Since the 12^th Five-Year Plan, China National Offshore Oil Corporation (hereinafter referred to as CNOOC) has achieved fruitful oil and gas exploration results, such as new records of reserve discoveries in domestic oil and gas exploration, continual successes of exploration expansion in mature regions, and constant exploitation of new exploration areas and fields. The innovation of petroleum geology theories and improvement of exploration technology was the important guarantee for CNOOC’s exploration breakthroughs during the 12th Five-Year Plan. A series of major theories are innovated, including differential hydrocarbon enrichment in active fault belts in Bohai Sea, hydrocarbon accumulation in deepwater at the continental margin of northern South China Sea, and the HTHP (high temperature and high pressure) gas accumulation in western South China Sea. Furthermore, a series of technical innovations are realized in terms of geophysical data acquisition and processing for complex seismogeological conditions of China’s sea area. These theoretical and technical innovations lead to a series of major hydrocarbon exploration breakthroughs of CNOOC in Bohai Sea area, deepwater in northern South China Sea and HTHP strata. During the 13^th Five-Year Plan, CNOOC will continue to deepen its geological understanding and to develop key technologies, so as to make new contributions to the Company’s sustainable development and to ensure national energy security.

During the 12^th Five-Year Plan, CNOOC International Ltd. (“CNOOC”) achieved good results of overseas exploration through optimization of exploration targets sorting and combination after quantification of risks and potentials, with the guide of “strategy-driven and benefit-priority” philosophy. During the five years, CNOOC had stably-growing equity recoverable reserves overseas, with about 600×10⁶ bbl oil equivalent (boe) contributed by five large-scale-reserve zones in Sahara of North Africa, both sides of South Atlantic, the East African rift, Southeast Asia, and lithologic zones in West Africa. During the 13^th Five-Year Plan, when the international oil prices will remain at low level, CNOOC will continue to deepen the management system construction for overseas exploration projects. Strategically focusing on the countries with blocks discovered, CNOOC will make more efforts to obtain new projects and enhance regional exploration, so as to make more breakthroughs.

Through years of efforts, significant progresses have been made in gas exploration in the Yinggehai-Qiongdongnan (YQ) Basins. However, further breakthroughs in the medium-deep formations with high temperature and high pressure (HTHP) are constrained by unfavorable factors, such as poor quality data seismic, complicated geological conditions and unclear gas accumulation mechanism under HTHP conditions. In this paper, through detailed case study, the HTHP gas accumulation mechanism, main controlling factors and accumulation modes were comprehensively analyzed supported by the systematic theory of basin-forming and gas accumulation mechanisms and relevant exploration techniques under HTHP conditions in the YQ Basins. The results reveal huge gas resource potential in HTHP formations in the YQ Basins, providing abundant source basis for achieving great breakthroughs in gas exploration. Moreover, it is innovatively cognized that predominant provenances exist in structural transfer zones and unbalanced subsidence controls gravity flow distribution within strike-slip extensional basin. The theory of HTHP gas accumulation was upgraded and the HTHP gas accumulation mode of strike-slip extension controlling source and sands, hydrocarbons conducting through overpressure fissures, and multi-stage superimposition was established. The study results provide theoretical evidences for defining exploration targets in medium-large HTHP gas fields within the YQ Basins and carrying out oil and gas exploration in other HTHP basins. In recent years, some medium-large gas fields (e.g. X13-1, X13-2, L13-2, and L25-1) have been discovered and assessed successively, creating a new ground for gas exploration in HTHP formations within the YQ Basins.

In recent years, two 100 million-ton class subtle reservoirs were found in the Shijiutuo uplift within the Bohai Sea. Exploration practices have proved that new geological understandings provide theoretical guidance and scientific deployment for hydrocarbon exploration discoveries. The mechanism of ternary-factor (ridge-trap-sand) coupling for controlling reservoir in low-amplitude structure guided the significant discovery of the Neogene structural-lithologic reservoir in extra-shallow water delta in the Shijiutuo uplift. The differential hydrocarbon accumulation mode of subtle reservoirs in basin-margin fault transfer zone in steep slope belt facilitated the discovery of the first 100 million-ton class the Palaeogene stratigraphic-lithologic reservoir in the medium-deep layers in the Bohai oilfield. These new geological understandings further promoted technical researches and innovations. As a result, six key technologies have been developed, including the variable-velocity mapping for shallow low-amplitude structure, the description of lithologic traps in sand-rich extra-shallow water delta, the semi-quantitative fault-sand coupling analysis, the exquisite palaeogeomorphic restoration under the tectonic-sedimentary simulation in medium-deep layers, the identification of stratigraphic-lithologic traps in medium-deep layers, and the testing of complex lithologies in medium-deep layers. These technologies provide technical support and assurance for discovering large subtle reservoirs in the Bohai Sea, playing a prospective role for future promotion and application.

