In recent two years, PetroChina Company Limited (hereinafter referred to as PetroChina for short) has made 12 important exploration progresses in domestic oil and gas exploration in clastic rocks, marine carbonate rocks, foreland thrust belts, lacustrine carbonate rocks, mature fields and unconventional oil and gas fields. Based on comprehensive analysis of PetroChina own oil and gas resources, remaining oil and gas resources and their exploration degree, and the influence of geological understanding and exploration technology on oil and gas exploration, this paper concludes that the prospect of domestic oil and gas exploration is still promising, but also faces many challenges, such as complex underground conditions, deeper burial depths and poorer quality of remaining resources. In the future, on the assumption of long-term low oil prices, PetroChina will adhere to the strategies of resources and innovation, follow the principle of efficient exploration, focus on "four types of exploration" and strengthen "three fields of innovation", so as to make new discoveries and breakthroughs in oil and gas exploration for sustainable and steady development.

Since the oil price significantly reduced, China Petroleum and Chemical Corporation (hereinafter referred to as Sinopec for short) has been active to update oil and gas exploration and development strategy which simply set efficient exploration as a long-term objective, and focused on large-scale targets and developed key and first projects based on large basins; and specifically, continuously made oil-gas breakthrough and large-scale reserve increase in marine carbonate reservoirs, eastern continental faulted basins, middle and western clastic rocks and shale oil-gas. This active strategy has delivered a new profile of sustainable oil reserve and production, and rapidly increasing natural gas reserve and production. Analysis of Sinopec's remaining oil-gas resources indicates that oil exploration is in early and intermediate stages and gas exploration is in early stage, and both of which has the resource basis for sustainable development. At present, influenced by multiple adverse factors such as sluggish oil price and poor resource endowment, exploration and development strategy should be further updated by focusing on eastern continental faulted basins, marine carbonate reservoirs, middle and western clastic rocks, shale oil and gas and sea areas with advancing development of theory, technology, techniques and management to promote precise exploration in old fields, make breakthrough in new fields and provide sound resource basis for sustainable development.

In recent years, China National Offshore Oil Corporation (hereinafter referred to as CNOOC for short) has achieved fruitful oil and gas exploration results. Domestically, it sets up new record of oil and gas exploration discovery and its proved reserves has increased stably. It has acquired a series of major hydrocarbon exploration breakthroughs in the active fault belts in the Bohai Sea, the high temperature and high pressure areas in western South China Sea and the deep water areas of northern South China Sea. Abroad, its recoverable reserves of overseas exploration equity increases stably, and four large-scale reserve zones are formed, e.g. Sahara of North Africa and both sides of South Atlantic. As the progress of integrated exploration, development and production of unconventional oil and gas is sped up, its proved reserves and production increase fast. As for the domestic exploration in the near future, CNOOC will advance deep zone exploration in the Bohai Sea, proceed oil and gas exploration in the central south of the South China Sea in good time and make more efforts to research heavy oil reservoirs and low porosity-low permeability oil and gas reservoirs. And its overseas exploration will still follow the principle of operative exploration and stress strategic selection of potential areas so as to discover more and better quality reserves.

Shanxi Yanchang Petroleum (Group) Co., Ltd (hereinafter referred to as Yanchang Petroleum Group for short) has promoted theoretical and technical research of exploration and development of oil, natural gas and shale gas, in Ordos Basin. The coordinated development of various resources has achieved remarkable results. In the aspect of petroleum exploration, the theory of "alternate accumulation of oil" was put forward. Through quantitative simulation of oil transportation and aggregation, exploration breakthrough of Lower Yanchang Formation had been achieved, and proven geological reserves of 2.75×10⁸t had been found. In terms of oil development, by revealing dual mechanism of seepage-displacement of fractured ultra-low permeability reservoir, development technology of "modest and mild" water injection was established. The natural decline rate of oil fields decreased from 18.22% to 12.6%. Effective development of low-grade oil fields had been achieved. In natural gas exploration and development, by putting forward a new theory of " sand body distribution was controlled by the shoreline frequent migration in shallow water environment" and "the natural gas accumulation was controlled by the mature hydrocarbon source kitchen migration " in the Upper Paleozoic, the Yan'an gas field has been discovered and proved reserves of 6650×10⁸m³ in the southeastern Ordos Basin. In terms of shale gas, the geological cognition of lacustrine basin possessed shale gas accumulation conditions was proposed. Based on supporting technology research, breakthroughs of continental shale gas were achieved. In the future, Yanchang Petroleum Group would continue to adhere to the strategy of "stabilizing oil, boosting gas and intensifying unconventional resources". By keeping pushing forward the exploration and development mode of "integrated geological engineering", to ensure the sustainable development of enterprises.

