Strengthening exploration and development of oil and gas is crucial for mitigating China's reliance on oil and gas supply from foreign countries and ensuring national energy security. Since the 11th Five-Year period, fundamental research on the deep-water area of the northern South China Sea has been strengthened, along with the acceleration of technological innovation and increase in exploration investment. As a result, a series of major exploration discoveries were found. This paper presents five major learnings regarding the theories of exploration geology and two achievements in the innovation of exploration technology. It also discusses the new challenges and coping strategies for oil and gas exploration in deep-water areas, and prospects the exploration potentials of three major exploration fields—medium-deep layers, buried hills, and lithologic traps—in the deep-water area of the northern South China Sea. Our research shows that the detachment of the continental margin in the deep-water area of northern South China Sea controls the formation of large sags in the Pearl River Estuary Basin and the Qiongdongnan Basin. Three sets of large-scale source rocks were developed from lacustrine, terrestrial-marine transitional, and marine sedimentary facies. High yet variable subsurface temperature controls the rapid hydrocarbon generation from the source rocks in the sags. In addition, three different hydrocarbon accumulation modes were established: accumulation mode of large axial canyon channel in the deep-water area of the Qiongdongnan Basin, late natural gas accumulation mode of deep-water fan in the deep-water area of Baiyun Sag, and differential hydrocarbon accumulation mode jointly controlled by fault and ridge. Meanwhile, the broadband seismic acquisition and processing technology for three-dimensional source triggering and plow-like cable receiving was independently developed. Guided by several geological theories and innovative technologies, a series of large- and medium-sized gas fields represented by "Deep Sea No. 1" (LS 17-2) were discovered, which are of great significance to ensuring the energy supply of the Guangdong-Hong Kong-Macao Greater Bay Area, facilitating the green development of energy in the Hainan Free Trade Zone (Port), and promoting the increase of oil and gas reserves and production in China.
This study carried out the development and distribution prediction research on coal-formed gas accumulation belts from the perspectives of the development and distribution of the coal measure source rock and the thermal evolution degrees of those rock masses in marine-continental transitional facies fault basins. The purpose of the current study was to improve the accuracy of gas reservoir explorations in the sea areas. The methods used in this study were based on the systematic analyses of the drilling, logging, geochemistry, and seismic data of the Oligocene deposits in the Qiongdongnan Basin of the northern region of the South China Sea. It had been previously established that the sedimentary processes of the fault basin were mainly controlled by faulting, and (fan) deltas had often developed in the fault bending sections, tectonic transformation sections, and fault ends. The distribution along the fault zone was observed to be beaded. During the early Oligocene period of the Qiongdongnan Basin, there were strong extensional actions, and the marine and continental transitional facies strata had developed forming the northern fault depression and the central fault depression. Three (fan) delta belts were formed in the slope belt in the northern fault depression and also in the slope belts located in the north and south of the central fault depression. Due to the superposition of the humid and hot climate during that period, the majority of the coal measure source rocks were developed in each (fan) delta. Also, three macroscopic coal measure source rock belts were formed. The types of source rock had mainly included autochthonous coal, allochthonous coal, and terrigenous marine mudstone, with terrigenous higher plant organic matter accounting for more than 50%. After the deposition of the lower Oligocene period, the basin continued to sink, and the coal measure source rock masses continued to be heated. During that period, the maturity of the organic matter had continued to rise. In the northern slope belt of the northern fault depression, the maturity of the organic matter in the coal measure source rock belt were low and the gas generation abilities were weak, which was advantageous to the formation of potential gas accumulation belts. The coal measure source rock belt on the northern side of the central fault depression had a high maturity of organic matter and strong gas generation ability, forming a macroscopic gas accumulation belt. In addition, the Yacheng 13–1 large coal-formed gas field had been discovered. The maturity of the organic matter had varied greatly in the coal measure source rock belt on the southern side of the central fault depression. The coal measure source rock masses with high maturity had strong gas generation abilities and had also formed a macroscopic gas accumulation belt, and three large and medium-sized coal-formed gas fields (such as Lingshui 17–2) had been discovered. The gas accumulation belts on the southern and northern sides of the central fault depression were favorable areas for coal-formed gas explorations. Furthermore, the trap zones in and near the coal measure source rock belts with mature thermal evolution were determined to be the most favorable areas for coal-formed gas explorations.
As the main target of deepwater oil and gas exploration, submarine fans are also the frontier and focus of sedimentology research. Based on the latest three-dimensional (3D) seismic data covering the study area, as well as heavy mineral, rare earth elements, and other data and guided by the theory of seismic sedimentology, this study analyzes the lower Miocene provenance system and sediment dispersion system in the eastern deepwater area of the Qiongdongnan Basin, northern South China Sea, from the perspective of source-to-sink system research. Our research defines the sediment supply function of provenance, paleogeomorphology and valley transport system, sedimentation results and distribution, and the coupling relationship between the source–channel–sink system and comprehensively constructs a set of application guidelines of source-to-sink system research to guide the exploration and prediction of favorable play in the study area and other similar basins. This study shows that in the early Miocene, as controlled by tectonics and paleogeomorphology, uplift areas developed on both the northern and southern sides of the Songnan, Baodao, and Changchang Sags in the Qiongdongnan Basin and small provenances of near-sag uplifts were found in the southern part of the sag. The provenance area in the northern part of the sag was large but not unified, and it formed different provenance systems consisting of the Hainan Uplift in the northern Songnan Sag, Hainan Uplift in the northern Baodao Sag, and Shenhu Uplift in the northern Changchang Sag. A series of canyon systems that developed in the early Miocene were the main channels for sediment transport from the shelf to the slope in the northern Songnan, Baodao, and Changchang Sags. In the early stage of the early Miocene, due to regional sea level decline, the scale of the canyons was generally large, and the scale of the canyons on the northern edge of the Songnan Sag was significantly larger than that of the Baodao Sag. As the relative sea level rose, the canyons became small, narrow, and shallow in the late stage of the early Miocene. Under the dual control of the “source–channel” system, multiple “delta–canyon–submarine fan” depositional systems developed in the Songnan, Baodao, and Changchang Sags in the early Miocene, and large deltas in the depositional systems were mainly located along the northern part of the sag and prograded to the shelf edge of the northern Songnan, Baodao, and Changchang Sags from north to south, providing a good material basis for the development of deepwater sediments.