Oil enrichment of China’s offshore oil-bearing basins is jointly controlled by hydrocarbon source rocks and geothermal heat. Major hydrocarbon source rocks in offshore oil-bearing basins include mudstone of semi-deep to deep lacustrine facies distributed in the Paleocene, the Eocene and the Oligocene, especially in the Eocene formations. These formations predominantly contain sapropelic organic matters of various degrees in enrichment. Areas of semi-deep to deep lacustrine mudstone formations may vary from very small to a few thousands of square kilometers. Predominantly controlled by crustal structures, geothermal fields in these oil-bearing basins may vary from high to low. As far as interactions between hydrocarbon source rocks and geothermal heat in depressions are concerned, there are four basic couplings: sufficient source rocks and geothermal heat, sufficient source rocks but insufficient geothermal heat, insufficient source rocks but sufficient geothermal heat and insufficient source rocks and geothermal heat. Due to significant differences in types of hydrocarbon source rocks, abundance of organic matters, scale and geothermal heat in different depressions of various oil-bearing basins, the coupling types of hydrocarbon source rocks and geothermal heat were different, and oil-bearing basins with dramatically different oil-production capacities were formed. The majority of depressions in the Bohai Basin have sufficient source rocks and geothermal heat. With a large quantity of extremely rich oil-bearing depressions developed, the Basin has huge potentials in oil production. There are significant differences in properties of depressions in northern depression belt of the Pearl River Mouth Basin in northern parts of South China Sea and those depressions in the Beibu-gulf Basin. In addition, properties within the same depression may also dramatically vary. With sub-sags as basic components, all four coupling types can be found in these depressions. The majority of depressions have sufficient source rocks and geothermal heat. Accordingly, some depressions have rich oil reserves and great potentials for exploration. There are a large quantity of depressions in the South Yellow Sea Basin, but the majority of these depressions have sufficient source rocks but insufficient geothermal heat. Under such circumstances, only the deep trenches have significant oil-production capacities. Overall exploration potential of this basin is of general level. With insufficient source rocks and geothermal heat, the Paleogene depressions in North Yellow Sea Basin are poor in oil resources. Generally speaking, the Paleogene depressions in the Basin contain minor potentials for exploration. With a large share of residual oil reserves, the Bohai Basin shall be considered as the key for future exploration operations in China’s offshore areas. Offshore basins in northern parts of continental edge in northern sections of South China Sea also contain significant reserves. The South Yellow Sea Basin contains only a small proportion of residual oil reserves. Most depressions in the Bohai Basin and some depressions in near-shore zones of northern parts of the South China Sea contain rich oil reserves. Among them, the Tanluxi Depression Belt in the Bohai Basin can be classified as super-rich oil-bearing zone.

The Qiongdongnan Basin, situated in the west part of northern South China Sea, is one of the important deepwater exploration prospects in China. This basin has tectonic-sedimentary characteristics and petroleum geology conditions different from other typical deepwater basins around the world. Initially, the cooperative exploration in this basin was carried out in a slow progress. Through continuous efforts and independent innovations, CNOOC Limited (“CNOOC”) has made great breakthroughs during the 12th Five-Year Plan by addressing the bottlenecks in relation to basin formation, hydrocarbon generation and accumulation, etc. During this period, CNOOC discovered the giant Central Canyon gas field consisting of several large-medium gas fields. Natural gas is mainly enriched in turbidite channel sandstones of the Huangliu Formation in the lower section of the canyon, and also in submarine fans of the Yinggehai Formation in the upper section. According to this study, hydrocarbon accumulation of this giant gas field attributes to following conditions: (1) the marine-terrestrial and marine source rocks of the Yacheng Formation serve as the major gas source rocks; (2) the turbidite channel sand and large-scale submarine fan reservoirs are shallowly buried, with favorable physical properties; (3) turbidite sandstones developed in multi stages constitute a large group of lithologic traps; (4) microfaults and fissures vertically connect deep source rocks and shallow traps; and (5) thick bathyal mudstone cap rock developed in late stage has good preservation conditions. In general, the natural gas reservoirs are characterized by “multiple reservoirs independently existing and vertically overlapped”, and controlled by structural-lithologic traps. There are still more prospects in the Central Canyon, suggesting huge exploration potential.

