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.

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.

Paleogene Tectonic Evolution Controls on Sequence Stratigraphic Patterns in the Central Part of Deepwater Area of Qiongdongnan Basin, Northern South China Sea

In active rift basins, tectonism is extremely important for sequence stratigraphic patterns, affecting both the sequence architecture and internal makeup. Sequence stratigraphic framework of a Paleogene rift succession in Qiongdongnan Basin, northern South China Sea, was built using seismic profiles, complemented by well logs and cores. One first-order and three second-order sequences were identified on the basis of basin-scale unconformities, and seven third-order sequences are defined by unconformities along the basin margins and correlative conformities within the central basin. Through unconformity analysis and backstripping procedure, the Paleogene synrift tectonic evolution of deepwater area of Qiongdongnan Basin was proved to be episodic, which can be divided into rifting stage-I, rifting stage-II and rifting stage-III. Episodic rifting resulted in the formation of various types of structural slope break belts, which controlled different architectures and internal makeup of sequences. This study enhances the understanding of the control of tectonic evolution on sequence stratigraphic patterns and establishes relevant patterns in a typical rift basin, and further proposes the favorable sandstone reservoirs developing in different sequence stratigraphic patterns, which will be pretty helpful for subtle pool exploration in deepwater area of petroliferous basins.

Sedimentary Facies, Sequence Stratigraphic Patterns in Pre-Cenozoic Inland Compressional Basin: Example from Early Yanshanian Succession of Eastern Yihezhuang Salient, Jiyang Depression, Bohai Bay Basin, China

To improve the success rate of locating hydrocarbon reservoirs in pre-Cenozoic inland compressional basins, taking the Early Yanshanian succession of eastern Yihezhuang salient as an example, this paper studied the sedimentary facies and sequence stratigraphic patterns. First, through seismic profiles, well logs, cores and outcrops, the sequence framework was established and internal sedimentary facies were identified. Further, according to analysis of single-wells and connecting-wells, the vertical evolution and horizontal distribution of sedimentation inside the sequence frameworks were discussed. The following results were acquired:(1) meandering river characterized by dual structures superposing each other was developed, and the dual structures can be further divided into three kinds;(2) the entire Early Yanshanian succession was interpreted as one first-order sequence, composed of three third-order sequences, including SQ-Fz1, SQ-Fz2 and SQ-St from bottom to top. Each third-order sequence can be further divided into three system tracts;(3) in different system tracts, different types of dual structures developed separately, and sedimentary bodies showed different horizontal distribution scales and vertical superposition patterns. Finally, the model of sequence stratigraphic patterns was established. This study enhanced the use of sequence stratigraphy to inland tectonically active basins, and would be helpful to predict reservoirs in pre-Cenozoic residual basins.

Sequence Stratigraphy and Sedimentary Facies of Lower Oligocene Yacheng Formation in Deepwater Area of Qiongdongnan Basin,Northern South China Sea:Implications for Coal-Bearing Source Rocks

For unveiling coal-bearing source rocks in terrestrial-marine transitional sequences, the sequence stratigraphic framework and sedimentary facies of Lower Oligocene Yacheng Formation of Qiongdongnan Basin were investigated using seismic profiles, complemented by well bores and cores. Three third-order sequences are identified on the basis of unconformities on basin margins and correlative conformities in the basin center, namely SQYC3, SQYC2 and SQYC1 from bottom to top. Coal measure in Yacheng Formation of Qiongdongnan Basin were deposited within a range of facies associations from delta plain/tidal zone to neritic sea, and three types of favourable sedimentary facies associations for coal measure were established within the sequence stratigraphic framework, including braided delta plain and alluvial fan, lagoon and tidal flat, and fan delta and coastal plain facies associations. Results shown that, in the third-order sequences, coal accumulation in landward areas(such as delta plain) of the study area predominantly correlates with the early transgressive systems tract(TST) to middle highstand systems tract(HST), while in seaward areas(such as tidal flat-lagoon) it correlates with the early TST and middle HST. The most potential coal-bearing source rocks formed where the accommodation creation rate(Ra) and the peat-accumulation rate(Rp) could reach a state of balance, which varied among different sedimentary settings. Furthermore, intense tectonic subsidence and frequent alternative marine-continental changes of Yacheng Formation during the middle rift stage were the main reasons why the coal beds shown the characteristics of multi-beds, thin single-bed, and rapidly lateral changes. The proposed sedimentary facies associations may aid in predicting distribution of coal-bearing source rocks. This study also demonstrates that controlling factors analysis using sequence stratigraphy and sedimentology may serve as an effective approach for coal-bearing characteristics in the lower exploration deepwater area of South China Sea.

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.