Analysis and statistics of the world's oil and gas discoveries is helpful to understand upstream exploration progress and predict future exploration fields. By means of statistical analysis and petroleum exploration theory, 9010 oil and gas discoveries from 1996 to 2016 were analyzed, focused on the discoveries' reserve scale, types and distribution, basin types and operating companies in recent five years. The statistics show that the total recoverable reserve discovered from 1996 to 2016 is 6722 x108 barrels of oil equivalent, and 997 oil and gas discoveries have recoverable reserves of more than 100 million barrels each, accounting for 85% of the total recoverable reserves. Since 2014, the number and scale of new oil and gas discoveries have fallen dramatically, but natural gas discoveries are increasing. Africa boasts for the largest recoverable reserves form 2012-2016 in the world, followed by Latin America, central Asia-Russia, Southeast Asia, North America, Middle East and Europe. Oil and gas discoveries are mainly distributed in passive continental margin basins, rift basins and foreland basins, of which 70% were found in foreland basins by the main international oil companies and national oil companies. Our analysis indicates that newly discovered oil & gas resources become less and less in land, and more and more transfer to deep water and ultra-deep water environment; exploration in frontiers, new types, new depth and new sequence in matured basin is ongoing; and natural gas is becoming more and more important. Since the offshore oil & gas exploration will be an important development trend, Chinese oil companies should strengthen upstream exploration investment and cooperation with international companies to acquire appreciable deep-water assets, pay more attention to offshore exploration target assessment and selection, and advance technical reserve and innovation.

Data collection and analysis, investigation to typical oil and gas fields, and the exploration progress and trends of neighboring countries in recent five years have been carried out using statistical methods and based on the geological background in the South China Sea. The results show that:(1) The newly discovered oil and gas reservoirs are mainly distributed in the Pearl River Mouth Basin, the Qiongdongnan Basin, the Yinggehai Basin in the northern South China sea and the Meigong Basin, Wan'an Basin, Zengmu Basin and Brunei-Sabah Basin in the central-southern South China Sea; (2) Structural reservoirs in delta and fan-delta sandstones account for the most, but lithologic reservoirs in deep-water sedimentary bodies contribute to the largest cumulative reserves; and (3) Oil and gas are mainly enriched in the Middle Miocene reservoirs, followed by the Upper Paleocene and Upper Miocene reservoirs. Specifically, the results show that:(1) Deepwater area has become the main exploration battlefield and reserve succeeding area; lithologic reservoirs accumulated in deep-water sedimentary bodies and those in organic reefs have become the main contributor to new reserves in the South China Sea. (2) The Miocene organic reefs directly on source or source side are the most realistic exploration target in the central-southern basins; the Pre-Paleogene buried hill reservoir is an important potential exploration target in the northwestern South China Sea; and the lithologic bodies formed by differential compaction and the updip pinchout bodies are main traps for reservoirs associated with deep-water sediments in the continental slope.

