarse sandstone （CStpc） formed in meandering river or distributary channels of near-source deltaic plain. Planar cross-bedded medium sandstone （MSpc） formed in the siliciclastic beach with highto moderate-energy conditions. By the petrology and trace elements analysis, three relatively large-scale transgressions were revealed. Each transgression was reflected by the lower content of Ba and ratios of Fe/Mn, and the high content of B and ratios of B/Ga. The ratios of Ni/Co of all samples are all lower than 2, suggesting oxygen-enriched shallower water en- vironment during deposition of the Kuishan sandstones.

The classification and significance of fine-grained deposits of micro-laminae rich in unconventional oil and gas resources

In this study,an in-depth analysis of the types,characteristics,and formation mechanisms of microlaminae and microscopic laminae was conducted in order to precisely examine the link or intersection of stratigraphy and petrology.This study was essentially a sedimentary examination of the minuteness-macro and micro-tiny layers between laminae and pore structure,as well as the types of structures and sedimentation.The results of this study bear important basic subject attributes and significance,as well as practical value for the basic theories and exploration applications of unconventional oil and gas geology.The quantitative data were obtained using the following:field macroscopic observations;measurements;intensive sampling processes;XRD mineral content analysis;scanning electron microscopy;high-power polarizing microscope observations;and micro-scale measurements.The quantitative parameters,such as laminae thicknesses,laminae properties,organic matter laminae,and laminae spatial distributions were unified within a framework,and the correlations among them were established for the purpose of forming a fine-grained deposition micro-laminae evaluation system.The results obtained in this research investigation established a basis for the classification of micro-laminae,and divided the micro-laminae into four categories and 20 sub-categories according to the development thicknesses,material compositions,organic matter content levels,and the spatial distributions of the micro-laminae.The classification scheme of the micro-laminae was divided into two categories and 12 sub-categories.Then,in accordance with the comprehensive characteristics of spatial morphology,the micro-laminae was further divided into the following categories:continuous horizontal laminae;near horizontal laminae;slow wavy laminae;wavy laminae;discontinuous laminae;and lenticular laminae.According to the structural properties of the laminae development,the micro-laminae was divided into the following categories:single laminae structures;laminated laminae structures;interlaminar structures;multiple mixed laminae structures;cyclic laminae structures;and progressive laminae structures.The research results were considered to be applicable for the scientific evaluations of reservoir spaces related to unconventional oil and gas resources.