Abstract This paper investigates the origin and migration characteristics of petroleum in the northeastern part of the Baiyun Depression, Pearl River Mouth Basin (PRMB). The discovered petroleum in the study area is mainly located in the Lower Zhujiang Member (N 1 z 2 ) and mainly originated from the Enping Formation source rocks in the eastern sag. Active faults (vertical migration) and N 1 z 2 sandstones (lateral migration) acted as the petroleum migration systems. The fault activities in the Dongsha event controlled the episodic petroleum migration. Fractures in the fault zones provided effective conduits, and overpressure was the driving force. The vertical migration could not cross the fault zones laterally. The petroleum injection areas in the carrier beds were the contact zones of petroleum‐migration faults and carrier beds. The lateral migration was steady‐state migration, and buoyancy was the driving force. The migration pathways in the carrier beds were controlled by the structural morphology. Secondary petroleum migration in the study area could be divided into two parts: vertical migration along the fractures in the fault zones and lateral migration through preferential petroleum migration pathways (PPMPs) in the carrier beds. The petroleum migration behaviors, including migrating direction, driving force, and migration pattern, in the faults and sandstone carrier beds were quite different. This study provides a typical example for comprehending secondary migration processes and has great importance for determining future exploration targets in the deep‐water area of the PRMB.
Abstract Deepwater oil and gas exploration has become a global hotspot in recent years and the study of the deep waters of marginal seas is an important frontier research area. The South China Sea (SCS) is a typical marginal sea that includes Paleo SCS and New SCS tectonic cycles. The latter includes continental marginal rifting, intercontinental oceanic expansion and oceanic shrinking, which controlled the evolution of basins, and the generation, migration and accumulation of hydrocarbons in the deepwater basins on the continental margin of the northern SCS. In the Paleogene, the basins rifted along the margin of the continent and were filled mainly with sediments in marine‐continental transitional environments. In the Neogene–Quaternary, due to thermal subsidence, neritic‐abyssal facies sediments from the passive continental margin of the SCS mainly filled the basins. The source rocks include mainly Oligocene coal‐bearing deltaic and marine mudstones, which were heated by multiple events with high geothermal temperature and terrestrial heat flow, resulting in the generation of gas and oil. The faults, diapirs and sandstones controlled the migration of hydrocarbons that accumulated principally in a large canyon channel, a continental deepwater fan, and a shelf‐margin delta.
The Qiongdongnan Basin, located in the sea between Hainan Island and the Xisha Islands, is a faulted Cenozoic basin on the northern continental margin of the South China Sea. The Changchang Sag, situated in the eastern part of the central depressional zone in the deepwater area of the Qiongdongnan Basin, exhibits a near EW-striking morphology and represents an important potential target for oil/gas exploration. However, the age of the interface of the Lingshui Formation remains controversial, which hinders a comprehensive understanding of the tectonic evolution and hydrocarbon accumulation pattern in the Changchang Sag. This study focuses on well A, located in the depositional center of the Changchang Sag, and employs cyclostratigraphic analysis to identify cyclic signals of the Milankovitch cycles recorded in the sedimentary strata. Spectral analysis of natural gamma logging data from this well reveals the presence of 405 kyr long eccentricity cycles, 100 kyr short eccentricity cycles, 39.3 kyr obliquity cycles, and 20.58 kyr age precession cycles. By employing astronomical tuning, a “floating” astronomical time scale of the Lingshui Formation spanning 5.483 million years (Myr) is established. The top interface of the Oligocene in the International Geological Time Scale 2020 (GTS2020), with a geological age of 23.03 Ma, is used as the time anchor to establish a high-precision absolute astronomical age framework for the Lingshui Formation. The results indicate that the bottom interface of the first member of the Lingshui Formation is dated at 23.79 Ma, the bottom interface of the second member is dated at 25.08 Ma, and the bottom interface of the third member is dated at 28.51 Ma. Additionally, the average sedimentation rate during this period is estimated to be 9.261 cm/kyr. Furthermore, paleoclimate and paleoenvironmental reconstructions were carried out through quantitative analysis of spore and pollen assemblages, as well as foraminifera within the Lingshui Formation. These analyses suggest that the deposition of the Lingshui Formation occurred under warm and humid temperate climatic conditions. The results of paleoclimate proxy analysis and comparative fitting analysis of the astronomical time scale confirm that the climate evolution during this period was influenced by astronomical orbital forces, such as eccentricity and precession.
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