Based on previous studies, the latest processing and interpretation results of seismic, gravity, magnetic and electdrical data and the latest collection of foreign geochemical data, the tectonic evolution, sedimentation, geological conditions and distribution characteristics of reservoirs in major sedimentary basins in Nansha sea areas were summarized. The study indicates that the Cenozoic Nansha sea areas generally experienced three tectonic evolution stages and two basining periods during which four fault systems and three types of basin groups were devloped with obvious basin-controlling fault zones; three ancient river systems in South China Sea controlled the distribution of the Cenozoic sedimentary systems in Nansha sea areas where marine, lacustrine, delta, swamp, carbonate platform and organic reef facies were developed, and as the ancient South China Sea disappeared and the modern South China Sea expanded, sedimentary evolution took place from continental to transitional to marine facies in different types of basins; three sets of hydrocarbon source rocks were identifed, Which include lacustrine and transitional mudstones, carbonaceous shale and coal seams; three sets of reservoirs were identified wich the main reservoir of Miocene sandstone and reef limestone; three sets of source-reservoir-caprock systems were found, mainly shore-shallow marine carbonate rock, transitional clastic rock and marine clastic rock. Large and medium oil & gas fields in Nansha sea areas are characterized by rich oil and gas in outer sandstone reservoirs and rich gas in inner carbonate reservoirs, and the oil and gas distribution shows vertical zoning and horizontal segmenting. Nansha sea areas possess huge potentials of oil and gas resources-the accumulated initially in place resources of oil and gas are 201×10⁸t and 32.4×10¹²m³, respectively.

The Chang 6₁ reservoir in the Jiyuan area, the Ordos Basin, is of typically low porosity and low permeability. Its complex pore structure, high irreducible saturation and formation water salinity make the oil reservoir with low contrast and bring great difficulties to logging identification. The genetic mechanism of the low contrast Chang 6₁ oil reservoir was analyzed in terms of rock lithology, reservoir properties and formation water properties, and two types of genetic mechanisms are:First, the high content and complex distribution of the clay in the interstitial material in the Chang 6₁ reservoir resulting in complex pore structure and high irreducible saturation which leads to additional conductivity; second, high water salinity in the Chang 6₁ reservoir. These two factors contributed to the low contrast on logging data, and resulting in the low contrast oil reservoir. Then based on the identified genetic mechanisms, the normal resistivity distribution of apparent formation water and the Fisher discrimination and analysis method were used to identify fluid properties and solve the problem in logging identification. And finally a water productivity prediction model is established based on the abrupt change in the linear neuron Sigmoid function to calculate water productivity and conduct quantitative classification and evaluation of the Chang 6₁ reservoir, and the detailed classification and evaluation standards are proposed for fluid properties. Application to 64 wells in the H57 well area in the study area indicates that the calculated and evaluated water productivity results from using the standards proposed in this paper are correlated to the initial water productivity by a factor of 0.917. This proves that the model precision and classified results meet the need of practical exploration and production.

Less systematic studies have been focused on the evolution and distribution of the sedimentary facies in the Sangonghe Formation in the southwest Baijiahai Uplift, the Junggar Basin. To support the oil and gas exploration and development, and understand the evolution of the sedimentary environment, the evolution rule in the Sangonghe Formation was investigated using outcrops, cores, logging and seismic data and based on the analysis of regional sedimentary background and the 3 mid-term datum cycles divided by high resolution sequence stratigraphy. The study suggests two sedimentary facies in the Sangonghe Formation--braided river delta and lacustrine facies; braid-river delta front subfacies are developed in MSC1-MSC2, and shore-shallow lacustrine sediments are mainly in MSC3; and influenced by the fluctuation of the lake level, there are differences in the thickness and distribution of the sandstone reservoirs. The study states that favorable reservoirs of class I and class Ⅱ are distributed in the braided river delta front in MSC1 and MSC2, and favorable reservoirs of class Ⅲ are in the shallow lake sandbar in MSC3.The study on the evolution of the sedimentary environment of the Sangonghe Formation not only enriches the exploration and development achievements of the Jurassic Baijiahai uplift, but also determinesand expands future field of oil and gas explorationin the Junggar Basin.