Oil and gas exploration operations in Huanghekou Sag, Bohai Bay Basin have been mainly conducted in the structural ridge in the central and steep slope zone in the north for a long time. The slope zone is not well explored or understood, and hydrocarbon accumulation and enrichment there is not clear. Based on the characteristics and differences in terms of four aspects of the slope zone including internal fault-slope combination, sedimentary characteristics, slope gradient and tectonic evolution, the southern slope zone of Huanghekou west subsag and the southern slope zone of Huanghekou central subsag can be classified as two categories: synsedimentary micro-oblique ramp type slope zone and synsedimentary stable fault terrace type slope zone. Different structural styles are developed in different types of slope zones which have significant differential controlling effect on the distribution of sedimentary system and hydrocarbon transport, accumulation and enrichment. Synsedimentary micro-oblique ramp type slope zone is of the “horizontal and stepwise” hydrocarbon transport model, where Neogene Minghuazhen formation is the dominant hydrocarbon enrichment series. During stepwise hydrocarbon transport, hydrocarbon accumulation series was lifted step by step and hydrocarbon accumulation horizon is shallower in higher positions of slope zone. Synsedimentary stable fault terrace type slope zone is of the “vertical through-going” hydrocarbon transport model, where hydrocarbon accumulation is close to the source, with high hydrocarbon abundance and deep-shallow composite hydrocarbon accumulation characteristics. On the plane, the bent pressurized section in principal faults of slope zone is hydrocarbon enrichment zone. These understandings provide a basis for further exploration.

The processes of hydrocarbon generation, preservation and accumulation are influenced by geothermal fields of the sedimentary basins, thus hydrocarbon accumulation characteristics of low geothermal basins are different from high geothermal basins. In this paper, the geothermal discrepancies of sedimentary basins and the exploration practices of low geothermal basins in China were reviewed. Taking the Xijiang sag in the shallow water area of northern South China Sea as an example, the hydrocarbon accumulation characteristics of low geothermal rifted basins were discussed. Intensive tectonic activities during the rifting stage facilitated the formation of under-compensation lake basins, providing conditions for the development of high-quality source rocks. The low geothermal gradients could delay the thermal evolution process of the source rocks, thus postponing the periods of hydrocarbon generation and expulsion while extending the duration of oil window to facilitate more charge of crude oil. The deep sandstone reservoirs were more capable of anti-pressolution in low geothermal background, effectively protecting physical properties. In low geothermal rifts, the supercharging effect was not apparently sensitive to temperatures, leading to weak dynamics for hydrocarbon accumulation, in which circumstance most of hydrocarbons preferentially accumulated in near-source traps. Considering the exploration situation and hydrocarbon accumulation features of low geothermal basins, it is suggested that the petroleum exploration in such basins should focus on the hydrocarbon-rich sags, especially deep layers near the source rocks. Furthermore, efforts should be made to ascertain high-quality reservoir belts below the stably-distributed regional mudstone cap rocks. After elaborate evaluation of prospects, wells should be accurately deployed and drilled for significant exploration achievements.

The Beibu-gulf Basin in the northern continental margin of South China Sea is dominated by extensional rifts, which facilitated the formation of depression-rift basin with typical double-layer (rift-depression) structure and controlled the sedimentary sequence of “marine sediment after continental sediment” in this basin. The Cenozoic reservoirs in this basin were generated, distributed, enriched and contiguously accumulated in large scales, and present a complex framework with diverse horizontal and spatial distributions. In this paper, the structural, sedimentary and hydrocarbon accumulation conditions in the Beibu-gulf Basin were systematically analyzed, and the exploration and development practices were also reviewed. It is concluded that the existence of large-medium reservoirs is controlled by the hydrocarbon-rich sags, and the distribution of such reservoirs is controlled by the spatial matching between large sedimentary facies belts and favorable tectonic units in the hydrocarbon-rich sags. The tectonic-sedimentary evolution process of “depression after rifting” and “marine sediment after continental sediment” is essential for the formation of large-medium reservoirs.