The third Member of Shahejie Formation (Es₃) in Puwei area in Dongpu Depression is one of the most important exploration targets, where both conventional and tight reservoirs are developed, but how to effectively predict porosity and analyze the evolution of the tight reservoirs are very challenging. Based on the characteristics of the sediments, petrology, reservoir properties and diagenesis, a porosity evolution model is established in time and depth domains with the constraints of modern porosity and burial history. The model is composed of four parts in terms of diagenesis, i.e. shallow compaction, compaction and cementation, dissolution and post-dissolution compaction and cementation, Which can simulate the porosity evloution during the geological period. The model has been proved to be of high precision and consistent with actual geological conditions. Therefore, it can be used to simulate the porostity evolution in the reservoir with the same sedimentary facies, and will provide guidance for oil and gas exploration in Puwei area.

The adsorption characteristics of shale gas have great influence on its reserves prediction, seepage mechanism analysis and productivity evaluation. The adsorption model is one of the principal methods to describe shale gas adsorption characteristics. So far, scholars at home and abroad have different opinions on the selection of adsorption models for shale gas. It is revealed that the applicability of adsorption models to shale gas is not understood clearly. To solve this problem, the applicability of four common adsorption models (i.e., Langmuir model, BET model, D-R model and half pore width model) to shale gas was studied thoroughly by fitting the adsorption models to the experimental data of methane isothermal adsorption on shale. And the following research results were obtained. First, neither Langmuir model nor BET model considers the adsorption energy level difference of active sites on the surface of shale pores, so their fitting errors are larger when the pressure is lower. Second, D-R model considers the ultrahigh adsorption in the interval of low pressure, so it is only applicable to the shale possessing more micro pores. Third, half pore width model takes into consideration the adsorption potential theory and the pore diameter distribution of shale comprehensively, and it is well fitted with the experimental data. And fourth, the fitting errors between four common adsorption models and experiment data of methane isothermal adsorption on shale in Fuxian area of the Ordos basin are ranked from the low to the high, i.e., half pore width model, Langmuir model, BET model and D-R model.

The empirical production decline analysis methods inside and outside China were investigated extensively for further understanding these methods and promoting their application in development of shale gas reservoirs. The Arps, power exponential, stretched exponential and Duong decline analysis methods were elaborated in terms of source, decline model, typical curve, application condition, limitation and improved method, and finally the key issues related to empirical production decline analysis methods in future study were proposed. The study results indicate that all the empirical production decline analysis methods are capable of forecasting both production and EUR, but they can only analyze the production data under constant or approximately constant bottom-hole flowing pressure, and require continuous and stable production data without long-term shut-in. The difference in these empirical production decline analysis methods lies in different flow regime. Arps decline model and modified hyperbolic decline model are only available to boundary-dominated flow regime. Stretched exponential decline model and Duong decline model are useful for linear flow regime. Modified Duong decline model, modified power exponential decline model and power exponential decline model are applicable for both linear and boundary-dominated flow regimes. It is critical to identify and classify the flow regime in a shale gas well for application of empirical production decline analysis. In shale gas wells in China, it is hard to keep constant or approximately constant bottom-hole flowing pressure during production, then how to correct such production data to the data under constant or approximately constant bottom-hole flowing pressure is the emphasis in future study on empirical production decline analysis methods.

Nanosecond laser induced breakdown spectroscopy (ns-LIBS) is a new elemental analysis technology, and it has been gradually applied in various industries. It is superior to traditional elemental analysis technologies and it is advantageous with high analysis speed, simple sample pretreatment and availability for all elemental analyse. In this paper, the independently developed ns-LIBS system was used to analyze the elemental compositions and contents of cutting samples taken from oil wells qualitatively and quantitatively. First, experimental parameters were determined, especially the relative distance between the focus of convergent lens and the surface of sample. Second, the content difference of indicator elements was verified by comparing the LIBS of cutting samples with different lithologies. Third, the best analysis lines of external standard method for Mg, Na, Si, etc were defined. And fourth, the lithology was identified by means of analytic hierarchy process and chart analysis method jointly. It is indicated that ns-LIBS is applicable to analyze elemental compositions and contents of cutting samples.