The northwestern margin of Beibu-Gulf Basin, a significant area for hydrocarbon generation and accumulation, is characterized by main chasmic stage of Paleogene and deposition dominated by terrigenous clastic rocks. With vertical grain-size and color variation showing coarse-fine-coarse and red-dark-motley respectively, the sedimentary environment gradually transmits from continental alluvial, lacustrine, fluvial to delta environment, forming a whole sedimentary cycle. The sedimentary history was analyzed and Eocene Liushagang Formation depositional pattern is established. Moreover, the paper preliminarily considers that the dynamic principle determining sedimentary system formation and development is jointly affected by tectonic power, filling power and climatic power. From the Palaeocene to Oligocene, the tectonic power was continually decreasing and the climatic power kept stable; while adjusted by the climatic power, the deposition rates of the Weixinan depression and the Haizhong depression were more than 100m/Ma during the deposition of both 2th member of the Eocene Liushagang Formation and the Oligocene Weizhou Formation, creating the most two favorable sets of the Paleogene reservoirs.

The Zengmu Basin is a large gas field with trillion-m3 scale gas reserves. Its sedimentary systems have not been systematically studied. There is a controversy about whether the basin was deposited in a continental environment or a marine environment during the early stage. In order to clarify the characteristics of sedimentary systems and to provide the reliable basis for the future oil and gas exploration in the Zengmu Basin, the previous research results were reviewed. Then, the Cenozoic sedimentary systems were analyzed using outcrops, wells, seismic and sedimentary facies data, as well as the regional tectonic evolution, material source characteristics and lithofacies paleogeographic background. As a result, 7 types of Cenozoic sedimentary facies were identified, including fluvial, delta, coastal plain, carbonate platform (organic reef), littoral, neritic and bathyal facies. It was the marine environment all over the basin during the Cenozoic. Large delta was constantly developed in the south of the basin, and the carbonate platform (organic reef) and bathyal facies deposits were mainly developed during the depression stage (the Middle Miocene-Quaternary), but seldom developed during the faulted depression stage (the Oligocene-Early Miocene). The provenance of the Zengmu Basin changed as a result of the counterclockwise rotation of Borneo during the Middle Miocene. The sources mainly came from the southwest during the faulted depression stage and from the southeast during the depression stage.

With the progress of seismic exploration in deep water, one of the challenges for marine seismic data processing is how to suppress the surface related multiple of complex geological structures, such as slope break zones and rough sea bottom. However, the traditional 3D SRME (surface related multiple elimination) is obviously deficient in data reconstruction. In this paper, the 3D GSRME (generalized surface related multiple elimination) was developed to eliminate surface related multiple. Based on contribution gather and sparse inversion methods, the 3D GSRME can construct the whole surface related multiple data by using the focal transform and 3D curvelet transform. Finally, multiple waves are suppressed by adaptive matching subtraction. The application of 3D GSRME in actual data shows that this method can effectively eliminate the surface related multiple of complex geologic structures with steep slope angle so as to improve the imaging quality.

As the discovered offshore oil and gas fields are successively put into development and production, the requirements for seismic data quality are increasing. In this paper, based on conventional wide-azimuth observation system, and considering the actual geological conditions of the Xihu sag in the East China Sea and the situations of existing equipment, a new multi-vessel wide-azimuth observation system was designed to meet geological requirements by changing the voyage frequency and vessel deployment. In practical seismic acquisition, against the platform influence, geometry-variable design was also applied as a supplement to the survey lines that cannot be implemented by original observation system. Finally, an optimized design of multi-vessel wide-azimuth observation system was developed for seimsic acquisition in offshore production zones.

Mitigation of reservoir damage during water flooding is critical for efficient development of offshore oil fields. In this paper, analysis of reservoir sensitivity and SEM analysis of suspended solids and scale samples were conducted for the water flooding process in a block, the Bohai Oilfield, during which some problems like higher injection pressure and insufficient injection exist. The experiment results indicate that the reservoir is susceptible to moderate-stronger water-sensitive damage, or plugging due to swelling and dispersion of smectite and illite/montmorillonite interlayer to various degrees. The content of suspended solids in injected water is up to 16.7 mg/L, which is much higher than the standard requirement. The size of particles in injected water are chiefly about 30 μm, which may easily cause plugging in the reservoir. After water flushing, a large amount of carbonate particles scatter in the pores and throats of the reservoir, decreasing the reservoir permeability. On the basis of the defined reservoir damage mechanism, some protective measures (e.g. optimization of doping way and acidification) were put forward. In practical operations, injection pressure at the wellhead of water flooding well dropped by 7 MPa, and daily water-injection volume increased by 6 times, revealing the obvious unplugging effect by acidification.